Plans conversion - Wood to PVC foam

[Stefano Dilena]

I run a search on the forum on this before posting but could find anything, that's why the open question:

I have plans for a sailing plywood cat - 9m length. I want to build it in PVC foam sandwich.
I downloaded the book "The elements of boat strength" which goes through the scantling rules in good details.
Can I do some reverse engineering by calculating the scantling number that has been applied for the plywood plans, and use the Sn just found to re-calculate the PVC thicknesses required?

Is there an easier way to substitute 8mm, 6mm and 4mm okoume plywood without all the calculations?

Main reason for this is that being a small cat, I would like to get rid of all the stiffeners.
Also the weight saving would give additional payload..

Cheers!

 

[fallguy]

Not really. You need to provide the plans to someone capable of redoing them in foam. If they are copyrighted, do not post them here.

Even things like expected seas are relevant, ie bluewater vs nearshore, etc. Bulkheads and stiffeners are not arbitrary either.

Sorry to be a downer.

 

[DogCavalry]

If the boat was properly designed for the same intended use, you should indeed be able to back out the SN and reverse engineer to something workable in glass/foam. But heavier than something designed from the beginning by someone good with that construction technique.

Gerr's book gives nice strong capable boats, but sometimes a little heavy.

 

[bajansailor]

Stefano, can you post some details of your 9 metre cat design please?
Not the construction plans as they are copyrighted, but maybe a general arrangement drawing, or at least a side profile / plan view on her?

It should be 'do-able', re converting the plans to foam sandwich instead of plywood - Richard Woods' strip planked Sagitta is available as foam sandwich, but only professionally built according to his website -
Sailing Catamarans - Sagitta - 9m round bilge performance cruiser http://sailingcatamarans.com/index.php/designs-2/3-25ft-to-30ft-catamarans-designs/171-sagitta

 

[messabout]

It is not a sure thing that you would save any weight by using foam sandwich. That is particularly true if you aim to eliminate some of the bulkheads. In that case, you have to make the sheeting thicker and use more glass. It is a sure thing that the labor content of the boat will increase by a large proportion versus using ply.

Sure as shootin' the foam "meat", in your sandwich, will eventually absorb some moisture over time. There goes your aim for lighter construction.

 

[DogCavalry]

I myself agree with Gerr. The best technique for a one-off is strip plank composite. Although I would have saved a little time with plywood.

 

[Stefano Dilena]

Hello everyone! You can see below here the basic layout and BH arrangement.
The designer in question builds only in plywood.
LOA: 8.6m
Beam c/c: 4.3m
Displacement full load ( which for this design is WL): 2800kg
Max righting moment: 5400kgm
8mm bulkheads and outside panels
6mm long. bulkheads
4mm furniture and roof

All the boat is developable shapes, so no strip planking needed if building with foam.. flat infused panels will be the way to go..
Regarding the weight saving, the only way to know is to decide on foam thickness and recalculate weight, otherwise we are just talking air...

Cheers