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Old 11-12-2017, 15:26   #1
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30ft powered cruiser with an extra outboard motor?

I知 hoping someone here can help answer this question for me, I cant find it anywhere. How fast can a 25hp outboard motor push a 30ft powered cruiser with a planning hull?
What I知 trying to figure out is how much fuel I would burn using an outboard vs the large inboard engines. This might be a ridiculous exercise, but I知 curious and looking at the different options between a sailboat and powered cruiser. We like the powered cruiser for the better floor space, but the fuel economy is a real drag.
Can a 25hp outboard get a 30ft powered cruiser up to 4kts? If not how many horse power would I need to look at to get it moving about that fast?
I saw a post from someone with a 30ft sailboat that had a 25hp outboard that could get going 6kts and burn 1gph, which sounds great. But I don稚 know if a planning hull will be anywhere near the same. Does anyone have experience with an outboard on a powered cruiser? Is this a ridiculous idea? Why?
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Old 11-12-2017, 16:23   #2
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Re: 30ft powered cruiser with an extra outboard motor?

I see no reason why a 25 horse outboard wouldn稚 push a cruiser to 4 knots.

We池e able to push our 15000lb 32 sailboat to approximately 3 knots with a 5hp outboard at 50% to 75% throttle.
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Old 11-12-2017, 18:26   #3
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Re: 30ft powered cruiser with an extra outboard motor?

Displacement speed is not dependant upon hull design, only water line length.
Thus a typical planing hull will go at displacement speed (or close) using relatively small power in calm conditions .
But you will be unlikely to save much fuel using an outboard versus the boats own engine for the same hull speed and more importantly you will be unlikely to move against a sea or wind. not a good idea imo.
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Old 11-12-2017, 18:35   #4
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Re: 30ft powered cruiser with an extra outboard motor?

Quote:
Originally Posted by stone beach View Post
Displacement speed is not dependant upon hull design, only water line length.
Thus a typical planing hull will go at displacement speed (or close) using relatively small power in calm conditions .
But you will be unlikely to save much fuel using an outboard versus the boats own engine for the same hull speed and more importantly you will be unlikely to move against a sea or wind. not a good idea imo.


This is the truth.

Try running the oat at idle speeds with the existing inboard. Chances are it値l consume as much fuel as a 25hp outboard.
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Old 12-12-2017, 06:34   #5
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Re: 30ft powered cruiser with an extra outboard motor?

Our heavy 26-foot power boat gets up to 4.5-5 knots with its kicker, a 9.9hp hi-thrust outboard. A 25 hi-thrust should easily push a 30-footer at that speed or a bit better.

If you do this, make sure you go with a hi-thrust - lower gearing, wider flatter-pitch prop, made for pushing heavier boats at displacement speeds.
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Old 12-12-2017, 06:54   #6
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Re: 30ft powered cruiser with an extra outboard motor?

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Originally Posted by NewMoon View Post
Our heavy 26-foot power boat gets up to 4.5-5 knots with its kicker, a 9.9hp hi-thrust outboard. A 25 hi-thrust should easily push a 30-footer at that speed or a bit better.

If you do this, make sure you go with a hi-thrust - lower gearing, wider flatter-pitch prop, made for pushing heavier boats at displacement speeds.
In addition to NewMoon's good advice, you must get the outboard propeller down below the hull in clean water for it to be efficient. If the prop is shielded by the transom then you are wasting hp driving the boat. Mounting to one side will help here.

One other thought, yachts are thin smooth canoe shaped objects, well mostly. Your planning motorboat has a flat transom designed to peel the water away from the hull when travelling at speed on the plane. However, at displacement speeds that same transom works like a huge brake with masses of drag so you can't compare the different hulls.

Have you thought about running on one engine to get the required 6 knots.

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Old 12-12-2017, 09:02   #7
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Re: 30ft powered cruiser with an extra outboard motor?

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Originally Posted by burtonridr View Post
How fast can a 25hp outboard motor push a 30ft powered cruiser with a planning hull?

What I知 trying to figure out is how much fuel I would burn using an outboard vs the large inboard engines.

Can a 25hp outboard get a 30ft powered cruiser up to 4kts?

Not very, relative to the designed cruising speed.

I would suspect the existing inboard engine at 4 kts wouldn't burn enough fuel to worry about -- relative to the cost of acquiring, mounting, and maintaining -- a whole 'nother (outboard) motor on the boat. A big engine, loafing (possibly idling, at 4 kts), doesn't burn much fuel. A small engine, cranking, might end up burning about the same, or even more. Running a 30' boat at up to about 5.5 kts with the main installed power, when sea states permit with a planing hull, would likely be cheap. Even up to about 7 kts might not break the bank.

Yes.

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Old 12-12-2017, 09:16   #8
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Re: 30ft powered cruiser with an extra outboard motor?

I believe the formula is 1.34 x square root of the waterline in feet to figure hull speed. For argument sake, let's say the waterline on your 30 foot cruiser is 25 feet. Sq root of 25 = 5, 5 x 1.34 = 6.7 knots.

My 37 hp inboard will push my 36.4 foot cruiser at 7.3 knots (it's hull speed), so it seems likely a 25hp motor could push a 30 foot boat at 6+ knots. Bear in mind that displacement increases approximately as the cube of the boat length, so a 30 footer is considerably lighter than a 36 footer, like 30% lighter.

