Hi Vance. I've looked at several manufacturers of thru-hull connectors and all the products look good. The problem is the cost. With all the different types of devices we might put on our little subs, it can get pretty expensive to use these off-the-shelf gizmos. I have a design that is relatively inexpensive and has some advantages that dictated the design.
First off, I wanted to be able to isolate the connector from the inside with a valve so if a leak developed, I could close the valve and the welded connection would be the only worry. I'll be using schedule 80 316L stainless nipples welded into the hull, rather than female "weld-o-lets". The threaded valve connection on the inside can be tightened from inside if a few drops manage to get through. The nipples are modified with two shoulders welded inside to hold the epoxy inside the nipple. The brass rods have shoulders on them to hold them in the epoxy. Actually, I'm using threaded brass rod as the connector pins, so they can't move.
To isolate that connector in case of a major leak, I can pull out the wires (spade terminals that come with the motors ) and close the valve. This of course after I throw the breaker. There will be a little resistance at the electrical connection, but not much.
Next, I wanted to be able to see if any water was getting through into the cable on the outside, or in your case, if any oil was leaking out. Oil and air are both lighter than water, so the connection to the motor must be on the bottom. I will be using clear tubing with the wires inside so that if any water gets in there, it will rest at the low point of the cable run and be visible through the clear tubing, before it can get into the motor housing. Hose barb fittings, clear tubing, clamps, and an additional coating of sealant around the clamped hose ends. ( Vulcem, a liquid polyurethane used around and in swimming pools works really good). I've used that stuff for years and it lasts a long time, is impervious to pretty much everything, and is easy to apply and/or strip off and replace. Even takes direct sunlight for extended periods. I've used it in some tanks containing very caustic acids or solvents and never seen it leak.
All the little parts can be made on your bench, or purchased at the local hardware store. The cost for one of these set-ups is low ( like maybe $20? not counting the nipple and SS valve.) and if done right, should last for years.
Here's a little sketch of what I'm planning on doing. Worst case scenario, if it doesn't work, I can still use the welded nipple to attach a store bought connector. Take a look and let me know your thoughts on this. Frank D.