The "Aerowherry" a lightweight rowing boat (was going to be a canoe........)

Having rifled through my stock of hoarded "make come in handy one day" stuff, I've come up with the following list of unlikely looking boat building materials:

 - 25 yds of lightweight Polyfiber Stits aircraft covering Dacron, together with a few rolls of assorted finishing tapes, all left over from building aircraft in the past.

 - two 5m lengths of 1" x 18g 6082-T6 aluminium alloy tube

 - three lengths of 3/4" x 16g 6082-T6 alloy tube

 - five and a bit 5m lengths of 1/2" x 16g 6061-T6alloy tube

 - assorted short lengths of 3/8" x 18g 6063-T6 alloy tube

 - a large assortment of Avdel aircraft rivets, of various lengths and diameters

 - odds and ends of thin aluminium alloy sheet, mostly 6082-T6 or 6061-T6

I've long held the desire to build a skin on frame canoe, along the lines of Platt Monfort's Airolite designs (http://gaboats.com/).  The problem is that I'm not really much of a woodworker and much prefer working in metal, or with composites.  Having found the above pile of bits, I've decided to have a go at using them to build a SOF canoe with a slightly unusual twist; it'll have an alloy tube frame, rather like a light aircraft.

The first big issue was working out whether or not the aluminium I happened to have to hand would be stiff enough to do the job, whilst still keeping the frame as light as possible.  Unlike the aircraft world, where all of the loads and stresses on just about every part are relatively easy to work out, I couldn't find a single reference that indicated how the scantlings of a SOF canoe were derived.  I wanted to have the minimum number of frames possible (to reduce weight and simplify the build), yet needed to ensure that the stringers were adequately supported and wouldn't flex too much between the frames.  Because I'm working around what I have to hand, the stringers will have to be 1/2" x 16g tube.

I've ended up reverse engineering the sizes and frame spacing needed from wooden designs, using beam bending calculations and the relative Youngs Modulus between wood and aluminium alloy to come up with frame spacings that give the same deflection per unit distributed load for the aluminium tube stringers as for wooden ones of greater section.  It turns out that the thin aluminium tube construction is several times stronger than wood for the same deflection, even though the alloy tubes are much thinner than wood stringers would be.

The next problem was working out how to make the frames and join them to the stringers.  I toyed with the idea of lashings, small alloy plates riveted on and bonding with something suitably high tech, like Hysol 3423, but decided to try and experiment with some long (3/4") 5/32" Avdel rivets.  These are just big, strong pop rivets made from stainless steel.  The good news was that I could drill right through a 1/2" tube into one side of another tube and set the rivet without crushing the tube.  This makes joining the rubs to the frames very easy, each joint will take only a minute or so and will be at full strength immediately - no waiting for glues to set.  These rivets are ridiculously strong, too, so a single rivet at each joint will exceed the strength of the stringer.  

So, I have the beginning of a plan.  The keel will be of 3/4" tube, the gunwales will be either 3/4" or 1" tube (probably the latter), the stringers will be 1/2" tube, as will the five frames.  There will be a couple of alloy tube thwarts, just to hold the gunwales apart and stiffen them up and the bow and stern will be fashioned from thin alloy plate riveted to the keel, gunwales and stringers.  The breasthooks will similarly just be flat alloy plates riveted to the top of the gunwales.  The central three frames will have folded alloy sheet floors riveted to them, between the lower two stringers.  These will stiffen the frames up and provide a means of fixing some slatted floorboards (the only bits of wood in the boat).  I'll also probably add false floors under the floorboards, in between the frames, to further spread the load of my kneeling on them, maybe with a couple of small blocks that rest on the keel.The length will be determined by the 5m length of the gunwales, so around 4.9m overall.  Beam will be about 32" and depth amidships will be around 15".

I have a target weight of 10kg (about 22lbs in old money) and so far I'm up to about 8kg in aluminium and rivets.  The cloth, when painted, will be around 90 to 100g per m², so with luck the finished boat *might* come in under 10kg.  I will bring it along to Beale Park for the cordless canoe challenge and might enter it into the Amateur Boat Building competition, if only to see the expressions on the faces of the wooden boat purists.................

