overpropped vs underpropped - Page 4 - SailNet Community
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post #31 of 46 Old 12-17-2012
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Re: overpropped vs underpropped

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Coolant flow from a water pump with an impeller is linked to rpm in a linear manner over it's normal rpm range. IE double the rpm and you double the flow. RAW WATE PUMP

However flow from a centrifugal pump is NOT remotely linear. This type of pump is likely to be used to circulate coolant around the engine. At very low engine RPMs coolant circulation can be impaired.
I test pumps and fluid flowrates for a living. Centrifugal pumps and fans operate with direct proportional flow rate vs rpm.
Decrease a pump by 15% rpm and you get 15% less flow.

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post #32 of 46 Old 12-17-2012
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Re: overpropped vs underpropped

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My circumstance is a bit different as I have a screwy, older engine (Perkins 4.108) and this mechanic had messed around with my particular engine on my particular boat before. My engine is also undersized for my boat, but old, sturdy, and heard to break, so I've pitched for a slightly shorter lifespan in exchange for being ABLE to reach hull speed at 75% throttle.
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post #33 of 46 Old 12-17-2012
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Re: overpropped vs underpropped

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I test pumps and fluid flowrates for a living. Centrifugal pumps and fans operate with direct proportional flow rate vs rpm.
Decrease a pump by 15% rpm and you get 15% less flow.
Dat not wot my text books and test results said.

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Notice that larger pumps are inherently more efficient and that efficiency drops rapidly at specific speeds of 1000 or less. Jacques Chaurette p. eng.
Primary coolant centrifugal pumps would be classed as small?

However I know I am not a centrifugal pump expert.
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post #34 of 46 Old 12-18-2012
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Re: overpropped vs underpropped

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Dat not wot my text books and test results said.



Primary coolant centrifugal pumps would be classed as small?

However I know I am not a centrifugal pump expert.
Hi TQA, I can't scan my text books but this might work

Try this link Downloads & CDs
You can get to it through ITT Bell & Gossett website, click on "Online Calculators" scroll down and click on 'Flow / Head - Variable Speed Pumping Wizard" and try it out.

Start at 5 gpm at 10 ft of head and then look at what happens to flow at different speeds. Directly proportional.

Try 5000 gpm at 10, 100, or a 1000 ft of head.
Directly proportional.


I test small pumps and i test BIG pumps. I test them at different speeds.

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post #35 of 46 Old 12-18-2012
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Re: overpropped vs underpropped

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Originally Posted by Oaksail View Post
Hi TQA, I can't scan my text books but this might work

Try this link Downloads & CDs
You can get to it through ITT Bell & Gossett website, click on "Online Calculators" scroll down and click on 'Flow / Head - Variable Speed Pumping Wizard" and try it out.

Start at 5 gpm at 10 ft of head and then look at what happens to flow at different speeds. Directly proportional.

Try 5000 gpm at 10, 100, or a 1000 ft of head.
Directly proportional.


I test small pumps and i test BIG pumps. I test them at different speeds.

This is very easy to see with centrifugal bilge pumps. Drop the voltage and the output is also cut. It can be pretty dramatic...

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post #36 of 46 Old 12-18-2012
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overpropped vs underpropped

Back to the topic.
Having just replaced my three blade fixed with a four blade feathering. I find that I can only get to 2900rpm with the max WOT at 3200. But I can reach hull speed easily here.

Now I almost never run the engine at WOT. Probably 99% of the time at 2500 or less because I don't care for the motion of the boat over 8 knots. The engine is a Yanmar 4jh4HTE

Is the 4 blade adding more stress to the drive train than the three and am I doing damage at this prop setting?

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post #37 of 46 Old 12-18-2012
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Re: overpropped vs underpropped

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....Having just replaced my three blade fixed with a four blade feathering. I find that I can only get to 2900rpm with the max WOT at 3200. ....
This is confusing. If your new feathering prop won't allow you to get to max rpm under load, then it needs to be adjusted. If you're uncomfortable at any given speed, just pull the throttle back. But it should be able to get to max rpm.


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post #38 of 46 Old 12-18-2012
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Re: overpropped vs underpropped

I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.
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post #39 of 46 Old 12-18-2012
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Re: overpropped vs underpropped

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I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.
Well put. From the mechanics I've talked to, being ABLE to reach max RPM at WOT is the only real way you can tell if you have the correct amount of stress on your engine at any given speed.

How many hundreds of RPM can you fudge it and get away with? That depends on a lot of things and many unknowns. If you adjust your new prop so that you can reach max RPM at WOT and you can still hit your cruising speed at a reasonably efficient RPM then there is no downside.

At what RPM do you currently reach hull speed/cruising speed? What engine do you have?

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post #40 of 46 Old 12-19-2012
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Re: overpropped vs underpropped

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FYI

Yanmar 4JH5E naturally aspirated.
Hi
I note that you say your engine is a Yanmar 4JH5E and that it has a governed rpm rate of 3,400rpm. May I ask where you got this data from? The only data I can find for it from Yanmar is a maximum output rating of 53mhp at the crankshaft at 3,000rpm I'm asking because I'm trying to find a source to find out what is the maximum WOT rpm of my 3JH5E engine.
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