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Old 01 February 2012, 14:10   #1
Country: UK - England
Length: 3m +
Join Date: Jul 2009
Posts: 24
Outboards on inflatables - short or long shaft

Most inflatables in the specs suggest standard shaft. What is the issue with a long shaft on these boats? On offer is an air deck with a long shaft, which "works well on the boat" - says the current owner!

I imagine you have a bit more weight to carry? What else will be an issue?

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Old 01 February 2012, 14:16   #2
Country: UK - England
Town: chesterfield
Make: Quicksilver
Length: 4m +
Engine: Mariner 30
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Im not a hive of knowledge but i did buy a sib with what i didnt realise at the time was a long shaft engine.
2 biggest problems i found was when i was pulling it on the transom launch wheels the prop was very close to the floor even on full tilt and the other was when you were coming back in off the water you had to pull the engine up alot earlier as it would bottom out usually still being in waist deep water.
But then as i said im no expert

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Old 01 February 2012, 14:44   #3
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Country: Ireland
Make: Redbay Boats
Length: 9m +
Engine: 370hp
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Not the best of setups.

OKish if you want to use your small auxie to potter out to the big boat at displacement speeds.

Not good if you want dry landings and planing speeds. Transom ratings are for the appropriate shaft length, the extra leverage will "increase" the forces at play. The extra drag from the leg will sap power and probably result in a lot of splashing, if the SIB ever planes at all.
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Old 01 February 2012, 14:59   #4
Country: UK - N Ireland
Town: Bangor,Co Down
Make: Gemini/ Avon
Length: 4m +
Join Date: Jul 2003
Posts: 166
I remember reading a thread on this subject on here a few years ago in relation to bigger engines over say 20hp.
Two things I did remember were that on a planing sib a longshaft engine was dangerous because so much of the leg was in the water causing steering issues and the engines where the exhaust gases went through the prop hub did'nt run correctly due to there being too much back pressure.
One of the outboard/inflatable outlets around Milton Keynes had an article on their website.
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Old 01 February 2012, 15:19   #5
Country: USA
Town: Seattle
Boat name: Water Dog
Make: Polaris
Length: 4m +
Engine: Yamaha 60hp
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Posts: 1,152
On the smaller SIBs and RIBs the tubes aren't that big and there's not alot of deadrise (V in the hull) so the transom is of course proportionally not as tall.

+1 on what the others said about long shaft outboards being too deep to work as designed on a short transom boat.
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Old 01 February 2012, 18:06   #6
m chappelow's Avatar
Country: UK - England
Town: yorkshire
Boat name: little vicky
Make: avon ex RNLI
Length: 3m +
Engine: tohatsu
Join Date: Mar 2008
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Originally Posted by Pav View Post

What else will be an issue?
if you get a plaggy bag around the prop or it gets clagged up with weed you have an extra 6 inches or so to lean out and clear it .

on some engines you can have starting problems with too much back pressure but when underway should be no problem .

there can be spray kicked up depending what make of engine you owing to the leg profile .

you may loose a knot or 2 speed wise though that also depends how much power the engine is .

one good point is that the prop is always in water so if the sea gets a bit lumpy you always have power and you get less airiation .

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