Too Much Pitch?

[danhaun]

Hi All:

I just repowered a Pearson 323 with a brand new Universal M-25XPB (rated for 3K RPM) and Campbell Sailor prop 14X8 RH. Reduction is 2.13:1. We launched this morning to test.

1. Calibrated tach with photo-tach.
2. Ran WOT in neutral and got 3000 RPM.
3. In gear WOT while tied to dock 2500 RPM. No smoke.
4. Quick run in bay at 2000K- 5.8 knots

But thunderstorms ran us back inside.

Question: Do I know enough about being over propped or do I need to test more? The owner's manual says to test while moving through the water. Mechanic says "tied to the dock". I can't get back for testing for 10 days but I could line up a diver and get the prop repitched in the interim if this is a slam dunk diagnosis.

[s/v Jedi]

The mechanic is crazy. Tell him Jedi said that! ;-)

You really need to open up the throttle in the bay after making sure prop and hull are clean.

I think the prop is correct with that 2500 rpm while tied off.

ciao!
Nick.

[David M]

Tied to the dock is irrelevant. If your engine is healthy and your hull is clean and it cannot reach its maximum governed speed while making way then you are over propped. Anything less and you are lugging your engine which will result in a shortened life and a carbon buildup in the combustion chamber.

[markpj23]

Originally Posted by David M

Tied to the dock is irrelevant. If your engine is healthy and your hull is clean and it cannot reach its maximum governed speed while making way then you are over propped. Anything less and you are lugging your engine which will result in a shortened life and a carbon buildup in the combustion chamber.

I whole-heartedly agree with David.... if you can't reach rated RPM underway then it's over-pitched. Good news is that re-pitch is fairly inexpensive.

[Rossco]

A sea trial is exactly that , at sea .The ultimate test is sea trial.The engine should attain maximum revs at wide open throttle as the specification sheet states for that engine. It is imperative that it is not overloaded .
Being a yachtee , you have time and freedom on your side ,so dont hurry this test .It must be done correctly , otherwise your engine will have a significantly reduced life .

[danhaun]

Thanks all for the advice.

I drove back to boat today (2 hrs one way) and had a good weather window. Bottom is clean, and prop is brand new. WOT gave us 2800 RPM so I suppose I need to lose one inch of pitch.


Is that the proper analysis?

Also, could I move the boat home- a 20 hr trip if I have to motor all of the way--before repitching? My old Volvo finally gave up on a trip in May and I have been driving back and forth to replace engine. I would like to get the repowered boat home if possible but I do not want to damage the new engine.

[Pblais]

your engine is healthy and your hull is clean and it cannot reach its maximum governed speed while making way then you are over propped. Anything less and you are lugging your engine which will result in a shortened life and a carbon buildup in the combustion chamber.

Been there. At low rpms the engine will lug bad and shake. When I re-pitched to a proper pitch overall high end speed on the water INCREASED! Low rpms were smooth. Any prop shop can do the math for a sail boat knowing the engine and transmission. This stuff is pretty much precise engineering. Screw propellers have been studied to death for 150 years. There isn't any personal judgement left.

You can normally reduce the pitch on a prop without throwing the prop away up to a degree assuming it has not been done before or you don't have a power boat prop on it now. On smooth flat water you should get max RPM's unless you have an engine problem.

Short term if it is close to the proper pitch (probably is) you can be fine to get home. Mine was off about that much and it went a long time but after 8 years it did trash the transmission. I found it when I bought the boat and got a new transmission in the deal but waited a 2 years to repitch the prop. I didn't belive it then either until it was redone. Any serious proplleler shop can do it for you. I think the price was $150. New prop was $750 for a new three blade bronze.

[s/v Jedi]

With a new engine: no, I would not do that because you are overloading it a bit and that's exactly not what you want during the break-in period.

cheers,
Nick.

[Boracay]

I'd suggest checking with your manufacturer and your mechanic as to whether you should motor for 20 hours on your current prop.

It does look like you'd like a few more RPM, and you'll be running in a new engine so it's a call that needs a bit more expertise than I have.

I put a new John Deere into Boracay about 90 hours ago. The prop was definitely over pitched (only 1850rpm tops) so I had it reduced as much as possible (about 4.5").

I'm still only getting 2300 (of 2500) rpm but the engine seems to be feeing up a little so it's possible that when the engine is fully run in I'll get close enough to 2500.

Maybe one day I'll get that feathering prop I dreamed of...

