Max Prop performance

[Sail-Rag-man]

Hi Folks, new to me Pearson 365 with a 17 in max prop. Need help with someone who has had experience with this equipment. My problem is not being able to reach estimated hull speed (6.5-7.2 knts) under power. With the current prop pitch we can hit 5 knt @ 1600rpm (not bad). But, as engine speed increases, the hull speed remains the same or decreases . Yes, the hull is clean and not dragging a drough. I have not been able to raise a response from the
MaxProp website. One old sailor claims I am having cavitation ? Can it be cured by INCREASING or DECREASING the pitch on the prop?

[ItDepends]

Need a LOT more information to help...

What engine do you have? Which Max-prop? 2 blade? Three? What is the current pitch setting? Is this a new prop? Is this a new problem? If you recently changed props, what was the size of the old one? What was the performance of the old one?

Are you saying that the boat does 5 knots at 1600RPM and then goes SLOWER at 2000? How much slower?? What is the maximum engine speed achievable? What is the boat speed at that maximum RPM? Are you sure the dash tachometer is accurate?

My first feeling is that significant cavitation is very unlikely with a prop of that size on that boat.

[noelex 77]

Welcome to the forum Sail-Rag-Man .

My first thought is that rather than a prop problem the gearbox is slipping, but there are many members of the forum that have greater mechanical knowledge. I am sure they will chime in.

[PerfectPirate]

I am facing a similar issue with my new Ontario 32 and a 16-inch, 3-blade MaxProp, pushed by a Yanmar 2QM20. Yanmar's propeller recommendation for this engine was 17 inches diameter with a 12-inch pitch, but there was insufficient clearance from the hull for a 17-inch prop. The original owner had set this prop for a pitch of 13 inches, in an apparent effort to compensate for the smaller diameter.

Most propeller sizing recommendations run along the lines of "keep increasing size or pitch until the engine can only just reach recommended cruising RPM", which for my engine is 2600 RPM (the QM series generally runs at lower RPM than more modern engines). My theoretical hull speed is 6.9 knots. When testing the engine's ability, I found that 1800 RPM could push me at 5 knots, or slightly more, in flat water. However, as I increased the RPM up to 2200 RPM or more, the propeller would simply begin to cavitate, losing its "bite" in the water and spinning uselessly. So, my engine is powerful enough to push past the theoretical thrust/drag limitation that underlies the usual approach to propeller and pitch sizing, causing the blades to lose their purchase and simply beat the water. At higher RPM's, the water cannot flow past the hull and over the blades fast enough to match the current pitch setting. I had similar experiences with the Atomic IV engine in my previous C&C 27. It too had way more power than the prop could effectively use.

Since I do not have sufficient clearance from the hull to go to a larger prop, when I relaunch in the spring I will reset the pitch of my current MaxProp down to 12 inches, as was originally recommended by the engine manufacturer. Hopefully this will allow the propeller to spin at higher RPM's without losing its bite and thereby allow me to reach a slightly higher cruising speed. If I can cruise at 6 knots in flat water I will be quite content. If there are any engineers out there who have more experience in the field of propeller sizing, I would love to hear their opinion of this approach to the problem.

[Jammer]

What RPM can you achieve at full power?


What is the gear ratio of the transmission?


Are there any odd noises or dramatic loss of propulsion as you increase RPMs?


Does the transmission run hot?


I am unsure what the problem may be but I doubt if changing the prop pitch will solve it. If you're getting cavitation you will need a different diameter prop.

[Jammer]

Originally Posted by PerfectPirate

Since I do not have sufficient clearance from the hull to go to a larger prop, when I relaunch in the spring I will reset the pitch of my current MaxProp down to 12 inches, as was originally recommended by the engine manufacturer. Hopefully this will allow the propeller to spin at higher RPM's without losing its bite and thereby allow me to reach a slightly higher cruising speed.

Your engine should be able to reach max rpm, not just cruising rpm. This provides extra propulsion that you may need in heavy weather.


