Proa hull lines, suggestions/guidance

Every material and construction method has a limit to when it breaks. The faster you go, the higher the forces. Skin on frame is lightweight, but will not be as strong as other methods. Also, the fact that the shape of the hull with have concave sections creates more drag. If your goal is high speeds, this is not ideal. However, from your posts it seems that skin on frame is a constraint. That is something that the whole design has to be developed around. Otherwise, write a statement of requirements (SOR) and it will determine which method of construction is best to attain your requirements.

 

Gonzo on Platt Monfort's 13' Blivit open sailing dinghy he gave it a ply bottom with Dacron sides to overcome some of the issues you mention.

 

It works for a dinghy, but for a proa where there is no flat bottom it doesn't. As a personal preference, I choose not to mix methods of construction. They are usually more difficult and expensive. Some hybrid systems do work well.

 

Hi Gonzo

SOR - speed, capacity for 2 people, light weight, ease of construction, draft - about in that order. I am not set on skin on frame or the round bilge vs chine/ply. I am in fact playing with both in the freeship with my limited knowledge. Just learned how to do bulkheads and extract that info from the file via offsets . So this is a learning experience as much anything. I will post my lines soon but happy to build with ply if SOF will limit the speed. Will using longitudinal frames like a fuselage frame be more forgiving in terms of starved cow/resultant drag?

Thanks

 

That is a neat idea. There was someone who put a webpage for their project on a hybrid proa like that but there are no updates on it for about 10+ yrs. Will it save much weight in this hull form - doing a hybrid of ply/SOF? The draft is about 1 ft, so I might only save the weight of about 1ft/20ft * 2 = 40sqft less of ply - so about 1.25 ply panels equivalent weight, at 14lb per panel, so about 18lb less weight but needing the rest of the SOG structure and the Dacron/coatings etc.

 

Here is the site. Two of their sailing designs have ply bottoms.
Sailboats https://gaboats.com/collections/sailboats

I would suggest that you could use lighter ply for the sides and paulownia framing, and probably gain more than you lose. Plus with ply you can box off the hull ends and have flotation plus a crash bulkhead. With SOF, flotation is extra mass.

 

Of the 30 + boats I've built over the years, 2 were skin on frame. The first "Leastie Beastie" was a 16' sit on top attempt at a 10# boat . The second was a 24' tandem river racing sit on top built as the quickest, cheapest thing that might last a 100 miles.
The six proas I've built were all either plywood, One stripper or foam and fiberglass; none were skin on frame.

 

Hi Skip

Assuming that was a deliberate choice - was it for the want of rigidity or to lessen drag from uneven surface?
In my models, I was not losing enough displacement when I turned it to hard chine. So, I can potentially develop plates in freeship with chines and use the 4mm ply like the P5 - about the same size/a bit more displacement.

Thanks

 

Hi HelmutSheina, I don't believe I have enough skill to develop a hybrid model at my current experience level. So, unless I do a good job, I might end up overweight anyways. Interesting that you mentioned Paulownia. I was looking for that wood in central Florida when I was thinking of building Gary Dierking T2. That was before I started to itch for self design
Thanks

 

There's a broad blend of considerations before starting a new design/build. Besides the SOR regarding regarding use there's a blend of time, cost, effort and a few other particulars.

Leastie Beastie was easy early in my design/build journey. I wanted to build a usable sit on top kayak that weighed no more than 10#, only overshot by 20% ;-). Frame and stringers like an old Comet stick and tissue model airplane fuselage. Formers were 3/16" ply minimised as much as possible. Stringers were 3/8" square tight grained cedar and it was covered with winter window film. This was somewhere in the 1980's, before carbon fibre was available .
The second skin on frame was a quick and dirty craft for Chuck Leinweber and I to run in a new 100 mile race on the Colorado river in Texas. Once again frame and stringers utility ply frames and a central keelson from flat deck to keel for longitudinal stiffness. Stringers were 1/2" square cedar . First covered with translucent shrinkwrap film used to winterize boats. Film didn't work that well Duckworks - Outings https://www.duckworksmagazine.com/05/outings/cr100/index.htm

Regarding your question specifically, rigidity is a structural consideration that needs to be analyzed and most of it at our end of the spectrum is just reviewing prior art. Skin on frame using longitudinal stringers over transverse frames works very well, lots of prior art. Regarding lost displacement due to sag it's instructive to take the wetted surface of a design and multiply it by an estimated loss of area due to sag; you'll be surprised at how little displacement is lost.

The primary reason I haven't considered skin on frame for my proas or other craft is out in the real world a skin on frame craft is fragile. I'd consider it for a light easy to carry canoe but my foam and fiberglass 12' EasyB weighs 28#

Apparently you're early in a journey with Freecad, enjoy the trip and you'll discover that the building part is a different but also worthwhile animal, but @gonzo's right if you really want some experience buy something already built.

 

Glad to hear that the longitudinal stringers on transverse frames works well. I have a couple of books on that - although they are for kayak builds, I can at least look at what they did for framing.

I probably didn't explain well - I meant I was not losing much displacement for my purposes between chine or round bilge, so, I am ok with either.

Fragility is a concern, I will primarily be using it in the intercostal waters of Florida, fairly protected but sometimes you do see little driftwood and some logs/crab pots etc. I am hoping that the SOF can take that abuse. If not, I am open to ply build with chine too.

So, fragility and not increasing the drag due to skin deformation are two concerns with SOF build

 

Fragility in engineering terms is called toughness. That is the property of a material and/or structure to absorb energy before breaking. It is quantifiable, but takes complex calculation or destructive tests. However, traditional SOF used animal skins and were not ultralight. Those were fairly tough. Your design will not. Further, in small boats stiffness is the driving property driving a design. This is also quantifiable. Proas and other multihulls need attachment points to take the forces between the hulls and beams. Also, the hulls have to be stiff and strong enough to be able to sustain the concentrated forces from the beam attachments. Your design is not likely to fulfill those constraints.

 

Ok. Thanks Gonzo. I will try to come up with a design and post here. Even if I don't go for skin on frame design in the end, it will be educational for me

 

I think that you need to define the goal. Do you want a lightweight boat, that can be built within your budget, or do you want to experiment with SOF. Either one is fine. However, which one drives the design will give different results.

 

I am aiming to experiment with SOF. Light weight and speed are next priorities. I built a stitch and glue 11ft dinghy I bought the plan from a designer before and managed to build it pretty close to the plan. So, this is not for an experiment