How to 'convert' an existing plywood design to foam core?

I'd like to learn what would be required to take a known plywood/epoxy/glass design and convert it to foam sandwich construction. Specifically, an Angus RowCruiser Sail.

Here are some of my initial questions:

• how to choose the thickness and density of the foam core for each component? (bulkheads, skin, etc)
• how to select the type of laminate material for each component/section (oz. weight/weave/orientation/etc.)
• the sequence of layers/strand orientation relative to the design for each component
• is vacuum bagging/resin infusion absolutely required, or can we wet out/lay up by hand if we are careful to minimize excess resin?

I saw @fallguy mention in another thread "build to a specified laminate schedule and not try to create your own." Yes...this is what I want to do! Build to a known 'schedule' that will approximate the structural properties of Okoume plywood for a given part/component. But how do I find that 'schedule' on a component-by-component basis (bulkheads use this layup, skin use that layup, etc)

 

"Yes...this is what I want to do! Build to a known 'schedule' that will approximate the structural properties of Okoume plywood for a given part/component. But how do I find that 'schedule' on a component-by-component basis (bulkheads use this layup, skin use that layup, etc)"

You consult the original designer.
You may be able to get approximate calculations for panels to match Plywood by other means, and even the 3 layers of plywood that the Amas require.
BUT
The beams, connections to the beams, other stress points like mast and centreboards, all require expert calculations, t hat will be proprietary to the original designer.
If you get another naval architect to redesign the scantlings for foam, to the same design, you will be violating copywrite of the plans, as well as incurring a fair bit of expense.

I suspect that if you contact the original designer, you may find they are planning a foam design version already.

Question - why are you afraid of Plywood ?

 

@rwatson , the original designer is Colin Angus, who is more of a 'world class adventurer' who also happened to design a very specialized boat for his adventures. (you can check out the boat in the link from the first post).

He's he's not a card-carrying boat designer or naval architect, so the calculations and stress points for foam construction aren't something he has. I'm actually here posting as a bit of a fact-finding mission to determine the feasibility of a foam core RowCruiser, since it's a question he says he gets asked frequently.

>why are you afraid of Plywood ?
Oh, I'm definitely not! Right now I'm just trying to learn what would be required to convert a plywood design to foam core construction, to potentially achieve the lightest weight craft possible for the given design. Maybe I'll learn that it's not feasible/too hard to be worth it, or not enough weight reduction can be achieved to warrant the effort.

 

I'm borderline obsessed with blue water 26 as it imitates ply very closely but isn't ply. But it's expensive and caustic so there is that downside, I'm not sure if I've landed grinder to a material that irritates any open skin more.

 

Oh, you mean one of the Coosa boards
They don't look like you could do hulls with them, they look totally stiff and non fair able

 

Well, it would be pretty complicated, Eric,

The weight gains would be incremental and only if you drove some cost and did it in infused or wet bagged carbon. The issue is to make a puncture resistant foam; you need to add enough glass and glass is heavy. The way to reduce the weight of resin in glass is via vacuum.

Assuming the boat is specified in 6mm plywood with 6 oz skins now; you could probably make the thing in 4mm okume plywood with 5oz carbon skins and vac bag it. Weight 4'x8' panel is 12# plus 7.1yds time 5oz or 35.5 ounce skins and call it 20 oz resins for 55.5 oz sheatbing or say 3.5 pounds for 15.5 pounds. And you have a flimsier boat, but saved 6 pounds or more against assumed specs of 6mm ply and some heavier glass skins.

12mm 4# foam is 5 pounds per sheet which allows you about 10 pounds for glass and resins. 12 oz glass at 7.1 yds is 85.2 oz plus say 50 oz resins is 135 ounces or 25 ounces under the 10 pound constraint. So you make it but the savings is under 2 pounds. So then you get into using carbon skins on foam to save weight. And things get very expensive ito time and money to save a few pounds and make a boat that is not as R2 Alaska capable, perhaps. Lotsa assumptions here..but the issue is incremental gains for some added risks and a sort of experiment to build a tortured panel foam panel that is stiff enough for the ?jig while saving a couple pounds per 4x8 sheet.

Then as pointed out above; the hardpoints and beams all need to be made in something stronger with improved attachments vs foam..so all of a sudden; these incremental gains start to get wiped out by the akas and attachments for masts, etc. And all of that stuff needs to be embedded in the foam core before skins are applied; generally. And, because you are in a sort of risk mode; you also need to take risks with bonding. Bonding, or tying foam panels together requires more glass tapes than a stiffer ply panel; generally, as well. This adds back some weight, and can also wipe out gains.

In a small boat; do you want to make it weaker to save 10 pounds net?

I spent more time drafting this contribution than I care to, but hopefully you understand; this is the business of hair splitting and taking risks against safety margins; not simply following Gerr or others guidance for laminates which would be too heavy for a return of effort.

For this reason, foam boats typically have a practical cutoff around 18-20 feet; below which point not a lot is saved. Now, that said, I worked on a foam dinghy design with some others here that is in foam and we got the weights down, but the thing is not made for hard use either..but light enough to be pulled up onto a catamaran by the bow...but one not built yet... Every ounce gets considered.

So this type of thing can be done, but it becomes a lot of effort for small margins of weight savings and usually some tradeoffs in safety.

