380/15HP Prop suggestions

Nasher · 13 May 2014, 21:48

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

Locozodiac · 13 May 2014, 22:58

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.

Happy Boating

Max... · 14 May 2014, 08:15

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

Locozodiac · 16 May 2014, 01:54

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.

Happy Boating

Nasher · 16 May 2014, 22:48

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.

Locozodiac · 17 May 2014, 01:55

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.

Happy Boating

Max... · 10 July 2014, 17:20

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.

Max... · 28 June 2015, 14:20

Nasher, where did you end up with this one?

Kong · 13 March 2019, 19:28

Quote:

Originally Posted by Max...

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.

Hi Max sorry for the thread resurrection! I think I have the same set up as you, A380 plus 2 stroke Mariner 15hp with standard prop. My typical usage is also me plus just my 5 stone son.

Is 5750 rpm the maximum the engine revs to or just the highest you could get it to? If the latter then it seems strange that such a lightly loaded/lightweight boat can’t get up to the maximum rpm.

My payload is always going to be around the same so I don’t need to compromise. I’d like something with a slight bias towards top speed but without sacrificing too much acceleration. But from what you’ve said above it may be slightly over-propped as standard anyway?

Fenlander · 13 March 2019, 22:54

When I ran the Aerotec/Mercury 15 2-stroke combo with a standard 9" prop I thought it ideal in all circumstances. With one adult plus light passenger (about 7 stone in my case) I thought it super responsive and lively.

If I understand you are thinking of propping down to get more revs on the OB I can't think that will be very noticeable on an already super lively combo when taking off... and would just add to the noise and stress on the OB with revs nearer the maximum.

Something I've noticed with the Aerotec is that top speed isn't its thing compared with some other sibs/boats. I found upping the power by 30% to 20hp only added a couple of kts so don't think you'll find much improvement above the already well suited 9" prop.

Kong · 13 March 2019, 23:55

Thank you for the reply Fenlander.

My original thought process was because it’s such a light boat with a light load that I could add an inch to the pitch for a bit more speed. But if Max’s isn’t reaching max RPM (says max 6000 on my outboard) then that wouldn’t work.

I think there’s a whole lot of messing about for minimal gain and, like you say it’s probably not going to do the motor any good driving on the redline constantly.

It seems the Aerotec is quite draggy and more power is the law of diminishing returns to some extent. I noticed someone on the forum trying to iron out the ‘hump’ before he transom using bracing, perhaps this would be a more useful pursuit.

Max... · 14 March 2019, 10:22

I think if max is 6000 and it's reaching 5750 then that's pretty ideal, the extra 250 isn't going to have much effect in all reality. All props are a compromise and I would not want to gain 1/2 knot or less but lose the holeshot (which is much more important).

As you say it is a draggy boat that drives a little like a RIB and dropping off power will see it come off plane easy so pick up is the most important aspect IMHO.

Not sure if you have read the 'maiden voyage of a bastard son of a Y class' (or something like that!) but that is one of many Aerotec topics with lots of useful info on set up, splash guards, raising the transom etc.

Fenlander · 14 March 2019, 11:30

Ahh sorry Kong I made a mistake thinking you wanted to pitch down to reach higher revs. Yep re pitching up I agree with Max so little to be gained if anything at all and possibly a negative at other points in the speed range.

I'd be interested to know an actual measured speed with a 25hp because as I said above the 20hp didn't much improve things re top end.

And as is so often said with a sib the frequently found UK sea conditions prevent maximum speed running on most trips.

Kong · 14 March 2019, 11:38

I don’t want to think about it. I had a peach of a Mariner Marathon 25hp left over from when I sold my Quicksilver which would’ve made the bombard rapid. Unfortunately it was a slipped-disc waiting to happen and would have made the difference between being able to beach launch by hand and not! :’(

Biggy · 17 March 2019, 00:28

As said, you really need a tach on to get a correct reading at wot.
With my old T38ie & Suzuki DF15, i ran a 9.25 x 10 Stainless Steel.. A slight top end gain over the standard DF15 ally prop 3x9.25x9"