Wall Density

I am trying to calculate the weight of a dingy i know the area of all of the pannels on it and i know they are made of layers as follows

0.5mm grp
8mm of foam
0.8mm grp (including gell coat)

does anyone know rough densities of these?

 

It depends on the density of foam you buy. The fiberglass depends on the ratio of resin to fibers.

 

Gidday Hamish

Is it an existing dinghy or one you want to build?

GRP stands for glass reinforced plastic which is somewhat misleading and tends to suggest any one of a number of different composite construction methods. A heavy duty production dinghy would use lots of chop strand mat (csm) whereas a performace dinghy would use pvc foam core with 1-2 layers of cloth set in either polyester or preferably epoxy resin.

PVC foams such as divinycell or airex would typically be somewhere between 60-100kg/m3 for a small racing boat, with higher density in load points (or substitution with plywood or solid laminate)
This doesnt include fairing and filling. e.g. If you are laminating on foam you will need to fill the surface with lightweight fill first or else all your resin will soak into the foam and you will need more than the ideal amount to fully wet out the cloth.

For a cloth laminate (not csm) for example I would expect 100gsm dry cloth to give approximately 0.15mm of completed laminate. With hand layup you may get 40-50% fibre content if you are experienced.

As an example 0.3mm laminate gives 200gsm cloth which if divided by fibre contect at 40% gives 500gsm finished laminate, not including filling, fairing paint or gell coat.

Im not experinced with gell coat but can say it is much heavier than paint, in general use it will last longer, if light weight is a focus stick with paint. I recall there is a thread going on in Sailing Anarchy website discussing weight of coatings (in the Dinghy Anarchy forum). Also try the search function here, it will have been discussed 10 times already.

Please treat this as an example only as there are many factors that affect the final result and the numbers suggested are very general. Most people tend to use more resin than is really required (more weight), then there is tabbing, reinforcing of high load areas, stringers etc etc. Typically you could expect to double the basic panel weight to arrive at the finished hull weight even before fittings are applied.

To be certain why dont you make a sample piece and weigh it.

 

The best approach, if doable.

If not, you can get a ballpark estimate as follows:
Density of the foam varies, but you can assume around 100 kg/m3 as a first estimate.

Glass fibers' density is 2500 kg/m3
Resin, both epoxy and polyester, are around 1200 kg/m3

By doing few calculations you'll se that for fiber contents ranging between 0.25 and 0.35 (assuming a hand lay-up), laminate densities will vary between 1500 and 1650 kg/m3. So, for a first estimate you can assume a mean value of around 1600 kg/m3 for a laminate.

At this point, the calculation of the weight per unit area of the sandwich panel is straightforward:

Density, sandwich = 1600 x t1 + 100 x tf + 1600 x t2
- where t1 and t2 are thicknesses (in meters) of the outer skins and tf is the thickness of the foam. The resulting density of the sandwich is in kg/m2.

If you want to use millimeters instead, then the equation becomes
Density, sandwich = 1.6 x t1 + 0.1 x tf + 1.6 x t2

In your particular case, the calculation gives a value of 2.88 kg/m2 - you can round it to 2.9 kg/m2.

Hope it helps. Cheers!

 

The easiest way to calculate the weight of a dingy is to weigh it.

Poida