Patrick you will need a drill and a jigsaw ideally, as well as a rasp. Drill a large hole, then use a jigsaw with a special blade for cutting worktops etc, this blade reduces splintering and is available at B&Q. Try to cut the hole slightly under size as you can finish it of with a rasp to get a nice fit.
If you do not have a jigsaw you can drill holes all the way round very close together then use a stanley knife to cut between the holes, finishing off with a rasp.
You could invest in an adjustable hole cutting attachment for an electric drill. I bought one quite cheap (and fairly nasty). It has a chunky drill bit which makes a central hole then a crossbar with 2 adjustable cutters which slide along to give the required radius each side. Worth testing it out on a piece of scrap first to make sure you have the hole diameter set just right. If you have a strong, steady hand and nerves of steel it works quite well and the resulting hole needs minimal tidying up. It is easier on a horizontal surface than a vertical one.
You can get a padsaw type blade for a Stanley knife. (this is a thin saw blade with a free end) I drew a circle to suit the plastic hatch fitting using a compass, then drilled a 6mm hole just inside the circle and sawed round the circle (carefully). The plywood is thin and easily sawn by hand
I Need to fit inspection hatches to carry out a repair to my Boat. When it recently blew over it fell onto the trolley ,making a 4 inch split into the wood under the side tank.I need access and it seems the best from this post are 6 inch. It may seem obvious,but where exactly in the side and if required the rear tanks do i fit them?
There are no actual rules and boats have them in different places. The most common is to put them half way along the tank in the cockpit as this allows you to reach most places in the tank, some people put them where they need to make a repair if there is no inspection hatch already. Try to avoid putting it on the deck as you are inviting problems if the water seeps under the bottom ring in time an causes rot.
To create a water tight fit we smeared a little epoxy on the ring and also in the holes where the screws go in, just smear a little vaseline on the screws before screwing them in so that they can always be removed
Thanks for the swift response. I guessed it would probably be directly under the thwart, but was,nt sure if there any rules that apply.I currently have the little 'bungs' , that were fiited from new 20+ year ago by Bell.Is directly above these, as they are the lowest point acceptable. Do you agree the 6 inch are the best?
As Paul said, were ever is most convienient for you to do your repair, bearing in mind future work. Below the thwart is acceptable as long as you can get your arm in and preferably be able to see a little of what you are trying to repair.
Thanks. The damage i have is quite central, so under the thwart, near the existing 'drain' sounds good.I believe i can effect a reasonable repair from the outside of the hull, but i am concerned about any invisible damage that may take in water. If i fit the hatches i will be able to stick a torch in and have a look see. And if required stick some SP and if required some tape or wood patch.
You do not need to go for a low point as the drain holes are still used for normal draining, taking into account what Bob Towler said about repair area etc. I would go just behind the thwart and half way up the side of the tank as it will be easier to to get your arm in to enable repairs.