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Old 11-11-2013, 14:31   #31
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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Originally Posted by GPSocal View Post
foggysail,


I've always wondered if free wheeling one prop shaft would cause excessive wear on either the transmission (V-drive or straight) or the shaft bearings. Free wheeling is common on sailboats but not so much on powerboats. Any theories on that?

Potential transmission problems were a concern prior to my adopting the one engine routine. So I called Borg Warner and asked if harm would happen to their Velvet transmission by running with only one engine.

They put me through to their engineering/applications department where I repeated my concern. NO, I was told, as long as one keeps his speed below 10K or so everything would be fine. I rarely go much over 7K and do so using both engines.

I also thought about having the props retuned for higher inches/revolution although this would be a radical method to further improve fuel economy. Recently, Professional Boatbuilder had an article discussing such methods. But as for now, I intend to leave the props alone.

Foggy
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Old 11-11-2013, 14:36   #32
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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Originally Posted by deckofficer View Post
For real powerboat fuel efficiency, Larry Graf of Glacier Bay fame has designed a new powerboat that at first glance looks like a powercat but the hull is a proa, where the driven hull is about 35% greater beam than the non driven proa hull. What this does for cruise efficency is instead of the common 1 to 1.5 nm per galllon, it is 5 nm per gallon.

Aspen Power Catamarans | 28', 32', 36', 48' Cruising Catamaran

Yes but that is of no help to those of us who own older designs. If fuel was my only concern, I would have remained with sail.
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Old 02-12-2013, 12:21   #33
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I have cruised a 1987 Tollycraft CPMY e/w Crusader 454 gas engines for 9 seasons on the Great Lakes out of Toronto. One of the best additions I made to the boat was a Flow Scan meter to monitor fuel consumption. Having graphed burn rates I now cruise at 8 or 9 knots. This seems to be the sweet spot just before she starts to climb up on plane and the consumption rate rises exponentially. She has a top speed of 18 knots. I watch the Flow Scan meter to keep my consumption at or below 5 ltrs/nm. That translates to 1 to 1.5 NM/US Gal at +/- 2000 rpm.
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Old 22-01-2014, 12:07   #34
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

Just to add my observations re. fuel usage. I own a 1979 40' Trader Trawler Yacht. It has twin 120hp. Ford marinised engines. We been based in the Ionian Greece since arriving here last year from Spain. Our cruising speed is whatever we achieve at 13 to 1400 rpm. This can vary from 5 to 7knots depending on sea conditions, current etc. Our optimum speed is usually 6nms. per hour. At this speed we use about 8lts. per hour. This works out that every 100 nms. covered we use 120lts. One has got to say that if we increase the rpm to over 1500 the fuel usage increases quite considerably.
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Old 03-05-2016, 10:16   #35
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

Following up on Capt Tom, who gave you the formula for hull speed. Above that speed your transom is no longer supported by your first wave past your bow wave, so you are climbing the bow wave but not planing. That's horribly expensive and produces a monster wake. We're diesel, but at 42' waterline and cruising at 7.5 knots, we get 2 NM/G, or just under 4 gallons per hour while pushing 60,000 pounds. Stay under your hull speed, and fuel costs whatever your engine will not be a problem. I've met a sport fisherman skipper who plowed the Tennessee-Tombigbee, Mobile Bay, to Carrabelle (our base) doing 1,800 miles and spending $25,000 in fuel. He still had to wait for the locks....
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