View Full Version : PVC foam core thickness?

01-28-2005, 08:01 AM

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?


Daniel Peeters

01-28-2005, 08:26 AM
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?

01-28-2005, 08:31 AM
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

01-28-2005, 08:55 AM
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.


Daniel Peeters

01-28-2005, 09:39 AM
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.

01-28-2005, 09:52 AM
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.


Daniel Peeters

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

01-28-2005, 10:04 AM
Maybe I'll have a look at it, but sailboats is more "my thing" :-)

01-28-2005, 10:11 AM
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 :-)


Daniel Peeters

01-28-2005, 01:17 PM

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.


01-28-2005, 03:16 PM
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

shear strength 147Psi
tensile modulus 231Psi

Could this be enough to determine some more?


Daniel Peeters

01-29-2005, 07:41 AM
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
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?

01-29-2005, 10:05 AM

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


Daniel Peeters

01-30-2005, 11:55 AM
Please let us know how it goes :-)

01-30-2005, 04:09 PM
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.


Daniel Peeters

01-31-2005, 07:12 AM
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?


Daniel Peeters

02-01-2005, 04:53 AM
Glass in the middle of the lamination will not do any good. It's a waste of money. When a laminated panel er beam or whatever flexes or bends, there is no deformation in the middle. There is pressure on one side and tension on the other and zero in the middle. Do you see?

I (and Mr Gerr :-) think it's better to have the same core thickness all over the hull and then put on one or two more layers (depending on weight) of glass in heavily loaded areas.

02-01-2005, 10:28 AM
Waste of money it is!

Thx for the replies it was verry usefull.

Any hints are well apreciated


Daniel Peeters

02-01-2005, 11:46 AM

Given the DWL of 6.1m. and dividing by 6 i get 1.0 frame spacing. This will be the panel length. Assuming breadth at waterline is 90% of molded breadth, i get 2.8*0.9=2.52m. Placing two longtitudinals per side, i will have 2.52/2/3=0.420.

Estimated panel size without deduction on stiffener base i have 1,000 mm. span x 420mm. breadth.

Aspect ratio would be 1,000/420=2.38.Ar then would not require a correction factor.

Bottom pressure was calculated as;

@30 knots @20 knots
62.7 kN/m2 (9.0 lbs/in2) 31 kN/m2 (4.5lbs/in2)

Estimating core thickness, we have;

phi= 0.144 (constant)
ks= 1 (correction factor for panel aspect ratio greater than 2.
b= 420 mm.
P= 62 kN/m2 Bottom pressure @ 30 knot, 2.5 tons
Etps= 11,000 N/mm2 tensile modulus minimum for sandwich facings.

phi*ks*b*cube root (P/Etps)

=0.144*1*420*Cube root (62/11,000) in mm.
=23.67 mm. (use 25mm. core)

Depending on the governing rule in your area, there is a minimum inner and outer skin for cored laminate. The outer(wet) skin being constantly under compression, hence thicker, plus the requirement to meet shore hardness due to slamming forces.
LR rule requires 5.5 mm. for outer skin and 3.5 mm. for inner skin.

Laminate schedule would approximately be (without correction factor for 50% glass content requirement),
Ply Glass Gc Layer Thickness Total
Inner WR600 50% 4x
CSM450 33% 1x 3.8 mm.
Core PVC 1x 25 mm.
CSM450 33% 1x
WR600 50% 5x
Outer CSM450 33% 1x

Veil cloth 5% 1x
Gel coat 700 to 850 gr/m2
CSM should be laminated on both sides of core.
CSM on wet for water barrier and prevention of "print thru"
Veil cloth and gel coat not included in calculation as they are non structural but should be included.

02-01-2005, 11:53 AM

At this point, i have not calculated the stresses on the core as the face of the sandwich is sufficient to carry the stresses.

I still have to do some conversion on the published data to run a full scale analysis.

Will send as soon as i finished.

By the way, the rule of thumb is great. Saves a lot of time and money. As soon as i get comfortable with it, i will use it. Have to practice on it.

02-01-2005, 11:59 AM

LR rule is 4.5 mm. outer and 3.5 mm. inner. Sorry, sent it without editing.

