Through Hull Fitting

[ark]

I am replacing the log impeller.The new one is smaller than the existing and so I wish to reduce the size of the hole in the fibreglass hull. Has anyone done this and can give any tips.
I have done the usual search but without a result.

 

[damo]

One method briefly as follows....Glass over the old hole, fill and fair it. Stick on a backing pad (I glass over mine) then drill a new hole the right size. Seal the inside surface of the cut eg with epoxy. Then fit the new through hull and the jobs a good un /forums/images/graemlins/smile.gif

Depends on the size difference though - if it isn't much then I would replace the backing pad and fill the hull hole with epoxy filler. Fair it then drill (the new backing pad provides a centre for the hole saw to give a clean cut)

 

[ark]

Thanks for that. At the moment not sure I understand differance between the two suggestions but will work on it . Thanks again.

 

[damo]

Well if the second fitting is only 5-10mm smaller, then that isn't much of a difference is it? So some filler will make up the gap without loss of strength - the outer flange of the fitting will still be able to grip the original outside surface of the hull. 20mm difference is a lot, so you need strength in the filling ie grp. The hole saw needs a solid centre to drill through, so if you are re-cutting then the easiest way is to replace the backing pad IMO.

There are various tricks and techniques to avoid some of the above - I told you it was a brief description /forums/images/graemlins/grin.gif

 

[oldsaltoz]

How big is the existing hole and small will the new be?

What is the hull made of, solid glass, composite, timber etc.

 

[Caer Urfa]

Hi Ark
There are a number of points to remember when filling GRP underwater holes or you may run into problems later and you do not say how big a hole you have, my guess it will be about 25mm dia.

You first need to chamfer/counterbore 'both' side of the hole making a larger hole like a saucer with your hole in the middle, (using a small grinding tool in a drill is best)if your hole is say 25mm dia the chamfer should be at least 50mm dia this will reinforce the hole when you fibreglass it and stop the risk of it being pushed or knocked out or cracking and leaking!.

Depending on your hull thickness you will need to fibre glass it with about 3 layers at a time until you have built up the thickness each side then grind back.

If it is a small hole make sure you get the right epoxy filler! many people use the standard stuff used topside make sure you get 'underwater' epoxy filler like say Internationals 'Watertite'.

Externaly if possible you should then coat the new filled hole after grind/sanding smooth with say Gelshield for hull protection.

Using a backing pad internaly is also good practice for fitting anything through the hull,BUT, make sure you fit it perfectly flat onto the inside surface, again glass it in to make a sound job.

I would also fit your new log in a slightly differant spot to the filled in hole.

If your going to do it do it right!

On the other side I used a log impeller in my last boat and as normal after a short time it got 'clogged up' and packed up, I filled the hole in and use the plotter speed its near enough!

Good luck
Mike

 

[ark]

I have not been around today hence delay in replying.
The existing hole is 50mm and I need to reduce it to 42mm.The hull is solid fibreglass and has an encapsulated backing plate with a total thickness of about 16mm. The domed external head of the new skin fitting is 70mm in diameter and covers the existing hole if the tube part of the fitting is positioned cental to the hole. I will be using West epoxy mixed with cold temperature hardener and my plan,which in the light of CaerUrfa's advice I will reconsider,was to shamfer both sides of the hole to 70mm,install a temporary tube 42mm dia. place a smooth cover against the hull(inside and out) and inject the resin through a hole in the outside cover. I have some micro balloons I intended to mix into the resin.