Thread: GPH efficiency?
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Old 05-24-2018, 08:37 AM   #80
Ski in NC
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City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
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Quote:
Originally Posted by Benthic2 View Post
So.....this may be a little off topic.....but.....
If you were repowering a boat with a hull speed of...say 9 knots.....and it took ( random guess here) 100 horsepower to do that speed....would the ideal be to pick an engine a little greater than 100 hp to compensate for headwind...current....bottom growth and so on ? I just have such a hard time fathoming ( did you see what I did there ? ) big boats with small engines. I had a 16 foot, 500 pound boat that had a 25 horse engine....I have a 17 foot boat that has a 140 hp engine.....and some of you guys have serious boats with less hp than my little bowrider.....Its hard to wrap my head around this concept. I know displacement boats are a different animal than planing boats....but .....not to single anyone out...but scrolling back I see Mark has a 14 ton boat with an 80hp engine. it blows my mind....sorry for the off topic rant.
Displacement hulls certainly are different animals than a planing bowrider. Completely appropriate to spec a low hp engine for those.

A typical 40' trawler will use like 40-50hp to run hull speed of 7-8kts. But you don't want to spec a 50hp engine as it is not good for it (nor quiet!) to run an engine at max power. So typical practice is to spec one so it runs at around 50% of its rated power at like 2/3 rated rpm. For most engines this is a "happy spot". So you shop for a 100hp engine...

Now getting to pick an actual engine gets complicated. There are not infinite choices out there. JD makes a nice 4cyl in the 80-150hp range, but many don't like the low rpm shake of a 4cyl. Their six is very nice, but like Cummins, a little overkill. Ford Lehman 6cyl 120hp is still available, but not from the original packager, which complicates things. Cummins lowest hp six is a turbo at 210, way overkill on hp, but still ends up being a nice trawler engine just with lots of headroom. And then there are little car based engines and low market share vendors that have another set of issues.

Few of us actually get to pick our engine. We get to pick a boat that comes with an engine and hopefully you like both!!
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