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Old 13 May 2014, 16:48   #1
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380/15HP Prop suggestions

Need to consider what to do about the prop on the Suzuki DT15 I'm rebuilding for our Bombard 380.

The one with the engine is quite rough, with part of a blade missing, but is the std short shaft 9 1/4 x 11.

They come in sizes from 9 1/4 x 8 to 9 1/4 x 11.

What size props are others using on 380s with 15Hp engines?

Thanks

Nasher
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Old 13 May 2014, 17:58   #2
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What's the Bombard max HP rated for ? If 15 is top which would doubt probably will need a prop maximization (play with pitches) to bump up revs.

Will need to test that standard prop with a induction tach and determine if inside min-max safety wot parameters factory stated for that engine to start with.

Then pass to a +,- pitch to be near the max wot parameters. Hard to tell with a prop with issues as won't be able to test it correctly to deliver full engine rpm.

Usually you test standard delivered prop and then go from there maximizing prop pitch to match your needs.

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Old 14 May 2014, 03:15   #3
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Max on the Aerotec is a 25. I (and I guess 99% of others) just use the standard prop that comes with the 15 (same on the Tohatsu 9.8) on their SIB - consensus is for normal use (ie 1 - 3 up, speed around 16-20 kts), from previous discussions this seems to give the best compromise. On the Mercury/Mariner 15 it's the A12P9 (9" x 9).

Page 13 of the manual here gives the 15 2 strokes prop options:

Boatinfo - Mercury Service Manual for 6-8-9,9-10-15 hp
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Old 15 May 2014, 20:54   #4
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You could be lugging a under powered engine at wot without knowing it, specially if running a 15 when the sib needs a 25. You would need to maximize your wot numbers in the event that sib will be used with more than 3 sumo passengers. Sorry, no short cuts on this one, will need a tach, are cheap a must have one

To get the idea. My 430 rib rated for min 30 HP engine is used with a Tohatsu 18 HP which gives at wot 30 rpm less than max factory stated, that's with me alone and lightly loaded rib, if add a extra mate, wot rpm will drop at about middle wot range, if add a thirdd mate rpm will drop to near min rpm which isn't good.

As don't like being on the low extreme of its rpm for long time periods just about lugging the engine, simply dismiss the third mate. This means that can take him for a ride at any other throttle setting except wot.

It's bit silly to change a factory delivered prop for any other as per blind recommendation, checking the owners manual without having even tested the original one.

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Old 16 May 2014, 17:48   #5
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Thanks guys, I know this isn't an easy one.

However, as it stands I don't actually have a good prop, the std size one that came with the engine is knackered, I'll have to TIG weld it and spend a lot of time reshaping it, so I'm on the lookout for another.
While I'm about it, and if I get a choice on the size of a replacement, I'd rather be better informed.

Nasher.
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Old 16 May 2014, 20:55   #6
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Ideal is to check wot numbers with original one when new with a tach, then will know if opt for your application or if under or over propped. Much easier to play with pitch props.

Try to get that prop on shape again to work and tach it. If underpropped go for at least one less pitch to bump rpm towards the max wot rpm factory stated for that engine.

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Old 10 July 2014, 12:20   #7
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Nasher - finally got round to doing some runs with the tacho on today.

Mariner 15 with standard factory prop.

Best trim position was 2 and achieved 5750 rpm max at WOT. Touched 19 kts lightly loaded with myself 12 stone and young son around 5 stone.
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Old 28 June 2015, 09:20   #8
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Nasher, where did you end up with this one?
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