Photos will follow soon, as soon as I've finished making the frame jigs (dead simple really, just a board with wooden pegs on, around which the frames will be bent).  I believe that the build will be pretty quick, I will almost certainly have to spend more time making a strongback and frame jigs than I will building the canoe itself.

Although I'll be using leftover materials from other projects (aircraft) this wouldn't be a particularly expensive way to build a canoe or small boat even if the materials had to be purchased.  I would estimate that there's around £70 worth of alloy tubing, plus maybe another £50 in covering material, plus paint etc.  Only time will tell as to whether or not it will work out OK, though.

I have to give credit to Dave Gentry for his SOF boat designs, as they were the ones that made me realise that you don't need a structure as complex as the Airolite Geodesic boats to work well (see here: http://gentrycustomboats.com/ and particularly this lovely boat here: http://gentrycustomboats.com/RUTH%20page.html )

Jeremy

Jeremy,

Seeing your "BoM", you might be able to combine the  - very beautiful - Ruth design with the construction used by Tom Yost for his DiY folding kayak designs. (http://yostwerks.com/MainMenu.html)

Thanks for the link to Dave Gentry, he apparently stayed "under my radar" until now.

Regards,

jaap

I did look at the Yost kayak designs, but decided they were a bit too heavy, plus the HDPE sheet for the frames is pretty expensive to buy.

I really wanted to make a boat like Ruth, but to be honest thought it would be sacrilege to build such a beautiful shape using alloy tubes.  Ruth is just too long for stock 5m length tubes, too, so would need to be trimmed down by a couple of feet, which might spoil her lines.

There's a couple of threads on the Wooden Boat forum describing the build of Ruth, here: http://forum.woodenboat.com/showthread.php?99017-My-SOF-sliding-seat-rowing-wherry-project.&highlight and here: http://forum.woodenboat.com/showthread.php?99286-I-love-it-when-a-plan-comes-together-RUTH-launched!&highlight

Jeremy

From: "Jeremy [via UK HBBR Forum]" <[hidden email]>
Date: Fri, 4 Feb 2011 06:43:29 -0800 (PST)
To: Tim O'Connor <[hidden email]>
Subject: Unusual lightweight canoe

Having rifled through my stock of hoarded "make come in handy one day" stuff, I've come up with the following list of unlikely looking boat building materials:

 - 25 yds of lightweight Polyfiber Stits aircraft covering Dacron, together with a few rolls of assorted finishing tapes, all left over from building aircraft in the past.

 - two 5m lengths of 1" x 18g 6082-T6 aluminium alloy tube

 - three lengths of 3/4" x 16g 6082-T6 alloy tube

 - five and a bit 5m lengths of 1/2" x 16g 6061-T6alloy tube

 - assorted short lengths of 3/8" x 18g 6063-T6 alloy tube

 - a large assortment of Avdel aircraft rivets, of various lengths and diameters

 - odds and ends of thin aluminium alloy sheet, mostly 6082-T6 or 6061-T6

I've long held the desire to build a skin on frame canoe, along the lines of Platt Monfort's Airolite designs (http://gaboats.com/).  The problem is that I'm not really much of a woodworker and much prefer working in metal, or with composites.  Having found the above pile of bits, I've decided to have a go at using them to build a SOF canoe with a slightly unusual twist; it'll have an alloy tube frame, rather like a light aircraft.

The first big issue was working out whether or not the aluminium I happened to have to hand would be stiff enough to do the job, whilst still keeping the frame as light as possible.  Unlike the aircraft world, where all of the loads and stresses on just about every part are relatively easy to work out, I couldn't find a single reference that indicated how the scantlings of a SOF canoe were derived.  I wanted to have the minimum number of frames possible (to reduce weight and simplify the build), yet needed to ensure that the stringers were adequately supported and wouldn't flex too much between the frames.  Because I'm working around what I have to hand, the stringers will have to be 1/2" x 16g tube.

