I/O Performance Advise

[Ripcurl]

I have a Silverhawk 24 i/o and have a prop slip problem I'm trying to solve...~30 - 35% depending on load. I know it's not a perfect comparison, but there are more Seacrafts out there than Silverhawks so giving it a try here.

Details below... my question is, can anyone with an i/o Seacraft 23 give me a sense of where you're WoT slip % is? Any comparison is further complicated by fact that I have a Yanmar 4lha 240hp that turns up 3550 max RPM. Only Seacraft I've seen with a diesel is that redone Potter hull going for ~100k+.

Many thanks,

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Engine: Yanmar 4lh
HP: 240
WoT RPM Range: 3450 - 3550 (Heavy / Light load)
Sea trial WoT RPM: 3550 - 3650 (Heavy / Light load)
MPH WoT: 34 / 35
Drive: Bravo I: 1.65
Prop: Stock four blade Bravo I 26P

[martin]

Perhaps to much pitch.. or maybe the prop is to small in diameter
Sooru wish I could be mor help

[Blackfin26]

Guessing a 22 or 24 pitch prop would be what you need... I'd go with a 22.

[Ripcurl]

Thanks for the replies...

Regarding diameter, I can only go up to 15 3/4... maybe 16 with the Bravo One sterndrive.

On pitch / other props I've tried, here's the history:
- Boat came with a 24P Bravo that had been altered somehow... looked like diameter was reduced and some cup was added... performance was slightly better but RPMs were hitting redline at 3750; yanmar spec is 3550 max propped rpm.
- I strapped a 24P stock on and lost ~5mph... RPMs still hitting redline
- Moved to 26P stock... slight increase in top-end, RPM's at 3650 / 3550 with light / heavy load, respectively so still need to bring down ~100rpm. Further, boat is a pig getting over hump

[bitsamonkey]

I'd have that prop measured to be sure it's a 26" prop. Speed seems about right for that combo.

[Ripcurl]

It's a stock Bravo, so probably coming in around 25". New, out of box this week so not a condition issue. I'm ok with top-end of 35mph, but the slip just seems too high based on what I read here / other boating forums. Beginning to think it's just the nature of my engine / drive / hull set up

[Blackfin26]

Ripcurl, My 22 pitch prop was assuming it was a bravo 3 duo prop.

[bitsamonkey]

There's still something wrong with that equation. No way do you have 30-35% prop slip.

[FishStretcher]

Can you confirm the final drive ratio?

[Ripcurl]

It's a Bravo One 1.65:1 outdrive with a Bravo One 26 / 15.25 wheel. Bravo One wheels are typically 1" lower pitch than stamped, so I ran 25P and 26P through the calculator. Results as follows:

25P
WoT RPM=3650
Theoretical Speed = 52 MPH
Actual Speed = 35 MPH
Slip = 1 - 35/52 = 33%


26P
WoT RPM=3650
Theoretical Speed = 54 MPH
Actual Speed = 35 MPH
Slip = 1 - 35/54 = 35%