Propellor rpm tied up vs under way.

[GILow]

Originally Posted by Jammer

I think you're close. Assuming you have a suitably large 3 or 4 blade prop, I would actually want it running 100-200 rpm slower than it is at the dock, so that the 2700 rpm target is ordinarily reached only downwind or surfing a wave.


With a 2 blade prop or a smaller prop of course you'll see less difference between bollard pull rpm and rpm under way.

Yep, three blade feathering. Not the most efficient of props, but plenty of grip when you need it.

[rslifkin]

Originally Posted by GILow

Weird, I cannot understand how the revs could be the same tied up and under way, unless there’s a governor involved limiting the revs under way.

Either a prop that slips and cavities readily enough that it doesn't build much load when tied down. Or with a diesel it's under propped and hitting the governor in the injection pump and it's never being limited by load.

[skipperpete]

Originally Posted by GILow

I don’t know what the certifier will want, but leaving the dock isn’t an option. Unless I want to run straight ahead into the boats on the next finger.

It’s a good ten minute motor to open water and a derestricted area from here if i did have steering so I assume the certifier is used to doing the numbers based on being tied up. Otherwise he couldn’t do his job on windy days for a start.

I was hoping someone has some numbers based on their own testing. It feels right to me, and the guy who set the initial pitch on the prop knows his stuff. But it would be good to know more now. If I had credible reasons for thinking I am over pitched I’d bite the bullet and don the wetsuit, but it’s b—-dy cold now and getting colder by the day.

Maybe I should have invited one of those Dark Mofo guys around.

I think the rpm difference is satisfactory, even if not, the certifier will be aware that you’re willing and able to re adjust the pitch after returning to the sea again. I suspect that a certifier will be more interested in things that are capable of harming the engine if poorly executed or installed and the top of this list would have to be the raw water intake and wet exhaust with siphon break followed closely by engine mounts and alignment and finally, fuel system supply up to the two connections on the rear of the engine. The problem with going to full fuel against the docklines is vibration , sometimes this can be substantial while doing the bollard pull but disappears when the vessel is free and able to “run away” from the prop.
A good certifier/mechanic will also check the control cable adjustments, particularly the arc of travel on the gear selector arm but also the throttle lever for full travel and high idle rpm.

[skipperpete]

Originally Posted by rslifkin

Either a prop that slips and cavities readily enough that it doesn't build much load when tied down. Or with a diesel it's under propped and hitting the governor in the injection pump and it's never being limited by load.

I agree, that post was a bit confusing. The cure as stated was to reduce pitch to achieve 2800 rpm which sounds counter intuitive, maybe it was a typo and should have read “adjust” pitch.

[skipperpete]

Here’s a page from the Yanmar seatrial suggestions

[Panope]

Tied to the dock, PANOPE's engine turned 3300 rpm. Running free, it would be just under the 3800 rpm max rating.

[GILow]

Originally Posted by Panope

Tied to the dock, PANOPE's engine turned 3300 rpm. Running free, it would be just under the 3800 rpm max rating.

I was actually trying to find where you’d posted this info before.

[GILow]

Originally Posted by skipperpete

Here’s a page from the Yanmar seatrial suggestions

Yeah, that’s all blindingly obvious. No help here at all of course.

[Dockhead]

Originally Posted by GILow

Weird, I cannot understand how the revs could be the same tied up and under way, unless there’s a governor involved limiting the revs under way.

Maybe the prop?

I have a variable pitch prop and I get redline both underway and at the dock, both cutting through calm water and bashing up against a gale.

Assuming you have a fixed pitch prop, it sounds like you've approximately nailed it, but you won't know for sure until you can take her to sea.


P.S. -- the pro way to do this is with exhaust gas temperature.

[s/v Jedi]

When you get same rpm either way, you are probably at max rpm and the governor limits it. How many rpm in neutral gear? How about while surfing down a wave?

[CSY Man]

Years ago I heard 90% of max RPM should be the target when tied to a dock.
Looks like you are in the ballpark.

[GILow]

Originally Posted by CSY Man

Years ago I heard 90% of max RPM should be the target when tied to a dock.
Looks like you are in the ballpark.

Yes, that tallies with Steve’s numbers too.

[Cooper37]

I just re-engined my Cooper 37 with a Beta 50. Sea trial was last week and went very well. My boats hulls speed is 6 kts. Engine replaced a Perkins 50 hp. I went to a courser larger diameter prop. Here's what I found. Under way, 1500 rpm 5 kts , 2600 rpm 8 kts.
I think it depends on what your objective is. I wanted a prop that gave me slightly better cruise/fuel performance, because where I sail, the wind seldom blows from the direction you want to travel, so you end up powering to destination. You'll get slightly higher RPM when underway (200) with the hull up to speed. With my courser pitch I do get a bit of prop cavitation when the power is initially increased until the hull comes up to speed. Once the hull is moving you can feel the acceleration more with than with the fine pitch. Like an aircraft, finer pitch is better for takeoff, course pitch for cruise. To bad the pitch isn't adjustable when underway.

[Rothblum]

Originally Posted by GordMay

A static pull [bollard test, boat tied to the dock], will lug the engine down, and it will never make rated rpms.

Have you had this actual experience? From a theoretical standpoint, I think the opposite would happen. There will be less loading of the prop and shaft if the boat is not permitted to move forward. The torque is proportional to the thrust, roughly, and at zero boat speed and max rpm, the flow over the prop is likely to be fully separated or cavitating. The lift on each prop blade will be close to zero, or much, much less than the lift generated by fully attached flow, so the torque will also be greatly reduced. I have learned that what I think should happen is often not borne out, so if you have had practical experience, I can't argue with that.

[GILow]

Originally Posted by Rothblum

Have you had this actual experience? From a theoretical standpoint, I think the opposite would happen. There will be less loading of the prop and shaft if the boat is not permitted to move forward. The torque is proportional to the thrust, roughly, and at zero boat speed and max rpm, the flow over the prop is likely to be fully separated or cavitating. The lift on each prop blade will be close to zero, or much, much less than the lift generated by fully attached flow, so the torque will also be greatly reduced. I have learned that what I think should happen is often not borne out, so if you have had practical experience, I can't argue with that.

Cavitation changes everything, but Steve on SVPanope has done the actual testing and documented his findings in a few YouTube videos.

If the prop doesn’t cavitation then the bollard pull revs will be lower. How much lower was the interesting question.

10% seems to be the expected reduction.