I've ended up reverse engineering the sizes and frame spacing needed from wooden designs, using beam bending calculations and the relative Youngs Modulus between wood and aluminium alloy to come up with frame spacings that give the same deflection per unit distributed load for the aluminium tube stringers as for wooden ones of greater section.  It turns out that the thin aluminium tube construction is several times stronger than wood for the same deflection, even though the alloy tubes are much thinner than wood stringers would be.

The next problem was working out how to make the frames and join them to the stringers.  I toyed with the idea of lashings, small alloy plates riveted on and bonding with something suitably high tech, like Hysol 3423, but decided to try and experiment with some long (3/4") 5/32" Avdel rivets.  These are just big, strong pop rivets made from stainless steel.  The good news was that I could drill right through a 1/2" tube into one side of another tube and set the rivet without crushing the tube.  This makes joining the rubs to the frames very easy, each joint will take only a minute or so and will be at full strength immediately - no waiting for glues to set.  These rivets are ridiculously strong, too, so a single rivet at each joint will exceed the strength of the stringer.  

So, I have the beginning of a plan.  The keel will be of 3/4" tube, the gunwales will be either 3/4" or 1" tube (probably the latter), the stringers will be 1/2" tube, as will the five frames.  There will be a couple of alloy tube thwarts, just to hold the gunwales apart and stiffen them up and the bow and stern will be fashioned from thin alloy plate riveted to the keel, gunwales and stringers.  The breasthooks will similarly just be flat alloy plates riveted to the top of the gunwales.  The central three frames will have folded alloy sheet floors riveted to them, between the lower two stringers.  These will stiffen the frames up and provide a means of fixing some slatted floorboards (the only bits of wood in the boat).  I'll also probably add false floors under the floorboards, in between the frames, to further spread the load of my kneeling on them, maybe with a couple of small blocks that rest on the keel.The length will be determined by the 5m length of the gunwales, so around 4.9m overall.  Beam will be about 32" and depth amidships will be around 15".

I have a target weight of 10kg (about 22lbs in old money) and so far I'm up to about 8kg in aluminium and rivets.  The cloth, when painted, will be around 90 to 100g per m², so with luck the finished boat *might* come in under 10kg.  I will bring it along to Beale Park for the cordless canoe challenge and might enter it into the Amateur Boat Building competition, if only to see the expressions on the faces of the wooden boat purists.................

Photos will follow soon, as soon as I've finished making the frame jigs (dead simple really, just a board with wooden pegs on, around which the frames will be bent).  I believe that the build will be pretty quick, I will almost certainly have to spend more time making a strongback and frame jigs than I will building the canoe itself.

Although I'll be using leftover materials from other projects (aircraft) this wouldn't be a particularly expensive way to build a canoe or small boat even if the materials had to be purchased.  I would estimate that there's around ￡70 worth of alloy tubing, plus maybe another ￡50 in covering material, plus paint etc.  Only time will tell as to whether or not it will work out OK, though.

I have to give credit to Dave Gentry for his SOF boat designs, as they were the ones that made me realise that you don't need a structure as complex as the Airolite Geodesic boats to work well (see here: http://gentrycustomboats.com/ and particularly this lovely boat here: http://gentrycustomboats.com/RUTH%20page.html )

Jeremy

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-----Original Message-----
From: Timmo [via UK HBBR Forum] [mailto:[hidden email]]
Sent: 04 February 2011 17:37
To: tony waller
Subject: Re: Unusual lightweight canoe

Personally I can't wait to see it. Sounds a lot of fun and could be an approach to boat building that will appeal to many not as interested in wood. The example could help broaden the boat building community. I'm all in favour. 

I particularly like the fact there seem to be no smelly, dodgy chemical processes and no varnishing! Though I'm not sure how the Polyfiber Stits aircraft covering Dacron needs finishing. 

Tim.

From: "Jeremy [via UK HBBR Forum]" <[hidden email]>
Date: Fri, 4 Feb 2011 06:43:29 -0800 (PST)
To: Tim O'Connor <[hidden email]>
Subject: Unusual lightweight canoe

Having rifled through my stock of hoarded "make come in handy one day" stuff, I've come up with the following list of unlikely looking boat building materials:

 - 25 yds of lightweight Polyfiber Stits aircraft covering Dacron, together with a few rolls of assorted finishing tapes, all left over from building aircraft in the past.

