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Old 20-06-2024, 20:23   #1
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Propellor rpm tied up vs under way.

Hi all,

I am commissioning the new engine in the pen at the moment. The boat has no steering so sea trials are some time away yet.

What I am trying to work out is whether I have the prop pitched correctly. (It’s adjustable).

I’m getting 2400 rpm tied up to the marina, the engine should achieve 2700 WOT. The boat would probably do about 7.5 to 8 knots at that power.

What do we think, close? Or over pitched ? I’d like to get it roughly right now if I can, the water is getting colder every day so putting off diving on it has some downside.

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Old 20-06-2024, 20:27   #2
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Re: Propellor rpm tied up vs under way.

I'm not an expert but it sounds like it is in the ball park. The engine will be getting loaded up by the prop when the hub speed is zero. The question is - by how much?

Can you advise the hp being developed by the engine at 2400 & 2700 rpm.
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Old 20-06-2024, 22:09   #3
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Re: Propellor rpm tied up vs under way.

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I'm not an expert but it sounds like it is in the ball park. The engine will be getting loaded up by the prop when the hub speed is zero. The question is - by how much?

Can you advise the hp being developed by the engine at 2400 & 2700 rpm.
Curve for the Beta50. Same engine, different paint colour.
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Old 20-06-2024, 23:03   #4
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Re: Propellor rpm tied up vs under way.

If that chart is correct it appears the difference between 2,400rpm and 2,700rpm is ~3Hp. if that.
Will your boat know the difference in 3Hp? Will you?
3Hp is hardly a small lawnmower.

Once you're up close to hull speed 3Hp is chasing a rainbow.
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Old 21-06-2024, 02:54   #5
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Re: Propellor rpm tied up vs under way.

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If that chart is correct it appears the difference between 2,400rpm and 2,700rpm is ~3Hp. if that.
Will your boat know the difference in 3Hp? Will you?
3Hp is hardly a small lawnmower.

Once you're up close to hull speed 3Hp is chasing a rainbow.
Totally true.

But I have to have the installation certified for the warranty and I’m not sure what the certifier would want in terms of rpm.
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Old 21-06-2024, 03:31   #6
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Re: Propellor rpm tied up vs under way.

Does the certifier actually want to test the engine while tied to the dock?

I would think you would reach 2700 under way in calm conditions.
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Old 21-06-2024, 04:46   #7
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Re: Propellor rpm tied up vs under way.

A static pull [bollard test, boat tied to the dock], will lug the engine down, and it will never make rated rpms.
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Old 21-06-2024, 04:56   #8
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Re: Propellor rpm tied up vs under way.

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Does the certifier actually want to test the engine while tied to the dock?

I would think you would reach 2700 under way in calm conditions.
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.
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Old 21-06-2024, 04:57   #9
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Re: Propellor rpm tied up vs under way.

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A static pull [bollard test, boat tied to the dock], will lug the engine down, and it will never make rated rpms.
Thanks mate. I know that, hence the question.
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Old 21-06-2024, 05:15   #10
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Re: Propellor rpm tied up vs under way.

I can tell you that my engine, tied to the dock, will not achieve full rpm, like yours, it will fall about 300 rpm short.
Once underway, it reaches full rpm no problem.
I believe that is more less typical for most diesel engines on sailboats.

Just curious about something though. You show the engine spec, but what is the reduction gear ratio ? Same as old engine ? Different ?

One last question. Did your old engine do the same thing ? ie, lug down at dock, but run up to rated rpm once underway ?
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Old 21-06-2024, 05:22   #11
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Re: Propellor rpm tied up vs under way.

300 RPM short while tied to the dock doesn't seem concerning to me at all. Load on the prop will be higher with 0 forward boat speed, so the power required to reach full RPM would be higher, hence why you can't. I'd think you're at least close prop-wise, possibly even a little under-propped.

The only boats that would be propped to reach full RPM in a static pull would be boats that regularly pull heavy stuff around, like a tug with a fixed pitch prop. Then they'd want the extra pulling power and abuse-proofing in exchange for a potentially lower top speed.

I've never tried it with the big boat (no way am I running even 1 of the 340hp engines above idle against the lines), but I did test it at one point with the dinghy. It goes from reaching 5800 - 6200 RPM at WOT (depending on load) to only reaching a little under 4000 RPM with it tied down and not moving.
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Old 21-06-2024, 10:12   #12
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Re: Propellor rpm tied up vs under way.

I have a new Beta 50 and a Jprop. I had to decrease pitch to #8 setting to get the right top RPM. I found that tied up and out in open water I got the same top RPM. I now get 2800. Just a few notches made a big difference on the Jprop.

If you have any other questions about the Beta 50 install you can PM me. I have done a few.
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Old 21-06-2024, 10:31   #13
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Re: Propellor rpm tied up vs under way.

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.
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Old 21-06-2024, 17:00   #14
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Re: Propellor rpm tied up vs under way.

Quote:
Originally Posted by MicHughV View Post
I can tell you that my engine, tied to the dock, will not achieve full rpm, like yours, it will fall about 300 rpm short.
Once underway, it reaches full rpm no problem.
I believe that is more less typical for most diesel engines on sailboats.

Just curious about something though. You show the engine spec, but what is the reduction gear ratio ? Same as old engine ? Different ?

One last question. Did your old engine do the same thing ? ie, lug down at dock, but run up to rated rpm once underway ?
Good to know, thank you.

Yes, the other engine has the same 2:1 gearbox ratio, however it’s no good for comparison as I’ve deliberately over pitched it and it only gets 2400 at WOT and about 2100 tied up. (Similar difference though.). I came up with this over pitch setting after a couple of years of use, it produced the best power and economy at my preferred cruising revs of 1950 rpm. There’s a sweet spot there on the Kubota V2203, and it turns out many of their generator packs are tuned to that speed.
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Old 21-06-2024, 17:01   #15
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Re: Propellor rpm tied up vs under way.

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Originally Posted by NorthCoastJoe View Post
I have a new Beta 50 and a Jprop. I had to decrease pitch to #8 setting to get the right top RPM. I found that tied up and out in open water I got the same top RPM. I now get 2800. Just a few notches made a big difference on the Jprop.

If you have any other questions about the Beta 50 install you can PM me. I have done a few.
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.
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