I/O Performance Advise
 

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

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

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

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

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

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

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

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

Can you confirm the final drive ratio?

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%

I think the issue is your drive ratio. I'm running the same ratio (if memory serves) on a 350 mag mpi that hits a rev limiter at 5200 rpm.

Take your prop pitch relative to top speed and compare it to an outboard or Gas I/O on the same or a similar boat. I think you'll find boats topping out at 35mph running 15P to 17P max props.

Now ask yourself why. Engines are typically geared so they'll turn the most efficient prop at the right rpm. It's generally more efficient to turn a lower pitch prop at a high rpm than a high pitch prop at a low rpm. But you've got a diesel with a ton of torque delivered at low revs. To convert that torque to propulsive power, you've got to have a prop with a lot of bite turning fast, or a prop with a lot of slip turning slow.

I'd check with a trusted Mercruiser dealer and see what he thinks is the right drive ratio for that engine. I'll bet he'll recommend a ratio that will allow you to prop down to a 15p or 17p.
Dave

So the drive ratio on my bravo 3 is on a sticker- and it is bleeding away at that. it makes me wonder if yours might be a 1:2.0 labelled as a 1:1.65?

BTW, I have a 1:2.20 on a 350 MAG MPI bravo 3 at 22" pitch both props on the counter rotating pair. It would seem that 26" pitch is ambitious for your setup.

Bravo 1 drive ratio info here:

http://www.go-fast.com/sterndrive_gear_ratios.htm

It's a reduction gear ratio, so the higher the number, the lower the prop rpm compared to engine rpm. I think the diesels usually run the 1:1.36 ratio. I checked again and mine is a 1:1.50.

The Bravo 3 ratios are higher because the counter rotating props are more efficient even when turning a high pitch prop at a lower rpm.

Dave

Hey Blue Heron, I think you got it backwards. A higher pitched prop turning lower rpm is more efficient than the other way around. OP, you're propped for a theoretical max speed of 54mph which you know won't happen. I remember towing my swamped seacraft with my big block powered inboard and the motor was able to "slip" the prop with no increase in speed. Maybe you should try a prop pitched closer to a realistic top speed and see what happens.

Back on this... Thanks to all for the insight.

My mechanic has a B3 laying around that he's going to let me try out. I don't care so much about the top-end, the issue is as I load up or run in wind / chop, the wheel doesn't bite enough and the change in performance is material. Assuming the B3 bites alot more, this effect won't be as great and I'll get better economy and stress the engine less when running offshore with a load.

One issue I'm keenly aware of is that if I don't have enough slip w/ the B3 when I throttle up, the turbocharger won't spool... basically the opposite problem to the one I have now. This can be solved with less pitch, but then I'm increasing slip again.

Clearly a narrow potentially expensive corridor to navigate.

I suspect you will be ok. I don't know if those are waste gated turbos or not, it sounds like maybe not?

I did a little counter rotating prop specification last year, working with an OEM. I seem to recall that extreme pitch ratios are less efficient, generally speaking. I seem to recall P/D in the 1.2-1.6 as desirable. I think that a 26P hasn't gotten so far in P/D ratio as to be a concern.

Bravo 3 props are stainless only, I am told. Based on the local prop shop, he isn't a fan of stainless for modification, he has seen too many shed blades!

But it seems like a bravo 3 at the standard 22" pitch probably is a good starting point. But it might still be slow over the hump, depending on the turbocharger trim.

Edit: I strongly suspect you have the 230HP waste gated model. I still think 22" is a good place to start for pitch.