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Old 27-05-2013, 11:52   #1
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Outboard Shaft Length?

I am getting a new outboard . . . to be used on a 3m (10') normal avon inflatable (With inflatable floor). Simple question - what shaft length do I want - 15" or 20"

The transom is 41cm (16") from top to bottom .

Our current outboard we bought in Iceland, and is metric measurements and looks to have a 45cm shaft = 18"
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Old 27-05-2013, 12:19   #2
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Re: outboard shaft length?

Short shaft nominally 15" from prop shaft to transom mount, is generally the length for a dingy motor.
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Old 27-05-2013, 13:20   #3
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Re: outboard shaft length?

Evans, you don't mention the hp of the engine, but I'd be inclined towards the "short shaft". My concern is that the inflatable floors are not really very rigid, and the torque on the boat generated by a long shaft would be more likely to bend the floor in use, compared to the short shaft where the lever arm is shorter. Or am I purely wrong in this assumption?

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Old 27-05-2013, 13:57   #4
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Re: outboard shaft length?

You want the cavitation plate to be 3-4" under water when the boat is at rest with the engine installed. Generally, this will place the cavitation plate 0-2" below the bottom of the dinghy hull.

The shaft length is measured from the clamp where it rests on the top of the transom to the cavitation plate.

Does Avon recommend a shaft length? If not, I'm thinking the 15" will work.

Mark
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Old 27-05-2013, 14:02   #5
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Re: outboard shaft length?

ok, great. Thanks guys (and gal) I got a 15".

Ann, 6hp . . . pretty small by modern standard, but we can plane with both of us in the dinghy (if it's dry).
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Old 27-05-2013, 15:34   #6
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Re: outboard shaft length?

whatever you get the cavitation plate should just be at or just below the lowest point of the transom.

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Old 27-05-2013, 16:07   #7
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Re: Outboard Shaft Length?

If your transom is 16" how is a 15"shaft going to work?I bought a short shaft Suzuki 4hp,4stroke thinking it would fit my 8ft. inflatable.Had to cut the transom 1.5 in. so it wouldn't cavitate.
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Old 28-05-2013, 21:38   #8
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Re: Outboard Shaft Length?

This begs the question.. can a shaft on a modest HP outboard be too long? And would it not be better to err on the long side of ideal?

We have a 5HP Yamaha with long shaft (this was all that was available on the day) which drives our 8ft soft floor dinghy. Seems to work ok, but we can only plane with one on board, sitting as far forward as possible - I put this down to the drag of the soft slatted floor.
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Old 28-05-2013, 23:17   #9
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Long shafts are nice because the prop is less apt to suck air at high power, in chop, whatever, without the extra extended fins.

Not so nice in shallow coral and rock.

Cavitation plate should be somewhat below the bottom edge of the transom.
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