PVC foam core thickness?

[Danielsan]

Hi,

Could there be any sort of rule of thumb for realising a sandwich foam core
for a 7.50m x 2.80m planning (cruiser)?

Let's say seen from outer to inner hull:
- gel(top)coat
- 2or3 layers of fibre not defined (hints are welcome)
- PVC foam core (???mm)
-forgot one 1or2 layers of fibre
- light cloth for finishing.

It would be an average thickness, nothing to do with the reinforced spots around engine, and others.

Could it be a good idea to make a one piece hull + deck?

Greetz,

Daniel Peeters

[Raggi_Thor]

First of all, you should have approx the same thickness to outer and inner skin. One rule of thumb (from Dave Gerr) is to make the outrside skin 40% and the inside 30% of what a solid grp hull would have. Maybe Gerr tend to be on the safe side?

[mistral]

you'd better follow Raggi Thor advice, take a good look at Gerr's book, then you can decide to do it on your own, or hire a pro designer to set up the scantlings of your boat.

best regards
Mistral

[Danielsan]

I am already waiting for a longtime to get the book, still waiting,...

So following Gerr, a 2-xx-2 or 3-xx-2 combination would do fine? that let's us with the thickness of the foam core schould this be 3mm, 10mm, 20mm is there some rule for it?

I will more tend to make it a 3layers -10mm foam core-2layers.

greetz,

Daniel Peeters

[Raggi_Thor]

3 layers + 10mm foam + 2 layers may veruy well be OK, but I don't know the weight of the boat, the engine pwer, the intended speed, the weight of one layer, what kind opf fibre, what kind of resin, what kind of foam.
Don't you want to buy some drawings including scantlings?
Stcok drawings for boats like this seem to cost a few hundred Euro for a boat that will cost some tens of thousands.

[Danielsan]

I will try to get more precise
LOA 7.50m, Beam 2.80m, LWL?6.10?
displacement 2.5ton, planning mono hull, 22deg constant deadrise for 2/3 of LOA
would like to use jetdrive, Hamilton 274, ultradynamic 251,... HP250-300
hope to get it in the sea, North sea or mediteranian sea,

Fibres dont know it yet, it will be using marine worthy polyester resin, PVC foamcore 50kg/m3.

The thing is I am designing it myself as I did not find a model or size that I liked.
So buying would be a nice plan but designing it myself is a fine challenge.

Tonight I will put some pics in the pic gallery, I'll post the link.

Greetings,

Daniel Peeters

PS: no need to appologize for the english, mine isn't that good neither, nor is my Norsk
:-))

[Raggi_Thor]

Maybe I'll have a look at it, but sailboats is more "my thing" :-)

[Danielsan]

I must say sailboats must be more charming than powerboats but they look more difficult to build I think, rigging, mast(s), keel,... more delicate to mount correctly, etc.

I saw on your website that you are more into sailboat, that's where I see that my Norsk is not good at all :-)

Greetz,

Daniel Peeters

[rxcomposite]

Danielsan,

There is no rule of thumb but there is a formula to find the approximate thickness.

First, the bottom panel pressure has to be calculated. To do that we need the speed and the displacement of the boat.

Second we need to know your planned panel size so that shear calculations can be made.

Third we have to find the core shear strength. This is published by the manufacturer. We have to find the correct shear strength to suit as not only the foam core comes in several thicknesses but in several densities as well.

Regards

[Danielsan]

Thx Rx,

You can find earlyer on the discription of the boat.
I didnt mention the speed, I would like to get it to 20-30knt cruising,
I dont have the exact draft yet, but it will be around 0,40m.

The idea was to use Core cell "A" grade from SP syst.
More precise the A500 with a dens. of 80Kg/m3. panel size is not defined as it wil depend of the curvature of the hull to be covered.

specs for the core cell
http://www.spsystems.com/solutions/s...A-Foam0904.pdf

shear strength 147Psi
tensile modulus 231Psi

Could this be enough to determine some more?

Greetings,

Daniel Peeters

[Raggi_Thor]

Well, Gerr describes one thumb of rule, but of course you should calculate this mre carefully to get an optimum weight and strength. Anyway here's Gerr's rule(s) of tHumb:

Skin thickness for solid grp: 1.8mm + LOA(m)/1.8
Sandwich,
Core: 2.2 times the solid laminate
Outer skin: 40% of solid
Inner skin: 30% of solid

Add 15% to bottom and subtract 15% to the topsides.
Deck ~topsides but with 50% thicker core.
Add 1% for each knot over 10knots, that is 20% for 30knots.

Now, this is for traditional comboination of woven rowing + chopped matte and polyester.

All of the above is from "The nature of boats". Te bokk called "Boat Strength" or simmilar goes into much more details. Actually too mujch detail thinks MacNaughton (that I read the other day :-)

If you need 4mm skin that is about 6kg/m2 and 1/3 is glass while 2/3 is polyester, so the glass is about 2kg/m2 (this is just an example, put in your numbers and see what you get.)

What will the weight of your hull and deck be using this rule?
Approx 1/5 of the total?

[Danielsan]

Thx,

This should help to make the calculations for quantity and price.

Greetings,

Daniel Peeters

[Raggi_Thor]

Please let us know how it goes :-)

[Danielsan]

Ok thx,
When I start building I will put the entire process on my website. So who ever wants can have a look at it.

Greetz,

Daniel Peeters

[Danielsan]

Other thing,

when creating a hull formed upon wooden stations beginning from inner of the hull with the foam core, to the outer hull the glass, you would have difficult to create multiple thickness? Unless the stations are deigned to compensate the multiple thickness. Which to me are a bit unlikely, because of not knowing the exact thickness of the glass part? up to (0,5-1mm)
Would it be a problem to start with a general layer of foam covered with the glass and systematicly increase, decrease glass thickness where needed and then, when you pull off the hull from the stations to lay up some extra foam where needed. and then glass the inner of the hull. Same as you would do with reinforcements?
Would it be a problem to bind the two pieces of foam?
If this should be one couldn't you apply it like a real reinforcement so you would have from inner to outer: 1Glass-1Foam-1glass-1Foam-3glass for the reinforced places?
Or can't it be done this way at all?

Greetings,

Daniel Peeters