Propellor choice dilema - please help.

[roycruse]

I own a small 4.2 meter humber assault.

On the back I have a suzuki dt55 2 stroke.

When I bought the boat a few months ago - it had a suzuki 11 1/2" diameter 14" pitch prop. This prop was great it that put the boat on the plane in about a nano second (i think the previous owner did some skiing behind it). But at WOT the revs were up at 6000 with speed at 33 knots and im sure the high rpm wasnt doing the engine any good.

I look around for a suzuki 11 1/2" diameter 15" pitch prop but no one has one in stock so rather than wait for one to be ordered in i foolishly went for a third party propeller (a turning point hustler).

At first I was recomended a 10 1/2" diameter 15" pitch prop - I questioned whether this would actually give any benefit as i thought the reduced diameter would cancel out any lowering of rpm that the increase in pitch would give - but I was assured that would not be the case.

Sure enough when I tested the boat went 3 knots slower and no change in WOT RPM.

The supplier kindly sent the only other prop that he had which was a 12 1/2" diameter 15" prop but when I tried this I find that my rpm has now dropped by 1000 to 5000rpm and my boat speed has decreased by 2 knots to 31.

My questions are :-

Is 5000rpm at WOT an indication that the prop is too big or too steep and am I labouring the engine or is 5000rpm ok ?

Should I just send the lot back and order a proper suzuki 11 1/2" X 15" ?

I was looking to get rpm at WOT to about 5500, is this about right or was 6000 not too much in the first place ?

Thanks for any advice anyone can give...

[Noddy]

Hi Roy
I bought a Propulse adjustable-pitch Propellor, there is a post on it but no-one seemed very interested? Its just the job on my 60hp Merc.
http://www.propulse.se/
Martyn

[Hightower]

Roy, what rev range does your handbook recommend?

If it was me and your range was 5000-6000rpm I'd be looking for around 5700rpm with full fuel tanks and just yourself as crew. That way if you load heavier (with extra crew) your engine revs at WOT souldn't drop below the lower recommened revs.

Assuming 5000-6000rpm is given as your rev range in your handbook.

Andy

[roycruse]

If only I had the handbook !!!

Finding info on this engine is prooving difficult.

The only info I can find is that anything from an 11 1/2" X 9" up to a 11 1/2" X 17" are optional props for this engine.

I got this from the 2004 suzuki propellor brochure.

[ADS]

Just looked in my Suzuki Clymer manual and the max engine operating rpm is 5000-5600 rpm.

Regards
Alex

[roycruse]

Originally Posted by Noddy

Hi Roy
I bought a Propulse adjustable-pitch Propellor, there is a post on it but no-one seemed very interested? Its just the job on my 60hp Merc.
http://www.propulse.se/
Martyn

These seem to recomend a 10.4" diameter prop - same diameter as the turning point one - so im not sure that would be an improvement over that.

shame though - looks like a great idea - especially if you are having trouble and need to experiment or you occ#asionally need to pull a skier and newed some extra kick from a stand still.

[roycruse]

Originally Posted by ADS

Just looked in my Suzuki Clymer manual and the max engine operating rpm is 5000-5600 rpm.

Regards
Alex

Thanks for that - confirms that i need the 11 1/2" X 15" suzuki prop.