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Old 09-08-2019, 13:42   #1
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Outboard pull double duty

I am saving and trying to buy my first tbd sailboat of considerable size. Looking in the 26 ft range and ideally trailerable. My most immediate use would be day sailing and weekend use thus Id be happy with just using an outboard to get on and off dock/trailer. I already own a 15HP motor I have on a tin boat. I have been wondering if I could use this motor on the tbd boat. My current motor is not long shaft. Is that totally necessary? I understand the prop probably should be below the hull bottom to be most efficient but if the only motoring is on and off the dock Id think you could use a non-long shaft outboard. Also would be nice to have a sizable motor on the dinghy for exploring once you get where you are going. Any thoughts would be greatly appreciated. Thanks.
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Old 09-08-2019, 13:47   #2
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Re: Outboard pull double duty

Need to know more about the 15hp outboard you already own; not to mention the boat you may buy. If the boat already has a sled (or similar mounting device) it may well work. If you have to build one yourself it may be a bigger job than you realize.
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Old 09-08-2019, 13:59   #3
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Re: Outboard pull double duty

15 hp two stroke Yamaha. not sure about the shafts length from mounting bracket to cavitation plate. my guess would be two feet or just shy of that. In my mind the motor would mount on a bracket that can raise and lower. Sounds like that may be called a sled. Does anyone know if the propeller just becomes inefficient if it is not in clean water below the hull or does it become useless.
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Old 09-08-2019, 14:04   #4
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Outboard pull double duty

It will work fine assuming itís mounted so that the anticavitation plate is at least at water height or ideally lower, as long as your in calm water, and itís meant for slower speeds maneuvering around and not for long distance traveling.
For that you will likely need a long shaft motor
It wonít likely work very well in any significant Ocean swell.
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Old 09-08-2019, 14:36   #5
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Re: Outboard pull double duty

My boat spec'd a 20" shaft but can also take 15". It's on a lift/lower bracket anyways...



How to Find Your Shaft Length | Southcentral Outboards
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Old 09-08-2019, 14:48   #6
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Re: Outboard pull double duty

The three outboard leg lengths are short @ 15", long @ 20" and XL - extra long @ 25"


My first boat had a short leg outboard fitted by a PO. He'd built a slider track and small lifting crane to allow the outboard to be slid down the track for operation and slid up when not in use. You can also buy heavy duty auxiliary outboard brackets that offer multiple positions and use a spring to reduce effort when lifting up that do pretty much the same thing.



Generally on a sailboat you'd want a minimum of a long leg because the boat pitching can not only pull the prop out of the water but can also dunk the powerhead in the drink too. Obviously, the mounting height should preference keeping the powerhead dry regardless of leg length.
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Old 09-08-2019, 16:07   #7
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Re: Outboard pull double duty

My motor which is a 5 hp 4 stroke 25" extra long shaft Mercury Outboard does great in waves to near 3' tall depending on several other conditions.

The motor when the bracket is all the way down is even with where the diesel's prop used to be.

I've been using it now for the last 8 years. I'm on my second bracket.

It still wouldn't help much though if I tried to motor upwind in the conditions in this video. Raymarine ST2000 Autopilot is doing the driving and you can hear it at the start of the video. Winds were gusting to 29 knots....

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Old 10-08-2019, 22:34   #8
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Re: Outboard pull double duty

In a former life, I was the engineer on a dredge. We had several Johnson 40hp outboards and one Boston Whaler. The outboards would be swapped out as needed so that we could keep working. One of the motors had a short shaft. Almost totally useless against the deep hull of the Whaler. We'd manage Ö say Ö 20 knots with a long shaft outboard, but about 3 knots with the short shaft outboard. The cavitation would kill most forward motion.

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Old 11-08-2019, 12:05   #9
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Re: Outboard pull double duty

Remember the water intake and exhaust go through the leg. U need to ensure water intake is always in water to ensure cooling, also that the prop is always in water to prevent overrevving if the engine if it lifts out of the water. Failure if either can be very expensive. Best get right engine and shaft length for your boat- safety first!
Also, motor must be high enough to prevent it being duncked as the boat pitches. Check also that the engine u pick has a reasonable charge rate for your battery and electrics.
Ask your local engine supplier for advice, then decide.
Engine is often last resort if in trouble - get tge right one
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Old 11-08-2019, 15:29   #10
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Re: Outboard pull double duty

Quote:
Originally Posted by epatant View Post
Remember the water intake and exhaust go through the leg. U need to ensure water intake is always in water to ensure cooling, also that the prop is always in water to prevent overrevving if the engine if it lifts out of the water. Failure if either can be very expensive. Best get right engine and shaft length for your boat- safety first!
Also, motor must be high enough to prevent it being duncked as the boat pitches. Check also that the engine u pick has a reasonable charge rate for your battery and electrics.
Ask your local engine supplier for advice, then decide.
Engine is often last resort if in trouble - get tge right one
Todays engines have overrev protection so that isn't a problem anymore depending on how old his engine is

As far cooling, that's basic stuff but you should check the tell tale stream every 15-20 minutes if your impeller is 2 years old or more
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Old 11-08-2019, 15:53   #11
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Re: Outboard pull double duty

20" shaft at a minimum and even then only if the mounting allows the propeller to be below the bottom of the hull (cavitation plate in line with the hull bottom, at a minimum!).

Even of you are thinking you will just use it to get on and off the trailer a short shaft will cause you problems, like when you expect to stop at the dock and your crew member goes to the bow and the prop comes out of the water...oops, no reverse. And you hit the gas in reverse and then run to the front to fend off, and wrow! the engine is revving in the air.

Then suppose, just suppose, you are out for a day sail and a few miles from the ramp when the wind goes completely dead. You crank up the motor but it just cavitates when you speed it up a little, especially in any waves.

If you are really short of cash to buy another motor, see if you can find a extension for the lower unit of you motor. They are easy to install.
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Old 12-08-2019, 05:20   #12
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Re: Outboard pull double duty

Many thanks to all for the thoughtful responses and the different scenarios to consider. Lots here I did not consider before. I certainly have a better picture of what I will need in the coming months. Thanks again!
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Old 12-08-2019, 07:04   #13
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Re: Outboard pull double duty

I have an adjustable bracket along with a 25" extra long shaft 5 hp 4 stroke outboard

This video has deteriorated a bit but you can see the waves are pretty good size to still be motoring with an outboard. I was motoring across in a calm and a cloud line passed over bring quite a bit of wind and waves with it.

The bracket is on the 3rd hole down out of 4. I have motored in larger waves but no much larger. The smaller view of the video seems to be best.

I unfurled the jib after the wind came up.....and worse case, I could have shut down the outboard and raised the main, but I only had 6 miles to go to get in.

There is usually a way to get around the problems outboards cause. The worst case it steep waves on the bow in a narrow channel where you don't have room to sail out.

Sometimes though you can motor sail it with the main only set extra tight

Autopilot is doing the driving in the videos below and since it's the Simrad TP10 you can hear it.



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