[Maine Sail]

Originally Posted by Pblais

Any prop shop can do the math for a sail boat knowing the engine and transmission. This stuff is pretty much precise engineering. Screw propellers have been studied to death for 150 years. There isn't any personal judgement left.

In my experience precise is anything but with prop calculations. After having worked on many power and sailboats when I worked in boat yards, and having a friend who runs a prop shop, and having purchased many of my own props, for my own boats, I can assure you that matching the prop to the MAX rated RPM on a boat is anything but precise. Many folks now ASSUME it is precise but only because they are clueless about proper prop sizing and having a prop that allows you to spin MAX RPM on a displacement hull. Close yes, but hitting the MAX RPM within 25-75 RPM or so is actually much more rare than you think.

A number of years ago, when I was helping a friend who is a surveyor, after his partner moved out of state, we found that nearly four out of ten sailboats were over propped. We noted this on the surveys and even checked with a photo-tach to make sure it was not a tachometer error. Most surveyors never even check this critical measurement but the good ones do. I think one of the reasons we see sooo many boats with failing exhaust elbows is from so many boats being over propped.

This past spring I purchased a Campbell Sailor for my current vessel. Even after discussing the closer to actual vessel displacement, engine specs and the current prop the prop calculator still failed Campbell and myself. I went from a 16X12 Michigan Wheel to a 16X10 Campbell and it was still over pitched and 300 RPM off MAX rated. What I wound up with was a 16X9 with less cup and I am now within 25 RPM of MAX rated. That 275 RPM made a HUGE difference as it always does.

The Campbell is a very, very efficient prop and more than once I have heard of them coming in too aggressive.

If you need to run your boat while waiting for Norm at West by North to send you a new prop just run her up to hull speed and stop the RPM at that point. If you are 300 RPM off MAX back your cruise speed off by 300 as well. Less smoke and less over loading of the engine..

[Northeaster]

I had read that removing 1" of pitch wshould result n about 200-300 rpms higher. Before having my prop modified, I would get heavy black smoke at 2200 rpm, reaching near hull speed. After having 1" of diameter and 1" of pitch taken off, I actually gained 1000 rpm. I now get 3200 rpm and about 0.5 kts more than before, with no smoke. I believe that is because the smaller/ less pitched prop allow the engine to rev higher, which in turn produces more HP at the higher rpm, therefore having more power. At 2200 rpm, I likely only had 8 hp at my disposal. At 3200, I have about 13hp.

I don't think being a bit overpropped will hurt to take it home. Like Maine Sail said, juts drop your cruising rpms back a bit. Maybe run it at full throttle for 10 minutes, every couple of hours.

You may only need 1/2" of pitch taken off.

[danhaun]

HI All:

I spoke with Norm about re-pitching and he recommended that we break in the engine with the current prop. He has a lot of confidence that this is the right prop for the boat and he says that we are within 90% of WOT RPM. He thinks that we will pick up the RPM as we run the engine in.

I was skeptical so I called the engine distributor and he told me the exact same thing. He is not worried about being slightly over-propped and that we should re-assess at 50 hours.

Anyway, I wanted to close the loop on this discussion and to thank you all for your expertise. This is great forum.

[Ex-Calif]

Your bottom and prop are spanking clean. As the hull and prop get a little fouled RPM will decrease due to drag.

I respectfully disagree with your engine and prop guy and would aim for a lower pitch and a little higher RPM.

Out boat gets 2800 clean and when the prop and bottom are fouled we struggle to get 2200.

I would however have no concerns about 20 hours of motoring at this time and doing the prop later.

[Blue Stocking]

What are the opinions on when a sailboat is overpowered and can attain the designed theoretical hull speed with much less than optimum (90% of max?) rpm, without the engine overheating, smoking or showing any other signs on distress.

[Pblais]

What are the opinions on when a sailboat is overpowered and can attain the designed theoretical hull speed with much less than optimum (90% of max?) rpm, without the engine overheating, smoking or showing any other signs on distress.

Ours is. We have a westerbeke 63b. It's a lot of engine for this boat. For the most part it does nice at 1800 to 2000 and it's quiet and efficient. If you pour on the throttle it is over powered, unless you are fighting heavy chop. At that point going to 2400 or a little more will yield more speed against rough water because you are no longer doing hull speed. Westerbekes don't mind running long at a slightly lower RPM. Some engines do.

If you are over powered and use too much throttle you are wasting fuel. I can see it in the exhaust if I do it. The blade gravitates once you are at hull speed.