Originally Posted by Polar Opposite
So, the experience (mine and others) with this engine so far is that it WILL overheat when pushed above the rated 2500 rpm. It happens slowly, creeping up over an hour or two until the water temp gage is bumping the upper limit. When I keep it at the rated RPM of 2500, it stays at around 195 for hours. And at 2500 rpm, in light chop, I'm doing about 5.2, nowhere near the hull limit. Is that a prop issue, a cooling
issue, or an underpowered boat?
Let's break it down.
At 2500 rpm the engine produces 25hp. Because of the v-drive and prop bearings, the prop is seeing 23hp. That is enough power to drive your hull to 6kts on a windless, calm day if your bottom is clean and properly propped. The fact that you're reporting only 5.2kts can be a host of issues that I can't diagnose from a distance.
Here's where I'd start to narrow the issues down. Have you checked the accuracy of your alternator-triggered tachometer with an optical tach? How accurate is your speed through the water measurement and SOG is not STW.
The tach accuracy is essential to determine if you have the correct prop. You indicate that you can rev above 2500, so how far can you go above 2500? Perhaps you are underpropped and that would reduce your top speed.
above 2500 but runs all day at 2500 can mean several things. First to mind is that Westerbeke installed a heat exchanger
sufficient only for the caloric load of the engine at 2500 which I doubt since the same heat exchanger
was used on the W50. So other items that can reduce the caloric load handling ability of the cooling system to be checked a: When was the last time you serviced the heat exchanger tube stack, raw water
impeller, thermostat, and pressure cap? What does the raw water
flow through the oil
cooler look like? Has it ever been removed and serviced? Another cause of the overheating
could be excessive exhaust
back pressure which could be caused by exhaust elbow
blockage, hose collapse/restriction or poor initial/subsequent installation
. Ever perform an exhaust backpressure measurement? Westerbeke has a maximum of 39" H2O or 1.4 PSIG. That's a very low number.
There's a lot to mull over here but at the end of the day, 23hp at the prop is only going to deliver 6kts under ideal conditions. That's should be enough to get you off a lee shore so it comes down to how you use the boat and how much money
you want to spend to get those remaining 0.7kts. A professionally done repower
could exceed 50% of the value of your boat. BTW you never said what type of v-drive you have which is important info to determine if a bobtail installation
is economically feasible.
(PS on edit) You may know this but the BMC 1.5L (4-91) was used in London taxis for years. Its durability and economy are legendary. I'd work
on getting that engine running at peak and making sure you have a 2500 rpm max prop fitted.