Engine RPM's low, wrong pitch?

[mrybas]

I'm buying a catamaran that the owner installed new Nani 3.21 diesels in 2015. The boat has adjustable pitch max props set at 22 degrees. When motoring at full throttle, the tach reads 2200 RPM's. The Nani manual says max rpm is 3800.
I'm going to have the boat hauled to change the pitch to 16 or 18 degrees. Before I do this besides making sure the fuel and air supply is good, could there be any other cause for the low RPM?
How much damage/ wear and tear does it cause running the engines while miss proped?

[Suijin]

22 degrees would be very aggressive for a monohull...can't comment on a cat. My Maxprop is pitched at 16 and I get 3000, the rated max RPM for the motor.

I'm no expert but I believe that seriously over propping can lead to engine overload and damage over the long term, if you're running it at full throttle. It will burn dirtier (richer). If you're not seeing excessive black smoke then you are likely not overloading your engine, but that does not mean it's not overpropped.

[J Clark H356]

I have a Hunter 356 with Yanmar 3GM30F and a MaxProp. I had 22 degree pitch until this last summer when I had the prop refurbished. I changed it to 20 and am going to change back to 22. I was able to get 3650 max RPM at 20 while at 22 was getting 3400. My speed went down from 6.2 knots at 2750 RPM to 5.8 at 2750. Went down in speed and max RPM going lower on pitch.

MaxProp told me 2 degrees was about 300 RPM. Was pretty much spot on.

[Wotname]

Originally Posted by mrybas

...... Before I do this besides making sure the fuel and air supply is good, could there be any other cause for the low RPM?
......

Dirty bottom and / or fouled prop.

[jimbunyard]

What is maximum RPM in neutral?

[Neptune's Gear]

Check the revs with an accurate tachometer first. Factory tachos are often wrong, especially if they are alternator driven ones, and the alt is non standard.

[Just Another Sa]

Originally Posted by mrybas

I'm buying a catamaran that the owner installed new Nani 3.21 diesels in 2015. The boat has adjustable pitch max props set at 22 degrees. When motoring at full throttle, the tach reads 2200 RPM's. The Nani manual says max rpm is 3800.
I'm going to have the boat hauled to change the pitch to 16 or 18 degrees. Before I do this besides making sure the fuel and air supply is good, could there be any other cause for the low RPM?
How much damage/ wear and tear does it cause running the engines while miss proped?

Before hauling the boat you should check all the things already said by others, and then check the gear ratio and boat speed at max throttle. Then calculate the slip or ask someone else calculate it for you. If the slip is low, reducing pitch is the wrong thing to do even if over propped. Changing gear ratio or reducing diameter would be correct ones, but cost a lot more than reducing pitch. You should know this and make the decision before hauling out, not during. And one more possibility although not a likely one, if the boat is very old, there could be something wrong with bearings of prop shaft consuming power.

[mrybas]

Originally Posted by Wotname

Dirty bottom and / or fouled prop.

Clean bottom and props.

[mrybas]

Originally Posted by jimbunyard

What is maximum RPM in neutral?

Port engine RPM in neutral is 3900 RPM

Starboard engine in neutral is 3500 RPM, but the throttle control hits the bulkhead, likely preventing it from hitting 3900 RPM. I'll have to see if I can make an adjustment or notch the throttle control.

[a64pilot]

Many, many people overprop because it makes for great motorsailing and lowers engine RPM and noise. If the sails are providing a lot of thrust, then the engine really isn't overpropped, however during engine only of course it is, and you have to exercise good judgement or you can damage the engine.
My boats engine max RPM is 3600, when I bought her it was propped to achieve only 2800. Was great for motorsailing and I liked that, but I didn't like the possible engine issues to go along with it, so I put on an Autoprop which will self pitch to some extent and add pitch when I'm motorsailing and decrease pitch when I am not.

I do not own a Cat, but would assume you would not want to be overpitched as I think it's pretty common for you guys to motor on one engine? If your overpitched with two running, I can only imagine it gets worse with only one?

[mrybas]

Originally Posted by Just Another Sa

Before hauling the boat you should check all the things already said by others, and then check the gear ratio and boat speed at max throttle. Then calculate the slip or ask someone else calculate it for you. If the slip is low, reducing pitch is the wrong thing to do even if over propped. Changing gear ratio or reducing diameter would be correct ones, but cost a lot more than reducing pitch. You should know this and make the decision before hauling out, not during. And one more possibility although not a likely one, if the boat is very old, there could be something wrong with bearings of prop shaft consuming power.

Gear ratio is 2.00
Boat speed was about 7 knots. I'll try and go for a ride today as there is little wind and get a more accurate boat speed. Are you looking for boat speed with both engines in forward at max RPM?

How do you calculate slip?

[DougR]

Calculate slip as follows:
- Divide engine RPM by transmission reduction ratio = propshaft RPM
- Multiply shaft RPM by inches of prop pitch= theoretical inches of forward advance per minute
- Divide forward advance per minute by the constant 1215= Theoretical speed of advance in knots
- Divide actual speed in knots by theoretical speed in knots and subtract this from 1 = amount of slip

Unfortunately this doesn't work well on flat bladed props like a Max prop because the pitch ANGLE is constant across the entire blade. This means that the prop pitch will be correct at only one point as you move out from the hub toward the tip. Everywhere else it will have either too much or too little pitch angle.

A properly designed propeller will have different pitch ANGLE as one progresses from the prop hub to the blade tip, and this produces even INCHES of pitch across the entire blade surface.

This lack of correct pitch is why flat bladed props tend to be noisy and less efficient than props with properly designed blades.

DougR

[mrybas]

Originally Posted by DougR

Calculate slip as follows:
- Divide engine RPM by transmission reduction ratio = propshaft RPM
- Multiply shaft RPM by inches of prop pitch= theoretical inches of forward advance per minute
- Divide forward advance per minute by the constant 1215= Theoretical speed of advance in knots
- Divide actual speed in knots by theoretical speed in knots and subtract this from 1 = amount of slip

Unfortunately this doesn't work well on flat bladed props like a Max prop because the pitch ANGLE is constant across the entire blade. This means that the prop pitch will be correct at only one point as you move out from the hub toward the tip. Everywhere else it will have either too much or too little pitch angle.

A properly designed propeller will have different pitch ANGLE as one progresses from the prop hub to the blade tip, and this produces even INCHES of pitch across the entire blade surface.

This lack of correct pitch is why flat bladed props tend to be noisy and less efficient than props with properly designed blades.

DougR

Calculated slip is .37
What does that mean?

[DougR]

It means that the vessel is moving forward 63% of the distance that the prop would move it if the prop was screwing itself thru a solid medium. Typically would be described as 37% slip.

This isn't necessarily good or bad. Props will always run with some degree of slip......they cannot run without it as water is not a solid medium.

Typically a well designed power boat with a well designed propeller running 25 or 30 knots will carry about 15 to 17 % slip. A displacement hull sailboat can easily carry 40% slip, or even more. If your boat was tied to the dock and the engine was running with the transmission in gear, your slip would be 100%.

DougR

[a64pilot]

Originally Posted by mrybas

Calculated slip is .37
What does that mean?

For what you are trying to achieve, nothing. Its interesting to know though.
You need in my opinion to adjust blade pitch until your close to rated max RPM with the boat loaded they way it will be used. My preference is to be a little overpropped a couple of hundred RPM, but that is just my preference.
Thankfully you have an adjustable prop, and I thought a maxprop could be adjusted without pulling the boat?