Strong wood joint advice please

[Paul H (admin)]

MilliBee's new keel has 55mm wide Iroko on the case sides and 60mm wide Iroko going forwards to the bow. The sides are screwed from inside with 6 coach bolts and 2 external screws at the back.

The 3 pieces of wood have to fit together in a strong way and I will protect them with 50mm x 3mm stainless strip, screwed down with countersunk screws.

I prototyped a joint with softwood in the pictures below (angles not perfect). The case sides were bevelled to reduce drag, leaving the case sides full width for strength when hitting the beach. Then I cut the softwood to fit. Then I realised there could be fillet, drawn in blue in the 2nd photo.





Maybe I could glue/screw a fillet of Iroko in the blue triangle? Perhaps there is a traditional keel joint for this area? This is a vulnerable area when hitting a sandbank, especially if the sand is rocky!

I'm planning to fit a SS plate across the 3 pieces, bending the sides/corners over on the leading edge.

All suggestions welcome, nothing is glued yet.

Cheers
Paul

[Timmo]

As you say Paul, there may be a traditional joint for this... but I don't know it.

The thought I had, just to make it harder, was that you make one piece both incorporating the joining piece and  fillets into a single central block and extend that central block forward with cheeks that fit round the keel piece.. The fillets could then be carved/filed/sanded so their leading edges are a nice organic curve from the keel to the outside of the centre board cheeks.

Bit like this (Plan view)



though I couldn't draw the curve as I wanted to. I picture it curving in three dimensions. But I like curves,

That would create a strong joint between the bits round the centreboard slot and the forward keel.

But it will be a very fiddly bit of woodworking.

Will you be countersinking those screws?

Tim.

[Paul H (admin)]

Tim,

That part could work, but its fiddly like you said. I'd be concerned the forward U shape edges would break away.

Around Chichester harbour and the Solent there are some really bumpy sea bottoms, which I tend to discover at low tide! CW recommended covering the Iroko with SS strip to protect it, with good reason.

I was planning on cutting the 60mm wide Iroko down to 25mm to slot between the side cheeks. There would be about 60mm overlap which gives a large area of glue joint. What you can't see are the forward 6 x 70mm coach bolts that secure the cheeks about 50mm from the end, making that area strong.
There will be coach bolts holding the forward keel member, bolting through the thick hog.

That made me think the fillets could be added later, as you said rounded to give smooth water flow. But I'll need the central area to be flat for the SS strip. The simple 45 degree bevel on the cheeks was so I could bend the cheek SS strip 45 degrees over the bevel; hopefully the bevelled SS would slide over rocks and not get damaged. Think of the brass rails on a home made 1970s taboggan!

Yes the screws will be countersunk and counterbored about 5mm, enough to seal with epoxy. My countersunk bit has just the right diameter to counterbore a 6mm screw head. The screws you see in the photo are temporary and will be removed when bolted from behind. There will only be external screws on the last 200mm of the CB case, which is underneath the cockpit floor and very difficult to reach from inside. Also the skeg will have external screws.

Here is the inside, you can see the coach bolts from the main bulkhead, on the right, forward. 9mm ply was laminated onto the 6mm bottom plank to strengthen it and Red Mahogany from your ex-Heron widens the hog for the chunky cheeks.

Four coach bolts :)

This gravity defying picture was taken with the hull rolled 90 degrees to Port!



The scratches show why a strong keel is necessary, for another 15 years of sailing.

-Paul

[John P]

I would just endorse the idea of adding stainless steel wear strips to such parts of a boat.  Our sailing dinghy has two 'runners', like very shallow bilge keels that it sits on when grounded.  Strips of stainless steel about 15mm wide by 16g (1.6mm) were screwed to the bottom of these runners when the boat was built and 40 years later they are still providing good protection.   I have seen similar runners on both wood and grp boats that have worn away quite quickly.  I suspect that any metal, bronze, stainless steel or maybe even aluminium, will last much longer for this purpose than wood, even if the wood is sheathed with glass and epoxy.

[Paul H (admin)]

Thanks John,

Did you use marine grade A4 SS strip?  It's difficult to find A4 grade strip on eBay, also 3mm seems to be the minimum thickness.

There is plenty of A2 (304) SS strip though at 1.5mm/1.6mm; its debatable if A4 is needed on a trailer sailor.

I'll try to find A4 screws though, as they could be the weak link.

-Paul

[Randonneur]

I'd get Leo to cast you a bronze strip, and then hand peen some rivets. 😉

On Sun, 18 Oct 2020, 10:19 Paul (admin) [via UK HBBR Forum], <[hidden email]> wrote:

Thanks John,

Did you use marine grade A4 SS strip?  It's difficult to find A4 grade strip on eBay, also 3mm seems to be the minimum thickness.

There is plenty of A2 (304) SS strip though at 1.5mm/1.6mm; its debatable if A4 is needed on a trailer sailor.

I'll try to find A4 screws though, as they could be the weak link.

-Paul

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NAML

[Timmo]

You've got to admire Leo's knees!

On 18 Oct 2020, at 11:36, Randonneur [via UK HBBR Forum] <[hidden email]> wrote:

I'd get Leo to cast you a bronze strip, and then hand peen some rivets. 😉

On Sun, 18 Oct 2020, 10:19 Paul (admin) [via UK HBBR Forum], <<a href="x-msg://1/user/SendEmail.jtp?type=node&amp;node=4031857&amp;i=0" target="_top" rel="nofollow" link="external" class="">[hidden email]> wrote:

Thanks John,

Did you use marine grade A4 SS strip?  It's difficult to find A4 grade strip on eBay, also 3mm seems to be the minimum thickness.

There is plenty of A2 (304) SS strip though at 1.5mm/1.6mm; its debatable if A4 is needed on a trailer sailor.

I'll try to find A4 screws though, as they could be the weak link.

-Paul

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NAML

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[Port-Na-Storm]

Yes, so much bronze. 😎

I realised too late that I should have crowd funded the Morbic.

Sent from my Xperia by Sony smartphone

---- Timmo [via UK HBBR Forum] wrote ----

You've got to admire Leo's knees!

On 18 Oct 2020, at 11:36, Randonneur [via UK HBBR Forum] <[hidden email]> wrote:

I'd get Leo to cast you a bronze strip, and then hand peen some rivets. 😉

On Sun, 18 Oct 2020, 10:19 Paul (admin) [via UK HBBR Forum], <<a href="x-msg://1/user/SendEmail.jtp?type=node&amp;node=4031857&amp;i=0"; target="_top" rel="nofollow" link="external" class="">[hidden email]> wrote:

Thanks John,

Did you use marine grade A4 SS strip?  It's difficult to find A4 grade strip on eBay, also 3mm seems to be the minimum thickness.

There is plenty of A2 (304) SS strip though at 1.5mm/1.6mm; its debatable if A4 is needed on a trailer sailor.

I'll try to find A4 screws though, as they could be the weak link.

-Paul

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NAML

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[Paul H (admin)]

I decided on a simple joint, like a mortice and tenon cut in half. Trimmed with a very sharp Marples chisel:



It's a friction/mallet fit, good enough to support the entire front keel




Triangle pieces will be fitted later, about 2:1 slope, for streamlining. With a wide edge to support the triangle it should survive compression forces as she scrapes along the bottom. Stainless strip could be bent over the triangle for extra protection . I'll see what happens when I get to that stage.

Either side of CB case are glued and screwed (6 internal coach bolts + 2 external screws each side). Top side (Starboard) is back filled with epoxy, I'll have to flip her 180 degrees to back fill the Port side - haven't a clue how to do it easily!

-Paul