Rudder Failure - builder error!

[Frogsider]

In an effort to economise on materials when finishing off my Selway Fisher Greenshank, I made the fundamental mistake of using 9mm marine ply instead of  the specified 12mm for the rudder blade.  I just couldn't believe that 12mm was really necessary, anyway, and laughed inwardly, as I shaped the elegant profile of my slim foil, at what I thought was Paul Fisher's ultra cautious over-engineering.

I'm not laughing so much now, as yesterday the rudder snapped cleanly off along the line of the lower edge of the rudder stock cheeks causing an instant, and, no doubt, spectacularly funny capsize.

The wind was blowing about 15 knots, quite fresh for an 8ft dinghy, and I had been enthusiastically short tacking against the incoming tide in the estuary, but I never dreamt that the rudder blade could be under such a load.  I certainly didn't feel it through the tiller (made slightly shorter than designer spec. to gain space inboard)  I know better now - the designer was right - it's the builder who is to blame!

Obviously I have to make a new blade, but to fit a 12mm blade I will have to alter the stock to take  the extra thickness.  This leads me to two questions.

1. I could presumably simply replace the 9mm blade with a new 9mm blade made of stronger material.  But what material?  Obviously steel would be one possibility, but are there any other lighter materials of adequate strength that I could cut and shape myself, using ordinary woodworking tools?

2. If I have to fit a 12mm blade I will have to either make a new stock or disassemble and alter the existing stock to accommodate the extra thickness.  The stock was glued up with thickened epoxy.  Is there any way to unglue it without breaking it?

Any suggestions will be welcomed.

[Jeremy]

Sorry to hear of your accident.  Marine ply is already pretty much as strong as you're going to get using timber in this thickness, but you could make a new rudder blade from 6mm ply with a composite skin on either side to increase the strength and thickness up to 9mm to fit your stock.  It would take a fair bit of work, using epoxy and glass cloth, best laid with the weave diagonal to the major axis of the rudder for one layer, with the next layer laid so that the weave goes along the major axis, then alternating this pattern until you have around 1.5mm thickness laid up on each side.  Best to shape the 6mm ply core first, so that there is little need to shape the epoxy glass afterwards.

Alternatively, you may be able to take the epoxy glued stock apart.  Epoxy softens at around 100 to 120 deg C and then becomes fairly easy to unbond.  The problem is going to be getting enough heat through the stock to the epoxy.  I'd suggest trying something like a steam chest, leaving the stock in there for long enough to get the heat right into the epoxy, then trying to gently prise it apart.  Depending on the area of the bond you may find that this is the best way forward, as if it works it would be less work than making a composite sheathed rudder, I think.

Jeremy

[alopenboat]

On 15 Sep 2011 at 1:12, Frogsider [via UK HBBR Forum] wrote:

> 2. If I have to fit a 12mm blade I will have to either make a new
> stock or disassemble and alter the existing stock to accommodate the
> extra thickness. The stock was glued up with thickened epoxy.  Is
> their any way to unglue it without breaking it?
>

Could you simply saw the box down the middle and epoxy in a piece to
get the right width? In a complimentary wood colour perhaps so you
could pretend it was intentional.

--
Hoping for calm nights

Alastair Law,      
Yeovil, England.
<http://www.little.jim.freeuk.com>          

[Frogsider]

Thanks to you both above for your advice and suggestions.  I'm not sure I want to do all the glass and epoxy build-up, and cooking the stock sounds a bit complicated.

Slicing through the stock is potentially workable, but I've now realised I would then have to buy new rudder fittings to fit the much thicker rudder stock.

What do you think of having a plain steel or aluminium plate cut of, say, 6mm thickness, then skinning it with thin ply, bonded on with epoxy, profiled and shaped at the edges to match the old blade?  Can you bond ply direct to metal?    

[Port-Na-Storm]

The good news is, if there was enough pressure to break the blade but no apparent pressure on the tiller, then the boat must be really well balanced.

If the two options already suggested seem too much of a faff then I'd cut my losses and get some 12mm ply.  Half a sheet of good WBP should do it. You can use your existing rudder and stock as templates, buzz round with a jigsaw or bandsaw and glue her up. Job done.  

