Considerations when converting plan to skin on frame

Hi all

I am looking for advice on what to consider when converting a plan to skin on frame construction. sailing and rowing designs upto 16' long. I'm thinking of boats like a peapod, or shetland yawl etc. I will be using an unstayed lug rig. Skin on frame due to ease of construction, material cost, and most importantly light weight for home storage/car topable etc etc

So far I realise that they are obviously a lot lighter, so would sit higher in the water and thus potentially be unstable, and they are obviously more flexible.

I have built a skin on frame canoe and was pleased with the result so am confident I understand some of the limitations etc.

So far I have thought of the following:

• Fair out any hollows in the hull design
• Utilise water ballast bags to put some weight back in the hull when on the water for cog and ballast
• Widen the waterline beam slightly and reduce any tumblehome, reduce sheer height perhaps.
• Experiment with Double gunwale ala curragh for stiffness when rowing, or perhaps tradional inwale/outwales with a thwart riser below
• Flat keel designs will need to have some rocker added for skin tensioning - a small external keel fitted after the skin is installed could return some tracking?
• Hollow birds mouth mast and spars to reduce cog
• Consider Kevlar tow diagonal bracing like in aerolite boats (although I haven't tried this so far, it seems like a good way of stiffening the boat while retaining impact flexibility)

Are these thoughts along the right lines? Can anyone please add/rubbish it.

Thanks
Sam

 

Over 30 years ago I watched Platt Montford, the designer of aerolite boats capsize his sailing skiff sailing on a breezy day. They are very tender in nature and flex under each stroke of the paddle or oar.

 

I'm all in for light weight, two of the craft I designed/built over a few decades were skin on frame.
But neither were sailing craft.
The forces involved are far greater than a paddle craft. In order to sail to windward at all rather than just being blown downwind is going to require some structural stiffness between your mast and means of leeway prevention. Two approaches come to mind. One is a mast/daggerboard assembly which could be fairly simple but seems out of place here. The other is using a leeboard, you really only need one especially for a more canoe style approach. I added such a combo to my 14' EasyB canoe once and it worked OK 27 s.f. lug sail on a c.f. mast, fairly minimalist but that was a setup for an expedition style trip (Texas 200).
Attention to detail is important here, it's easy to add unintended weight.

 

Sgf,

Welcome to the Forum.

Great idea but build to an existing, proven, successful plan.
It takes all the guesswork out.
If you can't find an existing, proven plan, then it's not a good idea.

 

Absolutely, I envisage a double ender, as easier to create structural strength for a mast near the bow, and the lug will keep the forces lower. I think I'll experiment with a dagger board but also some of the old British boat designs used a long rudder that followed the angle of the stern post and tilted under the hull which may be worth a shot too.

In regards to converting the actual lines to being more suitable for SOF though am I missing anything?

 

I do see your point but I've the time and inclination so want to try. I'm am experienced sailor so fully aware of the dangers and won't be venturing out to sea with anything untested.

There are not many designs published for sof boats, but there is evidence of them (Corey Friedman from skin boat school has a 20 foot sailing umiak for example)

 

Double gunwales or inwale/outwale setups both work well depending on the vibe you’re after (and how much weight you're okay adding). Adding a bit of rocker to help with tensioning is also a good move—I’ve seen people do exactly that and get great results.

 

Thanks for the feedback, yes I'm thinking of trying the double gunwale out on a rowing boat, and staying with the inwales/outwales on the sailing dinghy. My main things I'm not 100% sure on are which hull shapes would be bad for this conversion - i.e a boat that's more full towards the bows wouldnt cut through the water well because its now much lighter than designed etc

 

You will need to add buoyancy bags to the hull to compensate.

I have almost finished Dave Gentry's "Ruth". In that case I built it with no rocker over most of its length.
The tension from the lowest chines pulled the polyester tight to the keel.
Although you are probably right to have some rocker as the frames have been known to distort and become hogged.

Sailing puts a lot of torsion into the frame which has much less resistance than a plywood hull.

 

Congratulations on building Ruth - how did you find the process?

 

I will collect my thoughts and reply in a few days.

 

Thankyou id appreciate that.

I'm leaning in the direction of experimenting by reducing a design by volume. Say, for the sake of argument/example, I calculate my design will be 50% lighter construction, then if I take a design that is 20' long, apply ∛(0.5) and my new design will be about 16' long and so fourth.

Its worth a shot on the drawing board at any rate

 

The weight you use needs to be weight of the boat PLUS weight of the occupants and gear. Lighter construction does not mean the occupants and gear will be proportionately lighter.

 

Noted thankyou. I'm in process of reading John Teales 'How to Design a Boat' and saw that bit there too. It's an interesting exercise indeed.