Rx :o

02-01-2005, 01:50 PM
Is this your hobby or your work. It looks great to me. I could get along with that for a start

Daniel Peeters

02-02-2005, 03:34 AM
Interesting, compare with Gerr's rule of thumb:

LOA 7.5
Nom. Thickness Solid GRP: 6.0
Speed add. 20 %: 7.2
Topsides 85 % 6.1
Bottom 115 % 8.2
Core, basic 220 % 18.1 Use 20
Core xtra 150 % 27.2 Use 25
Outer 40 % 3.3 Use 4
Inner 30 % 2.5 Use 3

02-02-2005, 04:14 AM
Looks fine,
So in a general way I could get along with this data. It will permit to do some more price calc to estimate the cost of the project.

Thx for the great info.

PS you guys don't state any disclaimer of reliability. So if my boat sinks I can sue you???? LOL But I wont :-))

If it sinks it will be my fault. I think with the foam core it even wont sink, there would minimum be around 1.8m3 of foam without extra bouyancy foam?

02-02-2005, 04:36 AM
DISCLAMER: All advice is given without any expressed or implied responsibility....

1.8m3 shoul be around 70m2 of 25mm foam, is that right?
Then you have some floors, frames, longitudinal stiffeners etc, etc.

02-02-2005, 05:06 AM
Yes indeed around 75m2 but I will get more foam,
as for the deck I will be using a bit thicker foam I think


02-04-2005, 10:05 AM

Too expensive to be a hobby. I'm a boatbuilder and do the preliminary design analysis (design spiral). I'm more into composites and do hull structure analysis, cylindrical tanks, aero parts, and cars.

If you would be interested, i can send you the simplified powerboat design spiral. It is a guide but as it gets tighter, it helps you keep track of your changes.



02-04-2005, 10:18 AM

Rule of thumb is great because it gives you a foundation on where to start instead of guessing. I did not know there is one for foam.

I use it most of the time in preliminary design work. that includes powering and prop selection.

It is only when that extra level of confidence is needed (like you could get sued, :) hahaha, do i run a full scale analysis.


02-04-2005, 12:17 PM
As you say to expensive to be a hobby.
I am an amateur you 're the pro.

I will learn of the mistakes I make, but this will cost money.

So all help I can find or get is highly appreciated. So the calc you are talking about is welcome.

Kind regards,


02-06-2005, 10:26 AM
the calculations are they based on linear or crosslinked foam?

02-07-2005, 12:28 PM

The calculations does not say what type of foam.It is only required that mechanical properties be met.

I ran some stress analysis on your proposed design. I am not too comfortable with the 25mm. thickness even at 200kg/m3 density. There is failure on core shear even at 900mm. panel length. The only solution was to reduce bottom pressure by reducing max speed at 2.5 tonnes displacement.

However, by going up to 30mm. for the bottom shell i can use a 180 kg foam core. I used Divinycell H data sheet. These are the resulting data.
Inner skin 3.9 mm.
Core 30mm., 180 kg/m3. (or 200 kg/m3)
Outer skin 6.5 mm. (5.5 will do but i used 2x CSM450 nearest gel coat for geater water barrier)

Side shell- Not much pressure as it is above waterline(except on slamming zone) Use a lower density foam.
Inner skin 3.9 mm.
Core 20 mm.,130 kg/m3
Outer skin 4.8 mm.

Weather deck- assume designed only for walking and not for cargo storage.
Inner sKin 3 mm.
core 20mm.., 80 kg/m3
Outer skin 3.5 mm.
Watch out for beam stiffness on center of frame because of the long span. Use pillars to halve the beam length/span. Pillars, although obstrusive on small boats but do provide you with something to hold on.

In general, keep frame spacing to less than 2m. use secondary stiffener or shear ties in between frames/bulkheads to maintain panel length of less than 1 m.

Above analysis although optimized but still you have to comply with local regulating body if needed. Strss analysis varies from station to station. Data was analyzed at midship where the greatest stress concentration is at.

Nice boat design you have. Happy building. :)

02-07-2005, 12:37 PM
Thx for the extra info, as I said more than welcome.

I am not sure about what you mean whith panel length?

thx for the compliment, If I can establish to make what I am drawing that would be great !!!



