Strength of fitting bolted to GRP

[B27]

Can anyone suggest authoritative sources for how to work out the strength of a fitting bolted to e.g. a GRP deck?

Where should I look for dicument 'rules' about how much reinforcement or backing pads are needed behind things like a chain plate or a mooring cleat?
The fitting in question is a bit like that, in that the loads will mostly be close to being parallel to the deck, but not exactly.

 

[Boathook]

How thick is the grp deck and what is the lay up. Type of boat, plus where moored, etc will assist with answers.

 

[William_H]

I am sure from a theoretical engineering point of view figures are available for strength and failure loads. Just as there is much available about metals. Just keep looking if that is what you seek. However most on this forum will be happy to over engineer backing plates on a boat. As said the thickness of the layup and density of glass (or carbon fibre) to resin will affect ultimate strength. Plus of course you must look at attachment of GRP to the rest of the structure. So for chain plates you must look at not only strength of GRP in deck but strength of the whole load circuit from chain plate to keel ballast. Or for mooring cleat the load path cleat to the bulk of boat mass, bouyancy and major wind resistance. ol'will

 

[Trident]

Proper load calculations may be difficult unless you have the complete lay up of the boat. You need to know if its cored or solid, which core material, which resin too (vinylester is stronger than polyester) and then what type of mat was used , woven, chop strand etc. There is really no such thing as "fibreglass".

The simple rule of thumb is as big as you can get in the space really. If its a deck attachment you'll find the edge of the deck is quite close to the position you want so a backing pad will never be as big as you'd like so just fill the room available to some degree. Also think about what type of backing and make sure its attached in such a way that doesn't make a hard point. Check the integrity of the GRP around the area first too.

If the chainplates secure to the hull then you can not only put in a backing pad but if you are worried lay up more glass inside with unidirectional fibres radiating down the hull in an arc to transfer load in to a wider area but this may be overkill on a smaller boat or one that is not highly stressed

 

[B27]

I was really looking for something between 'finite element analysis' and 'guess, then chuck some more material in'.

 

[Freebee]

a quick google will show you that this question has been asked many times on many different boating forums and never gets a definitive answer, there is lots of advice about how to shape backing pads and how to apply them. i think its too hard a question for theoretical calculations, there are too many variables...in terms of load ,direction ,grp lay up, how the grp was laid up, is there a core or stiffener in the area. so you probably think of a number and double it.

 

[B27]

a quick google will show you that this question has been asked many times on many different boating forums and never gets a definitive answer, there is lots of advice about how to shape backing pads and how to apply them. i think its too hard a question for theoretical calculations, there are too many variables...in terms of load ,direction ,grp lay up, how the grp was laid up, is there a core or stiffener in the area. so you probably think of a number and double it.

I think I'm really looking for a guide to 'scantlings', i.e. dimensions of structural bits.
AIUI, things like 'Lloyds' have tables of dimensions of what's considered adequate for various sized boats.
I guess a textbook with some worked examples might suit my aged brain?

Take for example the mainsheet on my little boat. It's 4:1 and I sometimes heave on it, so we could guess I'm putting 400lb on the u bolt, which seems to be on a fairly small backing pad and about 5 mm of GRP? It's basically in the middle of the cockpit floor, an unsupported panel.
The loads on that must be quite high in a gust that heels the boat sharply.

So, who's got a book in their library that they might recommend?

 

[simonfraser]

look at other matching fixings / backing plates on your boat and repeat

 

[Refueler]

LLoyds and other Reg bodies have MINIMUM figs ....

Second unless specifically engaged by BUYER - they do not supervise building of an individual boat. Their Certificate is based on a prototype example submitted / observed .... then all buildings after (subject to maintaining standards of course) is certified.

OK back to original question .... backing pads etc. - I suggest you compare to other fittings that have survived the years ... and if you are in any way cautious of that - add to the area ... keeping it within reasonable bounds of course. Often the size of backing pad is dictated by the area available to use.

 

[alahol2]

There are several tables of laminate (glassfibre) physical and mechanical properties in Ian Nicolson's Boat Data Book 3rd Edition.
Whether you can make any predictions about your own vessel from the data in the tables is another thing entirely.

 

[B27]

There are several tables of laminate (glassfibre) physical and mechanical properties in Ian Nicolson's Boat Data Book 3rd Edition.
Whether you can make any predictions about your own vessel from the data in the tables is another thing entirely.

Thanks that's helpful.

 

[pandos]

Elements of boat strength by David Gerr.


I think this may have what you want. Unfortunately, although I have it on my bookshelf, I am away at the mo so I can't be certain.

 

[ylop]

I think I'm really looking for a guide to 'scantlings', i.e. dimensions of structural bits.
AIUI, things like 'Lloyds' have tables of dimensions of what's considered adequate for various sized boats.
I guess a textbook with some worked examples might suit my aged brain?