Switching to a four blade prop would give you more blade area without increasing diameter.

[PerfectPirate]

Originally Posted by Jammer

What RPM can you achieve at full power?
I am unsure what the problem may be but I doubt if changing the prop pitch will solve it. If you're getting cavitation you will need a smaller prop.

Jammer, I do not see how a smaller prop would solve a cavitation problem, especially since my prop is already smaller than the engine manufacturer's recommendation. In my experience cavitation arises when the engine overpowers the prop, forcing it to spin faster than the water can flow over the blades, creating air bubbles and thereby losing thrust. This generally happens when the prop is too small for the weight of the boat which it is trying to push, or the prop is too close to the surface of the water. Since the propeller pitch effectively equates to the application of torque, reducing pitch should be a practical equivalent to reducing the torque of the engine, which in theory should reduce the ability of the engine to overpower the prop.

My engine is a Yanmar 2QM20, developing 20 Horsepower at 2600 RPM (full emergency power is 24 HP at 3000 RPM), driving the shaft through a Kanzai YP-7M transmission with a 2.2:1 reduction ratio. At full throttle, the engine immediately overpowers the propeller and spins up to max RPM.

[rslifkin]

If the prop doesn't have enough blade area to absorb the load and is cavitating at higher power settings, there are 2 primary solutions. Either larger diameter, or if there's no space for that, a prop design with more blade area.

[jamhass]

I would contact PYI and ask for their recommendation. I have found them spot-on when it comes to knowing various boat/engine/prop variations.

[Tsuru]

Originally Posted by jamhass

I would contact PYI and ask for their recommendation. I have found them spot-on when it comes to knowing various boat/engine/prop variations.

I agree, start with a call to PYI, I've had good responsive service.

[Cheechako]

Originally Posted by Sail-Rag-man

Hi Folks, new to me Pearson 365 with a 17 in max prop. Need help with someone who has had experience with this equipment. My problem is not being able to reach estimated hull speed (6.5-7.2 knts) under power. With the current prop pitch we can hit 5 knt @ 1600rpm (not bad). But, as engine speed increases, the hull speed remains the same or decreases . Yes, the hull is clean and not dragging a drough. I have not been able to raise a response from the
MaxProp website. One old sailor claims I am having cavitation ? Can it be cured by INCREASING or DECREASING the pitch on the prop?

What is the max rpm you can reach wide open?
What engine do you have?

(revised)
-That seems like a fairly big prop for that boat. Do you have adequate tip clearance to the hull? 15%, 20% is better. Too little clearance can cause cavitation.
-I think the 44 ft boat in my avatar had an 17-18" Max Prop and reached 8.25 kn in flat water.
-Calling PYI will get you a response.

-One wonders if your transmission is slipping...

[rslifkin]

Originally Posted by Cheechako

-Your boat has a max speed based on waterline of 5.48 kn. That's with everything clean and perfect in flat water.

I'm calculating 7.34 kts for hull speed on a Pearson 365, based on a listed 30 foot waterline.

[Cheechako]

Originally Posted by rslifkin

I'm calculating 7.34 kts for hull speed on a Pearson 365, based on a listed 30 foot waterline.

yeah sorry, slipped a digit there!

[Sail-Rag-man]

Thanks to all for input. Some F/u info that I can add: The boat came with the current MaxProp. Do not know the original prop and pitch. Gear box is a Paragon, believe 2:1 reduction gear. Have checked the operating temp on the gear box and Walter V-drive and both only slightly above ambient air temp. Engine is the standard Westerbeke 4 cyl (Perkins 108). Runs well at low rpm and max rpm is about 2400. No noise or slipping is heard. But, the hull speed drops to about 4.7- 4.8 knts as rpm exceeds 200 rpm. Do not know what pitch the prop is currently set at. Boat recently in winter storage, so I will try to find
the pitch. May try PYI for more info...thanks again..

[Sail-Rag-man]

Also to add it is the 3 blade MaxProp