Hull deforming, advice needed

[vaddoc]

Dear all

I would be very grateful for a bit of advice. I am building a boat that has carvel planking with 42 steam bend ribs each side. Initially during building there are some plywood frames that hold the shape until they are replaced by the ribs.

I have installed almost all the ribs and only 2 temp frames remain in place. I was advised as a precaution to have some cross-spalls to avoid hull deformity and I installed a few. However, I noticed today that the sheer plank has deformed and is now a bit wavy. The photos show that in the gaps between the cross spalls and frames, the ribs are pushing the sheer plank outwards. The photos exaggerate the deformity due to lence and distance issues but it is there.It took a few days for this deformity to develop.

 

 

 

 

 

In the plans, the next step would be to install a stringer low in the bilge and a beam self. However, I still need to do some work in the interior (sanding, floors, mast step etc). Also I need to first remove the remaining two frames which will probably lead to more deformity.

1. What will happen if I remove all frames and braces? Will the sheer plank short it self out with minimal overall deformity or will it massively deform?

2. Should I put even more braces and pul the sheer back into shape and then install the stringer and beam self? But then I will loose all access to interior. Still the beam self should add a lot of rigidity to the sheer.

3. I this what happens with all steam bend ribbed hulls?

 

Any advice would be greatly appreciated as these are uncharted waters for me

Regards

Vaddoc

 

 

Edited December 9, 2017 by vaddoc

[druxey]

Ideally internal stringers would solve the issue (while still retaining the cross-spalls) However, in your case perhaps an external jig will be required to maintain hull form until you can install stringers and permanent cross-members.

[Jim Rogers]

Just my opinion and It may not have any impact on your situation but certain species of wood when kiln dried resist holding a steam bent shape ( Redheart as an example). Steam bent wood may take days to fully dry naturally and as it dries it tries to straighten back out. I have had this issue in my own builds. I leave mine in a plywood form and hit it with a heat gun to assist in drying, then steam it again followed by drying. It is a pita problem to solve.

Edited December 11, 2017 by Jim Rogers

[Louie da fly]

I would advise against taking off all the reinforcements and hoping it will sort itself out. It won't.  

 

I had a similar problem - my dromon had bentwood frames which spread outward when I took it off the plug. My idea was to use crossbeams to hold the sides in, but I also had a lot of work to do inside the hull before I could glue the crossbeams in place.

So I made some temporary "clamps" out of popsicle sticks and bamboo skewers, rather like your braces, to hold the hull shape while I worked on the rest (see page 9 of my build log - posts from Aug 27 to Sept 7 of 2017). Each clamp was sized to sit in one particular place on the hull (and marked accordingly). The beauty of these things is they are so easy to put on and take off.

 

Your problem is more difficult but I believe putting in more temporary clamps would help a lot. You can take off one clamp at a time to work inside the hull without the hull deforming too much (particularly if you work fairly fast!). And adding the stringer and especially the beam shelf should solve the problem permanently.

 

Steven

 

 

[Louie da fly]

On further consideration I should also say that there'll be no permanent harm done by loosening off the clamps - you'll just have the sides spring outwards, which can be remedied as above.

 

But before you do, make the temporary clamps so you can push the hull back into shape when it's time to glue in the stringer and the beam shelf. It's best to make them while the hull is still being held "shipshape" so you have a shape to return to. Do as many as you need to, probably at fairly close intervals, and ensure the hull and the clamps are marked to show where each one goes, as the distance apart of the bits of bamboo skewer varies depending on the shape of the hull.

 

Good luck with it.

 

Steven

[lehmann]

You definitely need to straighten the hull before adding stringers or clamps.  Adding them as is would lock in the shape.  

 

Since the frames are still above the shear line, you can put,  on the outside of the frames, a heavy temporary pre-bent strake, or a sawn mold.  As Druxey suggested, another possibility is to build external "shoring", something like is done in a dry-dock. The external support would bend the hull back, but still give you access to the inside.  

 

Good luck.

[vaddoc]

Dear Druxey, Jim, Steven and Bruce

Thank you for your suggestions. I have now installed more braces and removed the last two temporary frames.

 

 

 

Steven, your boat is very nice. 132 frames though! And I though my 84 were a lot. Still, your boat even without stringers held its shape. My frames are 4 x 2 mm beech and despite they bent nicely after heated through, they are stiff and they seem to push hard against the planks.

I am really not sure what I should do. Building a scaffold externally to support the hull seems to be the more comprehensive way but means an awful lot of work. I find that I can squeeze my hand enough to do some work and possibly I will be able to sand the frames and install the stingers and beam selves.

 

I think that I may have overcorrected a couple of areas when installing the new braces as I had to push rather hard to install them. I ll have another look. Certainly though the hull will need to be supported.

 

This was a useful lesson. I think that hulls with steam bend frames are lighter but probably not as strong as hulls with proper solid frames.

 

I ll post updates in this thread when I find theme to do some more work.

 

Regards

Vaddoc

 

[druxey]

All you need is a ply sheet with the correct shape cut out and mounted at the correct height on a building board. The model sits vertically inside the cut-out.

[vaddoc]

Dear all

I think the problem is solved. I decided against building the interior so no huge access needed. Still, the boat is so large that I can squeeze my arm in the hull!

Most importantly though, I added the first strip that will make the beam selves (It will be laminated from 3 strips 3 x 3 mm each. I feel that the hull immediately stiffened up a lot.

I ll add the rest of the strips for the selves and also the stringers and I will report back when I take the braces off.

Many thanks for your advices.

 

Regards