 - two 5m lengths of 1" x 18g 6082-T6 aluminium alloy tube

 - three lengths of 3/4" x 16g 6082-T6 alloy tube

 - five and a bit 5m lengths of 1/2" x 16g 6061-T6alloy tube

 - assorted short lengths of 3/8" x 18g 6063-T6 alloy tube

 - a large assortment of Avdel aircraft rivets, of various lengths and diameters

 - odds and ends of thin aluminium alloy sheet, mostly 6082-T6 or 6061-T6

I've long held the desire to build a skin on frame canoe, along the lines of Platt Monfort's Airolite designs (http://gaboats.com/).  The problem is that I'm not really much of a woodworker and much prefer working in metal, or with composites.  Having found the above pile of bits, I've decided to have a go at using them to build a SOF canoe with a slightly unusual twist; it'll have an alloy tube frame, rather like a light aircraft.

The first big issue was working out whether or not the aluminium I happened to have to hand would be stiff enough to do the job, whilst still keeping the frame as light as possible.  Unlike the aircraft world, where all of the loads and stresses on just about every part are relatively easy to work out, I couldn't find a single reference that indicated how the scantlings of a SOF canoe were derived.  I wanted to have the minimum number of frames possible (to reduce weight and simplify the build), yet needed to ensure that the stringers were adequately supported and wouldn't flex too much between the frames.  Because I'm working around what I have to hand, the stringers will have to be 1/2" x 16g tube.

I've ended up reverse engineering the sizes and frame spacing needed from wooden designs, using beam bending calculations and the relative Youngs Modulus between wood and aluminium alloy to come up with frame spacings that give the same deflection per unit distributed load for the aluminium tube stringers as for wooden ones of greater section.  It turns out that the thin aluminium tube construction is several times stronger than wood for the same deflection, even though the alloy tubes are much thinner than wood stringers would be.

The next problem was working out how to make the frames and join them to the stringers.  I toyed with the idea of lashings, small alloy plates riveted on and bonding with something suitably high tech, like Hysol 3423, but decided to try and experiment with some long (3/4") 5/32" Avdel rivets.  These are just big, strong pop rivets made from stainless steel.  The good news was that I could drill right through a 1/2" tube into one side of another tube and set the rivet without crushing the tube.  This makes joining the rubs to the frames very easy, each joint will take only a minute or so and will be at full strength immediately - no waiting for glues to set.  These rivets are ridiculously strong, too, so a single rivet at each joint will exceed the strength of the stringer.  

So, I have the beginning of a plan.  The keel will be of 3/4" tube, the gunwales will be either 3/4" or 1" tube (probably the latter), the stringers will be 1/2" tube, as will the five frames.  There will be a couple of alloy tube thwarts, just to hold the gunwales apart and stiffen them up and the bow and stern will be fashioned from thin alloy plate riveted to the keel, gunwales and stringers.  The breasthooks will similarly just be flat alloy plates riveted to the top of the gunwales.  The central three frames will have folded alloy sheet floors riveted to them, between the lower two stringers.  These will stiffen the frames up and provide a means of fixing some slatted floorboards (the only bits of wood in the boat).  I'll also probably add false floors under the floorboards, in between the frames, to further spread the load of my kneeling on them, maybe with a couple of small blocks that rest on the keel.The length will be determined by the 5m length of the gunwales, so around 4.9m overall.  Beam will be about 32" and depth amidships will be around 15".

I have a target weight of 10kg (about 22lbs in old money) and so far I'm up to about 8kg in aluminium and rivets.  The cloth, when painted, will be around 90 to 100g per m², so with luck the finished boat *might* come in under 10kg.  I will bring it along to Beale Park for the cordless canoe challenge and might enter it into the Amateur Boat Building competition, if only to see the expressions on the faces of the wooden boat purists.................

