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Old 07-02-2019, 11:37 AM   #21
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Sandpiper is 40' LOA, 36' LWL, 12' beam at waterline, 40,000#, full displacement, built in 1976.

Engine is a single 120 Lehman with 6,000 hours, 2.57 gear.

The boat was equipped with a 24X22 DynaQuad 4 blade at purchase in 2000. Told by several prop shops and engine "experts" that the boat was over propped. Could not reach WOT in gear. PO replaced the 3 blade OEM prop with the 4 blade in the late 80's.

With the 24X22, fuel burn @ 1700 rpm - 8 knots was 1.8 gph - 4.4 kmpg

Had the DynaQuad repitched to 24X20, which was calculated to be the ideal size prop. The engine was able to reach WOT in gear.

With the 24X20, 1700 rpm yielded 7.3 knots, 2.1 gph - 3.5 kmpg
At 1800 rpm, speed was 7.5 and fuel burn was 2.3 gph - 3.3 kmpg

Repitched back to 24X22 in 2002.

The speeds were measured on a measured mile, opposite direction runs, back to back at slack tide. Fuel usage was with a graduated fuel container.
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Old 07-02-2019, 01:05 PM   #22
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Quote:
Originally Posted by djmarchand View Post
An over propped condition is when the engine won't reach rated maximum rpm at wot due to the prop having too much pitch. Let me give an example:

The Cummins 6BT engine often used in trawlers from the late 80s to 90s is rated for 210 hp at 2,600 rpm. If the prop has too much pitch, it will take too much of a bite in the water and the engine won't have enough power to reach its rated 2,600 rpm. It is like going up a hill in a heavily loaded car in high gear. The solution is to downshift which is analogous to reducing pitch on your prop.

So say it only reaches 2,200 rpm at wot. If you only run it at 1,600 rpm or less, that is way down on the prop power absorption curve and probably won't harm the engine. You will get a little better fuel economy and lower noise by running in that condition.

But don't try to run that engine at 2,200 rpm which would be fine if it weren't overpropped. It will put too much load on the engine and will result in reduced life.

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Old 07-02-2019, 01:46 PM   #23
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Originally Posted by djmarchand View Post
The Cummins 6BT engine often used in trawlers from the late 80s to 90s is rated for 210 hp at 2,600 rpm. If the prop has too much pitch, it will take too much of a bite in the water and the engine won't have enough power to reach its rated 2,600 rpm. It is like going up a hill in a heavily loaded car in high gear. The solution is to downshift which is analogous to reducing pitch on your prop.

Just an ignorant guess, but wouldn't an alternate solution for a heavily loaded car in high gear be to simply not drive up a hill? Likewise in a boat that is overpropped simply be to not run at high power?


On my pre-purchase survey, at full throttle, I'm guessing 1/2 fuel and water, and a relatively clean bottom, my Cummins QSB 5.9 only achieved 2925 rpm at full throttle instead of the rated 3,000 rpm. So I am slightly over propped. However, I normally cruise at 1,400-1,500 rpm and never run at WOT. So in my application, not likely an issue.
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Old 07-02-2019, 01:53 PM   #24
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With the 24X22, fuel burn @ 1700 rpm - 8 knots was 1.8 gph - 4.4 kmpg

With the 24X20, 1700 rpm yielded 7.3 knots, 2.1 gph - 3.5 kmpg
At 1800 rpm, speed was 7.5 and fuel burn was 2.3 gph - 3.3 kmpg

8.0 kt at 1.8 gph versus 7.3 kt at 2.1 gph

That’s a huge difference with just a repitch. Are you sure those numbers are correct?
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Old 07-02-2019, 02:10 PM   #25
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8.0 kt at 1.8 gph versus 7.3 kt at 2.1 gph

That’s a huge difference with just a repitch. Are you sure those numbers are correct?
Of course they are correct. I spent several days making two way runs on a measured mile and measuring and filling the calibrated fuel container before and after the repitch and again after pitching back.

And, the repitched 24 X 20 remained on the boat for two years which resulted in documented higher fuel usage.
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Old 07-02-2019, 02:25 PM   #26
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if you operate your boat exclusively a trawler speeds. 7 to 9 knts over propping your boat has the advantage of loading your motor more and hopefully allowing it to get up the proper operating temperature
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Old 07-02-2019, 03:31 PM   #27
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On the same subject....
Can someone explain what an EGT gauge should show when properly propped ??

