Prop size for better slow speed performance

[bobk]

Brian's posts on over-propping for better economy at displacement speeds continues to intrigue me, so I have decided to try it before I take the Chateau de Mer to Florida in October. However, I'd like to over prop it by increasing pitch rather than diameter because it is easier and cheaper.

Here are the details: Boat is a 1981 48MY series I, stabilized. Dry weight 45000 pounds, cruising weight not measured but likely 55000. Engines are 6V92 TA rated at 425 HP (rebuilt). Installed props are 26X22 four blade with a slight cup. When I bought the boat, she turned up 17 knots at proper rpm under load with 6 men, one gal and three quarters of fuel and water, but all owners stuff was off the boat. After we loaded her for extended cruising I struggled to get much past 12 knots, although rpm still comes up. BUT, that's not an issue as I'm happy at 8-8.5 knots (being an ex-trawler guy).

Hatteras literature says it should have 26X23 inch wheels so I expect I can at least go up one inch. The question is how much to increase the pitch when I'll only run 1000-1250 rpm? And will this get me the kind of increased mileage as Brian reported, or do I really need to increase diameter as well.

One additional point. Walter P has the same boat with larger 6V92's (500+ hp?) and the 26X23 inch wheels, and he makes at least 0.5 knot faster at 1000 rpm and easily turns up 17 knots..

And yes I know that I will destroy the engines if I try to push them hard when they are over-propped. I'm not going to install the controls on rpm as Brian did, just exercise self control.

Thoughts please??

Bob

 

[REBrueckner]

good plan...as long as you don't need hi speed...

If your boat turned 17 knots at proper WOT when new and now makes only 12 knots at the same WOT RPM, I'm immediately suspicious. Very suspicious!!! If you really think it's all the stuff onboard causing that, I'd remove everything I could...that's a 29% loss in efficiency!

You should also consider computer prop scanning if you'll be doing a lot of running as you can pick up a few percent efficiency that way...and lower prop speeds means you can in general use less clearance....maybe larger diameter props. Increased diameter gives you the best efficiency improvement, if you have the underwater clearance.

Dave Geer (Prop Handbook) says three bladed props are usually more efficient than four, so check that out with the manufacturer of your choice.

Somebody with some formulas can likely give you prop size estimates, and I'd use that to confirm prop manufacture recommendations... a CRUDE start is that 1" of diam or pitch usually changes RPM about 100....so if your new WOT RPM was say 1600 RPM vs 2500, a 900 RPM change, you'd want roughly say 1" more diam (if you have room) and 8" more pitch...

As Brian noted in his post on this subject, he calculated the available HP at the cruise RPM he selected....you want some spare shaft HP there, maybe 20% to 30%, and also the ability to increase engine RPM....

 

[Jackman]

You'd be really suprised at how much just going through your boat and removing uneeded weight will save you. I only have a 32' FB and I went through and got rid of everything I did not need. I cut 1500lbs off of the boat by doing this...verified by a calibrated scale. It was noticable once everything was removed. I didn't write down any numbers but I can tell you there was a definite difference in speed and how the engines responded.

 

[bobk]

The potential increase in speed if I unload the boat doesn't concern me... it will only be run at 8-9 knots. I like the efficiency and the tranquility of slow speed and I run lots of miles each year. What I'm trying to do is load the engines a bit more AND gain a bit of efficiency.

Bob

 

[REBrueckner]

Bobk...my prior post was perhaps not clear ....you can reduce fuel consumption by around 29% at WOT!!!! something is screwed up!

So maybe that's only 10% or 15% at 8 or 9 knots...in any case, something is wrong if your top speed in a 48MY slipped from 17 to 12 knots...something that will cause a significant fuel burn increase at lower cruise speeds as well.

good luck,,,,

 

[Brian Degulis]

Adding diameter is better cause you get more efficiency with a larger prop at slower speeds. But most of the gain comes from loading the engine more at lower RPMS. So adding pitch is a good and alot less expensive than buying props. Typicaly 2" can be added which will make a noticable diffrence.

Brian

 

[bobk]

Thanks everyone. I'll be taking them to Milers Island propellers this week and also get their input. Will post the results when I gewt them installed.

Bob

 

[bobk]

WOW! What a difference some props make.

Rob at Millers Island Propellers suggested tuning the spare props to 26"X23.5" and so I did since it seemed to be a conservative compromise to the Hatteras original props (26X23), what was on them and Brian's recommendation. The props on the boat were 26X22 four blade with a cup (small cup??). Millers Island said a cup has the effect of wasting fuel and power if propeller RPM is less than 1600 and the 6V92's with 1.5:1 Allisons don't quite get there.

So with the new props, Chateau de Mer comes right up on plane as the rpm is pushed up. No bogging down as she had been doing. It was marvelous to see the boat respond to the throttle. Top speed at WOT was 18.5 kts, 2300 rpm. And there was no smoking as I had previously, and the vibration at 1700-2000 rpm was gone. And the engines were at 182F (helm gauge) in 78 degree water.

At lower rpm, she is idling at 5.5 kts, speed at 1000 rpm is 8.5 kts (up about 0.6-0.7 kts) and she makes 9 kts at 1150-1200 rpm. So now I can't wait to see what the fuel burn is. I'll post that along the way to Marathon.

PS for those who will ask, the props were printed. Almost identical!

Bob
Chateau de Mer
1981 48MY
woopee!!

 

[Walter P]

Bob,

Wonderful result. Makes me wonder what was wrong about the other props. You said the boat comes out of the hole better now and still pins at 2350. Maybe I'll give it a try as well... perhaps raise the pitch to 24"

Walt

 

[chris]

Bob
That's a nice improvement. . Is your boat stabilized, I don't remember.

Chris

 

[bobk]

Bob
That's a nice improvement. . Is your boat stabilized, I don't remember.

Chris

Yes, it is stabiliized.

Bob