Does gas mileage go up when using small outboard on 25 foot speedboat?

Mark_BC

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Mar 5, 2015
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Say you had a 25 foot speedboat with a Mercruiser 454. That is a pretty big gas pig. The most efficient cruising for these setups is usually around 20-30 knots. If you wanted to get better mileage and didn't need to get there fast could you just use a smaller outboard, say a 4 stroke 20 hp or something, and putter along with that, making your way along much like a sailboat under power? And would this save you a lot on fuel? I know speedboats have planing rather than displacement hulls, but at these low speeds could they just function like a displacement hull? It would be with a boat like a Searay 25 Sundancer or similar.

I am asking because I am looking to buy a boat large enough to live on for extended trips up BC's Inside Passage, but I want it trailerable. So that eliminates most sailboats. Catamarans are too expensive. It would also be nice to have some high speed capabilities if needed. But I don't want to be sucking back huge gas bills all the time, and if I can power the boat with a smaller outboard and just go say 5 knots or something as I poke my way along the waterways, that would be a decent compromise.

Is there something to this, or am I out to lunch?
 

Sea Rider

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Sep 20, 2008
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Welcome to Iboats,

Do you plan to keep the internal engine for speed and install an OB kicker for fast displacement speeds ?

Happy Boating
 

Georgesalmon

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Apr 14, 2012
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It would work, your talking about a "kicker". Many use them for trolling. I doubt you would need a 20hp, probably a 10hp would do the trick.
 

Faztbullet

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Mar 2, 2008
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A 454 with some sort of fuel injection will get better mileage in gear and fast idle(1200rpm) than a kicker. The reason being is the larger motor turning a 23 pitch prop will move boat farther and faster than a kicker as it smaller prop and would have to run at higher rpm to move boat. A 454 MPI @1500 rpm will use 3.5 gal per hour so depending on water conditions and up/down stream current you could go 16-20 miles in 1 hour and use 3.5 gal of fuel. A small kicker would have to run at a much higher rpm to push boat and would be less efficient as prop slip would be higher, this will result in smaller motor using more fuel to go same distance....

http://www.boat-fuel-economy.com/mercury-mercruiser-454-7.4-fuel-consumption-us-gallons
 

Mark_BC

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Mar 5, 2015
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Thanks. Faztbullet, I was wondering about that. One boat I am looking at actually comes with a 9.9 hp Nissan kicker.
 

JimS123

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Jul 27, 2007
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According to Merc, their 20 HP 4-stroke consumes 2.4 gph at wot. Figure about 80% of that at reduced rpm to just maintain hull speed. Nice to hav a kicker anyway. You need o decide if it benefits you.

I don't think a 9.9 would give you what you want on that Searay.
 

bassman284

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Jun 24, 2006
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Thanks. Faztbullet, I was wondering about that. One boat I am looking at actually comes with a 9.9 hp Nissan kicker.
Well, there are a lot of people here who know more than I do, but 16-20 miles per hour at 1500 rpm sounds like a lot, even with a 454. I would expect maybe 8-10 at best because, depending on the shape of the hull, that bad boy is going to start trying to plane at around 7 or 8. Could be wrong.

I think the 9.9 would be able to get you to 5 or 6 mph at about half throttle or maybe even less. My 9.9 pushes my 17' Lund at about 5-5.5 mph at about 2000 rpms. I can crank it wide open and not get much more because the nose starts coming up at about 6.
 

Chris1956

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Mar 25, 2004
Messages
27,036
Speedboats of that size usually have a small vee berth, a head and a bar. I am not sure that equates to liveaboard. You might try a different hull style. To be trailerable means the beam is 8 foot or less. You could get a 25' cabin boat with those dimensions, if you look around.

A 25' cabin boat could be powered by a SBC, if the hull is not too much of a deep vee. A 10 HP 4 cycle kicker might (might) save some gas over the SBC powerplant, at idle speed. I am not sure I would bother with the kicker. I would just run the V8 motor at idle. It will take lots of patience to keep your speed down.
 
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