20 SF Prop numbers
 

1972 20 SF. 2001 yamaha 115 two stroke. Put in at local pond for shake down and anted to get some input on numbers I received via the Garmin GPS. All speeds are in knots. Prop tested is a yamaha aluminum 13 1/4 X 17 semi clever. I feel like im hitting the rev limiter at 5700 as I have throttle left and there is definetly a noise change when bouncing off 5700.

Trimmed all the way down:

3400 RPM 17.3 kts
4000 RPM 22.7
4500 RPM 26.2
5000 RPM 29.5
5700 (WOT) 33.4

Trimmed up:

3400 RPM 16.8
4000 RPM 22.2
4500 RPM 26.7
5000 RPM 29.9
5700 RPM 34.3

How do these numbers look? Surprised that speed did not increase by even 1 knot when trimmed up. Thoughts on other props? Thanks.

Those numbers look decent to me. I get 28-29kts on a F100 on a 20MA with a heavier motor.

I Would try to find a 3 blade 19 pitch.

I'm so confused about props. I have some telling me my results are good and others are saying I'm getting 25% slip which is pretty bad. Some are saying go down to a 14 pitch while others say go up to a 19 pitch. I think I'll have to call prop gods.

I am running a 13 1/8 X 19 SS Michigan on a 115 Opti and can top out at 41 mph. GPS. I am a little shy of max rpm but like the way boat runs. Do not have mid range numbers recorded will get those when ice breaks up.

So Ken at prop gods reccommended the Powertech SCD3 with 18 pitch. The other option would be the SCD4. I know the four blade is a stern lifting prop. I don't need to use tabs hardly at all but was wondering if the 4 blade would give me less slip and better economy. Anyone running somthing similar to these on a 115 yami?

34.3 knots is 39.5 mph. That's not bad. I calculate more like 16% slip. There might be room for modest improvement? I have a Solas amita 4 blade 17"p you could try. It fits the F100- should fit the 115 yamaha. It is a tiny bit too much for the F100

Fish stretcher, where in Boston are you? I'll actually be in Boston tomorrow at the coast guard sector around 3:30 or 4.

If you are hitting the rev limiter now with the 17 Pitch you defiantly don't want to go down in pitch to a 14p! You would be better to increase it slightly to a 18 maybe 19. If a 3 blade 17p is on the limiter try a 17p four blade, it will give you some rpms back and be more like a 19 3 blade. Ken at prop gods knows his stuff. Listen to him- He knows

Thanks frizz. I'm waiting to hear back from fishstretcher to borrow his 4 blade for a test. I hate spending money on props only to discover I need a different one and half to sell it for a big discount cause it's used.

I have to dig the prop out of the boat in storage this weekend. I was going to pull the tarp anyways. I will PM you with details. I actually only get as far south as Chelmsford regularly but I will be in Quincy Dorchester area the weekend after.

It is a $100 aluminum prop. I have some M gearcase props from a 175 kicking around, I think. 19" pitch. Let me look. I think that might be too big a gearcase.

Pretty sure prop gods has an exchange program also

Chance yer numbers sound pretty good. The SCD prop is basically PT Props variant of the yamaha semi-cleaver. They come in 3 and 4 blade. pitch wise it sounds like you're where you wanna be. Semi cleavers like Stainless SCD are stern lifters. They will be a little thinner blade the your aluminum so it might have less drag therefore the 18p might get the last bit of throttle in play. They also have a 4 blade SCD that will give you more lift and control.

Another to consdider is the MQF series from PT Props. It a black max clone in stainless, has more blade area and is still the best stern-lifter. Also comes in 4 blade. SCD might give you more top end, the MQF maybe a little more economy at cruise. Both are easy to turn and very good for the SC. I often see SCDs on ebay for cheap.

Let us know about your results.

Finally got a chance to test out a new (to me) stainless steel prop. This one is a yamaha K18 3 blade stainless. Numbers below are trimmed all the way down

3400 RPM. 20 kts
4000. 24.2
4500. 27.8
5000. 30.5
5500. WOT 34

Trimmed up

3400. 20.3
4000. 25.1
4500. 28.5
5000. 31.5
WOT. Never wrote it down.

Overall I'm pretty happy with the numbers. At cruise looks like an increase of almost 2 knots which is fine by me.

So, same diameter as your original prop (13.25), but up 1 degree in pitch to 18 and SS instead of Aluminum?

Interesting.

I'd like to lower my cruise RPMS as well.
Not really concerned with top end. Maybe Ill swap the 13.25 17p aluminum I'm running on my 18sf (Merc 2 stroke 115) for an 18P SS as well.

It's a 13 diameter I believe. I usually cruise around 4k so I'm pretty happy with 24 knots or roughly 27 mph.