02-07-2005, 06:00 PM
I think I have to get some rest?

a laminate whith a 25 mm core has a lower density 542Kg/m3 with a 40/60resin ratio
than a laminate whith a 20 mm core? 579Kg/m also same ratio

Just as I want to determine the weight per m3



I am gooing to sleep, designing from 10 till 24 is more than enough! :o :o


02-09-2005, 10:46 AM
When you draw the profile of the boat, you will subdivide it it into equal transverse divisions. This is called stations. These stations may either be bulkheads or room divisions. On areas where there is no divisions you have to put in frames to support the laminate. The frames i usually use is the "tophat" design.

When you draw the midship of the boat, you will have to subdivide it longitudinally. These are called longitudinal stiffeners. These are usually smaller in dimensions than the frames.

Normally you would divide the boat so that each panel size is bounded by a longitudinals on each sides and frames on the ends. The center to center distance between frames (or longitudinals) are called frame spacings. For optimum design, the length should be twice as long or grater than the breadth.

This is true whether you are designing with a cored panel or single skin.

The panel bounded by the frames and longi is the panel size. Panel size means the inside length and breadth. Do not confuse the panel size by the spacings.

Density of the core is as defined by the manufacturer. When you add a laminate of both sides, this is called laminate density. The more laminate (skin) you add, the greater the laminate density but the core density stays the same.

I forgot to tell you that in my analysis, the fiberglass was holding up to the stresses but the core was a wee bit high on shear stress. A touch on the keys (30 instead of 25) solved the problem. I then backed down to using 180 gr/m3 and there was no problem. I can go to a thicker core and reduce laminate thickness but that is fine tuning. when you get all your dimensions final, lets see what we can do.

Have some sleep. ;)

02-09-2005, 11:40 AM
Ok I am at the same wave length as you are. I think
So the panel length is the distance between two stations: so when making tophat stations (40x40mm) with the centers spaced at 500mm it will give a panel length of 460mm?

As yo calculated the panel length at 1000-900mm it is shure it will have to endure more severe loads. But in order to reduce the stresses couldnt we go to 500-600mm
spacing for the stations? I have to say that the bow to midschip will be filled with bouyancy foam to create the floorplate and some extra bouyancy in case it would sink. I hope not. Will add pic when at home.

The thing is I calculated the price using foam core with a much lower density I think 80Kg/m3 and I allready get a high price so what if I would have to use 30mm with 180-200Kg/m3 then I will have to stick to traditional glass construction.

Seen from stern to midship I will have heavy parts.(jet + engine + feul+ watertank)
So I intend to put more logitudinal reinforcements from the stern to past midships So the hull wont crack in two in the midship. Waht about this?

THX in advance for your precious time.


02-09-2005, 04:22 PM
Hi RX,

I did some more drawing concerning the stiffners I put the stations at 600mm
the longitudinals are put a bit random.

I would think that this is way to much for a liitle thing like this?
what do you think?


I don't see my attached files, so here are the links

Frames A
Extra buoyancy foam
Frames B

02-10-2005, 12:46 PM
Yes, you are correct. Spacing minus the base width of stiffeners. That is panel length. Goes the same way for panel breadth.

I usually divide the LWL by equal spacing. Grat for simpsons rule. If i have unusual placement of bulkhead in some areas, i divide still the station to get half station. so 6.1 m DWL/6= 1.0something right?

If you divided your waterline by 12 and have 500mm. spacing, then it will be more rigid and i will have to recalc again because then a thinner laminate and lower density core will do at the expense of more work putting up all the added stringers. This is what design spiral is about. Number chasing does not end until you get the optimum cost and weight.

Well, you will be producing only one boat but in production, we cannot afford to be a hobbyist. Less work is always better. and we cannot afford to let the part stay in the mold too long as the second unit is waiting to be laid up.

Dont worry about the stern. I calculated at Midship where the greatest stresses is located. There is a reduction of stress at the aft and the bow.

Will take alook in your drawing tommorow. hope you have midship drawing.


02-10-2005, 01:05 PM
THx for all the help.
Today I received some books, but it will take some time to read them and understand them completely.
I got the: Elements of boat strength By Gerr, understanding boat design by Brewer, fiberglass boats by du Plesis, elements of yacht design by Skene, the last one is for when I try to make a sailing yacht in the distant future :-)


02-10-2005, 01:45 PM

Try to obtain a copy of Sorensens guide to Powerboats. First part explains all types of hull form and behavior of boats. Great overview of what a boat is and its variation on design approach.


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