Scantling question for beginner

[Brands01]

Hello, I'm new here.

I'm interested in boat design - read a few books (Nature of Boats, Elements of Boat Strength, Devlin's boatbuilding, Designing Power & Sail, Understanding boat Design, along with anything I can get my hands on on the net, ie lectures on the westlawn site etc .. ) - I've got others on order, but in the meantime I've got a few questions.

After all this reading, I thought as an exercise, I would try and design myself a boat from start to finish. Obviously there is heaps I don't know, but I'm finding the process very interesting and rewarding. I've taken some time and drawn the preliminary hull on freeship.

I have settled on a hard chined epoxy plywood displacement powerboat - around 9 metres LOA, 3 metres max beam, designed for weekend coastal, harbour and river cruising. I've settled on the plywood for ease of construction and price, and I've built a small stitch and glue dinghy, so am familiar with the methodology.

I've been using Dave Gerr's Elements of Boat Strength for scantlings.

After this big intro, I have a fairly simple question. Gerr covers scantlings for exterior sheathed ply, cold molded sheathed inside and out - or composite construction. He doesn't cover plywood composite - so plywood sheathed with glass on the exterior and interior. My assumption is that I could use scantlings for foam sandwich construction but using plywood as the core. However, my experience with boats is that if you assume, things go wrong.

Can I substitute plywood as the core for foam sandwich scantlings?

Many thanks in advance for any advice:)

[gonzo]

Use the scantlings for strip planked with fiberglass in and out.

[Brands01]

Thanks for the quick response Gonzo.

Does this mean for plank thickness I use:

1. strip planked scantlings with interior/exterior sheathing; or

2. Strip planked scantlings for wood-epoxy with exterior only?

Option 1. has a thinner plank thickness.

Thanks again for the quick response!

[Tim B]

Have a look at the ABS rules, they are freely downloadable, and have a set of rules pertaining to your design.

http://www.eagle.org/absdownloads/index.cfm

[CapKos]

Sorry for the question, am not designer either, but why sheathing the interior? This is substantial work, especially when considering the sanding involved.

All the best,
CapKos

[Brands01]

Hi Capkos,

This could be a case of the blind leading the blind, however I think with sheathing the interior the structure becomes a monocoque structure which relys less on the strength of internal stiffeners (such as frames and bulkheads) than the hull structure itself. It also means I can use fewer bulkheads, and theoretically should be lighter.

If I just use exterior sheathing on plywood, I will need heavier hull planking, and more heavier bulkheads = heavy but weaker boat.

Any more experienced people on this forum might comment to confirm or set me straight!

[Brands01]

By the way, Thanks TimB for the pointer to the ABS site - very interesting - that should keep me busy for a while!

[Willallison]

The ABS rules will almost certainly give you a lighter structure, particularly for a boat of the sort of length you are considering. (Elements of Boat Strength ers on the conservative side).
Having said that, I wouldn't bother with the complexity of the calculations used in ABS rules for your project - I'd just go with Gerr's stuff.
Having said that, they will give you a better understanding of the stresses involved and the way in which scantlings are calculated.

As Gonzo suggests, you would use the scantlings for strip plank, just substituting ply for planks. So - assuming you will be applying FRP to both sides, that's the system you should use

[Brands01]

Thanks Will, that is exactly what I was after. I think I'll save the ABS stuff for a rainy day :)

[Willallison]

Don't get me wrong - there's nothing wrong with using the ABS rules - quite the opposite in fact. If you were trying to build a lightweight planing hull, that's what I'd suggest.
But for your displacement boat, Dave's rules will give a strong, conservative structure.

[Raggi_Thor]

Regarding ABS, I found a lot of interesting pdfs there, but nothing about plywood or sheated strip planking.
Is it there for free download (after registration)?

[CapKos]

Hi,
Plywood is one of the best materials on stiffness/weight ratio basis, so I don’t think you could improve the weight here. Gougeon Brothers made some tests and found that sheathing the interior improve greatly the impact resistance, which is important, but they said the weight (as well as the price) of such construction should increase, which indicate they didn’t consider decreasing in the scantling.

All the best,
CapKos

[Brands01]

Will - I notice you're here in Australia - do you know of a similar site for Australian standards?

Re Dave Gerr's rules, a strong conservative structure is exactly how I would want to build a boat of this type, so that suits me well - his book's alot more user friendly than the ABS documents as well :)

Thanks again for everybody's input.

[MikeJohns]

Brands


The Oz rules are Called Uniform Shipping Laws (USL) We can use DNV or ABS here as well. All about to change to a new system.
USL is free to download but you can also pay for it....funny system. Search for USL downloads. USL is pretty much the same as ABS.

Always make sure you use current rules as older ones were often defficient and have been corrected in light of earlier failures.

[Brands01]

Thanks Mike, free sounds good :) I'll try and track them down.

[fcfc]

Regarding ABS, I found a lot of interesting pdfs there, but nothing about plywood or sheated strip planking.
Is it there for free download (after registration)?

And nothing neither for "small" powerboats ie smaller than 80 ft.

Or did I looked bad ?


Thanks.

[Willallison]

There are Standards Australia rules that cover smaller boats in Oz. I can't recall the AS numbers off the top of my head, but just do a search and you'll find them...

[gary1]

G/Day Brands01,
Mate I think the standards for Australia which may be of help to you are AS4132 parts 1&3. Although dont quote me on that
Gary

[Brands01]

Thanks Gary, you're right on the money there. For non commercial vessels are there any legal requirements to use the standards?

[gary1]

G/Day Brands01,
Mate I'am a total amatuer when it comes to boat building the only knowledge I have on the subject is what I have learnt by word and mouth, that being said. I don't know if there are any legal requirments to build to those specific standards AS4132 parts 1&3. But the questions is why wouldn't you not want to build to them. The way I see it these standards have been hopefully worked out by a lot of highly educated people with a shitload of years experience and scientific know how to draw on. So that if someone is using a boat built to these standards as a minium requiremment, and that boat is being used in a commercial sense say pro fishing where the boats often have to take a lot of constant hard work and abuse in all sorts of conditions. Then hopefully it will give the owner of that boat peace of mind that the boat can stand up to it structually.
When I got my boat designed I asked the guy who did it to come up with the strongest designed boat that he could, using the method that I was going to build with "Stich n Glue". Which apparently he has done and I realise it will cost me more in materials, because of what I asked of him. But I reckon if I do my part on the building side of things that I will feel totally confident in my boat structually. How it handles well I'll just have to see come the time but I'm confident it'll be good.
Mate go the AS standards I reckon it will pay off in the long run in a lot of ways.
Stay Safe
Gary

[Willallison]

The scantlings in Gerr's Elements of Boat Strength will almost certainly exceed those by AS 4132. And for that matter, most of the others - loke ABS, ISO, USL, Lloyds, etc.
Elements is, by the authors own comment, a conservative set of formulae. It will meet or exceed those mentioned.
As I've previously said, if you were after a lightweight structure, I'd go with ABS or similar. As you aren't Dave's rule's will work fine for you.

But going through the process of calculation by one of the others will give you a better understanding of the stresses involved. Why not calculate your scantlings by both and compare them? The work involved in doing that is WAY less than you will expend on actually building it....