Vertical foam plank design conversion and technique

I’ve been reading on here for a long time and normally keep quiet but I’m getting close to starting what to me is a big project.

I’ve decided to build a 23 foot 7000 lbs displacement cruising catboat designed for construction in unsheathed wood strip plank construction. At first I was planning to build it sheathed strip to MacNaughton’s scantling rules but the more research I do the more I’m considering building foam core, not to reduce weight but to have a low maintenance and mostly wood free hull with any weight saving being used on cruising stores. It’s more of a learn something new build than anything else.

I’d likely use formulas as laid out by Gerr and am interested in other resources and reading material of foam core construction as I’ve never worked with the material. Do folks here have any suggested reading lists or resources?

I’ve become most interested in lofting her out and building female half molds laying battens down horizontally then vertically stripping with foam using heat as needed and exterior screws to hold the foam to battens, sheath the inside, add plywood bulkheads in place fillet and glass, roll the hull and sheath and finish the exterior before rolling back for interior fit out and decks.

Sounds kind of complex and maybe not as pleasant as working with cedar. But I think it might be fast? I’m looking for a long lived low maintenance hull. Thoughts?

Any recommendations for foam choice and fabrics? I plan to use epoxy throughout.

I think the removal of foam and and the use of solid glass at the stem, keel and hard wearing areas are the most difficult to wrap my mind around are there other methods such as using a laminated timber stem instead?

Again I’m all ears for good books on the subject and helpful hints.

Thanks,
Close Call

Welcome to the Forum Close Call.
@fallguy is your man for any discussion about foam cores - he has built a wonderful Woods Skoota power cat with this material, and she looks immaculate.

There are lots of good books on Amazon about fibreglass boat building - I have a copy of this edition of The Fibreglass Boat Repair Manual by Allan Vaitses, and it is very good. Although I am astounded by the 2nd hand price mentioned. I shall hang on to my copy.....
https://www.amazon.com/Fibreglass-Repair-Manual-Vaitses-1990-09-03/dp/B01K8ZA49I/ref=sr_1_7

By extension, Vaitses' book about one off fibreglass boatbuilding should also be good, however it is a bit older (but I would hope that it still includes foam sandwich construction).
https://www.amazon.com/Boatbuilding-One-Off-Fiberglass-Allan-Vaitses/dp/0877421560/ref=sr_1_8

Have you designed the hull lines (and the rest) yourself?
If so (or even if not), can you post any copies of the design sketches / drawings of the catboat please?

Thanks Bajan. Kudos to @Richard Woods

It is not wise to modify the build plan based on reading Gerr. A lot of people think they can just read a book and extract a boat. Please don't. It is really folly.

Building a boat is a difficult job. Messing up a design by an amateur is easy.

Foam is slower than cedar, but if you glass the foam as a monocoque type structure it is pretty fast. A lighter boat allows for more wine onboard!

Much slower is wet bagging foam components.

Areas of loading typically are specified in higher density foams, but this is also tricky to spec.

I encourage you to either hire help or find a different design in foam.

A cedar hull is self supporting. A foam hull is not. Even jigging or placing bulkheads is a requirement.. My boat was drawn by Richard, and he spent quite a bit of time making it work in a full female mould which was really ideal..

A hull, when glassed on one side, may not be strong enough to hold her shape when released from the mould.

Anyhow, no desire to stop you from building, but designing is relatively impractical.

Thanks for the book recommendations I’ll try to track down some copies.

As for the design it’s catboat from the board of a passed on designer. Not a whole lot of choices in foam core catboats. They are usually carvel or strip which I have zero interest in. Sheathed strip plank I can swallow as I think in a well built boat it is relatively low maintenance construction technique.

Just to be clear because sometimes it’s not. I’m talking New England catboat, one mast forward in the bow kind of boat. Not a multihull.
The design has the scantlings of a tank not an airplane. (BTW I think Richard Wood’s cats are great and he’s a super nice guy)

I’d happily hire an engineer or a NA do to the calcs and scantlings in foam core but I don’t really know who to ask and didn’t think NAs like to mess with other designer’s work and I’m not really interested in commissioning a new design as this cat has some unique features that really fit my brief already.