Photos will follow soon, as soon as I've finished making the frame jigs (dead simple really, just a board with wooden pegs on, around which the frames will be bent).  I believe that the build will be pretty quick, I will almost certainly have to spend more time making a strongback and frame jigs than I will building the canoe itself.

Although I'll be using leftover materials from other projects (aircraft) this wouldn't be a particularly expensive way to build a canoe or small boat even if the materials had to be purchased.  I would estimate that there's around ￡70 worth of alloy tubing, plus maybe another ￡50 in covering material, plus paint etc.  Only time will tell as to whether or not it will work out OK, though.

I have to give credit to Dave Gentry for his SOF boat designs, as they were the ones that made me realise that you don't need a structure as complex as the Airolite Geodesic boats to work well (see here: http://gentrycustomboats.com/ and particularly this lovely boat here: http://gentrycustomboats.com/RUTH%20page.html )

Jeremy

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There is some smelly stuff involved, as the adhesive for the Polyfiber is thinned with acetone or MEK, as is the sealant that has to be applied to the fabric to make it airtight (or in this case, watertight) after it's been heat-shrunk to the frame.  The fabric also needs painting, or else it will degrade from UV exposure fairly quickly (inside a year if left outdoors).  I'm planning on being wholly unconventional and painting it silver, primarily because that's the colour of the best UV blocking paint but also because it may as well shout out that the construction is a bit unusual.

I've no idea whether or not boiled linseed oil would work as a sealant for the fabric.  It might well do.  My only concern would be UV resistance, although the other Dacron skinned boats around don't seem to worry about this (on aircraft we do, because the fabric strength decreases fairly quickly with exposure to the sun).

Most of these lightweight fabric skinned boats just seem to varnish the fabric, so maybe that's all that's needed if the boat is kept undercover much of the time.  On an aircraft we're aiming to get 20 years or so of life from the fabric, but perhaps that's impractical on a boat, where the inevitable knocks and bumps might lead to a need to re-cover it more frequently.

I managed to get the frame lines drawn up this afternoon, so should be able to make a start on the jig tomorrow.  This will probably be the longest part of the build, together with making a strongback.

Jeremy

Another skin on frame designer is Kudtz Craft. he has a thread running just now on WBF. His Sweetbriar design is a little shorter than Ruth.



http://forum.woodenboat.com/showthread.php?126117-Sweetbriar-*-Rowing-design

http://www.kudzucraft.com/designs/index.php

also Tom Yost's hybrids only use material and some alloy tubes.


http://foldingkayaks.org/phpBB/viewtopic.php?f=27&t=2998




   

Thanks Brian, I've been following the Sweetbriar thread on the WBF.  The only slight problem I think he may have is when covering the stern, those hollows will pull out flat as the fabric shrinks, which is why Dave Gentry went for the slightly unorthodox transom shape on Ruth.  Although using Dacron to cover a structure is surprisingly easy, there are some basic rules that have to be applied, one of which is limiting the use of concave surfaces.  I've covered a wing that had a slightly concave lower surface and it never really looks neat, as the fabric always wants to pull away from the bonded areas (the tension when it's heat shrunk is pretty high - the test that you have it tight enough is to bounce a coin on it, the coin should bounce back almost to the height it was dropped from).

The Tom Yost designs I looked at were all a bit on the heavy side for what I was looking for.  When I did the calcs on the tube sizes and frame spacing I was surprised how small a tube section, and how wide the frame spacing could be, to get the same deflection as a typical 1 1/2" x 1/2" western red cedar stringer as used on the Dave Gentry boats.  It turns out that 1/2" x 16g 6082-T6 alloy tube is almost identical in terms of deflection, and many times stronger in terms of failure load, than the cedar stringers.  The Tom Yost designs use much bigger and heavier tubes, but I think that maybe because of the need to have the frame come apart (in the case of the folders) and also because his designs are intended for hard use at sea, with a very heavy vinyl covering.

