Making a hole in the boat

westhinder

westhinder

Well-Known Member
Joined
15 Feb 2003
Messages
2,656
Location
Belgium
Visit site
The new bilge pump requires a larger diameter hose and hence a new through-hull fitting. What would be the safest method to enlarge the existing hole? How can I prevent the gelcoat around it from cracking? Do I start on the outside or on the inside? Does a through-hull fitting for a 28 mm hose need a backing plate? If so, how do I fit one?
Probably all reasonably simple things, but advice is warmly welcomed.
 
A rough grinding stone, the type that will fit in a drill will take the hole out to the required size.
Use masking tape on the outside to protect the gel coat.
You would be better with a backing plate of good quality marine ply,1/2" thick, perhaps coated in epoxy, left to harden sanded rough on the side to mate with hull and bonded to same with epoxy, or sikaflex, if fitting it at same time as skin fitting.
Make sure there is a backing washer under the backing nut.
 
Trouble with a backing plate is that the hull is quite sharply rounded where the hole is. So that would mean shapeing the backing plate to fit. I was hoping to get away without, but you are probably right.
What do you think of this suggestion: put a wooden taper in the hole and drill that with the appropriate diameter drill?
 
Either drill a series of small holes at the required diameter the cut out with a padsaw and file the edges smooth or knock a tapered wooden peg in the hole from the outside, cut off flush with the hull and use a hole saw of the right diameter, frill from the outside slowly to avoid cracking the gelcoat, the backing pad as before. Just reread the previous post about a backing pad - so fit the pad first with epoxy and drill through hull and pad in one go. You may have to slightly profile the pad to fit the hull nicely. Use a decent hole saw as the cheapy ones may not cope. depending on your hull thickness you may not need the plug the original hole first.
 
tape over the existing hole on the outside ,fill the existing with car body filler from the inside.
when the filler has set remove tape & drill new hole with a "hole saw" in a battery drill.
i would make a small backing plate from 12m/m ply shape by using say a 50m/m hole say to cut a disc then drill the disc with the 28m/m hole saw you then have a plywood washer apply suitable sealer to the skin fitting & washer donmt over tighten allow the sealer to cure then a final tighten
 
G'day Johan,

If you use a 25mm hole say you can file the last bit and remove any small chips though most pump outlets have a cover ring so you would not see them anyway.

If you decide to fit a backing plate, avoid ply, it rots; a stainless steel off cut of 4 to 6mm with some micro balloons behind it will avoid any shaping and be a perfect fit.

Avagoodweekend......
 
I have used ply with chamfered edges, bedded on filler on the inside of the hull. Then glassed or epoxied over. The hole on the outside was also filled, then the hole cut with a hole saw. The exposed edges were epoxied.

The ply can't rot if it doesn't get wet
 
You won't need a backing plate on a fitting above the waterline that is connected with flexible hose as it won't come under strain or threaten the integrity of the hull . All other recommendations as above.
 
Thats the way most Pro`s do it. Temp plug hole use the right size hole cutter, personally I mask up then drill from the outside useing the slowest speed speed and LIGHT pressure until the surface of gel coat has been penetrated.
 
You must log in or register to reply here.

Other threads that may be of interest

Halo
Cuuting a 115mm hole for Wema tank hatch
Replies
9
Views
361
pete
pete
Y
The Seafarer 700 and NASA clipper sounders saga
Replies
10
Views
425
Dellquay13
Dellquay13

Share this page

Top