How strong is GRP?

[FlyingDutchman]

Most likely a stupid question from a newbee:
When working on my GRP boat underneath the floorboards (around the engine, propshaft etc.) I always try to avoid stepping on the hull itself and try to stay on the beams. My fear is, that my 75 kilo's are too much for the GRP and i am afraid to step through it!
Is this fear realistic or can you just walk around without damaging anything?

 

[MIKE_MCKIE]

Do not know the boat, but with that size engine, I would hope that the hull can easily take your weight, if not you are in trouble! If you consider a boat clobbering the quay, the impact point takes the whole weight of the boat (several tons?) hopefully without fracturing, so your 75kgs should not be much of a problem, in my opinion. Just dont sue me if you do fall through!

 

[FlyingDutchman]

I am sorry I did not mention anything about the boat.
It is a Norwegian built boat with a round stern with a length of 7,2 meters. The GRP hull looks like it is made of wooden planks. I am Dutch, so I don't know the English wording for it. The weight is around two tonnes and the engine is a 4 cylinder Volvo Penta Diesel with 59hp. Hope this makes it more clear. I do have a picture on my PC, but I don't know how to post it?

 

[snowleopard]

if you can walk around on the bottom of a mirror dinghy which is 6mm of cheap plywood, the bottom of a GRP cruiser is no problem. nor would it be for an elephant!!! GRP is very strong. if you want to prove it to yourself, find a piece in a boatyard's waste and try hitting it with a heavy hammer - just be careful that it doesn't bounce back and hit you on the head.

i managed to get a hole in the bottom of my boat but it happened when the boatyard dropped all 6 tons of it onto one small sharp stone.

 

[richardandtracy]

Being needlessly technical & answering the question that was initially asked...

A good rough & ready answer for most polyester chopped strand matting layups is 50N/mm^2 for a long term load and 115 N/mm^2 for a short sharp load. For woven rovings you can get up to 2.5x the strength, and a breaking strength of 1200 N/mm^2 along the fibres for unidirectional epoxy laminates [ie the same strength as cap head bolts!].

As for applying this info.. Provided that the supports are at least every 600mm (2ft), a 75 kg person will not over stress a 5mm thick CSM laminate when walking over it. I have stressed a 2mm floor from sheet aluminium & found that 2x100 kg people only stress it to 150N/mm^2 at 2g with 600mm supports. The strength is proportional to the cube of the thickness, so 3mm would pobably be strong enough - but feel disconcertingly flexible.

Regards

Richard.

 

[aitchw]

And that is for flat panels. If there is anything in the layup like the simulated clinker I think was described in the original post the stiffness will be improved in that direction at least similarly with the compound curves of a round bilge hull form.

Anyway, no problems with walking around on it. I frequently stand in my GRP Albacore dinghy hull which is nominally 5mm thick whilst on it's trailer with no apparent problems.

 

[FlyingDutchman]

Thank you all for the information!
It seems I am worrying over nothing.
On the other hand, I have no idea about the thickness of the hull and don't like to drill a hole just to measure it...

 

[Thistle]

[ QUOTE ]
don't like to drill a hole just to measure it...

[/ QUOTE ]

Plenty of holes already in the hull, I should think. A little bit of measuring next time you have the pipe off any of your skin fittings should give you the hull thickness.

 

[simonfraser]

go on have a go, you can allways put your finger in the hole