I've completed the frame templates and am currently puzzling over simple ways to join the tubes together at the stem and stern.  I don't much like the "flatten and drill" approach used on some designs, so I'm looking at trying to form some alloy sheet around MDF formers to make V shaped sections to which I can rivet the tubes.  I've made alloy fuel tanks this way and found it fairly quick and easy to hammer soft sheet to fairly complex curves.  It doesn't need to be a super smooth finish in this case because the fabric will be glued to it, with finishing tapes over the top for extra abrasion resistance.  It's another reason to build a canoe shaped boat, as one former will do both ends!

Jeremy

I agree about Sweetbriar's wine glass stern I prefer the simple stern on the earlier pink version on the thread. I have actually just posted that I feel the wine glass transom is a design affectation if no passenger is planned to be carried.

Ian Howick in New Zealand built a very light carbon tubed kayak and the engineering is first class. Well worth a look at his design for ideas.

 http://www.flickr.com/photos/50860028@N04/sets/72157624085895887/detail/


Ian is building a Goat Island Skiff just now and having an interesting time building with no metal anywhere. That is not even any rudder pintals etc.

Thanks again Brian.  The use of CF tube is interesting (but probably mighty expensive for something that I want to build from stuff I have around).

The rigorous approach to building without metal in your second link is more inspiring, as it has made me think about making this boat entirely wood-free.  It'll make things interesting, as I was planning on using wooden slats for the floor boards, so may now have to switch to using foam cored composite instead, but it might make the build just a bit more challenging.

Having now finished all the lines drawings, it does look pretty easy to switch the hull shape to something closer to a fast rowing boat, like the wherry.  I'm sort of wondering whether or not to switch the design to a rowing boat now.  It would be an interesting exercise in lightweight construction, as I think it would come out at less than half the weight of "Ruth", for a LOA of 5m.  If I go down this route it would mean extra work, as I'd need to make alloy outriggers and alloy oars, probably with composite sandwich blades.  It'd be the epitome of an anti-wood boat, though..................

Jeremy

Sounds like a really fun challenge to me.

If you do need any extra tubing we found this supplier was very well priced and delivered with no problems. A set of alloy tubes to make a Scow rig was far cheaper than buying good quality spar wood. All our local Scows have alloy rigs.

http://www.aluminiumwarehouse.co.uk/Aluminium-Round_Tube/c120_131/index.html

Brian

I've used them before, they are indeed reasonable value if you order enough alloy to cover the delivery charges and if they stock the particular alloy you need.  I usually use MG Metals in Chandlers Ford, just because they are local and have a free cutting service.  Their basic prices are slightly higher than the Aluminium Warehouse, but I can collect directly from them and they always provide a certificate of conformity for structural alloys.  The Aluminium Warehouse tend to carry a fair bit of the weaker, non-structural tubing, like 6063-T6 "architectural alloy" (no use for anything that's going to be load bearing, really) but not a lot of choice in the useful medium strength structural alloys, like 6082-T6 or 6061-T6.

It's worth checking that you're not accidentally using 6063-T6 tube for masts, as apart from being fairly weak (around 210N/mm² vs 310N/mm² for 6082-T6) it's more prone to stress corrosion and fatigue failure and is not recommended for any structural use.  It's a good alloy for things like window frames though (which is why it's often referred to as an "architectural alloy", as it anodises well and is reasonably resistant to corrosion.

Jeremy

Good evening Jeremy    
For reasons not immediately apparent but probably `Single Malt` fuelled  I have just recalled your comment in an earlier post    ".......and might enter it into the Amateur Boat Building competition, if only to see the expressions on the faces of the wooden boat purists................. "  
 As my memory never was much good and is now even worse (above reference to `Single Malt` refers) I have just had a look at Page 40 of Water Craft No 85 where Pete lists the entry conditions for this years events . 
 
  " In both categories, boats must be home-built in wood or plywood by one or more genuine amateurs..................."
 
Page 8   Cordless Canoe Challenge                ".................and they may be made of any material.........."   Looks like you could be in the market for a big old cordless strimmer.
 
Regards  John
 


 

---

Date: Fri, 4 Feb 2011 06:43:31 -0800
From: [hidden email]
To: [hidden email]
Subject: Unusual lightweight canoe

Having rifled through my stock of hoarded "make come in handy one day" stuff, I've come up with the following list of unlikely looking boat building materials:

 - 25 yds of lightweight Polyfiber Stits aircraft covering Dacron, together with a few rolls of assorted finishing tapes, all left over from building aircraft in the past.

