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Old 01-09-2016, 11:18 AM   #21
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I agree. All I care about is mpg. But how do we get that in the absence of accurate expensive fuel flow meters? That's how the thread started. Seems to me that the way to get there is to understand fuel consumption along the prop curve, then match to measured speed of the boat for a given hp/fuel input to create a chart that shows mpg by rpm. Once I've done that I won't care about gph. It's just an input into the final set of calculations.

I'm sure this has been done before, but I'm new to this and it's winter :-)
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Old 01-09-2016, 12:25 PM   #22
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or cruise and write down the miles covered and fuel bought.

doesn't work as well for all boats initially...but after awhile, even simplistic data becomes useful to the point that the other details are just fluff.

say you cruise 2000 miles a year...plus or minus....

after 5-10 years of doing it and the average trip costs XXX in fuel.....and not being able to change much...isn't that average number good enough?

not for stretching a tank to the next fill up...but for many...that doesn't matter....so when is data too much?

only personal preference matters there.....
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Old 01-09-2016, 02:30 PM   #23
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If I had a FD boat I'd agree. Determining an appropriate cruising speed is relatively easy, and there's not a big difference between fast and slow cruise. But with a SD hull like my Mainship 34 there is a much wider range of cruising speeds available. The PO of my boat chose to cruise at 8-9 knots. Other owners cruise at 6-7 knots, and some who have repowered are cruising at 12-14 knots. That's a big spread, and my goal is to fully understand the choices WRT fuel consumption and use that information to influence my choices. I'm a data guy, if that's not clear by now :-)
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Old 01-09-2016, 03:11 PM   #24
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Unlike car manufacturer's one company makes the engine and another installs it in their boat. We think in gallons per hour first because that is what the engine manufacturer's publish. They don't know what kind of boat their engine is in. A 200 hp engine could be in a 40,000 lb trawler that uses 1/4 of that typically or in a 6,000 lb go fast boat that uses 80% of it typically.

It is up to us to convert the numbers that the engine manufacturer's publish in their prop curves to NM/gal based on how fast and at what rpms our boat can go.

Also there is no EPA mpg testing and reporting requirements for boats.

OTOH, Mainship did pretty good job, while they were in business, of letting each engine manufacturer test their boat/engine and report fuel consumption vs rpm and MPG. They were always a little on the high side of speed and MPG because the weight was minimal, but if you adjusted for that, they were pretty good.

David
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