Securing fittings to deck

[MIKE_MCKIE]

Needing to fit various deck organiser/blocks & clutches to the coachroof in order to run lines back to cockpit.
Question is, how best to secure them? The deck/coachroof moulding is pretty substantial, however there is a moulded grp cabin liner, which is seperate, and there does not appear to be any insulating filler or anything between liner & coachroof. I do not particularly want to cut the liner in order to fit a doubling plate under the coachroof, and getting the liner out is a definite no-no.
Can I use bolt fittings of the hollow door type (albeit rather more substantial) and just fit from above, without any doubler, or is a doubler considered essential?
Look forward to your comments in due course!!
Brgds
Mike


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[oldsaltoz]

G'day Mike,

What material are we dealing with here, Ply, Balsa or foam core?

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[Viking]

Most GRP boats have areas which have been strengthened where such fittings are usually sited. Around the mast area and each side of the companionway. I have a boat manufacturers drawing showing way all such areas are. Its not a new boat by the way, built 1980/81.

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[richardandtracy]

For very low load fittings (20lb max - well, something of that order), you could probably get away without a doubler. Any more than that is an absolute no-no.

The doubler is intended to spread the tensile component applied to the fitting as a shear load perpendicular to the laninate (almost its weakest direction). Compare the perimeter length of a bolt head with that of even a tiny doubler (2" x 2") - shows why one is needed doesn't it?
The allowable shear strength of your GRP may be up to 30N/mm^2 (4320 lb/in^2) if CSM, or 150 N/mm^2 (21600 lb/in^2) if woven rovings. Personally I'd stick a safety factor of AT LEAST 2 on those allowables. Remember also that the gel coat & filler [if present] should not be included in the laminate thickness when calculating the strength. Then, knowing the load to be applied you can size the doubler perimeter (choose a sensible shape so that load can be spread to all the edges - an 18" x 1" doubler behaves like a 3" x 1" doubler unless properly stiffened!).
The shear component of the load to the fitting should also be taken out by the bond between the doubler & the hull - if using an epoxy adhesive you should be able to guarantee a bond stregth of 1N/mm^2 (145 psi) regardless of conditions of application.
Finally, ensure that the bearing strength between the bolt and the doubler is adequate (gives you the minimum doubler thickness). Can't give you strengths for this really - I don't know what you are going to use.

Finally, good engineering practice would have you using spacer tubes through the laminate to prevent crushing of the grp as you do up the bolts. I know this is rarely used, but it reduces the local damage and means that the fitting is more likely to hang on until the bolts break - preventing tear through of the doubler.


Sorry about your fancy headlining - but it's got to have some holes..

Regards

Richard.


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[MIKE_MCKIE]

g'day oz,
no core at all that I can find, just an airgap twixt coachroof & liner. (Old boat 1970)
Guess that I will have to bite the bullet, cut some holes in the liner & fit some nice hardwood doublers for the various bits.
Thanks to all for the advice, I guessed it would come to that, but hope springs eternal! Thought someone might have a cunning plan to save me the work. (Bone idle me!)
brgds
Mike

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[oldsaltoz]

G'day Mike,

I suspect your top will be solid glass given the age of the boat.

You can do a neat job by drilling the holes for the fitting through the cabin top and liner, then cutting a slot about an inch or so past the last hole, (use a very small cutting wheel) with a Dremel.
You should be able to flex the liner enough to slip in the backing plate, line it all up and screw together, the two bolt holes can be covered with plastic removable plugs, the slot can be filled and sealed, depending on what colour and texture the liner is, it should come up ok.

Failing this, cut a slot out big enough to fit the plate and nuts, then get some 'H' section plastic moulding and pop the cut out back into the slot, or some nice teak and make a frame you screw to the liner.

Hope this helps.



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[MIKE_MCKIE]

G'day Oz,
Thanks for that, it seems a good solution with minimum damage to liner & relatively easy to do. I'll let you know how it goes when I get on the job.
Brgds
Mike

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[vyv_cox]

There's a solution that works for Sadlers, which have the same construction. This was even used by the factory for some components, like the mainsheet track fixing bolts. I see that you have suggested using varnished wood doublers, so if your boat has a very traditional appearance below you may not like this one!

In Ikea and B&Q you can buy plastic plugs about 1.25 inches in diameter, used for covering the hinge fittings in self-assembly furniture. They can be painted to match the headlining/bulkhead colour. Drill the internal surface with a holecutter to accept this plug, install the fitting with penny washers or similar. Not something you would want to do in a very visible location but quite adequate and easy elsewhere. My anchor windlass is held down like this and the fittings are almost invisible.

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[MIKE_MCKIE]

Now that's an idea! I know exactly the things, in fact I probably have a small (large)box full in the garage left over from previous kitchens, bedrooms etc.
I'll need to match the peculiar shade of puce green (that's puce, not puke, although closely related!) of the liner, or alternatively paint the whole thing, but at least no big holes or lumps of wood in the middle of the grp.
Many thanks
Mike

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