Outboard prop sizing?

[prv]

Thinking about this boat...

The engine is a 1981 DT40, and the prop hub (probably original, or at least old) is now slipping. My mate's spoken to his local prop shop, who says that a rebush is only £10 less than a new prop, so might as well just buy new, which we probably will.

However, the prop man recommends changing from the current 11.5 x 13 to a 10.5 (not sure of pitch). To me it seems wrong to reduce the prop diameter - surely the bigger the prop (within reason) the more grip on the water? I know that you can over-prop a shaft-drive engine on a big boat, but surely Suzuki would have put the right size on? Are we just being sold whatever he has on the shelf (he has lots of 10.5 but no 11.5 in stock) or is he right that the smaller prop will be better (or at least not worse)?

Cheers,

Pete

 

[Spi D]

TBH the proper way to go is to know the achieved rpm at wot as that will tell if the engine is running under optimal conditions. You need to keep within the recommended rpm range, preferably near the upper limit.
Edit: 5200-5800 rpm for your engine.

Reducing prop pitch will result in increased engine rpm - and vice versa (1 inch = 200 rpm). In this case however, the power to weight ratio is different that most and testing is needed to find the best prop. If, at the same time, you want to remedy any unwanted behavior (believe you migh experience ventilation now and then) testing of multi blade or even surface breaking props would be way to go. Blade area and shape has a lot to do with performance.

To my knowledge Suzuki never supplied 10.5 dia props for this engine but that is a bit besides the subject. You go for the right prop and the dia will be fixed to match. Suzi offer up to 17" pitch for your engine (compatible with DT50).

Third parties may offer differently. Personally I'd stay away from composite props, adjustable and fixed.
You need a prop for 3 1/4" gear house dia and 13 splines.

 

[Spi D]

Thinking about it I'd guess 16-17" would be right for your setup. At least!

 

[prv]

TBH the proper way to go is to know the achieved rpm at wot as that will tell if the engine is running under optimal conditions.

Cheers - but it all sounds a bit academic for our redneck shoestring operation . At present our main focus is on just keeping the engine running; we're suffering a lot with water getting into the cowling and shorting out the HT leads. Gaffer tape is helping, but not fully solving

If we need to buy a new prop then we'd like to avoid buying something completely wrong, but squeezing the last drop of performance out of the rig is not currently a priority. Needless to say, our pull-start tiller-steered single-carb 34-year-old two-stroke does not have such refinements as a rev-counter!

It's annoying that the pitch of the proposed prop has dropped out of the Chinese Whispers between Prop Man and me - I'll try to find out what he's proposing. But should we believe him on the question of diameter?

Thanks,

Pete

 

[Spi D]

Point taken. In trying to keep the thing running, staying within rpm and load range would be a prerequisite. An universal, wireless, handheld tacho would set you back some £9..

But still, if you are going to get a new prop and the cost is the same for any, why not get the better? Your prop man certainly should allow you pay for one and then swap until happy (provided they are still as new after test). If he doesn't it might be time for a new supplyer or an ebay hunt for alternatives (make sure you look for the right rotation - RH is standard)

One to talk to could be
http://www.ebay.co.uk/itm/SUZUKI-OU...CH-2-STROKE-/331147663357?hash=item4d19f057fd

In theory a larger blade area will hold better in the water but the pitch will decide forward movement. Properties like cupping (raising the trailing edge of blades), blade shape, rake, tip design etc. are important too.

If interested, I've put a booklet about the subject here:
http://prop.spidybot.com/Quicksilver_Propellers.pdf

 

[prv]

Cheers - away on the big boat for a week now, so not going to be doing much prop stuff. But I didn't want your helpful post to seemingly drop into a black hole

I've advised my mate (who's not registered on the forum) to have a read of this thread in the meantime.

Pete

 

[Davy_S]

I cant comment on the dia of the prop, I am certain that 15 inch pitch would be the max on a standard prop.

 

[Spi D]

I'm certain that part no.
58100-88L72-019 - is a S1600 Propeller 3x11-1/8x16 by Suzuki Outboard and that
58100-88L83-019 - is a S1700 Propeller 3x11x17 by Suzuki Outboard

 

[prv]

Just to end the story on this one - we ended up with an 11-1/4 x 15, taking SpiD's advice on going for a higher pitch figure (even higher might have made sense, but I think the choice was largely based on what was available cheap!)

I had to fabricate a new tab washer for the prop nut from stainless sheet, as all the legs had broken off the old one and they seem to be hard to get hold of.

Prop works well, no slipping obviously and it feels like we're going a bit faster (no way to tell ).

Main drama this time was the throttle grip falling off - had to jury rig with a spanner to get back

Cheers,

Pete