Cutting a hole in a GRP hull for waste outlet skin fittings?

Ben998

New member
Joined
3 Nov 2004
Messages
153
Location
St.Lucia today, Martinique asap!
www.manateeofhamble.blogspot.com
Hi All,
Whats the best way of cutting a hole in a GRP hull? Im going to fit a holding tank ( got the Headmistresses book on order to read before I start ) and will have to cut a couple of holes.

Is it best to use a standard hole cutter? If so, whats the best speed for cutting through the GRP?

Secondly, obviously the hole will leave the edges of the GRP exposed where it passes through the hull. Do I have to treat the edges of the exposed GRP with anything, or will the sealant that I use around the skin fitting do the job?


I was going to use a silicone sealant that Ive used on ponds, marine tanks etc to seal the fitting, that seems to do the job without any drama, but if anyone knows any better then please let me know.
Any advice gratefully received,

cheers,
Ben
 

snowleopard

Active member
Joined
16 May 2001
Messages
33,645
Location
Oxford
Visit site
An ordinary hole saw on a low-speed setting will do the job just fine (the cheap DIY jobbies where a series of saw rings fit into a cast metal base aren't up to the job). Some people advise against silicone as it contains acetic acid. The alternative is sikaflex (polyurethane). There should be no need to seal the edges of the hole though if you're worried a little catalysed polyester resin will do no harm. On the other hand if the hull is sandwich construction it's a whole different ball game.
 

Evadne

Active member
Joined
27 Feb 2003
Messages
5,752
Location
Hampshire, UK
Visit site
It is possible to get silicone that doesn't release acetic acid, e.g. from RS but its nearly as expensive as Sikaflex anyway.

When they peel the gelcoat off for osmosis treatment, I was told it is common to lightly go over the hull with a grit blaster to open up the glass fibres that have been sealed by the heat of the cutting process.

I tried to drill through grp once (deck, not hull) with a multi-size hole saws and can confirm their wimpishness!
 

oldsaltoz

New member
Joined
4 Jul 2001
Messages
6,005
Location
Australia, East coast.
Visit site
G'day Ben,

You don't say if the hull is solid glass or a sandwich / composite, so we will assume it's solid glass for now.

Using a hole saw start your drilling from the inside, this ensures you get the hole in the right spot and have access to the fitting later.

Move to the outside when about half way through to finish off.

Check the skin fitting, some have lugs on the outside to prevent them turning in the hole, dry fit the fitting to make sure you have some clearance, it should not need more than palm pressure to push it through, you may have to cut (grind) 2 small sections out for the lugs to locate in.

Run a round file over the sawn area and seal it with at least 2, coats, 4 if you can, of epoxy resin, you can apply them wet on tacky, a small bit of masking tape under the hole that has only 20 mm of it's 48 mm stuck to the hull will provide a drip point for any resin that flows through when applying. Let this cure overnight.

Check the inside shape of the hull, if it has any curve it should be made flat to provide an even seal for the inner part of the fitting. I use fibreglass and epoxy resin to do this, some use timber to provide a little 'give' in the fitting. However I have yet to have fail after doing it this way for the last 20 years or more.

Just grind back to the hull glass before you start glassing, apply a patch that is oval in shape about 300 mm by 200 mm and each patch that goes over that should be smaller by about 40 mm. Make sure you don't leave too much resin in when using epoxy, you only need a 1 to 1 ratio of material to resin, not like wax resins when looking for a ratio of 3 resin to one material.

I also use only Sikaflex (Polyurethane) to seal the outer fitting.

Soavagoodweekend...
 

Evadne

Active member
Joined
27 Feb 2003
Messages
5,752
Location
Hampshire, UK
Visit site
Dear Oz

Have you ever thought of gathering together all the answers you have given to folk on the forum over the years and making them into a book? If you haven't already, that is. I'd buy it.
 

simonfraser

Well-known member
Joined
13 Mar 2004
Messages
7,512
Visit site
i used a cheap multi part hole cutter for mine, worked fine
sikkaflex will seal the hole no prob, but getting it off later is a major head ache
 

Birdseye

Well-known member
Joined
9 Mar 2003
Messages
28,497
Location
s e wales
Visit site
[ QUOTE ]
i used a cheap multi part hole cutter for mine, worked fine
sikkaflex will seal the hole no prob, but getting it off later is a major head ache

[/ QUOTE ]

Marine silicone sealants work perfectly but dont use the bathroom stuff. My Prout had all fittings sealed with silicone at the factory, no probs with leaks in 19 years and you wouldnt believe the tenacity of the grip when I had to get a Blakes fitting out.

On the other hand, I have had problems with both polyurethane and polysulphide sealants.

Incidentally, after experiencing corrosion of a Blakes unit used as the outlet for a holding tank, I would consider Marelon next time round. Its difficult to completely flush a holding tank system after emptying, and a mixture of **** and !!!!!! seems to be very corrosive. Prob SWMBO's cooking!
 

Ships_Cat

New member
Joined
7 Sep 2004
Messages
4,178
Visit site
You state "hole... ...for waste outlet fittings" and mention in the hull.

If you have an existing toilet installed then there is usually no need to cut any holes through the hull for the waste outlet, as the existing toilet outlet can be used.

That leaves just 2 above water holes to be cut, the one for the pumpout connection (usually in the deck, in which case you have to be careful that the deck is not cored) and the other (small, usually high on the topsides) for the vent.

Perhaps there is no existing toilet fitted?

John
 

oldsaltoz

New member
Joined
4 Jul 2001
Messages
6,005
Location
Australia, East coast.
Visit site
Re: Dear Oz

G'day Dave, and others supporter,

Thank you all for your support, I don't have a list of replies or a book, the truth is most common questions don't have a common answers.

You have to consider age, if manufactured during an early oil shortage the resins produced were of a lesser quality, so an epoxy would be the only resin likely to achieve a bond. Material, solid class, type of resin, or timber, ply or solid, clinker, carvel or other. Composite type, balsa, foam, duracore or other. The variations go on and on.

Ever noticed that a post with all the details will get better quality advice?

As for book..... Thanks for thinking I could produce one, but I think not, I'm just a bloke who plays with glass every working day and then some, I do my best to keep up with developments in the industry and have established a number of good contacts to keep abreast of what's around the corner, or what may have caused a problem with hindsight. I don't know all the answers, but after so many years I have managed to pick up a thing or two, and if I can help someone out, or better, save them from making what can be a very expensive mistake, I am only too happy to offer some assistance based on jobs I have completed myself in the past.

If I ever make up a collection of tips, tricks and techniques; you will all have full and free access via a web site.

Seems a shame that we can download anything from the net but nothing from head or heart.

Avagoodweekend......
 

aitchw

New member
Joined
18 Feb 2002
Messages
2,453
Location
West Yorkshire, UK
Visit site
I, OTOH, have managed well with the cheap DIY jobbies. Trick is to let the saw do the work and not apply much pessure. I have also used a trepanning type tool for larger holes. That takes a little getting used to but produces a very clean hole except on the back when it breaks through. If you can get to both sides it's better.
 
Top