A 25hp has about 32% less power than 37hp, so the math works for you to achieve 6 knots or so.
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Old 12-12-2017, 09:18   #9
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Re: 30ft powered cruiser with an extra outboard motor?

Quote:
Originally Posted by burtonridr View Post
I知 hoping someone here can help answer this question for me, I cant find it anywhere. How fast can a 25hp outboard motor push a 30ft powered cruiser with a planning hull?
What I知 trying to figure out is how much fuel I would burn using an outboard vs the large inboard engines. This might be a ridiculous exercise, but I知 curious and looking at the different options between a sailboat and powered cruiser. We like the powered cruiser for the better floor space, but the fuel economy is a real drag.
Can a 25hp outboard get a 30ft powered cruiser up to 4kts? If not how many horse power would I need to look at to get it moving about that fast?
I saw a post from someone with a 30ft sailboat that had a 25hp outboard that could get going 6kts and burn 1gph, which sounds great. But I don稚 know if a planning hull will be anywhere near the same. Does anyone have experience with an outboard on a powered cruiser? Is this a ridiculous idea? Why?
it's called a trolling motor or kicker. 5 kts is about the correct speed to troll for big game fish . Like salmon or tuna.
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Old 12-12-2017, 09:19   #10
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Re: 30ft powered cruiser with an extra outboard motor?

I am curious though, as to the make of the "cruising boat" that has a planing hull. I'm a sailboat guy, so maybe I'm not thinking about this right.
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Old 12-12-2017, 09:21   #11
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Re: 30ft powered cruiser with an extra outboard motor?

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Originally Posted by AJ_n_Audrey View Post
I am curious though, as to the make of the "cruising boat" that has a planing hull. I'm a sailboat guy, so maybe I'm not thinking about this right.
there are a lot of them from the little 25 ft bayliner to the 50ft plus tollycraft.
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Old 12-12-2017, 09:30   #12
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Re: 30ft powered cruiser with an extra outboard motor?

We used to have a 28 ft express cruiser (30 LOA). beam = 9'9". We could run at 6-8 kts and burn fuel like a trawler. If you want to burn less fuel and move a larger boat, look into diesel trawlers.
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Old 12-12-2017, 10:29   #13
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Re: 30ft powered cruiser with an extra outboard motor?

Quote:
Originally Posted by burtonridr View Post
[SIZE=3][FONT=Calibri] I知 hoping someone here can help answer this question for me, I cant find it anywhere. How fast can a 25hp outboard motor push a 30ft powered cruiser with a planning hull?
Hull speed, around 8 knots. In favorable conditions, you could reach hull speed with less than 25 hp and if trying to push faster would end up plowing the water and leaving a huge wake without getting up on plane. Think in terms of 1 gph.

Quote:
What I知 trying to figure out is how much fuel I would burn using an outboard vs the large inboard engines. This might be a ridiculous exercise, but I知 curious and looking at the different options between a sailboat and powered cruiser. We like the powered cruiser for the better floor space, but the fuel economy is a real drag.
Look at Ranger tugs or the Atlas Acadia, which are semidisplacement hulls that are set up for that. They are available with smaller, more fuel effecient, inboard diesels that are intended for use at low speeds.

There are also a few much larger full-displacement power yachts like the Kadey Krogen. Their smallest in current production is 44'. I believe they have made slightly smaller ones in the past. There are a couple of competing makers with similar boats.

Quote:
Can a 25hp outboard get a 30ft powered cruiser up to 4kts? If not how many horse power would I need to look at to get it moving about that fast?
I saw a post from someone with a 30ft sailboat that had a 25hp outboard that could get going 6kts and burn 1gph, which sounds great. But I don稚 know if a planning hull will be anywhere near the same. Does anyone have experience with an outboard on a powered cruiser? Is this a ridiculous idea? Why?
The setup is going to be hard to steer and noisy. Controls will be complicated. Fuel handling will be complicated. A 25hp motor requires a reasonably strong transom mount and will not fit on the transom of most inboards -- the available brackets are intended for smaller outboards.

Inboard cruisers have a higher surface area above the waterline which makes them more susceptible to wind. Most are designed as planing hulls which means they have an inefficient shape in the water at lower speeds. They are designed to outrun weather rather than withstand it. You'll not want to get caught out in a storm.
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Old 12-12-2017, 10:41   #14
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Re: 30ft powered cruiser with an extra outboard motor?

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Originally Posted by AJ_n_Audrey View Post
I am curious though, as to the make of the "cruising boat" that has a planing hull. I'm a sailboat guy, so maybe I'm not thinking about this right.
The nomenclature is a little fuzzy.

The "cabin cruiser" is a powerboat that is usually intended mainly for day or perhaps weekend use, distinguished from a runabout (or bowrider) by the presence of an enclosed cabin. I had assumed that this is what the OP is asking about.

Whether or not such a craft is suitable for cruising for a longer period of time is a separate question. I would suggest that it depends on the waters to be visited. Most are not suitable for bluewater cruising, but are fine for rivers, lakes, and coastal cruising in protected areas.
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Old 12-12-2017, 10:49   #15
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Re: 30ft powered cruiser with an extra outboard motor?

Looking at where the op is located any vessel with a cuddy cabin could conceivably be a cruiser. For long or short term.
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