Plywood on Frame Modification Questions

I'm thinking of building this V bottom dory or something similar, and I would like to build a lighter version. The plans don't show the finished weight of the boat but it does look to be on the heavy side for a boat this size which is 15' x 7'. The plan calls for 2x4 lumber frames spaced 18" apart, and this just seems like overkill to me. Does the boat really need this many frames? It also calls for 1/2" ply sides and 5/8" bottom. Could the number of frames be reduced a little without compromising the integrity of the hull?

I would also like some opinions on building this boat using 1x4 frames, 3/8" ply sides and 1/2" ply bottom, and possibly reducing the number of frames. Would this be practical and safe?

Of course the hull exterior will be glassed and epoxied. I appreciate any thoughts!

 

The scantlings seem ridiculous. I'd suggest copying those of another comparable boat.

 

Build something else,that is a really fat boat and the frame size is ridiculous.There are lots of designers out there and few of them would expect 5/8" ply to take the twist in the bottom panel.The smallest design listed here might be a suitable choice Fishing boat designs: 2. V-bottom boats of planked and plywood construction (Rev.2) https://www.fao.org/4/y5649e/y5649e00.htm . The instructions are quite detailed.If none seems quite right for your use then look at Duckworks or Glen-L as they offer reliable designs but the detail might not be as thoroughly explained as the FAO boats.

 

My first question would be what power is called for on that boat? If it is designed for heavy use, heavy loads and high horsepower, then the scantlings would make some kind of sense.
Yes you could reduce skin thickness and framing, but maybe not if you intend to have high loadings and a 200hp outboard.
I built a DK18 from 9/6mm plywood, ( 18′ Dinghy – Boatplans.dk ) absolutely flew with 25hp, but so light it would plane with 6hp and just me. The FAO boats are proven and work well.
There are also some flat skiff type designs in John Gardens book, building classic small boats.

 

Pretty normal and not that heavy really. They used to build like that in Alaska, but kept breaking stuff so they went to 2 x 6 frames. The material costs are not much if you are using big-box store lumber. Costs will go up if you try to reduce the weigh and substitute lighter, higher grade materials.

If this isn't the boat you want to build, just browse through the hundreds of designs out there until you find the one you do want.

 

Why are you wanting to use those particular plans?

 

Thanks all, I certainly will research the links provided. I'm not stuck on any plan yet.

Skaraborgcraft - 30hp max.

Size, shape and purpose. I plan on fishing the open bay from it. I like a wide, stable boat.

 

Lumberyard skiff? Lumber Yard Skiff — Old Wharf Dory

 

The reason the transverse and bottom are heavier is there are no longitudinals in the design. I would not reduce the bottom unless glass is not called for and you plan to add it, but more than a cosmetic amount.

I have a cold molded 1960 Carver Commander and it also has no transverse members, but some seats and stiff gunwhales and an inch thick keelson and the bottom is like 7/8” thick. Basically, the entire structure is stiff from the hull first.

The design actually seems appropriate and was probably drawn sans glass. There appears as only one 2x4 bottom and sides 18” oc, and it looks like a 2x6 on one other side, so each floor is a 2x6 and a void down the center and a 2x4, unless the bom says otherwise. This is done so the frames don’t lose their shape as timbers often will. The sole, if there is one provides final stiffness. If specified with no sole or planked sole; then it makes sense as well.

Fiberglass vs plywood .. I don’t know a conversion, but the same thickness of fiberglass and plywood are similar stiffness until you get into say G10. An 1/8 glass panel is pretty floppy like luaun. So, I’d say you’d want 1/10-1/8” glass for each 1/8” reduction in ply. If you sandwich the ply; perhaps a bit less. For example, a 5/8” ply spec is similar to a 1/2” ply spec with 0.100 to 0.125” of glass which is 2 layers of 1708 which is really a **** ton of weight and expense. Dropping down to a single layer of 1708 is probably enough, with a final thickness of 0.550” versus 5/8” ply at 0.594”.

Finally, lotsa sage advice here to ditch the plan. A monocoque stitch n glue design will outlast this one by about 30 years.

ps - probably the design puts the frames together first and so the frames must hold their shape well and not deform or it’ll end up in the final shape…frames made with 1x materials will not hold their shapes well at all even when made, let alone assembled and awaiting ply

 

It might be fun and wise if you are in love with the lines to determine how weights compare.

Each 1/8 of marine plywood weighs 9 pounds.

A panel is 32 square feet which is 32/9 square yards. A piece of 1708 weighs 17 oz plus 3/4 •9 or 27/4 oz or call it 7 oz for a total of 24 oz per yard and double it for hand lam resin to 48 oz per yard or call it 3 pounds per yard times 32/9 sq yards.. 32•3/9 is call it 11 pounds. So, my example above is heavier with 1/2” ply and 1708 than 5/8 ply no glass by 2 pounds. Get into okume and it’d be even more. The cost ito time, effort, and weight is all higher.. If you are gonna start to skin the boat; may as well do both sides.

 

T

The only people who use 1708 instead of 1700 on ply boats are those with shares in epoxy companies

 

Do yourself a favor and find suitable plans to start with.
When a fella finds himself making structural or other big change to plans, he needs to realize he has the wrong plans.
Too many good ones out there that have been optimized by the designer and don't need to have basic stuff guessed at.

Plans & Kits - Duckworks Boat Builders Supply
Plans & Kits Search | Wooden Boat

 

Yes I think it is good advice. Finding a stitch & glue plan is probably the way to go - simpler and lighter - I've seen some of those plans for a similar size boat that weigh only 100-200 lbs or less.

I do have a question about S& G plans for an open fishing boat. If I find one that I like but want it to be slightly longer or wider, is recalculating the measurements for the plywood pieces a difficult thing to do?

Thanks for the advice!

 

Recalculating is quite a task.I have known it done by a man who created formers for a model and wrapped thin cardboard round it to establish the developed shape of the panels,for scaling up.It helped that with a 4mm skin thickness,the ply was quite flexible.Much better to find a design that suits your requirements and get to building it.

 

I thought I might be able to do it this way for a flat bottom stitch & glue boat:

Increase the bottom panel by X number of inches, in width, measuring up from each point of measurement shown on the plan. Then measure the exact length of one side of the bottom panel from where it meets the transom to the bow. Then keeping the same transom and bow angle as shown on the plan, simply draw it out on the plywood using the new measurements for the bottom and sides. And maybe add a few inches to the height of each side panel and the transom.

Is this feasible or am I dreaming? Appreciate the responses.