Density of foam required for deck replacement in small runabout

[Ad Hoc]

It is all about the stiffness and the deflections... once understood...you can make any structure do anything!
It then comes down to practicability v costs.

 

[rxcomposite]

I was trying to ask if the foam density was sufficient . I finally was able to figure out that divinycell is about 5lbs/ cu ft. so I was wondering if the 20% decrease in density would be a problem or would the thickness of the core make up for the lower density?.

It is not about the density of the foam but shear strength. Rigid PU foam has about 0.5 N/mm2 which is nearly equivalent to DV H45 which has about half of the density. If you are looking at equivalent density, it will be DV H100 which has about a strength of 1.4 N/mm2.

Deck load is a live load and is constantly moving and add to the fact that it will be a concentrated load. Worst pressure of 180 lbs/(3" X 3") (going into English system) your weight divided by the ball of your one foot, with soft shoes on. Not to count the vertical acceleration or running on the deck.

Thus even with 3 layers of DV 1708, and 2" of foam, it will still deflect by 1.5" a little too much for the minimum 0.47" deflection. That is for a panel of 4' x 8'. We won't get anywhere without you posting the arrangement of panel size or a layout of the deck.

 

[fallguy]

I think by deck, the OP actually is referring to the hull inside liner which is technically wrong, but an important distinction.

The balance of your post is, of course, spot on. This is almost the same issue as the other guy who built witj no stiffeners.

 

[TANSL]

Worst pressure of 180 lbs/(3" X 3") (going into English system) your weight divided by the ball of your one foot, with soft shoes on. Not to count the vertical acceleration or running on the deck.

It can be even worse. When calculating loading conditions, heel due to personnel positioned as far as possible from the center line should be studied, assuming there are 4 people per square meter.
I know that is not the case with this boat. I mention this to point out that deck loads can be much higher than one might think. You cannot tell if a solution is good or bad without knowing under what conditions the structure works.

 

[rxcomposite]

It can be even worse

Like somebody wearing high heels.

 

[mudsailor]

Nobody has asked why?
It’s a small old boat, why spend lots of time and money on doing something that might (or probably won’t) work very well
The hull or deck doesn’t need to be sandwich construction. PU is very friable (falls apart easily, especially when subjected to lots of impacts
post a few pictures, but why not just rebuild it the way it was - most small runabouts are still built that way because it works well