Cut and pasted from this site:
If you are contemplating adding a through hull fitting you need to plan its position.
So before you start to drill that hole consider;
•Will it be readily accessible from inside.
•Will there be ample room to turn the valve handle.
•Will the new fitting set up any turbulence in front of your depth sounder or speed log impeller.
•Will the hose run from the fitting be a short and straight as possible.
•On the other hand, will that stiff hose be long enough to allow it to be bent for easy fitting and replacement.
Once you have chosen the position drill a pilot hole from the inside so you can be sure that it is centered on a plank and between frames.
Then using a hole-saw to suit the size of the fitting cut the required hole from the outside of the hull.
Using the hole-saw from the outside will give more room to control it and a crisper edge to the outside of the hole.
Once you are sure the hole is the correct size clean it up with sand paper and prime it with paint or varnish.
Use the same sized hole-saw to cut the aperture in a piece of hardwood for a backing plate.
The backing plate wants to be 2 or 3 inches larger than the flange of the seacock and shaped to match the inside curvature of the hull.
The backing plate is to reinforce the hull around the hole and to provide a flat surface for the seacock flange to bed onto.
The backing plate will also want to be primed to prevent rot.
If it is possible to plan for the hole to coincide with a butt block joint between planks, the valve flanges and securing bolts will add to the strength of butt and the butt will do away with the need for a backing plate.
However, the need for a new hole in the hull might possibly be avoided by fitting a tee-connector to an existing inlet or discharge line.
Read more: Through Hull seacocks below the waterline. http://www.diy-wood-boat.com/Through...#ixzz13hwMCNOZ
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