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Old 05-03-2015, 11:31   #16
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Some of the posts are confusing gallons per hour with miles per gallon. While running a fast boat at idle speed will drastically reduce your gallons per hour burn rate your miles per gallon rate could actually go up.
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Old 05-03-2015, 11:43   #17
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

If you go real slow with the main engine or the Kicker your fuel economy will be much better. However, as mentioned steerage etc must be dealt with by using a tie bar from the main engine to the kicker.


Other than that, the best fuel economy will most often be at cruising rpm 3500-4000..... you are sucking fuel but going 35 miles in an hour also.
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Old 05-03-2015, 11:56   #18
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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Yes, going slow will improve your MPG. General rule of thumb, keep the wake under 6". If you go slow but are throwing a 2-3' wake, you aren't going to see good fuel economy.


A simpler solution is to leave the engine alone and just go slow (you do want to make sure the engine gets up to full operating temp and also open her up once in a while). You will get 80-90% of the benefits just going slow. Unless you put a lot of miles under the hull, you will never justify the cost of repowering.


Also if you pull out that big hunk of iron, you will mess up the trim on the boat.
DITTO ... the big block engine can be surprisingly efficient at low RPMs.

FWIW, here two samples from 29' express cruiser, 8500 lbs loaded, with VP 8.1 L 375 HP engine ... MPH measured with GPS, GPH measured with flow meter in real time. The MPG calculated from MPH/GPH relation ... as noted elsewhere the GPH means nothing without knowing the distance covered over time (MPH). Hope this helps.

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Old 05-03-2015, 12:03   #19
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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Don't know what your budget is, or how many folks you'll be cruising with, but if you want an cruiser that's economical to own and operate, one you might consider is the C-Dory 22 or 25 Cruiser. They're outboard powered, can cruise at 16-20 knots, or putt along happily and even more economically at displacement speeds (as long as the outboard is 4-stroke, not 2-stroke). The 22 sleeps two or three adults, and is light and super easy to tow. With a 90-100 hp outboard and a 8 or 9.9 hp kicker it's well equipped. The 25 looks much the same only larger.

They're designed exactly for the rainy BC/Alaska coast, and are great simple and seaworthy little cruisers. We took our CD 22 from the San Juans up to the BC north coast, and all over SE Alaska. Here's a photo in Port Hardy.

Good suggestion. Another might be the Ranger Tugs. Or maybe the Nimble Nomads.

Sundancers have some advantages for some folks, but for us they're pretty "cave-y" inside.

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Old 05-03-2015, 12:40   #20
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

The Searay 25 Sundancer and similar designs are not particularly good in rough water.
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Old 05-03-2015, 13:09   #21
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

I wouldn't place a bet on any particular hull design, engine size, or speed. But the entire concept of a planning hull versus displacement, is that when you get the hull up OUT of the water, you eliminate all water drag on the part of the hull, vastly increasing the efficiency and theoretically, that has to mean better gas mileage in terms of miles covered per gallon burned.


That even applies to inflatable dinghies and RIBS that are often used 9.9-25hp outboards and getting up on a plane.


Some very similar hull designs have very different efficiencies. Just looking at them, who would suspect a Ford Fusion has a smoother ride than a Ferrari?
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Old 05-03-2015, 13:48   #22
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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I wouldn't place a bet on any particular hull design, engine size, or speed. But the entire concept of a planning hull versus displacement, is that when you get the hull up OUT of the water, you eliminate all water drag on the part of the hull, vastly increasing the efficiency and theoretically, that has to mean better gas mileage in terms of miles covered per gallon burned.


That even applies to inflatable dinghies and RIBS that are often used 9.9-25hp outboards and getting up on a plane.


Some very similar hull designs have very different efficiencies. Just looking at them, who would suspect a Ford Fusion has a smoother ride than a Ferrari?
True, but you also burn a lot of fuel keeping the boat on a plane. It takes a lot of energy to get enough lift from water to lift the boat, and that doesn't come cheap.

If you want economical speed take a look at a catamaran. Something like a cuddy cabin TwinVee. The long flat sides also help them track reasonably well at low speeds. But they will never be as efficient as a displacement hull running slow.
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Old 05-03-2015, 21:05   #23
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Thanks for all the replies. The cruiser boats look nice but they are a bot out of my price range. And I do't really like the layout of the Rosboroughs, they seem to be more for day outings. The Searay Sundancer 25 has a layout I like with everything I need for extended living, and believe me, I can handle cramped quarters, I kayaked halfway up the Inside Passage.

Some Searays are available quite cheap. I guess there's a reason for that and I am willing to do some restoration work as long as the structures aren't rotted. I'll get a survey done before buying. For their price I'm sure I could put in a new more efficient smaller diesel engine and still come out way ahead.
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Old 05-03-2015, 23:53   #24
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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I wouldn't place a bet on any particular hull design, engine size, or speed. But the entire concept of a planning hull versus displacement, is that when you get the hull up OUT of the water, you eliminate all water drag on the part of the hull, vastly increasing the efficiency and theoretically, that has to mean better gas mileage in terms of miles covered per gallon burned.


That even applies to inflatable dinghies and RIBS that are often used 9.9-25hp outboards and getting up on a plane.