Take for example the mainsheet on my little boat. It's 4:1 and I sometimes heave on it, so we could guess I'm putting 400lb on the u bolt, which seems to be on a fairly small backing pad and about 5 mm of GRP? It's basically in the middle of the cockpit floor, an unsupported panel.
The loads on that must be quite high in a gust that heels the boat sharply.

So, who's got a book in their library that they might recommend?

Does this help: http://alienboats.free.fr/download/docs divers/The Elements of BOAT STRENGTH.pdf

 

[B27]

Elements of boat strength by David Gerr.


I think this may have what you want. Unfortunately, although I have it on my bookshelf, I am away at the mo so I can't be certain.

Does this help: http://alienboats.free.fr/download/docs divers/The Elements of BOAT STRENGTH.pdf

Should have thought of Mr Gerr, as I've recently used his book on propellors!
That will keep me amused for a while.
Thanks.

 

[William_H]

Take for example the mainsheet on my little boat. It's 4:1 and I sometimes heave on it, so we could guess I'm putting 400lb on the u bolt, which seems to be on a fairly small backing pad and about 5 mm of GRP? It's basically in the middle of the cockpit floor, an unsupported panel.
The loads on that must be quite high in a gust that heels the boat sharply.

So, who's got a book in their library that they might recommend?

On my little boat 21ft with large mainsail and 4 to1 main sheet that is attached to a traveller track across the bridgedeck at the cabin entrance I found that the main sheet loads were very high with athletic crew. The track and bridge deck were bulging upwards under load. It was convenient to fit small stay wires from track mount screws under deck down to furniture in this case steps. So you could increase size of backing pad or provide more support. Note here if you fit a backing pad it should ideally come in different sizes firstly over large area then next layer a bit smaller area etc to avert sharp edge loading of edges of backing pad. ol'will

 

[rogerthebodger]

On my little boat 21ft with large mainsail and 4 to1 main sheet that is attached to a traveller track across the bridgedeck at the cabin entrance I found that the main sheet loads were very high with athletic crew. The track and bridge deck were bulging upwards under load. It was convenient to fit small stay wires from track mount screws under deck down to furniture in this case steps. So you could increase size of backing pad or provide more support. Note here if you fit a backing pad it should ideally come in different sizes firstly over large area then next layer a bit smaller area etc to avert sharp edge loading of edges of backing pad. ol'will

If you fit sone hardish rubber under the backing plate it will reduce the high compression stress between the edge of the backing plate and the underside of the deck

 

[geem]

If you fit sone hardish rubber under the backing plate it will reduce the high compression stress between the edge of the backing plate and the underside of the deck

How does that work? Once the rubber is loaded up and compressed it has no give. You still have a hard point and a backing plate that is moving in relation to the deck.
Surely reducing the strength at the edges of the backing plate by thinning the backing plate material removes the backing plate hardpoint? Glassing the backing plate in place and there is then no hard point or movement. It's the recommended way of building backing plates and has been done successfully for decades with grp

 

[rogerthebodger]

How does that work? Once the rubber is loaded up and compressed it has no give. You still have a hard point and a backing plate that is moving in relation to the deck.
Surely reducing the strength at the edges of the backing plate by thinning the backing plate material removes the backing plate hardpoint? Glassing the backing plate in place and there is then no hard point or movement. It's the recommended way of building backing plates and has been done successfully for decades with grp

Yes but the edges does have some dive and will spread the edge load and also spread the load over the ared and prevent hard spots being crushed

Think of a solid rubber tire on a concrete path against a steel wheel on the same concrete path''

Thee point pressure on a steel wheel will crush the hard concrete where as the rubber will spread the load thus not crushing the concrete

 

[Wansworth]

Look at it this way,for instance a cleat too much backing plate and I see the cleat dangling off the quay with a big hole in the fore deck…..correction boat floating without any foredeck!

 

[geem]

Yes but the edges does have some dive and will spread the edge load and also spread the load over the ared and prevent hard spots being crushed

Think of a solid rubber tire on a concrete path against a steel wheel on the same concrete path''

Thee point pressure on a steel wheel will crush the hard concrete where as the rubber will spread the load thus not crushing the concrete

I don't get that. A tyre has a pressure of say 30psi. It's full of air. Try the same thing with a solid rubber tyre. It's likely that the point load will destroy the rubber because its a point load. The tyre full of air spreads the load. The tyre also has a wide footprint. The edge of a Backing plate is just that. An edge. Once you have compressed the rubber, it's still a hard edge. With Backing plates, you don't want movement. Bolts have to go through the deck into the Backing plate. Any movement of rubber will cause the bolts to move and therefore water penetration. If the rubber isn't moving then its not flexing. You can't have it both ways