aitchw
New member
The GRP layup of older boats was often described in ounces, eg 6oz. Could anyone tell me what this refers to and can it be translated into an approx. thickness?
Thanks,
Howard
Thanks,
Howard
Old GRP layup spec
- Thread starter aitchw
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graham
Well-known member
The ounces refers to the weight of the glass mat used.It doesnt relate accurately to the hull thickness as it depends how much resin was used .Modern boat manufacturers control the ratio of glass to resin ,years ago it was mixed in buckets and slapped into the mould with brushes and rollers or sprayed on with the ratio of resin to glass being dependant on the skill of the operator(or lack of it.)
aitchw
New member
Cheers, Graham. I thought it might turn out to be one of those meaningless descriptions.
Steve_N
Well-known member
I would think it refers to the numbers of layers and weight of CSM (chopped strand mat) used? In imperial measurements this used to be sold primarily in 1 and 1.5 ounce per square foot thickness. Today's metric near equivalents would be the 300 and 450 grams per square metre CSMs.
So a 6oz. laminate would be 4 layers of 1.5oz. mat i.e. not a lot, probably in the order of 2.5mm depending on the factors mentioned in graham's post..
So a 6oz. laminate would be 4 layers of 1.5oz. mat i.e. not a lot, probably in the order of 2.5mm depending on the factors mentioned in graham's post..
Mudplugger
New member
The Westerly 33 of 79 vintage had a lay up of approx 38mm below the waterline and about 25mm above, which seemed to be fairly representative of the l;ay ups of that era. I think it all went Tits up when the fuel crisis of the early 80's forced the introduction of value engineering....?(got to make it cheaper)
oldsaltoz
New member
G'day Howard,
Many of the 'older' boats were moulded and the glass was sprayed on with a chopper gun that was fed by what looks like a rope, this was drawn into the gun, chopped and sprayed out with a mixture resin and catalyst, dye markers were put in the rope to indicate to the gun operator how much material had been added. This meant the operator could spray an area of say one square yard till the dyed glass showed, then move to the next square yard and so on.
However this method has nothing to do with a 6 oz lay-up, as Steve-N points out in his post, this refers to chopped strand MAT (CSM) not chopped strand from a chopper gun (CS).
The prime reason for older being thicker is the simple fact that in the early days of fibreglass boat building not a lot was known about it and more was considered safer, today the rule is less because its cheaper in some cases.
Some mistakes were made back then, toll little catalyst, too much resin, not enough glass, too much glass, humidity related problems, overheating due to thick application and the list goes on.
I hope this helps to define your question, and, you are not alone when it comes to how manufacturers, designers, engineers and boat builders describe their products and specifications.
Avagoodweekend……
Many of the 'older' boats were moulded and the glass was sprayed on with a chopper gun that was fed by what looks like a rope, this was drawn into the gun, chopped and sprayed out with a mixture resin and catalyst, dye markers were put in the rope to indicate to the gun operator how much material had been added. This meant the operator could spray an area of say one square yard till the dyed glass showed, then move to the next square yard and so on.
However this method has nothing to do with a 6 oz lay-up, as Steve-N points out in his post, this refers to chopped strand MAT (CSM) not chopped strand from a chopper gun (CS).
The prime reason for older being thicker is the simple fact that in the early days of fibreglass boat building not a lot was known about it and more was considered safer, today the rule is less because its cheaper in some cases.
Some mistakes were made back then, toll little catalyst, too much resin, not enough glass, too much glass, humidity related problems, overheating due to thick application and the list goes on.
I hope this helps to define your question, and, you are not alone when it comes to how manufacturers, designers, engineers and boat builders describe their products and specifications.
Avagoodweekend……
William_H
Well-known member
As already explained the weight of the cloth doesn't make so much difference because heavier cloth indicates more layers so the final thickness is what counts. Many boats were built with the chopper gun and seem quite OK.
For things like aircraft wings the strength to weight ratio is very critical. They use preimpregnated cloth and vacuum bag the layup with material to bleed out excess resin to ensure minimum resin max cloth. This gives a thin strong panel. However I think in boat hull stiffness in the fiinal product especially when not using a foam core is the primary consideration. This is only acheived by curves and thickness of panel and weight is not so much of a consideration. Hence the chopper resin gun was quick and gave a thick stiff panel albeit with probably more resin than optimum. Now especially for performance yachts and with the cost of resin there seems to be a return to laying cloth and using foam core to get the thickness hence stiffness. ( not to mention carbon fibre cloth or kevlar.
if youur question arises from a desire to replicate the original in a repair don't worry about cloth weight. Just use light cloth and put layer upon a layer until the thickness is made up. If however you wish you can buy woven roven which is just a super thick cloth which means less layers although it is easier to get a smooth finish with a light cloth on the outside. olewill
For things like aircraft wings the strength to weight ratio is very critical. They use preimpregnated cloth and vacuum bag the layup with material to bleed out excess resin to ensure minimum resin max cloth. This gives a thin strong panel. However I think in boat hull stiffness in the fiinal product especially when not using a foam core is the primary consideration. This is only acheived by curves and thickness of panel and weight is not so much of a consideration. Hence the chopper resin gun was quick and gave a thick stiff panel albeit with probably more resin than optimum. Now especially for performance yachts and with the cost of resin there seems to be a return to laying cloth and using foam core to get the thickness hence stiffness. ( not to mention carbon fibre cloth or kevlar.
if youur question arises from a desire to replicate the original in a repair don't worry about cloth weight. Just use light cloth and put layer upon a layer until the thickness is made up. If however you wish you can buy woven roven which is just a super thick cloth which means less layers although it is easier to get a smooth finish with a light cloth on the outside. olewill
aitchw
New member
Thanks, guys. It is more a curiosoty question than one with practical repercussions. I recently acquired a copy of an promo leaflet from around 1975/7 for the boat I own. Amongst other interesting titbits was a proud declaration that the hull layup was 6oz and at least double that in key areas (along the keel, for instance). My experience of the boat is that it is built like a tank and suffers no flexing anywhere. Where there is gel cracking it is clear that it has occirred through abnormal pressure/impact in the dim and distant and even now those panels do not flex when walked on. She is certainly heavy, around 1500lbs with basic kit on board but that gives her big boat feel in many situations.
The gel coat itself is very thick, a good 3-4mm where I have been able to see (small impact flakes from years ago).
So, as I say, just curiosity as I have no worries about her strength. She'll be going strong long after I'm dead and buried if anyone wants to take her on.
The gel coat itself is very thick, a good 3-4mm where I have been able to see (small impact flakes from years ago).
So, as I say, just curiosity as I have no worries about her strength. She'll be going strong long after I'm dead and buried if anyone wants to take her on.
