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Old 05 April 2007, 04:50   #1
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Country: UK - England
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AUX for 5.25

I am looking at getting a 4 or 5 HP Mariner, Mercury or Yamaha as an aux for my 5.25 metre Osprey.

I am hopeful of finding a 5HP as this would be close to the 1HP per metre rule however I may have to settle for a 4HP as 5's seem to be fairly rare to find second hand.

The reason I like the above engine is they are all single cylinder compact units and I dont think you can get a 6HP or greater in a single cylinder engine.

I have a couple of questions:

Should I be looking at a long or short shaft for the Osprey?
Can you buy props that will maximise the engine for use on a large boat (Hi Thrust)

Thanks

Chris
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Old 05 April 2007, 05:04   #2
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Originally Posted by Chris1573 View Post

Should I be looking at a long or short shaft for the Osprey?
Depends on where you have it on the transom. Get a tape measure and measure distance A below. If it's below 18" then short shaft, if above 18" or your boat rolls enough for the bottom of that measurement to come out of the water at displacement speeds (rare) then you need a longshaft.
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Originally Posted by Chris1573 View Post
Can you buy props that will maximise the engine for use on a large boat (Hi Thrust)
Best place to ask would be Redline Outboards in Abingdon for info like that on a specific engine-he's very helpful and not too far away. High thrust props are more used on river/canal boats.

Thanks

Chris [/QUOTE]
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Old 05 April 2007, 06:40   #3
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Quote:
Originally Posted by Chris1573 View Post
The reason I like the above engine is they are all single cylinder compact units and I dont think you can get a 6HP or greater in a single cylinder engine.

Can you buy props that will maximise the engine for use on a large boat (Hi Thrust)
Johnson/Suzuki do a 6hp four stroke single cylinder, same basic engine as the 4 and 5 but with different carb or something. It's what I use for an aux and v pleased with it.

From what I have read on here though, once you get to the "displacement speed" of a hull then it won't go any faster unless you pour a lot more power in. That tallies with my observation - at WOT I don't get anything like max revs on my aux, and I'm not really sure I am getting any more power out of it than I would be out of a 4hp ... I guess it probably tops out at about 4000, something like that (just a guess) and the extra power is developed right at the top end. I wondered about a "high thrust" prop too but I might also be getting a little boat just to play in so I am not sure about taking that route - and from what I found there are only 6" and 7" pitch available for mine anyway so it wouldn't make much difference.
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Old 08 April 2007, 17:40   #4
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Tohatsu also do a 6HP single cylinder 4 stroke which is pretty much the same size as my old 4HP merc single cylinder. I get just over 5knts on my RC 5.3 with this. Got the optional battery charge circuit so I could run lights etc if the main engine did give up (and I'd flattened the battery trying to start it).

High thrust prop is a waste of time on a RIB. Basically it gives added thrust in astern - mainly designed to stop larger displacement craft. On a RIB this is not required. The HT prop is also less efficient in ahead so you loose some forward thrust.

Note - in terms of positioning, I jammed mine right against the A-Frame with just enough clearance to allow it to lift. The prop thrust holds the tiller against the A-Frame (on a piece of padded pipe wrapped around the tiller arm). You can steer with the main engine which acts as a good rudder and allows you to drive in relative comfort. Turning circle is pretty good (at least on my setup) and I've no problems motoring around the marina. I normally motor out and back in on the AUX to give it some exercise.

Good luck!
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