Skin on frame sailboat stiffness

I am currently designing a 12ft skin on frame sailboat loosely based on a Peapod
We do not have an abundance of good wood here in the UK so going down the Dave Gentry ply frame route (4-5 frames) with 4 off 3/4” x 1” ash stringers per side plus keel and substantial gunwales. Dave has a number of good sailboats using this construction which appear to sail well, even completed the Everglades Challenge in his 15’ Chautauqua. All his boats have epoxied joints.
Home http://gentrycustomboats.com/

Platt Monfort with his Geodesic Boats glues the ribs to the stringers and then added Kevlar diagonal bracing which makes a lot of sense to me. Our Boats - Geodesic AiroLITE Boats http://gaboats.com/boats/

I have also looked at traditional methods, read ‘Building Skin-on-Frame Boats’ by Robert Morris and listened to Corey Freedman on Hooked on Wooden Boats from Skin Boats
HOWB 109 – Interview the Expert | Skin Boat School Owner Corey Freedman on Skin-On-Frame Building http://hookedonwoodenboats.com/howb-109-interview-the-expert-skin-boat-school-owner-corey-freedman-on-skin-on-frame-building/
Corey who has build some quite large sailing Umiaks is very addiment that even for sailing SOF craft flexibility is critical. This totally strikes a cord with me if you are building a lightweight structure allowing it to move and spread the stress throughout the whole craft has a lot of logic to it. This obviously goes against what your hard core sailor would want.

The boat I want is a cruiser for exploring areas not usually sailed, it has to be light, cartopped and reliable, as long as it sails overall speed is of little concern

This gives me a few directions I would value input on, remembering it has 4 - 5 plywood frames
A> As per Dave Gentry- epoxy all joints
B> Joints lashed as a more traditional design
C> Joints lashed and in addition add diagonal low stretch braces as per Platt Monfort
D> Continue search and try and find some suitable timber for a traditional build
E> Continue search and try and find some suitable timber for a Platt Monfort type construction

 

I recently built a skin on frame rowboat with transom and the back half of the boat was extremely flexy until the boat was covered and painted. I don't really understand why you would want a sailing boat to be flexible; that might pull apart any glued stringer/frame joints. Without having tried it, I would think that the Gaboats diagonal string bracing must work, or arrange the covering fabric with threads going +/- 45 deg. instead of 0/90 deg. would be an excellent way of both stiffening, and spreading the torsional forces throughout the covering. I considered this covering method for my boat but it requires more fabric, and I won't be sailing it.
(edit) ps. A double ended boat will be inherently stiffer than my transom boat as it is mainly triangles whereas the back of my boat is trapezoidal. Also, if you have a deck (mine doesn't) the boat will be stiffer.

 

Thanks, sounds like a greenhouse until you put the glass in it. I think a model is called for to test different string and fabric bracing.

Re flexibility the idea is that rather than one joint being stress in yeilds a little and the next joint takes the stress and so on. By not loading up one area and spreading the load about the boat is able to take more punishment. Most structures are designed with a degree about of flexibility. Planes would be too heavy to fly if the design criteria was a wind that could not flex. With a sailboat I would not be wobbly just yield a little

 

Made a very crude model, hope pictures work. Results not at all surprising but good the feel the effect different stiffening. Without any cross bracing the craft is VERY flexible. It is however easy to add stiffness this however is best on all sides, ie adding features where the deck is important.

 

I love the bracing model! Can you try it with skin (bedsheet?) at +/-45 Please?! Did you get closer to making a decision about what you will do re. construction?

 

Half an hour with a quick and dirty model can be very informative. Will model with a skin but it might not be until Monday.

 

The answer is:
F>take advantage of the fact that you have a double-ender hull and follow the concept of the traditional kayak construction with tall vertical boards at the gunnels (or wherever vertical is in your hull). You can use modern construction to lighten these boards by making "I" beams with solid straight grain stringers on the sides of a thin plywood shear web. Glue these beams solidly to the primary bulkhead, the frame at the back of the cockpit, and at each end of the boat. Also mount the dagger board trunk and the mast directly and solidly to the primary bulkhead. This may add a little more weather helm but a slightly larger rudder to smaller dagger will work great on such a small boat. A tall strong keel is not a bad use of wood -I prefer continuous straight grain strips on either side of the dagger case the full length of the boat.

The negative to the diagonal low stretch braces is that they are exposed to damage or buckling and/or they stress the stringers and the skin loosens or puckers. If you do it the way I layout above you have a solid sailboat with only the addition of 2 thin webs (2 layers of any veneer at 45 deg) and a little glue.

 

I was working on a CAD sketch along the same lines when you posted, haven’t yet completed the I beams yet but sure you get the picture this is one 5th of the hull 3.4 kg. Extra stiffness can be added to I beams with unidirectional fibres where neaded. Gunwale may be a L shape for comfort. Going to get a general construction arrangement sorted and see how that lays out from one sheet of 12mm ply. If that doesn’t quite make it I can substitute the sheer web bits with 4mm / sheet lumber

Planned on building keel first and build everything off that - no strong back required

Was having a ring bulk head at the mast directly attached to the dagger board and using a balanced lug rig to bring CE forward.

 

Very rough model, something went wrong with the width 18kg at the moment

 

Ball park DelftShip design
LOA 3.6m > 11’ 10”
Beam 1.2m > 3’ 11”
Draft 0.1m > 4”
Displacement 140kg > 308lb
Midships depth 0.4m > 1’ 4”

 

Lots of improvements to hull shape required but CAD model enough to try and nest all the 12mm ply onto one sheet

 

Feasibility layout of ply as a test done

Proves can all be got from 1 sheet 12mm ply

Space to join the frames ie port and starboard frame would slot into keel, not separate as drawn

Plus gunwales and boards etc

Modifications required to the actual hull shape next and then lots of detail design work

 

For your inspiration here's a group of people building big (10 person) skin on frame sail boats. They seem to use a clever "backbone" consisting of the keel, stem and centreboard case to obtain the stiffness. I like it a lot.

Photos Jeunes Marins Urbains http://www.jeunesmarinsurbains.org/photos.html

Cheers, Hans

 

Thanks for that like, certainly is a substantial keel, I am meeting progress, still to design the gunwale and internals

 

Tink,

Did you finish this design?