On another boating forum a friend of mine posted a tip I'd never thought about so I figured I'd repeat it here, albeit without the photos he posted.

So let's say you have a 2" hole through your gelcoat or whatever material, and you need to enlarge it to 3".

First step is to tape off the entire around the existing hole, covering an area at least an inch or two around the larger hole you want to make.

Next, on your drill mandrel, mount the 3" hole saw. Then, using the same mandrel, mount a 2" hole saw inside the first one.

Carefully center the 2" hole saw in the existing hole and very gently lower the bit into the hole. Keep it centered so it doesn't run away across the gelcoat.

When the 3" hole saw comes in contact with the tape hold the drill steady and VERY gently lower it until the 3" hole saw bit comes in contact with the gelcoat. Let the teeth on the 3" bit start to eat their way through the gelcoat. The 2" bit will keep the bit centered as the 3" bit works its way through the material you're cutting through.

After you've finished drilling the 3" hole stop the drill and pull it back out through the hole. Remove the 2" hole saw from the bit, then remove the collar of the material you've removed.

Bingo you just finished enlarging your hole.

Need to cut holes larger than your largest hole saw? You can see how I did that with a Dremel using a rotating blade. Find it here:

http://www.boatingabc.com/forums/ubbthreads.php/ubb/showflat/Number/565165/Searchpage/1/Main/38185/Words/%2Bdremel/Search/true/re-had-to-cut-some-holes-for-access-to-tower-lock-nuts-bolts.html#Post565165

Jack, that's a great way to cut a hole. Did you attach the Dremel to that piece of wood?

Some great tips on making circle. Have used one of the below to add some speakers on one of our past boats. Worked great.

https://www.dremel.com/en_US/products/-/show-product/tools/678-01-circle-cutter-straight-edge-guide

If you use bi-metal hole saws, you will probably need a special mandrel to follow the method GFC posted about. There are a few options, but the one I like the best is made by Starrett and replaces the center drill with a mandrel that allows mounting a smaller hole saw inside a larger one. The Oops Mandrel lets you use a smaller hole saw instead of a drill bit to center a larger hole saw. On the webpage I linked to, there is a link to a newer model that has a photo of how it works.

The timing of GFC's post was excellent because I have a situation where I need to enlarge a hole for a through hull and was thinking about Rube Goldberg ways to do it. This is a much better solution!

The float switch for my primary bilge pump failed recently and my boat had bilge water almost up to the carpet in the lower level. My secondary pump should have kept the boat from sinking, but I never found out as I discovered the problem before it kicked in. Luckily, I had a replacement switch and pump already on the boat waiting for a good time to replace the old one. I was surprised to find out that even though both pumps have a 1 1/8" output, the previous owner had used a 1" ID hose connected improperly to a 1/2" through hull. I had previously been worried about the water hose he used for the bilge pump that kinked at bends, but the small through hull was even worse. I will be installing a proper sized Marelon through hull and real bilge hose in the near future.

GFC
Yes, I cut and drilled the piece of wood, including holes every 1/2 inch (i inch diameter) for future use. I was quite pleased with the cutting action of the Dremel blade. Only small pieces were cut and the normal fiberglass shards were non-existent. Not splinters as all. But, the edges were a bit sharp.

Jack, what type Dremel bit did you use? Was it a router type bit?

I think this is the one I used.
https://www.dremel.com/en_US/products/-/show-product/accessories/561-multipurpose-cutting-bit

I didn't read the entire post but I drill my holes like Al taught me. I reverse the direction in the beginning and then go forward, I use painter's tape to mark the area being drilled and measure 100 times before I pull the trigger. I also take a couple of swallows of my favorite adult beverage to take a small portion of the edge off of me. Using this method has been pretty successful. The key item is I only drilled holes for screws to mount an aftermarket bimini top to my 18' boat.

Got it, thanks Jack.