I had an old peter and duff dovekie boat years ago and it was airex foam boat that was so stiff and light, I always wanted to try that build technique. Maybe I’ll stick to cedar.

Oh, I well know what a catboat is...

The thing with cedar is the density is about 21#/cuft...corecell M80 is 5 or so.

So you have a high density core in the entire boat. corecell's higher density core is 12# which is still less than the spec'd core

It doesn't mean foam can't work, but someone has to go through and work through the where to insert the HD core and someone needs to do the laminate schedule. And you have to decide how to deal with the lighter hull.

If you have concerns the boat is overbuilt, it may be of interest to hire someone no matter the core.

There are a few here capable. Not me.

I have built both wrc and foam core. Sheathing cedar can be a slow process, but there are ways to make it go faster. Working in multiple directions is helpful, so starting on a chine can work. Wrc is really expensive now. The quality is also not so wonderful. And this leads to lots
Of short lengths which slows the work.

I realize I am talking all the negatives, but you need to know the pitfalls.

Fallguy, thanks for all of the comments, I’ve thought through a lot of these pitfalls but only know enough to be dangerous. I know wood construction, foam is a bit of a enigma to me.

I have to admit I went and got a quote for the wrc, and prices are so silly right now it made foam seem more palatable in the current market.

I think I’ll just stick with wrc and hope the market settles a bit before I’m ready to start ripping strips.

Close Call said:

Fallguy, thanks for all of the comments, I’ve thought through a lot of these pitfalls but only know enough to be dangerous. I know wood construction, foam is a bit of a enigma to me.

I have to admit I went and got a quote for the wrc, and prices are so silly right now it made foam seem more palatable in the current market.

I think I’ll just stick with wrc and hope the market settles a bit before I’m ready to start ripping strips.

Click to expand...

why not ask on the forum if anyone would redo the boat in foam for you...is it a William Garden?

As fallguy mentions, the real problem is what to do with the massively lighter hull. The usual solution is to redesign the underwater part of the hull, wich is the same as creating a new design. The other solution is to match the original weight, wich can be best done with a solid fiberglass hull. One example is the Folkboat, where the fiberglass version is massively overbuildt to match the wooden boats weight and weight distribution.
If I understand correctly, you want a lower maintenance version of a wooden design, while maintaining its sailing characteristics. The simplest way, and achievable for the amateur, is to calculate the weight of the wooden scantlings, then convert this weight into fiberglass thickness, maintaining the distribution.
Example: wooden skin is 1" DF with 2×2" steambent white oak frames on 10" centers. One sqft of skin weighs 2.8bs plus one frame section 1.1lbs. The replacement fiberglass panel must weigh 3.9lbs. To convert this into thickness we take an arbitrary laminate, say poyester with one layer 2oz CSM + one layer 24oz WR, wich gives us approx. 0.7lbs/sqft and 0.098" thickness. The final laminate will then be approx. 1/2 inch thick.
The only thing left is to check stiffness, and see if we must have local stiffeners. Usually one keeps the bulkheads, and the floors in the way of the keelbolts, and sometimes the stringer.

Building can be done on a male mold, with polyester, cheap.

Hello Close Call,
I have helped build a trimaran using the vertical foam method, and the building of the hull was (relatively) easy. The vertical foam strip method worked well. It was a Farrier f22 trimaran. First one half hull is built in the female (stations and wood battens) mould, sheeted on the inside, and the main bulkheads and floor/bunks are added. The half hull is then stiff enough to be released and put aside. Next the stations are turned around, and you build the second half. Glue the two halves together, fair the outside foam (relatively easy as opposed to fairing wood) and sheet the outside.
My feeling is that for building it's not more difficult or messy than strip planking, as you have to sheet the wood hull anyway, and the foam is easier to work with.
Greetings, Hans

PS the blog of this build is here F22 trimaran building by Menno: Starting planking + question http://f22bymenno.blogspot.com/2007/09/starting-planking-question.html , and there are a lot more blogs to find for examples of this kind of build.