Courtney the Dancer
Join Date: Nov 2000
Location: San Juan Islands., WA, USA
Thanked 22 Times in 21 Posts
Rep Power: 15
Re: overpropped vs underpropped
TQA- your analogy is incorrect. You can't compare a car that generally has way more maximum horsepower that it will ever need to a sailboat that generally only has a bit more hp than needed to achieve hull speed. You also can't compare a care with a lot of horsepower leisurely coasting along at 65 mph to a sailboat that is just below it's maximum speed due to hull design and high drag through the water at that speed. To make your comparison somewhat more realistic you should have the car going up a mountain pass at 65 mph, which gear are you going to have to be in to get the rpm up high enough for the engine to develop the hp required to maintain the 65? It isn't going to 6th gear, probably not even 5th if it's a steep hill. And it still isn't a good comparison because of the hp difference. They don't make cars that barely have enough hp to get to 65 (anymore) but that would be a fairer comparison. You're not even close to comparing apples to apples. If you want to run your engine at a higher load than is recommended by every engine manufacturer on earth and every marine mechanic that knows what he's talking about fine, but don't encourage others to follow your advice. Some of the older heavy duty engines might do OK running over propped but the modern, lighter, higher revving, turbo engines are a different story. How much fuel do you figure you have to save to even pay for a minor engine problem like a blown head gasket? Is the small amount you save by running a few hundred rpms lower even measurable? Seems penny wise, pound foolish to me.
Ground- not knowing what your recommended WOT is I don't know if 240 rpm is a big deal or not. I do know that on a Volvo that has a WOT of 3800 the factory mechanic told me that I was asking for trouble only being able to hit 3600. I had the prop re-pitched. I would call a couple of marine shops that work on your engine and run it by them for opinions.
SV Laurie Anne
1988 Brewer 40 Pilothouse
Last edited by jrd22; 12-16-2012 at 10:58 PM.