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jwmiller33

running a short shaft on a 20 inch transom

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can you run a short shaft motor on a 20 inch transom? i am only wanting to do this for a couple days til my other motor gets its lower unit fixed. i have a 20 horse that came with my boat, and was the motor that was used on the boat before me, however, it is a short shaft motor and its on a 20 inch transom Lund. if i just wanted to putz around the lake for a couple days and not worry about getting on plane or driving too fast, are there any problems i will encounter by doing this?

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If you idle around it will work fine. Once the speed comes up it will cavitate like the dickens!

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If you idle around it will work fine. Once the speed comes up it will cavitate like the dickens!

what do you mean? the boat wont get on plain? or....?

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Cavitation means the prop is grabbing to much air and to little water.

If the prop isn't submersed in water your going to hear some slurping sounds, and you won't be able to stay up on plane. You also have very little control while the prop is out of the water.

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The term is actually ventilation. Air is pulled into the prop blades causing major slippage. Heck it's not even a matter of "pulling" air in, it will just plain be out of the water. Not only will it not plane, it will also over rev. If it pulls air in, it cant "bite" the water.

Bottom line is, no you cant run a short shaft on a 20 inch transom unless, as Hydro said, you only plan on idleing around only.

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but would it be fine if i was only going to go really slow speeds? its just a temp thing until i get a new lower unit for my 35 horse

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Like the guys pointed out, yes you can use it if you keep to trolling speed. Any faster and not only will it ventilate and over rev, you stand a chance of overheating because the water intake will be above water level.

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how far should i engage the throttle max? 50%?

i have heard of guys that can customize a motor or buy the correct piece to turn a short shaft into a long shaft... is this something that is easily done? how about with this 20 hp merc of mine? probably not even worth it?

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Let's put it this way:

You won't get up on plane. If you get above 25% throttle I'd be surprised.

If it rains, you're going to get wet.

You might be able to find an extension kit to turn your short shaft into a long shaft, but you'll probably have to find a used long shaft that has it installed and adapt it to your motor.

If you have another motor being fixed to put on this boat long term I wouldn't spend money on adapting the 20 horse motor now.

marine_man

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