 - two 5m lengths of 1" x 18g 6082-T6 aluminium alloy tube

 - three lengths of 3/4" x 16g 6082-T6 alloy tube

 - five and a bit 5m lengths of 1/2" x 16g 6061-T6alloy tube

 - assorted short lengths of 3/8" x 18g 6063-T6 alloy tube

 - a large assortment of Avdel aircraft rivets, of various lengths and diameters

 - odds and ends of thin aluminium alloy sheet, mostly 6082-T6 or 6061-T6

I've long held the desire to build a skin on frame canoe, along the lines of Platt Monfort's Airolite designs (http://gaboats.com/).  The problem is that I'm not really much of a woodworker and much prefer working in metal, or with composites.  Having found the above pile of bits, I've decided to have a go at using them to build a SOF canoe with a slightly unusual twist; it'll have an alloy tube frame, rather like a light aircraft.

The first big issue was working out whether or not the aluminium I happened to have to hand would be stiff enough to do the job, whilst still keeping the frame as light as possible.  Unlike the aircraft world, where all of the loads and stresses on just about every part are relatively easy to work out, I couldn't find a single reference that indicated how the scantlings of a SOF canoe were derived.  I wanted to have the minimum number of frames possible (to reduce weight and simplify the build), yet needed to ensure that the stringers were adequately supported and wouldn't flex too much between the frames.  Because I'm working around what I have to hand, the stringers will have to be 1/2" x 16g tube.

I've ended up reverse engineering the sizes and frame spacing needed from wooden designs, using beam bending calculations and the relative Youngs Modulus between wood and aluminium alloy to come up with frame spacings that give the same deflection per unit distributed load for the aluminium tube stringers as for wooden ones of greater section.  It turns out that the thin aluminium tube construction is several times stronger than wood for the same deflection, even though the alloy tubes are much thinner than wood stringers would be.

The next problem was working out how to make the frames and join them to the stringers.  I toyed with the idea of lashings, small alloy plates riveted on and bonding with something suitably high tech, like Hysol 3423, but decided to try and experiment with some long (3/4") 5/32" Avdel rivets.  These are just big, strong pop rivets made from stainless steel.  The good news was that I could drill right through a 1/2" tube into one side of another tube and set the rivet without crushing the tube.  This makes joining the rubs to the frames very easy, each joint will take only a minute or so and will be at full strength immediately - no waiting for glues to set.  These rivets are ridiculously strong, too, so a single rivet at each joint will exceed the strength of the stringer.  

So, I have the beginning of a plan.  The keel will be of 3/4" tube, the gunwales will be either 3/4" or 1" tube (probably the latter), the stringers will be 1/2" tube, as will the five frames.  There will be a couple of alloy tube thwarts, just to hold the gunwales apart and stiffen them up and the bow and stern will be fashioned from thin alloy plate riveted to the keel, gunwales and stringers.  The breasthooks will similarly just be flat alloy plates riveted to the top of the gunwales.  The central three frames will have folded alloy sheet floors riveted to them, between the lower two stringers.  These will stiffen the frames up and provide a means of fixing some slatted floorboards (the only bits of wood in the boat).  I'll also probably add false floors under the floorboards, in between the frames, to further spread the load of my kneeling on them, maybe with a couple of small blocks that rest on the keel.The length will be determined by the 5m length of the gunwales, so around 4.9m overall.  Beam will be about 32" and depth amidships will be around 15".

I have a target weight of 10kg (about 22lbs in old money) and so far I'm up to about 8kg in aluminium and rivets.  The cloth, when painted, will be around 90 to 100g per m², so with luck the finished boat *might* come in under 10kg.  I will bring it along to Beale Park for the cordless canoe challenge and might enter it into the Amateur Boat Building competition, if only to see the expressions on the faces of the wooden boat purists.................

