It turned out that the reason my boat's engine overheated on the way back from the boatyard wasn't the thermostat after all. When I crawled into the engine compartment to assess the job of changing out the thermostat, it came to my immediate attention that the water-pump belt was loose. It had been tight enough when the engine was first fired when the yard splashed the boat, but it continued to get looser because for some reason, the meck-a-neck loosened the alternator in it's bracket and failed to re-tighten it and correctly tension the belt. The alternator just kept getting looser until it wouldn't turn the water pump or the alternator.
A lesson here. If you're young, you might never have serviced an engine with actual V-belts. They are not made now for common uses such as automotive and boats, and have been replaced by those wide serpentine belts, which themselves have changed in the past couple of years. You don't need any tools to install the latest serpentine belts, they are stretchy and just slip onto the pulleys (heaven help you if you don't have the diagram of how they are fitted over which pulleys in which directions, though!)
So, I have a V-Belt to tighten, and no way to get the usual lever-bar onto the alternator to tighten it, because it's on the underside of an engine which has a front end laid out that doesn't permit access from the top, and there isn't enough room on the bottom to swing a long enough lever.
What to do?
I remembered that years ago, when I was wrenching on big ugly machines for the Air Nasty Guard, we had a tool in the tool crib just for this purpose. It had welded Y-ends on threaded rods, one left hand and one right hand, and a turnbuckle in the middle. It was used by fitting it to the side of the alternator or water pump, whatever was movable in an adjustable bracket, and bracing the other end on something fixed, usually the flywheel on the front of the engine. Turning the turnbuckle then lengthened the tool gradually, and the alternator was moved outward of the immovable part, and held until the correct tension was obtained, either by eyeball or tension gage, and then the alternator bolts were tightened to complete the job, and the tool removed, carried back to the tool crib, and exchanged for my stamped chit.
So, I go to NAPA, and ask the counter guy if he has such a tool for sale. He had never heard of one, but he was in his 20's, and I hadn't seen one of these myself for almost 20 years.
Time to design one.
Over at my nearby Ace hardware, they sell turnbuckles, so I after driving over from NAPA, I selected one which would take up 2/3 of the required span, which I had estimated at nearly 6" by comparing it to a dollar bill (you DO know that a dollar bill is exactly 6" long, don't you?). I got a stainless one, 4 times more expensive than the aluminum, but capable of exerting several times the force before deforming, too. I bought two stainless carriage bolts of that 5/16th size also, to replace the right-hand threaded side of the turnbuckle (which comes with eye-bolts). The two bolts gave me a total span of up to 8 inches on the 4 1/2" turnbuckle.
Back at the boat, I sat in the tiny space in the side bilge, fitted sticky duct tape to the lefty end of the turnbuckle, and selected the short carriage bolt and ran it out to the approximate loose-alternator size, then fitted it and began to turn the turn buckle to take up the slack. I reached hand-tight between the flywheel and the alternator with more than half the bolt's length left, then used a small adjustable wrench on the carriage bolt to take it up to the proper tension level, then snugged down the two alternator bolts. Job done! Except for the tight squeeze of the side-bilge on my ancient body, nothing was the worse for the 10 minute job.
For the future, I will get more turnbuckles, and have my son (who can weld any metal) weld up a variety of fittings for the turnbuckles. I will then have a set of these tools to work with.
The best tool in the toolbox is never IN the box, it's between your ears.