PVC foam core thickness?

[Raggi_Thor]

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.

[Danielsan]

Waste of money it is!

Thx for the replies it was verry usefull.

Any hints are well apreciated


Greetings,

Daniel Peeters

[rxcomposite]

Daniel,

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.

Formula:
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
Notes:
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.

[rxcomposite]

Daniel,

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.

[rxcomposite]

Correction

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

Rx

[Danielsan]

Is this your hobby or your work. It looks great to me. I could get along with that for a start

Greetz,
Daniel Peeters

[Raggi_Thor]

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

[Danielsan]

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.
Daniel.

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?

[Raggi_Thor]

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.

[Danielsan]

Yes indeed around 75m2 but I will get more foam,
as for the deck I will be using a bit thicker foam I think

Greetz,
Daniel

[rxcomposite]

Daniel,

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.

thanks

Rx

[rxcomposite]

Ragnar,

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.

Rx

[Danielsan]

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,

Daniel

[Danielsan]

the calculations are they based on linear or crosslinked foam?

[rxcomposite]

Daniel,

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.