Photos will follow soon, as soon as I've finished making the frame jigs (dead simple really, just a board with wooden pegs on, around which the frames will be bent).  I believe that the build will be pretty quick, I will almost certainly have to spend more time making a strongback and frame jigs than I will building the canoe itself.

Although I'll be using leftover materials from other projects (aircraft) this wouldn't be a particularly expensive way to build a canoe or small boat even if the materials had to be purchased.  I would estimate that there's around £70 worth of alloy tubing, plus maybe another £50 in covering material, plus paint etc.  Only time will tell as to whether or not it will work out OK, though.

I have to give credit to Dave Gentry for his SOF boat designs, as they were the ones that made me realise that you don't need a structure as complex as the Airolite Geodesic boats to work well (see here: http://gentrycustomboats.com/ and particularly this lovely boat here: http://gentrycustomboats.com/RUTH%20page.html )

Jeremy

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Bugger............   I hadn't noticed the ban on non-wood construction.  

The original intention was to use this boat for the cordless challenge, so I guess I'll just have to stick with that.

Jeremy

It'd be nice if there was that option.  As I'm working through the design I'm getting more and more enthusiastic about the idea.  The hard part is getting the templates absolutely spot on, as unlike wood there's no way of easily correcting a frame that's out by a mm or two.  Making the thing should be very quick and easy once I've finished sorting out all the exact dimensions for the frame bends, making due allowance for bending etc.

I've tested a sample of the 1/2" x16g tube and found that it bends fairly nicely without kinking when just pulled around a 1" diameter bar:



The result seems fairly stiff too, and I'm pretty sure that when combined with the gunwales, thwarts, stringers and some additional stiffening from formed sheet floors it should make for an adequately stiff structure.  

I'm trying to make sure that no special tools are needed, it should be possible to build the boat with just a rule, felt pen, drill, hacksaw, hammer, tinsnips, a pop rivet tool, a domestic iron and some paint brushes.  The build will need a strongback the length of the boat, plus some MDF and 1" diameter dowel for making jigs, but will probably be fairly unique in boat building in not really requiring many (or may be any) clamps.

Jeremy

Jeremy wrote

…The build will need a strongback the length of the boat, plus some MDF and 1" diameter dowel for making jigs, but will probably be fairly unique in boat building in not really requiring many (or may be any) clamps.

Jeremy

Jeremy   My attempts at boat building can best be described as "Agricultural" , based mainly on mixing the various contents of the saw bench box and the dust bags with copious quantities of Mas Epoxy to fill the gaps in my wood butchery, so please view my comments with this in mind.
 
I understand that the very early Skin on Frame boats were constructed from whale bone, drift wood and the odd reindeer antler covered in the skin of whatever poor creature got in the way of the hunters.  On this premise, I wonder if these "mm" that you refer to are rather a little too aeronautical?  I don`t really understand these foreign units but I gather you get quite a lot of them to a foot. 
I do know that about 20-25 years ago there was a GRP Sea Kayak that had a decided "banana shape" straight from the mould, thus resulting in asymmetric shoulder development in the paddler.  I also know from experience that the commercial moulding of the 22foot Sea Worker (c.1985) was 2 inches longer round one side than the other.  I can`t remember which side was which so it must not have been too safety critical.    
You may wish to study the course of the "Cordless Challenge" and get any inherent turning effect to work for you.  Anyway, you could tell the critics at Beale park that you intentionally designed any minor anomalies into your craft with this intention.
 
No doubt Graham, Paul or ToC will point me out to you at some stage and you will be able to extract suitable retribution!
 
Regards  John
 
 

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Date: Sun, 6 Feb 2011 07:48:54 -0800
From: [hidden email]
To: [hidden email]
Subject: Re: Unusual lightweight canoe

Jeremy wrote

…The build will need a strongback the length of the boat, plus some MDF and 1" diameter dowel for making jigs, but will probably be fairly unique in boat building in not really requiring many (or may be any) clamps.

Jeremy

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Are you going for a rowing boat? If so, perhaps with your spare alloy tubes you might even include a sliding rigger system. This would also keep all the stresses away from the lightweight gunnels. Here's a photo of Virus Rowboats alloy tubed sliding rigger design