If you are serious about such an endeavor; you need to understand the amount of square footage of hull and deck in the boat, the original layup plans and see whether there is any room for gains. If the boat is already specified in 4mm okume; then gains will be very hard to come by..

Should add, changing things like the crossbeams to carbon can be done, but the risk reward is not great for a typical user.. maybe if you wanted to do the R2AK

 

@fallguy thank you so much, this is the answer I needed to hear.

"The difference between successful people and really successful people is that really successful people say no to almost everything."
~Warren Buffet

This one's going to be a 'no'. Will be building in plywood, 100% based on that guidance. The boat is indeed specified in mostly 4mm Okoume.

Thanks.

(FWIW, I'm not planning on doing the R2AK in this boat, at least not any time soon. My short term goal is to build a sailing RowCruiser "Yaka II", retrace the Caribbean route of Frederick Fenger/Yakaboo)

 

Have you calculated the weight using low density lumber and light weight plywood? There are many things that will stay the same. For example, the crossbeams, rigging, masts and hardware. There used to be plywood with end grain balsa core.

 

It looks like this is a well thought out design and set of plans. I'd follow them to a T.

 

As fallguy said above, it is a game of compromises. If you are willing to baby it, it's amazing how little carbon is actually needed to keep the boat together. If the goal is to build the lightest possible version then it's worth paying the price.
Otherwise there isn't a lot to be gained the RowCruiser is 148lbs and the sail option adds 72lbs to that. If a "normal use" foam/glass version shaves 20-30lbs from the 220lbs of the ply version, is it worth the aggravation of having to build a mold?

As for how to convert, you take the lines, redraw them to round bilge and forget you ever heard about what thickness plywood was specified. Then you take a book on boat design and start calculating the structure from scratch.

 

For a boat that will be beached and sailed in shallow water, puncture resistance is very important. To get the same value with foam core construction, the weight will most likely be higher.

 

Unless you have some tooling to work with,forget it.Without a mould you have very little chance of creating a hull by infusion.Yes you can lay up flat panels and persuade them into place.Will the weights come out close to the plywood version?A laminator with a good deal of experience would have a rough idea of the resin/glass ratio he can achieve.Somebody who is learning infusion as he goes and beginning with the most critical parts of a boat's structure has little chance.A composite engineer could produce a laminate schedule but their services would take the whole exercise out of the realms of the affordable home built boat.If you were venturing into the world of series production and could amortise the cost over a few dozen boats it might make sense.The way to get afloat is to build what the designer has drawn.Using the project as a way to learn hull production by infusion will take longer and cause heartache.The other consideration that needs to be kept in mind is whether you would enjoy actually sailing the boat-have you actually sailed one?Spending 1400 hours over a few years to create something that turns out not to match the dream is a tragic waste of time and happens rather more often than you might think.I don't doubt that somebody will jump in and tell me I'm wrong and it can all be done,well it can be,but there may be a lot more expense and suffering than the usual daydreamer can imagine.

 

I'll cut a slightly different direction, I'd find a core supplier and get some samples. It becomes apparent pretty quick the different properties. I'd bet you could almost do a 1:1 substitution was something like the blue water 26, for the lower density you're probably going to have to go thicker or with more laminate on the top. I'd be willing to bet with samples in your hand and a little bit of understanding of basic working or handyman work you could get close enough.

Now if you want to get it 100% right the first time and you have very little experience hire an architect and maybe a builder. But the reality is most of us can fiddle around with these things and find something that works, or conversely find a deeper appreciation for things that already exist.

Friend was poking around a couple years ago looking for some guidance on some small boats he was wanting to build. Got similar responses as to what's in this thread. He pretty much ignored them all and pushed forward with his idea, first two boats were a little rough but he learned a lot and he figured he had in materials roughly what it would have cost to hire a good design. And they were good enough boats for his kids to bounce off rocks and use through the teenage years.

I'm home for the weekend and was out admiring his 11th build. It's spectacular both in performance and anesthetics. Asked him if all his research helped him out, and he felt like it did. But he would have turned his filter up about 95%. Said the internet helped guide him to products that were time saving coming from baseline knowledge but on the material front gave very little actual help. His big learning on that came from a 15 minute conversation that he had with an engineer that was provided at a composites show by a supply company. His take was that 15 minutes supercede the 100 hrs reading various forums. After that the learning came from doing, lots and lots of doing.

While the doing has a dollar attached to it, it's part of the fun.

 

I don't disagree with the general thrust of the previous post as far as trying things in order to become a better boatbuilder.If the OP wishes to get afloat in the boat of his choice,it will be quicker and less complicated to stick with what the designer intended.I wonder how many of the comments came from people who actually followed the link to the design and considered the details of the whole process.For instance,how to incorporate tougher inserts where loads are applied,how to bond the deck edges,attaching the chocks for the cross beams and actually working inside a quite confined space.I haven't mentioned puncture resistance or weight because that was covered earlier.Now our prospective builder has chosen to go with the known quantity of the original design,those matters won't crop up and he is likely to be sailing a lot sooner.

 

Fair points wet feet. Figured anyone wanting to make a very hyper specialized race to ak boat was already fairly well versed in both hyper endurance as well as sailing. Probably a mis judge on my part.