Some very similar hull designs have very different efficiencies. Just looking at them, who would suspect a Ford Fusion has a smoother ride than a Ferrari?
It's almost never the case that a cruising boat on plane will get better MPG. The primary advantage of a planning boat is that it can go drastically faster but that speed comes with a penalty of lower MPG.

The exception is the idiot who takes the boat up to speed just before getting on plane. The bow is pointed up 45 degrees and they are pushing a massive wake without getting much of a speed boost. Compared to that, on plane will be more efficient but drop down a bit and displacement will be more efficient.

To the OP: I agree some of the specialty boats people suggest are nice but yes, they are very expensive. I'm not a huge fan of searays and similar high volume boats but there are enough of them out there that you can usually find a good deal and for your purpose, it should work fine.
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Old 06-03-2015, 07:40   #25
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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DITTO ... the big block engine can be surprisingly efficient at low RPMs.

FWIW, here two samples from 29' express cruiser, 8500 lbs loaded, with VP 8.1 L 375 HP engine ... MPH measured with GPS, GPH measured with flow meter in real time. The MPG calculated from MPH/GPH relation ... as noted elsewhere the GPH means nothing without knowing the distance covered over time (MPH). Hope this helps.

This is an interesting post. 23 mph at 1400 rpm is pretty amazing. That's a really low rpm to go that fast. Is that supposed to be 2400? It's hard to believe you could more than double the rpm to 3000 & only increase the speed 3 mph. For the OP who appears to be very inexperienced the miles per gallon numbers here are about the best that you could expect in a boat this size & better than most.
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Old 06-03-2015, 07:47   #26
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

FWIW, C-Dory 22 Cruisers well equipped and in very good condition are available for us$ 35K-38K. If you'd like to know more about them, take a look at the C-Dory owners web site c-brats.com

They get about 4nmpg on plane, and a good bit better (6?) at 5-6 knots.
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Old 06-03-2015, 07:48   #27
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

[QUOTE=hellosailor;1766244]I wouldn't place a bet on any particular hull design, engine size, or speed. But the entire concept of a planning hull versus displacement, is that when you get the hull up OUT of the water, you eliminate all water drag on the part of the hull, vastly increasing the efficiency and theoretically, that has to mean better gas mileage in terms of miles covered per gallon burned.


In concept this sounds like it could be true. In actuality it's the opposite of what's true. I have a 5 ton boat with a displacement hull. I cruise at 7 knots (8 mph) & burn .5 gallons per hour. That's 16 miles to the gallon. A similar size planning hull, on plane, would be lucky to get 1 mile to the gallon. Often boats this size change to gallons per mile.
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Old 06-03-2015, 07:54   #28
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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Originally Posted by Mark_BritC View Post
Thanks for all the replies. The cruiser boats look nice but they are a bot out of my price range. And I do't really like the layout of the Rosboroughs, they seem to be more for day outings. The Searay Sundancer 25 has a layout I like with everything I need for extended living, and believe me, I can handle cramped quarters, I kayaked halfway up the Inside Passage.

Some Searays are available quite cheap. I guess there's a reason for that and I am willing to do some restoration work as long as the structures aren't rotted. I'll get a survey done before buying. For their price I'm sure I could put in a new more efficient smaller diesel engine and still come out way ahead.
I wouldn't count on replacing a gasoline engine with a diesel. That's going to be very expensive. Another thing to consider; outdrives don't do well in salt water. Corrosion becomes and issue.

I have friends with Sea Ray Sundancers and they enjoy their boats but their use is a lot different than what it seems you are intending. They boat on lakes and the AICW and most keep their boats out of the water between uses.

While we are on the subject, a Sea Ray is basically a Bayliner with upgraded appliances and trim. Same engine, same outdrive, etc. If a beat up Sea Ray fits your needs, consider a Bayliner as well.
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Old 06-03-2015, 08:03   #29
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

I agree with RWIDMAN as I've had both a 25' Searay, IO Mercruiser V8 and a 24" Bayliner with a Volvo 270 outdrive with a Volvo FWC diesel. I do disagree however, the Bayliner easily outshined the Searay both in quality and comfort. The Bayliner was also a much dryer boat (living in Juneau, AK0 that made a difference. The Bayliner I would take out for multiple day trips and come back in and only have to "top off my fuel" tank, 20 or 30 gallons.


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Old 06-03-2015, 08:05   #30
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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Thanks for all the replies. The cruiser boats look nice but they are a bot out of my price range. And I do't really like the layout of the Rosboroughs, they seem to be more for day outings. The Searay Sundancer 25 has a layout I like with everything I need for extended living, and believe me, I can handle cramped quarters, I kayaked halfway up the Inside Passage.

Some Searays are available quite cheap. I guess there's a reason for that and I am willing to do some restoration work as long as the structures aren't rotted. I'll get a survey done before buying. For their price I'm sure I could put in a new more efficient smaller diesel engine and still come out way ahead.
It's true that Rosboroughs & C-Dorys are quality boats that don't come cheap. However, they are good examples of boats that are being used for the same type of cruising that you want to do. Sea Rays are not. If I were you I'd look at older cabin cruisers with gas engines & straight shafts like Chris Crafts. I would avoid gas IOs.
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