I don't see any advantage and a lot of work involved in building a composite metal/ply rudder on a little dinghy.

Regards
Graham  

[Jeremy]

I tend to agree with Graham, it's likely to be less work in the long run to make a new one of 12mm rather than trying to make up some sort of composite.

If you were going to go for a composite rudder, then you'd want to put the stiffest/strongest material on the outside, as that's the bit that takes all the bending loads, rather than the middle,  The centre of the rudder doesn't contribute anything worthwhile to the strength and could be something like balsa wood (a bit extreme, perhaps) with a strong skin on either side to take the loads.

The problem with using any non-mouldable material on the outside is that you'd need to shape it to the profile of the rudder, so losing useful strength.

I mentioned using epoxy glass, as it's stronger than wood and could be laid up over a shaped core to get the right profile.  It's be a lot of work though.

Jeremy

[Port-Na-Storm]

I have to admit after your experience I'm wondering whether the 9mm centreboard I'm about to fit to Katie Beardie will be up to the job. She is a 16' sailing canoe which will have a 40sqft or so sail area. I don't plan to be out in strong winds, and if I am I'll probably drop the rig and run for it but you never know.
The case is made as is the board, and i've started the surgery to fit it into the hull so if I'm going to change my mind this is the time. My main consideration for using 9mm was to keep the weight down as I want to car top.

Of course I could build it to the specification on the plans, but as some of you know the plans aren't actually drawn yet. They must surely be in the designer's head but I fear they have been shoved into a dark recess by a certain 12ft premise and the need to navigate through bollards.

What dya think Cee Dubbs?      

[Frogsider]

My centreboard is 12mm and seems to be able to take the load.  Sail area is about 3.5sqm.

I'm wondering if I had cut the rudder blade on a diagonal from the ply sheet, instead of with its axis parallel to the edge of the sheet, it might not have snapped so easily.

Now I'm considering using a rudder blade of 8mm aluminium which I hear is easy to cut with normal hand or power tools.  It should be up to the job, and if not it should bend a bit rather than snap right off.

[WalneyJohn]

Hi Patrick,
I inherited a GP14 once that had no rudder, So I used 9/10mm Aluminium plate for the blade and it worked fine, I cut it out using a jigsaw then finished with a coarse file and belt sander. If using in the sea, it did need rinsing in fresh water and drying before storing as it soon salted up.

Regards

John

[Jeremy]

Frogsider wrote

My centreboard is 12mm and seems to be able to take the load.  Sail area is about 3.5sqm.

I'm wondering if I had cut the rudder blade on a diagonal from the ply sheet, instead of with its axis parallel to the edge of the sheet, it might not have snapped so easily.

Now I'm considering using a rudder blade of 8mm aluminium which I hear is easy to cut with normal hand or power tools.  It should be up to the job, and if not it should bend a bit rather than snap right off.

Cutting on the diagonal won't help, as it is only the outer plies on each side that are taking the bending load, the core will be doing very little other than keeping the outer, load bearing, plies apart.

I assume that you cut the rudder with the outer plies running vertically?  if so, then this was about as strong as you could hope to get from that thickness of plywood.

8mm alloy will be heavy and a bugger to cut to shape.  It can be cut with tungsten carbide woodworking tools, like a router, but will be a bit scary, throw sharp swarf everywhere and present a potential hazard from fast flying hot chips of alloy.  It will also be quite expensive.  Even a common, moderate strength, corrosion resistant alloy like 6082-T6 (probably the minimum acceptable in terms of strength and corrosion resistance) will cost a heck of a lot more than plywood.  It's also hard to buy plate offcuts in the sort of small-ish size you want - as standard it's sold in 2.5 x 1.25 metre sheets on the continent (or still in 8ft x 4ft sheets here in the UK).

[Frogsider]

Hi John,

I thought maybe if I painted the blade with epoxy it wouldn't have too great a tendency to dissolve in salt water!

Hi Jeremy,

I can get an offcut of the right size locally.  Yes, it's more expensive than plywood, but I'm giving the boat to my grandchildren, and the extra cost will be worth it for the peace of mind.

