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Old 21-01-2020, 01:32   #1
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Changing Pitch of My max prop

on a Beneteau first 47.7, with a volvo penta tmd 78 hp engine, and a 3 blade maxprop feathering...Im thinking of increasing the pitch by 2 degrees to increase speed and lower revs for long journeys. Can this damage my engine or heave negative results?
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Old 21-01-2020, 01:51   #2
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Re: Changing Pitch of My max prop

Yes, indeed when increasing the pitch by 2 degrees will lower the RPM. It may not advisable as one can overload the engine and the actual top/hull speed can not be reached anymore.
Before doing so, check the max revs of the engine and at WOT (wide open throttle), see what speed you get. If you can't get max revs, then you may need to reduce the pitch; if you reach max revs easily and even before WOT, than you may have a reason to increase the pitch.

How do I know this? I have a boat with a similar HP engine and a Max prop, and I thought I was smart doing exactly what you asked here..... Result was..... yes, a very quiet engine, and motor-sailing was even better but engine could not reach its RPM and top speed was about 0.5 to 1 knot lower. A week later I hauled the boat out again, and reduced the pitch back to what it was before. And even now I am considering to go 2 degrees lower still, next time I haul out.
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Old 21-01-2020, 02:24   #3
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Re: Changing Pitch of My max prop

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Originally Posted by HankOnthewater View Post
Yes, indeed when increasing the pitch by 2 degrees will lower the RPM. It may not advisable as one can overload the engine and the actual top/hull speed can not be reached anymore.
Before doing so, check the max revs of the engine and at WOT (wide open throttle), see what speed you get. If you can't get max revs, then you may need to reduce the pitch; if you reach max revs easily and even before WOT, than you may have a reason to increase the pitch.

How do I know this? I have a boat with a similar HP engine and a Max prop, and I thought I was smart doing exactly what you asked here..... Result was..... yes, a very quiet engine, and motor-sailing was even better but engine could not reach its RPM and top speed was about 0.5 to 1 knot lower. A week later I hauled the boat out again, and reduced the pitch back to what it was before. And even now I am considering to go 2 degrees lower still, next time I haul out.
What about cruisers who rarely see/dont care about top speed?

I understand it would be foolish to utilise all of the available HP at lower revs but could some of the power available beyond current prop load be utised to increase prop rpms, reduce fuel burn and noise?

Or is it just a trade off: higher constant load = less engine life?
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Old 21-01-2020, 02:41   #4
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Re: Changing Pitch of My max prop

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Or is it just a trade off: higher constant load = less engine life?
Possibly. Pdemgo has a turbo diesel (TD) engine which isn't going to like being over propped and unable to reach max revs. Likely he will be running it at 1/2 or 3/4 throttle so over fueling the engine creating extra internal heat and not allowing the turbo to work properly.

On a yacht it may not be really noticeable, but speak to the TD cabin cruisers who run at planning speeds, then is it really important to match the props to achieve full revs or they loose a proportionally large amount of speed and burn extra fuel.

If this is for long trips at sea, have extra stores, water and fuel been loaded on board increasing the weight? This could make the situation even worse.


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Old 21-01-2020, 02:59   #5
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Re: Changing Pitch of My max prop

Hardly a cruiser but a real world test. More pitch = lugging = more fuel:

"Our Bluewater was run and turned up with eight different stainless-steel propellers. Not only did fuel consumption vary wildly, but also the thirstiest set of wheels allowed the engines to rev up only to the low end of the range specified by the manufacturer. The most fuel-saving wheels allowed the motors to rev to near the maximum of that range, however, meaning they’ll not only save dinero at the pump, but will also pay dividends in increased long-term durability and reliability."

https://www.boatingmag.com/expert-tips-saving-fuel/
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Old 21-01-2020, 04:34   #6
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Re: Changing Pitch of My max prop

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Originally Posted by Pdemajo View Post
on a Beneteau first 47.7, with a volvo penta tmd 78 hp engine, and a 3 blade maxprop feathering...Im thinking of increasing the pitch by 2 degrees to increase speed and lower revs for long journeys. Can this damage my engine or heave negative results?
You might get better performance when motor sailing
Remember..you are attempting to achieve the best all round performance

When adjusting pitch you must consult exhaust temp .

