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Old 02-03-2014, 19:55   #1
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Outboard Motor Sizing

I've got a 6000 lb. boat, 27 feet long. If the Diesel engine gives out, what size outboard motor will I need to push it along at 5 knots.
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Old 02-03-2014, 20:26   #2
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Re: Outboard motor sizing

I would imagine the same horsepower used by the diesel engine to push it 5 knots. Remember an outboard will bob in and out of the water off the transom.
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Old 02-03-2014, 21:22   #3
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Re: Outboard motor sizing

Assuming you're not fighting the wind/current 6 HP will be plenty. I have a 27' 5200 LB boat with a 9.9 outboard and cruise at 4.5 knots at half throttle.
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Old 03-03-2014, 07:16   #4
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Re: Outboard motor sizing

For better grab on the water, I'd suggest a 9.9 hi-thrust. Larger diameter flatter-pitched prop, and lower gearing. Significantly better in reverse as well. Ours pushes our 26' 11,000 lb boat at 4.5 knots.
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Old 03-03-2014, 07:26   #5
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Re: Outboard motor sizing

My Bristol 27 (6600lbs) had a 352 lb 10hp Bukh diesel. After it failed, I installed a 2011 (58lb) 4 stroke Mercury 5 hp engine with a 25" shaft on an adjustable bracket on the stern.

It works fine especially coming and going from the slip.

The engine cost was $1500.00, and it is just as good on fuel as the diesel.


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Old 03-03-2014, 07:48   #6
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Re: Outboard motor sizing

6hp Tohatsu with hi-thrust prop pushes my 8000lb Triton just over 5 knots in calm water with clean bottom. About 4.5 in open water at 3/4 throttle. Use less than 1/2 gallon an hour.

Drops to under 3 knots into 25 knot headwind in protected water. Into a chop and heavy wind I doubt it would make progress -- but...it is a sailboat. I just use the motor in and out of the slip.
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Old 03-03-2014, 13:24   #7
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Re: Outboard motor sizing

I've used a 9.9 on a 27' Catalina. It moved no problem with that motor. It's nice to have a little cushion as far as HP go so you're not running it way up in the RPMs a lot.
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Old 03-03-2014, 14:30   #8
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Re: Outboard motor sizing

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Originally Posted by Mikerodrig27 View Post
I've used a 9.9 on a 27' Catalina. It moved no problem with that motor. It's nice to have a little cushion as far as HP go so you're not running it way up in the RPMs a lot.
I never run my 5 hp much more than 1/2 to 3/4 throttle on my 6600lb 27' Bristol since it doesn't help much with the speed anyway.

The main thing that helps with speed is to be sure the motor is perfectly aligned.
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Old 03-03-2014, 14:52   #9
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Why would you want two engines? If you dont trust your inboard so much why not spend the money on increasing your trust with the inboard instead? Also outboards make a boat look like crap
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Old 03-03-2014, 15:09   #10
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Re: Outboard motor sizing

a 5hp would push it fine in a light chop. The nice thing is you can use your dingy motor if you install an adjustable OB bracket on the mother ship.
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Old 03-03-2014, 15:58   #11
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Re: Outboard Motor Sizing

Neil

You didn't say were you are sailing or what type of boat and that will have a direct impact on what you would buy.

In my old stomping grounds of San Francisco Bay a 5 horsepower motor on a 6,000 pound boat wouldn't get you out of the marina because of the winds and currents. Here in Southwest Florida a 5 hp motor would probably be fine unless you got caught in a summer squall.

I had several friends with Catalina 27's in San Francisco who installed outboards after the inboard engine failed. They used 9.9 hp long shaft outboard motors on them.
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Old 03-03-2014, 16:04   #12
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Re: Outboard Motor Sizing

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Originally Posted by d design View Post
Why would you want two engines? If you dont trust your inboard so much why not spend the money on increasing your trust with the inboard instead? Also outboards make a boat look like crap
I think the key point there is spend the money. Why fix a say 1979 vintage diesel when you can have the reliability and technology of say a 2012 4 stroke outboard for around $1,500.

If you don't like the look, reach back and remove the outboard and store it once you are out of the slip.

You can get a Honda 2hp air cooled outboard that weighs 27 lbs for about $700. Easy enough to put up on the rail after getting away from the slip.
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Old 03-03-2014, 16:23   #13
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Re: Outboard Motor Sizing

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Originally Posted by Neil Holck View Post
I've got a 6000 lb. boat, 27 feet long. If the Diesel engine gives out, what size outboard motor will I need to push it along at 5 knots.
Key variables are getting an outboard with a shaft long enough of to get the right size prop deep enough.

A long shaft 9.9 Yamaha is the standard, but you can get by with a lower HP motor.

As others have posted if you are fighting strong current/wind and large waves a smaller outboard may be overpowered.

One problem boats with outboards mounted on the transom may have is going into the teeth of wind, current, and waves is that the boat can go up the face of a wave, be stopped dead in its tracks once the wind hits it, and slide backwards down the wave.

A good mounting system to raise up the outboard is well worth investing in for many reasons. It may well be more important than the choice of the engine.
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Old 05-03-2014, 18:15   #14
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Re: Outboard Motor Sizing

Thanks Guys,
We do 99% of our sailing in coastal Florida. We will probably go to the Bahamas next Winter. After that, we intend to go up the East Coast to the Hudson River and explore it. Some of the East Coast cruise will be ICW and some outside. The boat is a Compac 27/2 . The reason I would like a decent sized outboard is for the time, and IT WILL COME, when the engine gives out and I can't get to help by sailing. I Do have Boat U.S. Towing service, but for the Unlimited Towing Plan premium, after a few years, a good outboard would pay for itself.
Thanks and I DONT think an outboard looks bad hanging on the transom.
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Old 05-03-2014, 18:28   #15
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Re: Outboard Motor Sizing

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Originally Posted by Neil Holck View Post
SNIP

I Do have Boat U.S. Towing service, but for the Unlimited Towing Plan premium, after a few years, a good outboard would pay for itself.
Thanks and I DONT think an outboard looks bad hanging on the transom.
Never had to use it but a friend of mine was heading from Dry Tortugas to Key West and had some serious issues with his inboard motoring into the teeth of 30-40 knot winds. Bottom line is he called Sea Tow and got out of a problem he could not have avoided with an outboard.

Not to say a good motor is not a wise investment, just that I would not drop Sea Tow.
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