Weight of Steel Vs GRP

J

jetwake

Well-Known Member
Joined
7 Sep 2009
Messages
96
Visit site
Purely as an academic argument, my brother insists that Steel yachts will not weigh more than GRP. We are actually arguing over Colvic Watsons. I argue that steel WOULD be heavier and thus the GRP chosen by CW was the better option.

Am I right???

David
 
It depends on size. Steel weighs much more than GRP, obviously, steel being around 490 lbs per cu. ft. and GRP around 100 lbs per cu. ft. Because there is a minimum thickness of plate that can be used for a boat, around 3mm I guess, a small boat will be relatively very heavy compared to a GRP boat. As the boat gets bigger the plate thickness increases but not at the rate that the GRP layup increases, so the weight difference diminishes as the boat gets bigger.
But, a steel boat of the size we are talking about will be substantially heavier than a GRP boat.
Whether this means the choice of GRP over steel is the right one is not quite that clear cut. The heavier boat will need a larger rig and a larger motor to achieve the performance of the GRP boat, but steel is less likely to be damaged if you hit a reef or a floating container.
 
John, Thanks for the info. Yes I realise that the choice is a complex issue, but the weight thing was the argument...and you have resolved it for me. Thanks.

David
 
The problem with steel is that the rigidity of any structure depends on material thickness so in smaller boats you have to make a steel hull to a certain thickness which in turn means that it is heavier and stronger than it needs to be. To the point where you start to struggle to get enough ballast ratio in a small steel boat.

As the boat gets bigger you can start to design a steel hull to the material strength limit instead so a steel hull starts to become less lardy. Nothing to do with availability - we make hot rolled sheet down to 1.6mm thickness and cold rolled down to feeler guage and below.

However, if you start using grp composites and exotics, I reckon you will always be able to make a lighter hull than you would in steel. Ditto aluminium. Ever seen a steel aeroplane?
 
GRP/Aluminium/Steel Aeroplanes

I've only heard of aluminium (modern) aeroplanes. Does that mean aluminium alloy construction for a boat (undr say 70 feet) is likely lighter than grp/composite, dya think?
 
Lots of GRP airplanes

monowheel_breakout.png


- W
 
Its possible that a well designed steel MFV type yacht might be lighter than a similar sized fibreglass Colvic, but more likely it would be heavier.

Alastair Hunter designed and built a series of small steel cruising yachts - I met one of them, Shuffle, all of about 23' long, here after she pitched up in the ARC, singlehanded, no engine, with a very respectable passage time from the Canaries.
Later sailed in company with her on a 35' GRP yacht - we were a bit faster, but not a lot.
More about Shuffles here at http://www.naval-architect.com/Yachts.html

Re aluminium vs fibreglass, I think that generally ally boats will be lighter, across the whole size range.
Even at the bottom - look at the AB Inflatables.
http://www.abinflatables.com/i_producto.asp
They make three 8' ish dinghies - two with GRP hulls, and one with aluminium.
The Ventus 8VL GRP boat weighs 95 lbs, the Navigo 8VS (also a GRP hull) weighs 125 lbs, while the Lammina 8AL with an ally hull weighs 73 lbs.

Re the RNLI Mersey class lifeboats (launched from a beach off a carriage) - the first 4 or 5 boats were built of aluminium, while the rest were built of FRP (fibre reinforced plastic) - high tech vacuum bagged and cooked epoxy stuff. I cannot remember which was heavier, but I dont think there was a lot of difference between them.
 
jetwake said:
Purely as an academic argument, my brother insists that Steel yachts will not weigh more than GRP. We are actually arguing over Colvic Watsons. I argue that steel WOULD be heavier and thus the GRP chosen by CW was the better option.

Am I right???

David
Click to expand...

Not if you consider your finished product - being a sailing yacht, and not an aircraft ...

For any given design - providing it can be made from both materials without transgressing any design criteria, the finished product will weight exactly the same whether it is made from grp or steel (or even aluminium).

The difference will lie in the amount of ballast required to bring the hull down onto it's lines, which in turn will determine in part how stiff the sailing characteristics will be.
 
If you are discussing the hull and deck without ballast modern composite frp construction will be the lightest, steel the heaviest. In the middle would be aluminum and cold molded woo/epoxy construction. As far as strength steel looks to be the strongest but aluminum in a greater thickness will be just as strong as steel and weigh about 1/3 less than steel. A steel boat can be weight competitive in sizes over about 50'. All told aluminum can give the best combination of strength without unnecessary weight. Cold molded wood/epoxy is lighter than conventional frp construction with the same strength/stiffness or more. But modern cored frp with carbon/kevlar used in the layup is the lightest of all. Where metal really wins is abrasion, being better than any other material. As far as cost wood/epoxy and aluminum are probably the best choices for a custom design as no mold is required and in the case of aluminun paint is not required above the waterline. Where a steel boat loses is ballast/disp ratio as already mentioned. And ongoing maintenance. Aluminum, especially unpainted and wood/epoxy are the most maintenance free followed closely by frp.
 
You must log in or register to reply here.

Share this page

Top