Through Hull backing pad, how big?
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coopec
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Praxinoscope
Well-Known Member
Can I suggest that anyone who has a GRP boat and is either thinking of doing some work on it or just wants to learn more about good practice on GRP they find a copy of 'Fibreglass Boats' by Hugo Du Plessis, Ihave an old 1978 ediition, which has been revised extensively over the years, but IMHO it is one of the best sources of information on GRP and how work with I have found
it covers most aspects including backing pads.
it covers most aspects including backing pads.
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coopec
N/A
I think that is excellent advice.
Hugo Du Plessis, was a yacht surveyor (Died 2018) so he knows his business. That would be far better than taking technical type advice from a person on a forum who knows stuff all and is just regurgitating mis-information.
I take advice from David Pascoe who is a (now retired) Yacht Surveyor but even then I have reservations regarding his technical advice e.g. on cored hulls because he only ever seems to consider balsa core. He does not differentiate balsa from Klegecell, Divinycell, Airex for example, they all "sus" in his view. He seems to overlook the fact that some of the best yachts like Hallberg Rassy, C & C, Brewer, Swan, Discovery, Contest.... have cored hulls. I think these days his advice is "dated" as well.
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LittleSister
Well-Known Member
A 1.5" nominal size fitting will require a larger hole than 1.5", and Tru-Design and other similar GRP fittings will have significantly thicker walls than their metal counterparts. Thus if you want to have hole diameter plus 100% this will be larger than 3".
coopec
N/A
Are you aware you can have thru hull fittings above the waterline?
I'm not sure what you are getting at?
PaulRainbow
Well-Known Member
It was a simple question, what core does your boat have in its hull ?
Seems a little unlikely that it will be cored above the waterline and not below.
coopec
N/A
Paul
I checked today and it is cored for about 30cm below the waterline.
With the core being Airex I'm not worried about that.
rogerthebodger
Well-Known Member
Clive
I have Bruce Roberts book on Boat Building dated 1994.
It says on page 25 Non - cored arias of the hull
There are certain areas of the hull where it is not desirable or necessary to use Airex or similar PVC core. Such areas include the skeg, keel and area below the cabin sole line.
It does not say anything about "below the water line" sa the cabin sole on your boat and mine the cabin sole will be below the water line.
The principal is that it is not desirable to have a core hull where water could lie and soak into the core by osmosis.
Again we have people who pick up an a comment without knowing the engineering reasons therefore misinterpret the fact.
Nothing wrong with core/insulation below the waterline a in your case.
PaulRainbow
Well-Known Member
Interesting. So you can use backing plates for the sea cocks below the waterline, but not for any through hulls above the waterline (or to be more precise, above the cored area).
rogerthebodger
Well-Known Member
Paul you know very well how to fit a seacock through a cored hull, it has been discussed on here before
Once the hole is cut the size through the hull you remove the core around the hole and fill with epoxy filler to strengthen the hull before fitting the skin fitting.
You an also fit a backing plate of what ever material you wish.
Of cause as my boat is steel I only need to drill a hole and fit the skin fitting unless there is foam insulation in which case I just remove it around where the fitting is located but that's not essential as some times I drill and tap the hull and bolt from the outside into those tapped holes.
PaulRainbow
Well-Known Member
Of course i do.
So why are you telling me how to do it ?
Is there a point here Roger ?
northwind
Well-Known Member
Plastic chopping boards.. Loads of them around, ikea for example: LEGITIM white, Chopping board - IKEA
pvb
Well-Known Member
Most chopping boards are low density (LDPE). Better to buy small sheets of HDPE.
rogerthebodger
Well-Known Member
So that any newcomers who find this thread will have some info on both GRP foam cored boats and steel boats.
If you are nat interested just ignore what I post or like pvb put me on ignore.
PaulRainbow
Well-Known Member
I asked Clive a question, out of curiosity, he answered me. Not sure why you constantly feel the need to comment on my posts.
pvb
Well-Known Member
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coopec
N/A
Paul
I don't think you understand cored yacht construction.
You say
(or to be more precise, above the cored area).
My hull and deck are cored also so now I am confused
Do you mean the mast? ?I am bewildered......... Your comment #27 and your comment above don't make any sense at all. Sorry. I'm beginning to think you are playing games ?
Where I have a thru-hull for raw water it is below the waterline and where there is no core material. The hull is about 3/4 in. thick at that point. Do you think I need a backing plate? ?
If I install a thru hull fitting where there is core I remove the core and re-lay glass. Maybe these posts by experts on another forum will help?
billknny
You can remove the inner skin for a distance around the hole, and lay up glass to replace the core. That is a certainly a premium job, but is likely overkill.
The core can easily be dug out with a variety of tools, and the the resulting gap between the skins filled with thickened epoxy. That's the way 95% of all properly installed holes in cored decks or hulls are handled.
Another option is to remove the inner skin, install a piece of G10 in thickened epoxy, and glass over that to replace the inner skin. Another very strong way of doing the job.
Nicholson58
As to the cored hole, it depends how great the core thickness is as to how far in you dig. I prefer 1.5 times the thickness if possible and I use random glass fiber as the filler and the silica as a modifier to the resin. On a large hole such as this I like to see glass fiber because there could be bending loads on the new port. If it is filled with neat resin and silica only it will be hard but brittle. Your installer was not competent or they took an enormous short cut. The original job would eventually leak into your core.
Thru-Hulls in cored fiberglass. Is core removal and backing required? - Cruisers & Sailing Forums
I don't like David Pascoe's (Retired yacht surveyor) approach
If the hull is cored, dig out the core around the hole to hollow an area at least as large as the flange of the seacock you are installing. Fill the hollow area with epoxy putty and allow it to fully harden before proceeding. The epoxy provides a solid base for the through-hull and prevents water from reaching the core material.
I don't know where your comments are heading but if you do have a genuine interest in cored hull construction I suggest you do some reading of excellent (very basic) articles on fiberglass yacht construction published on the internet.
I hope this helps
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PaulRainbow
Well-Known Member
This forum is fast becoming an alternate reality.
You said you had a cored hull and could not use backing pads. Out of curiosity i asked what boat you had, cored hulls below the waterline aren't exactly common.
You answered the question, that'll do.
You say
(or to be more precise, above the cored area).
My hull and deck are cored also so now I am confused
Do you mean the mast? ?I am bewildered......... Your comment #27 and your comment above don't make any sense at all. Sorry. I'm beginning to think you are playing games ?[/quote]
Seriously ? What's wrong with you ?
So your hull isn't all cored then and you can fit backing pads if needed, not that i suggested you could or should, did i ?
So you can fit backing pads if needed to those areas too, not that i suggested you could or should, did i ?
I think i can survive without any help from your Google searches, but thanks all the same.
I don't know where your comments are heading but if you do have a genuine interest in cored hull construction I suggest you do some reading of excellent (very basic) articles on fiberglass yacht construction published on the internet.
I hope this helps
My comments were heading nowhere, i asked a simple question, you're obviously paranoid about something.
rogerthebodger
Well-Known Member
I didn't know making a comment on someones thread was against forum rules. You is it against your rules.
As pvb said it's must be an ex colonies thing
You can some the problem by putting me on Ignore.