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Old 02-12-2010, 12:59   #1
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Prop Pitch Adjustment

I searched and read a number of prop threads here and couldn't figure out an answer to this question. I think I spent an hour and a half searching. So what I ended up doing was calling PYI. They passed me to Fred and he answered my question easily/quickly.

Here is the scenario -- I have a classic three blade Max prop. Last time I hauled I adjusted my prop by myself. I didn't find it difficult. The problem is that when I made the change after repowering I thought the settings would need to change and the guys at PYI gave me the new setting. I set the prop to their specs. My boat speed went down. My fuel consumption went up. I could only get 2600 RPMs and I am supposed to hit 3000. And the bottom paint just keeps on working so I haven't had to haul.


I told Fred my problem 2600 RPM's, Yanmar 4JH4E, didn't know the setting he said move it one setting down and you should get 2950 rpms. I then told him that my fuel consumption had gone up and my speed had gone down. He informed me that the over propped situation was making the engine work harder but that the extra work wasn't translating into boat movement. Getting the pitch right should improve speed and decrease the fuel consumption.


What he is saying makes sense, but I would like to get your opinions on this.
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Old 02-12-2010, 15:58   #2
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Charlie:

Firstly you are overpropped. Your engine should reach 3,000 rpm at WOT and you say it will only reach 2,600. That isn't then end of the world and a little overpropping won't hurt anything.

But an overpropped condition is generally more efficient than being propped right. There are less parasitic losses (internal friction) at lower rpm and props are more efficient at lower rpm.

So decreasing pitch will hurt fuel consumption as it will let the engine work at a higher rpm, but should let the boat go a bit faster.

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Old 02-12-2010, 16:24   #3
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I agree it sounds like you have the pitch set too high, ie overpropped. This is like trying to drive your car up a steep hill in 4 gear. To make the car go up the hill in high gear you have to give it a lot of gas.

Same with your boat, so you might actually get better miles per gallon at the same speed with a correctly adjusted prop. You will probably also get a higher top speed but the mpg may decrease.
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Old 02-12-2010, 16:46   #4
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If you're overpropped, you may find that running at full throttle for a while results in higher-than-normal engine temperatures.
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Old 02-12-2010, 16:59   #5
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Originally Posted by skipmac View Post
I agree it sounds like you have the pitch set too high, ie overpropped. This is like trying to drive your car up a steep hill in 4 gear. To make the car go up the hill in high gear you have to give it a lot of gas.

Same with your boat, so you might actually get better miles per gallon at the same speed with a correctly adjusted prop. You will probably also get a higher top speed but the mpg may decrease.
I am hoping this is the case. I used to be able to go faster with less fuel consumption and if I adjust the pitch correctly then hope to have that situation again.
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Old 02-12-2010, 17:08   #6
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[QUOTE=djmarchand;570648]Charlie:

Firstly you are overpropped. Your engine should reach 3,000 rpm at WOT and you say it will only reach 2,600. That isn't then end of the world and a little overpropping won't hurt anything.

But an overpropped condition is generally more efficient than being propped right. There are less parasitic losses (internal friction) at lower rpm and props are more efficient at lower rpm.

So decreasing pitch will hurt fuel consumption as it will let the engine work at a higher rpm, but should let the boat go a bit faster.

David[/QUOTE
If the boat is going faster fuel economy improves. You are covering a greater distance for the same fuel consumption
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Old 03-12-2010, 12:54   #7
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Hi cfarra

Would mind reading my thread in powered boats subject hot running engines and tell me what you think.

Thanks
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Old 03-12-2010, 13:05   #8
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[QUOTE=perchance;570692][QUOTE=djmarchand;570648] But an overpropped condition is generally more efficient than being propped right. There are less parasitic losses (internal friction) at lower rpm and props are more efficient at lower rpm.
d[/QUOTE

I'm not sure this is so. What if the engine is overfueld (and putting out black smoke) in order to keep the rpm's up? In this case, being overpropped will reduce, not improve fuel economy.

To the OP: are you getting black smoke?
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Old 03-12-2010, 19:12   #9
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And the bottom paint just keeps on working so I haven't had to haul.
You don't have to haul to change the pitch of your Max Prop. Call PYI and they'll happily recommend a diver local to you who can do it at your slip.
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Old 03-12-2010, 19:22   #10
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I keep hearing that it's possible to change the pitch of a Max Prop classic underwater, but I've never herd anyone who actually achieved this.

I've done it on land and that thing explodes into a mess of cotter pins, allen bolts and cogs.

Anyone really done this ?

Duncan
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Old 03-12-2010, 19:56   #11
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All this talk about more or less speed based on more or less pitch is confusing when you are at hull speed for a cruising sailboat. I believe the OP had to be at or just pushing past his hull speed at 3000rpm. If he over pitched his prop so he's achieving hull speed at lower rpm, and that is what he wanted, then everything that's happening is to be expected. If the OP is trying to go faster than his hull speed, I believe he's wasting fuel.
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Old 03-12-2010, 20:13   #12
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If your marine diesel is not making rated RPM it is being overworked, is running a hot exhaust gas temp and is inefficient. your cooling water">engine cooling water temp may not show it but a pyrometer would. PYI has it right.

By all means change your pitch. As to changing the pitch on a max prop classic in the water, good luck! seems too tricky to me.
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Old 03-12-2010, 21:01   #13
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In this regard I am concerned about my Pearson323. (purchased this past June) I've been reading about this issue but cannot come to a conclusion because of my lack of experience in this area.

It's a Yanmar 3GM fresh water, with fixed three blade of unknown-to-me pitch and diameter. It may have 1500 hours in this configuration, but I'm not sure if the prop was ever changed after the initial engine conversion.

I can get about 6.5 knots at only 2100-2200 rpm. That's close to hull speed but I just haven't tried to push it to 7+ yet. At first I thought that was a good thing. I assumed less engine wear and better fuel economy.

But I didn't know anything about diesels. Though I have rebuilt engines for Harley's, Chevys, VWs, Triumphs, and most recently a '57 Ford 800 tractor, I now realize I don't know everything about engines.

I've never seen any smoke, black or otherwise, of any significance. I realize being over propped is the same as a hot-rod car being over geared, so I understand. However in a car no damage is done until the situation is so severe the car is hardly drivable. I'm unsure if I need to make any changes to my boat.

I think I'm either over propped or just haven't pushed it hard enough to know. It may take 2700-2800 rpm to get to hull speed.
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Old 03-12-2010, 21:37   #14
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Quote:
Originally Posted by duncan_ellison
I keep hearing that it's possible to change the pitch of a Max Prop classic underwater, but I've never herd anyone who actually achieved this.

I've done it on land and that thing explodes into a mess of cotter pins, allen bolts and cogs.

Anyone really done this ?
Many, many times.
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Old 03-12-2010, 21:42   #15
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My new 4JH4-TE (75hp)Yanmar installation was completed 2 months ago. I put the boat in the water to get some hrs on the engine and verify my boat speed/rpm predictions with the new prop (20x18) and 2.57 tranny. I'm happy with the results: 5.5 kts @2200 rpm, hull speed 8kts @ 2650 rpm, WOT 8.2 kts @ 2750rpm. My only problem is that Yanmar wants me to be able to be able to achieve 3200 rpm @WOT in order to maintain warranty. Not being able to achieve required WOT leads to glazing, etc. I'm negotiating with Yanmar to not reduce the pitch 2". I agree with four winds that I'd rather cruise at a lower rpm. I'm not sure Yanmar will go along with me.
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