So my e32-2 has a universal 5416 (16hp at 2,800 rpm) and a two bladed prop. but at just 1,000 rpm the boat moves along quite nicely at about 5 knots. it seems silly to run the engine at 80 percent of max rpm given the the boat is already moving right along. i understand that diesels like to run at 80 percent of max rpm to keep warm. and i can understand that, since at 1,000 rpm the engine is only running at about 150 degrees. the recommended operating temp is 175 to 190.
so i wonder if i have the wrong propeller, or maybe the wrong thermostat to get the temp up and move along at a reasonable clip?
a leaking packing gland has kept me from going beyond 1,500 rpm. but the questions remains. is it reasonable to push an e32-2 along at 8-10 knots?
As covered in some recent threads, you first want to determine that your tach is reading correctly, or at least close to it. Getting one of these will help:
https://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html Then, there is also the question of whether your speed readings are right. You need accurate readings that also take into account whether you are being helped along (or hindered) by any current.
So, for the sake of argument let's assume your tach is at least in the ballpark....
I'm not sure why a *leaking* packing gland would keep you from hitting your top RPM. (One that is severely too tight could possibly cause problems like that, but that packing gland would be running really, really hot. But you are talking about one that is apparently loose, not too tight.) Rather, your installation shows at least some of the telltale signs of being over propped. Significantly over propping your engine is one of the worst things you can do to it. Diesels like to run at 80% of max because that puts them in an advantageous part of the torque curve. If you are severely over propped you will shorten the life of your engine.
Here's a rough way of knowing if your prop (and/or engine functioning) is close to right. You should be able to hit the top rpm that your engine is designed to hit, give or take maybe 150 rpm or so. If the engine hits top rpm too easily, i.e., when you open up the throttle it just sort of zooms right up to peak rpm, close to the way it does when you are in neutral, then you are probably under propped. If you are truly only able to hit 1,500 rpm, then either you are WAY over propped, or something is keeping your engine from producing its rated horse power, or the prop is severely fouled, or maybe some combination of all of those. If the prop and engine are performing as designed, you should be hitting close to max rpm with the engine having to work to squeeze out those last couple of hundred rpm.
Don't mess with the thermostat; figure out what is going on with the prop. Do you know what the specs are for the prop--specifically, the diameter and pitch? If you had a survey done when you bought the boat it is common for the surveyor to note that in his or her report. You can then contact a prop shop and they will tell you whether what you have sounds right. They will need not only the specs for the prop but certain specs from your engine, such as your transmission's gear ratio. But they will tell you what they need, assuming they don't already have the info for your particular engine on file.
Hope you get this sorted out. If you truly cannot hit more than what is actually only 1500 rpm with that engine then something is seriously wrong. However, assuming the engine itself is fine, getting your prop re-pitched is no big deal and you'll be happy with the performance improvement if you do.
Good luck and report back.