Patrick

[BrianP]

With a 9mm board only half the ply's are configured vertically to prevent bending, that's not much. Boards normally snap where they enter the case. I use some unidirectional glass that John Claridge has in stock (costs very little) when glassing boards to make sure they are stiff enough. Is there any chance you have room between the board and the case sides for a glass coating on each side, or even just one side? Brian

[Chris Waite]

I think Grum

That it should be 12mm.  'Rosie Mae' has sixty square feet and her centreplate is 18mm; it ripped to splinters when a current took her sideways over a mound of dredgings.

I was pretty amazed at the damage sustained

CW

[simplesimon]

Just been out and measured an Oppie daggerboard & rudder. (Sail area 38 sqft, sprit).

Both 12mm ply, and need to be robust (being school boats). Dagger is 48" long, and I've used one as a leeboard before now. Case is about 10" deep.

Simon

[simplesimon]

Frogsider wrote

I'm giving the boat to my grandchildren, and the extra cost will be worth it for the peace of mind.

They might find a plywood blade easier to carry.

Thin rudders stall easily too.

Simon

[Paul H (admin)]

MilliBee's rudder is 2 x 9mm laminated - 9mm ply is remarkably strong when laminated and her optional centreplate is 18mm.

Illusion's transom is 2 x 6mm laminated and strong - make a test piece of 6mm and 4mm laminated for Katie and compare it to 9mm. I think you will be surprised how strong it is.

-Paul

[alopenboat]

On 17 Sep 2011 at 11:04, Port-Na-Storm [via UK HBBR Forum] wrote:

>
>
> I have to admit after your experience I'm wondering whether the 9mm
> centreboard I'm about to fit to Katie Beardie will be up to the job.
> She is a 16' sailing canoe which will have a 40sqft or so sail area. I
> don't plan to be out in strong winds, and if I am I'll probably drop
> the rig and run for it but you never know. The case is made as is the
> board, and i've started the surgery to fit it into the hull so if I'm
> going to change my mind this is the time. My main consideration for
> using 9mm was to keep the weight down as I want to car top.
>

When he was at school my elder brother built a 15ft Percy Blandford
canoe and fitted it with sails. It sailed perfectly well on lakes and
rivers but every time it was sailed on the sea the lee boards broke.
We came to the conclusion that it was not the leeway load that was
the problem but the surging due to the waves.

Strong winds may only be a part of the problem.

--
Hoping for calm nights

Alastair Law,      
Yeovil, England.
<http://www.little.jim.freeuk.com>          

[Port-Na-Storm]

Right then, having given Frogsider, the advice to cut his losses and start again, guess what I've done!

The Board now has an extra laminate of Alec Jordan's finest packing case which comes in at a thickish 4mm so we are now up to 13/14mm which will just have to do. I'll attack it with a plane in a couple of days and try to get some kind of profile into it, which should at least lighten it up.

The case has been cut open with the pull saw and widened to suit. Its now all glued up. I took the opportunity to try a few different pivot points to try to get the optimum position and I'm fairly happy with  the way things are now.  

Cheers Graham

[Anders]

Interesting

I´m in the mental preparation stage for a 11´ sail and oar dink.... It´ll have to be a cartopper because of Spanish stupidity.
I´m discussing endlessly the centerboard/daggerboard dilemma.  I prefer centerboards, but the hull weigh more than a hull with a daggerboard case and the daggerboard taken out.

So a few  questions: How thick should a centerboard be in order to be strong enough? And btw. how thick is the centerboard in other dinks same size like the Coot etc. I was thinking about something like 15 - 18mm

[james gilchrist]

Hi Anders,

This weekend I finished the centreboard on Coot.  The finished width was 21mm.  I also took the opportunity to put some lead low down in the centreboard.  It is made up of strips 38mm wide.

I also came up with a way of making foils which helps me get a decent finish.  So I shall share it with you all.

Firstly make up your board etc.  Then mark the shape of the board in plan format and cut out to shape on the bandsaw or with a jigsaw etc.

Then I get a slot cutter on the router and router out the leading and trailing edges to their finished width.  I then fill the slot with thickened epoxy (silica and graphite).  This gives a tougher leading edge than just wood.

Then I mark the board in plan showing the limits of where the board should be full thickness.  It is then an easy job to shape down until the leading and trailing edges just show.  

Then on this one I have covered the whole lot in graphite epoxy.

Cheers

James