Weight guesstimates.

[whb]

Hi All,

Trying to design a flat bottomed river skiff. it will be fibreglassed on account of the various gravel bars etc. it will encounter.

Question is does anybody know roughly what a square foot of say 6 oz. cloth with the right amount of resin might weigh.

Also, I have been using 25 pounds per quarter inch of thickness for a sheet of fir plywood. I think I got that number from someplace reasonable but can't find it again.

Thanks,

Howard

[Bruce Taylor]

From Skene:

Weight in lbs. per sq. ft for a given thickness of fiberglass (30% glass, 70% resin):

1/4" - 2.0
3/8" - 3.0
1/2" - 4.0
5/8" - 5.0

Fiberglass, weight in lbs / cu. ft: 96

Fir plywood: 36 lbs / cu. ft

[Bruce Taylor]

Oops...just noticed I'm addressing a countryman. Amend "fiberglass" to "fibreglass."

[whb]

Thanks Bruce,

We do have a split personality when it comes to re/er don't we.

Boy, that fibreglass stuff is heavy I wonder UHMW plastic is lighter. The main goal is protection of the bottom of the boat from wear.

Thanks,

Howard

[LongJohn]

whb -

John Winters suggests some numbers for this at the Green Valley Boat Works site: What Will My Boat Weigh? (http://www.greenval.com/weight.html)

He says a single layer of 6 oz cloth with resin will weigh about 1.34 oz/sq ft.

- John

[whb]

Thanks John,

Between the site you posted and Bruce's numbers I should be able to come up with pretty accurate guesses on my boats weight. I am trying to get to a 3" draft unloaded and a 4" draft loaded so I will be estimating fairly closely.

Thanks again,

Howard

[Bruce Hooke]

No offence Bruce, but for coating with 'glass cloth I would focus more on John's figure. A 1/4" or more of laid up glass & poly resin is a somewhat different beast from a single layer of 6 oz. cloth in, I assume, epoxy resin...

[James R]

A layer of 6 oz cloth saturated in epoxy with a further 3 coats of epoxy to fill the weave should weigh about 0.14 lbs/sq.ft. The 1.34 oz/sq.ft (~.084 lbs/sq.ft) from the What Will My Boat Weigh? site doesn't include the extra coats.

According to Skene's a cubic foot of fir ply weighs 36lbs. A 4x8 sheet of ply should weigh about 24 lbs or .75lbs/sq.ft.

[ 01-03-2003, 12:48 AM: Message edited by: James R ]

[whb]

Bruce the second.

I actually think I will benefit from both sets of numbers. Part of the boat will be a normal fibreglass and epoxy covering. Other parts will be much more heavily coated to withstand the anticipated relationships :D with gravel bars.

But I agree that most of the boat is likely to be lighter than a 1/4" coating.

Howard

[JimConlin]

To answer a bit more completely,
Oz. glass/ft^2 Lbs./ft^2
4 0.07
6 0.10
12 0.21
18 0.31
24 0.42
30 0.52
36 0.63
42 0.73
48 0.83
54 0.94
60 1.04
66 1.15
72 1.25

This assumes 40-60 glass resin ratio which you can better with careful squeegeeing. Vacuum bagging will be lighter still.

FWIW, if the area of the bottom equals the are of the waterplane, 1.25 lbs./ft^2 will sink the boat 1/4" deeper.