Ask to borrow a exhaust temp sensor ..most engineers have them

Over temp means you are over pitch
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Old 21-01-2020, 12:32   #7
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Re: Changing Pitch of My max prop

My 41' Cheoy Lee Pedrick had a Max prop that was over pitched, never allowed the vessel to hit wot, and when we got close saw lots of black smoke. When the enging needed to be rebuilt, my mechanic buddy told me carbon build up was excessive. The law of unintended consequences.
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Old 21-01-2020, 13:33   #8
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Re: Changing Pitch of My max prop

increasing the pitch will reduce your torque.. or conversely will make the engine strain more (this point has already been made)... now imagine trying to motor head into 3-5ft waves and see if you can start to sympathize with your engine.

if not, then go ahead and do it..................
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Old 21-01-2020, 15:20   #9
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Re: Changing Pitch of My max prop

I am no diesel expert, but I believe I have read that over-pitching the prop results in higher combustion chamber pressures. Which probably is not good for the life of the engine, and I think some manufacturers have warranty exclusions for over-pitched props.
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Old 21-01-2020, 17:02   #10
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Re: Changing Pitch of My max prop

We’ve got a MaxPro Prop that’s been tweaked’ as described on our Moody47 that’s new to us. We’ve got a Cummins 3.9 4BT that produces 150bhp @2500RPM (ridiculous I know). The previous owner had adjusted the pitch based on the exact same theory. Now we’ve had her for a few months it’s clear that the engine’s not happy and won’t rev up properly. In drive it tops out at 1800RPM. As a result the turbo isn’t spooling up properly which will definitely cause damage. Although the boat does 7 knots at 1100RPM you can tell it’s really not happy. We’re going to crane out soon and one job is to reduce the pitch so the engine can rev up properly. I get the theory but our experience is it really does load the motor and doesn’t do the turbo any favours.
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Old 21-01-2020, 17:10   #11
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Re: Changing Pitch of My max prop

Depends on how you use the boat. If you are a weekend sailor, you will want all the power you can get to punch into a headwind on Sunday afternoon. If you are a world cruiser, you will want a bit more gas mileage when motoring through the doldrums at low speed.

The usual rule of thumb is to see if the engine will just achieve maximum rated rpm at wide open throttle. The next degree of sophistication is whether that was tied to the dock or running in smooth water with a clean bottom. You are not going to fall off a cliff by changing things 2 degrees.
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Old 21-01-2020, 22:23   #12
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Re: Changing Pitch of My max prop

I have a 3Gm30F and my MaxProp was 22 degrees when new in 2003. It needed a rebuild in 2016 and I thought I would decrease my pitch to 20 to run lower rpm and save fuel. What happened is I lost .5 knot, used the same amount of fuel because it took longer. Call MaxProp and get their recommendation for your boat. I just changed mine back this past summer ton22 degrees and am happy again. My .5 knot is back.
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Old 21-01-2020, 22:53   #13
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Re: Changing Pitch of My max prop

Being able to finetune the prop is one of the beauties of the maxprop.
As a rule of thumb a change of 2 degree gives you 10% change of revs. In most I Installations this should not harm the engine, if the installation had been calculated correctly in the first place.

A look at the diagram (hp vs rpm) for your engine will tell you more. In an ideal world your current setting should be that max speed is achieved at about the max portion of the linear portion of the graph In many of today’s boats engines are oversized and thus achieve max speed at 3/4 or less of max revs. Cutting their revs even further will have even more negative effect on engine life than the current system already has.
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Old 22-01-2020, 04:53   #14
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Re: Changing Pitch of My max prop

Contact PYI. Give them the relevant information, and they will tell you what pitch your prop should be set at.
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Old 22-01-2020, 14:33   #15
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Re: Changing Pitch of My max prop

Quote:
Originally Posted by Pdemajo View Post
on a Beneteau first 47.7, with a volvo penta tmd 78 hp engine, and a 3 blade maxprop feathering...Im thinking of increasing the pitch by 2 degrees to increase speed and lower revs for long journeys. Can this damage my engine or heave negative results?
My experience has told me there is only one way to properly pitch a prop. Trying to squeeze an extra bit out of it can just lead to more problems.

1. Find out the max RPM for your engine from the manufacture and the transmission's ratio.

2. With a clean hull do a test run on flat water with no current and note the RPM and speed at WOT...did you reach max RPM? Were there any vibration, cavitation, or other issues?

Note that it has been reported that one negative tradeoff with the MaxProp is some slight loss of speed. Additionally, your net resulting speed is also affected by the transmission ratio; is it low, can it be increased at a reasonable cost?

My boat came with a fixed Michigan 18" prop. I upgraded to a MaxProp for better sailing speed and to stop prop rotation while sailing...a better reverse was not a major reason but does exist. After talking with PYI I ordered and had install a conventional 18" three blade feathering prop. I ran a test afterwards and did not like the results (vibration and well short of max RPM) which were diagnosed as cavitation by PYI...I lived with it until the next haul to avoid the hauling fee. The issue required that the prop be cut down by PYI at a fee to 17" and a different pitch. Afterwards I ran a test and found it solved the cavitation issue but I was short at WOT by about 200 RPM from the max...PYI said to stop there.

The MaxProp adjustments for my prop come in two degree increments so there is not any more room for adjustments...one way leads to cavitation the other way is 200 RPM short of max RPM.

I don't believe in screwing around with prop pitch. Find what works best overall, set it and leave it.

Practical Sailor currently has an article on prop pitch called OFFSHORE LOG: A pitch for Adjusting Pitch worth viewing.

Good Luck.

~ ~ _/) ~ ~ MJH
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