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Old 05-11-2013, 12:44   #16
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

jm21You may have to curtail your cruising range to 800 miles or less to keep within your fuel budget. Just throttle back and enjoy the journey. Fuel is only one expense to be aware of if you are buying an old(er) boat with hours on the engines. Things wear out or break in a salt water environment. Moorage, haul-outs and continual maintenance are continual expenses.
Tolly made a fine hull and there are lots of them around the Pac NW. They also have an owners' forum which you should visit. www.tollycraft.net
If you cruising on Puget Sound and up into the San Juans and Gulf Islands, you don't have to go far for some spectacular boating adventures. While you can't beat the Southern California weather, the boating experience here is not the same. I'd move back but my wife says I'd miss her.
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Old 05-11-2013, 19:33   #17
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jm21

An earlier post has not yet arrived on this forum for some reason. I suggest you look at the Tolly owners forum for answers to your questions about fuel consumption. Look at Tollyclub.com
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Old 06-11-2013, 10:32   #18
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I will check out the tolly forum.

My thinking was just trips in for gas, water, groceries, etc., would add up pretty fast as far as mileage goes. But maybe with a really nice dinghy you could cut that expense down by just going in with the dink.
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Old 08-11-2013, 18:56   #19
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I switch over to one engine after getting underweigh. I cruise with the running engine set to between 1750-1800 RPMs. My flowscan tells that the fuel consumption varies between 1.4-1.6 m/g. Speed is about 6.5K. I swap the engines for the return trip back.
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Old 08-11-2013, 23:13   #20
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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?
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Old 08-11-2013, 23:22   #21
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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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?
We always engaged the transmission to stop auto-rotation, and aligned the two-blade propeller with the sailboat's keel to reduce drag.

Heard that some transmissions don't like free-wheeling. Also, some shafts depend on the engine running to provide water lubrication.
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Old 09-11-2013, 04:00   #22
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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is 1.5 or even 2 nmpg/hour unrealistic?
completely. expect 40-60GPH burn rate. going slow works out the same. gasoline engines are not good at low speed. i did an experiment once, i went 40 miles on the erie canal in my powerboat, at no wake speed. then i did the same 40 miles on plain. fuel burn was the same.
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Old 10-11-2013, 20:53   #23
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Fuel burn is related to engine load and RPM. Look at some of the tests in boating magazines, there is a significant difference in GPH between low and high speed. Most tests will show an optimum efficiency range.
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Old 10-11-2013, 21:31   #24
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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gasoline inboard engines use about 0.6 pound (272g) of fuel per horsepower generated
per hour, and diesel engines need about 0.4 pound (181 g) per horsepower generated per
hour.
Would you mind sharing your information source for these basic parameters? I'd like to do a few more calculations for an unrelated project.

Thanks,
Jim
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Old 11-11-2013, 10:11   #25
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

I performed tests on about 50 boats of 250-500 hp gas powered. Other than slow idling.. (or at least not pushing any noticeable bow wave at all..) the best speed for fuel burn was about 4000 rpm fast cruising, yeah the engines were sucking a lot of gas, but the boat waas going 40 mph.... turns out that was the most economical to get from point A to point B. One of the least economical is trying to "push" the displacement speed. ie: if the displacement speed is say 7 knots.... and youre trying to go 10-11 knots... it's terrible for fuel consumption over a given course.
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Old 11-11-2013, 10:32   #26
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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
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Old 11-11-2013, 11:26   #27
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

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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
I wouldn't get too excited about it being a "proa". From the web site:

"The 28' C90 weighing in at 9,200 lbs. (with gear and fuel) cruises comfortably at 16 knots, burning a stingy 4.6 gallons per hour."

That's no where close to 5 nmpg. While they come up with an interesting explaination, I don't buy the idea that you don't have to counter steer to account for the engine being offset. Prop walk just doesn't create that much turning force.

If you move up to the 38', it says:

"Design cruise speeds are 16-18kts with a top speed of 22-24kts. Expected fuel consumption is 8.5gph at 18 kts, in other words, 50-75% less than competitive cruising boats. At trawler speeds, it burns 2.6gph, which will get you 630 miles on 180 gallons of fuel. Cruising at 18kts, you will travel 350 miles with 10% reserves"

At cruise, that's a hair over 2mpg. Not bad but again no where close to 5mpg. They don't say what "trawler speeds" is but if you back calculate from the range and tankage number it's around 3.8 nmpg.

Compare that with say a PDQ trawler and they produce similar numbers:

PDQ 34 Performance

Since the web site seems to be talking about theoretical performance, expect real life to be less.

I'm a fan catamarans but keep in mind, you do lose storage space and they are affected much more so by being heavily loaded which is typical for cruisers.
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Old 11-11-2013, 11:34   #28
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Re: nmpg per hour for 35-38' powerboat with twin gas engines at displacement speeds?

When I think trawler speed, I'm saying 6 ~ 8 kt speed. At 6 kt it does 6 nm per gallon.
http://www.aspenpowercatamarans.com/...M2013Breed.pdf

Also real owners in a PNW group cruise "...In fact, several Aspens taking part in a
rendezvous during the summer reported
covering 291 miles on about 58 US gallons
of fuel. That’s 5.02 miles per gallon."
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Old 11-11-2013, 11:49   #29
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My experience on my own twin screw MV is that there is little advantage to running on one engine. Between the drag of the other prop (either freewheeling or locked) and the crabbing angle induced through the water, you really don't save fuel.

As for freewheeling versus locked, i believe several boating magazines published studies on this and found locked produced the least amount of drag.

If you plan on freewheeling, you may need to fire up the freewheeling motor about once every four hours ( that was the advice I remember seeing for hydraulic transmissions) to put cooling water through the transmission cooler to help prevent overheating the bearings.

Having performed a gas to diesel refit on my boat which I use as you intend (planing hull at displacement speeds), I can tell you the gas boat is cheaper upfront but costs considerably more to operate even at displacement speeds.. I'd recommend getting a diesel trawler from the start since that seems to be what you want It will be cheaper in the long run.

My fuel management has changed from doing calculations of how much fuel I have onboard and how many more gallons I need for the weekend trip out to worrying if I can burn all the diesel on board before I have to add more biocide.
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Old 11-11-2013, 14:27   #30
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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
When I think trawler speed, I'm saying 6 ~ 8 kt speed. At 6 kt it does 6 nm per gallon.
http://www.aspenpowercatamarans.com/...M2013Breed.pdf

Also real owners in a PNW group cruise "...In fact, several Aspens taking part in a
rendezvous during the summer reported
covering 291 miles on about 58 US gallons
of fuel. That’s 5.02 miles per gallon."
And I can say trawler speed is 4-5kts but without knowing what they are basing it on, we can speculate all day long and still be wrong.

Also, I just noticed that 32' model is only a 10' beam so even if you ignore the lost bilge space, it's going to be narrow also. Compare that with most trawler cats and they gain back the lost bilge space with extra beam.

I might be proved wrong but it seems like a gimmick boat and I like idea of a proa. I have a proa dingy half built.
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