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Old 05-03-2015, 00:02   #1
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Does gas mileage go up when using small outboard on 25 foot speedboat?

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, that would be a decent compromise.

Is there something to this, or am I out to lunch?
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Old 05-03-2015, 00:11   #2
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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.
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Old 05-03-2015, 00:18   #3
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Thanks. This would be in addition to the mercs. I'd still have them there for when I need or want the speed, but for saving fuel when poking along the waterways I'd just use the little outboard.
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Old 05-03-2015, 05:51   #4
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

You might look for a boat set up for fishing. A lot will already have a kicker OB on the stern for trolling purposes (the big engine at idle goes too fast for fishing).

You can certainly add a small outboard but I wouldn't expect it to ever pay for itself. Might make a nice get home engine if the big one dies.

Especially if you are are towing the boat to your destination, I believe you are overthinking the problem.
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Old 05-03-2015, 05:57   #5
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Quote:
Originally Posted by Mark_BritC View Post
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.

Quote:
Originally Posted by Mark_BritC View Post
Thanks. This would be in addition to the mercs. I'd still have them there for when I need or want the speed, but for saving fuel when poking along the waterways I'd just use the little outboard.

Adding an outboard to an inboard boat probably wont be workable; they need decent support to deal with potential torque. And then how would you rig it for steering? And so forth...

Running slow on the existing inboard engine is indeed a decent option. You'll find the hull may or may not be comfortable at slow speeds in all sea states -- especially beam seas -- but you can usually work around that by either getting up on plane for that period, or by tacking, or by waiting out the weather, etc.

Two other shopping options: look at boats built for outboards; there are several with decent below-decks accomodations... although I'm not sure many can offer as much internal space as a Dancer model. Or look at Sea Rays (or similar) with newer/more efficient single inboard engines compared to the older 454s (I think in that size range, they're usually an IO installation). Running either at slower speeds will save you fuel costs, without having to buy a separate outboard that might come with it's own mounting/operating issues.

-Chris
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Old 05-03-2015, 06:02   #6
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

One other consideration if you find a boat with a trolling OB already mounted. Make sure it's a newer 4 stroke. The old 2 strokes tend to be fuel guzzlers.
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Old 05-03-2015, 06:02   #7
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

check out the Magregor line of trailable sailboats These can come with a 50hp outboard give reasonable accomodations can sail and if in a hurry will plane.

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Old 05-03-2015, 06:56   #8
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

As mentioned adding a kicker outboard is your best bet for better fuel economy. Fishermen who troll for hours on end use a kicker for it's efficiency. The best solution is to add a lift bracket which gets the kicker up and out of the way as tilting put too much stress on it when using the main engine on plane.

Depending on the size of your boat and how fast you want the kicker to push it, your best option is probably a high-thrust (lower geared) 9.9 like this Yammie:

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Old 05-03-2015, 06:59   #9
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Quote:
Originally Posted by Mark_BritC View Post
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, that would be a decent compromise.

Is there something to this, or am I out to lunch?
I don't think you'll find a 25 foot "speedboat" that you will be able to live on for more than a day or two.

I think you'll find that a trawler style boat has the most living space for its size. Traditional trawlers are not very fast but there are some newer "fast trawlers" and some trailerable trawlers. Home | Rangertugs is one brand.

Some of these will "get up and go" if you don't mind the greatly increased fuel consumption. They do fine at 7 knots or so.
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Old 05-03-2015, 07:09   #10
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Diesel.
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Old 05-03-2015, 07:43   #11
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

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As mentioned adding a kicker outboard is your best bet for better fuel economy. Fishermen who troll for hours on end use a kicker for it's efficiency. The best solution is to add a lift bracket which gets the kicker up and out of the way as tilting put too much stress on it when using the main engine on plane.

Depending on the size of your boat and how fast you want the kicker to push it, your best option is probably a high-thrust (lower geared) 9.9 like this Yammie:

Are you sure on the fuel efficency?

Growing up on the Great Lakes with several family and friends into salmon/lake trout fishing, it's about speed not efficiency. At idle, the big engine might have you going 4-5mph. The salmon/lake trout like it down around 1.5-2mph. The common solutions:
- Trolling plate that partially blocks the thrust from the main engine (obviously not efficient but avoids buying a second engine). The plate tilts up out of the way while on plane.
- A smaller HP trolling motor (typically 5-15hp range like the picture) allowing you to dial in the speed down to as little as 1mph.

I wonder about the MPG efficency as the kickers are typically offset meaning you must constantly turn to counter it's effect. Effectively dragging the main drive leg sideways thru the water. Of course, they are dragging the main leg thru the water no matter which way it is turned.

If it is about cost efficency, I just can't see it ever paying for itself.

Going with a trawler or a diesel as some have suggested is a nice option but now you have really deviated from cost efficency idea as the OP posed the question.
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Old 05-03-2015, 08:02   #12
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Quote:
Originally Posted by Mark_BritC View Post
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, that would be a decent compromise.

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

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.



We cruised our CD22 for 7 summers, then decided to upgrade to a 26-foot diesel powered boat that weighs three times as much. More room and creature comforts, but four times the cost, sucks a lot more fuel, and requires a BIG diesel truck. We can still cruise 18 knots if we wish, but mostly do 6 knots, for 3X the fuel economy.



There are several other small cruisers, built more for the rain coast than the Sea Ray, that you might consider as well.

You might want to check out the little book I wrote for folks who are looking into a trailerable cruiser for the Inside Passage, "Cruising in a Big Way". Previews and books are available on the publishing web site lulu.com, and on Amazon.
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Old 05-03-2015, 08:15   #13
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Brother used to own a 36' Sportfisherman with twin Merc 454's
First, the most economical speed is NOT on plane, I thought that just as she broke over on plane that would be her most economical speed, but it wasn't even close. I had flow meters on the engines and I graphed the speed vs fuel flow. Most economical speed was by far idle, one engine idling actually, the faster you went, the more fuel you burned, period. Now you could get worse mileage if you insisted to run it right just before plane with her bow in the air and throwing an enormous wake. But you will burn 1/3 or less the fuel for the distance traveled at hull speed than you will on plane.

Like was said earlier, keep the wake flat, all that energy used to make that wake, came from your gas tank.
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Old 05-03-2015, 09:26   #14
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

I'm assuming this is an I/O?


There are numerous brackets built specifically for mounting an outboard on the transom. There are also arms that tie the outboard to the outdrive for steering purposes.


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Old 05-03-2015, 09:50   #15
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Re: Does gas mileage go up when using small outboard on 25 foot speedboat?

Running a planing hull, typically deep V, at displacement speeds does not make it a displacement hull. With no rocker the transom is well underwater & you are dragging the ocean behind you making it very inefficient. Another problem with cruising a planing hull at slow speeds is that there is no directional stability. The boat wanders & you are constantly correcting your direction. The answer is a semi-displacement/semi-planning hull. Lobster boats are a good example. Fine entry with a keel, soft chines & flattening out at the stern. The Rosborough 246 is a good example. Sam Devlin's Surf Scoter is another cool design that would work for you. The C-Dory is also a good boat that would work but it's a different flat bottomed dory design. Nimble made some neat small outboard powered trawlers that you might like but they don't plane.
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