Highest temp ? Lowest temp ? Steady ??

Or maybe how to use an EGT to run an engine...
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Old 07-02-2019, 04:38 PM   #28
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I think that an EGT is particularly useful for high output, turbo charged engines. Turbo charging crams more air into the cylinders and at high hp loadings which means lots of fuel, EGT climbs.

EGT values are somewhat dependent on where the probe is located, either before the turbo or after. After will drop the temp anywhere from 200 to 300 degrees.

On a recent trip with a pair of Cummins 6BTA 370 hp engines, we were running at about 2,500 rpm and over propped 150 rpm (running medium hard) and saw 900 deg F on the EGTs. At idle it was near 400 F. The probes were after the turbo.

As a rough rule of thumb, when EGTs get to 1,000 (after turbo) you are running very hard.

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Old 07-02-2019, 05:25 PM   #29
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dhays wrote;
“my Cummins QSB 5.9 only achieved 2925 rpm at full throttle instead of the rated 3,000 rpm. So I am slightly over propped.”.

I’m about exactly the same w a 3000rpm engine that makes 2950. I’ve been thinking about taking out an inch of pitch. But I’ll be (probably) at 3100 WOT rpm then. I was once there and like the way the engine ran. Technically speaking 3100 would be great but at the next haulout I could put in just a tad of cup if it went to 3200. Or (instead of re-pitching) just cut out a small bit of blade area removing some metal from the LE relying on the prop man’s experience to get it right. I wonder how accurately a good prop man can remove just the right amount of metal to up 100rpm?
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Old 07-02-2019, 06:25 PM   #30
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A well set up engine doesn't need any gauges. Gauges are to indicate when something is out of expected range. The likelihood of that happening dictates how many gauges you need.

Personally I never put an EGT on more than just the single exhaust for the engine, but I did so because I was running a set of engines close but not exceeding their design spec. The EGT caught the cylinder cracking 2-3 hours before the engine room was splattered in oil. It didn't prevent anything, but it did diagnose early. Not early enough to avoid anything though. It's greatest value was in setting a baseline for how the engines were treated. THAT was the difference in finding the proverbial needle in the haystack and caused us to look far enough to find an obscure failure point. As in, we just suffered a heat related failure, but we have a clean history of not running in an overheated condition. That was VERY valuable information and we would have blamed operation as the most likely cause and suffered even more failures without this valuable tool.


A clarification on running over-propped. It's not just an overload at high rpm's, its technically an overload at all rpm's on a boat with a fixed transmission ratio. The difference is that at low rpm's your engine likely has some extra cooling capacity that you can take advantage of. Being overloaded on most boats near cruise to wide open is a good recipe for a heat related failure. One of the most non intuitive methods of cooling an engine is not the raw water/antifreeze circuit, in many ways that's a backup. The primary way an engine cools itself is in the very combustion process in which the engine exhausts its spent exhaust and inhales a fresh and cool supply of air/fuel. The water cooling system simply cools the secondary heat remaining in the block. If you run the same HP and fewer rpm's, then you are operating at a higher heat load per cycle. Which is why "load" on an engine is so critical. The engine may be more "efficient" at this higher load, but it is less forgiving. A well running engine has some built in margin to deal with this. But keep in mind that over time, your heat exchanger gets a bit more gummed up, your oil loses some of its lubricating potential at the end of the season, your valve train may drift a little out of spec, and your fuel may lose a bit of its cetane rating, your old impeller moves just a bit less coolant. All these things taken together, put you a little closer towards the margins as well. I've had my current boat for over a dozen years now. I've had a bad season where I hardly saw my boat, did not change the impeller, had a dirty bottom. I personally like to have my safety margin in place at all times. Those who know what they are doing, are perfectly fine, so am I. As someone who literally measures things for a living, I'm always a bit skeptical whether the "measurable" fuel improvements are truly measurable in real life. Some do, and I've certainly witnessed a few I would trust. Most are measuring background noise. It's not a night and day difference anyway. Make your own decisions.
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Old 07-02-2019, 07:06 PM   #31
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What ghost said. I cant add to that.
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Old 07-02-2019, 07:24 PM   #32
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Thanks Ghost. Your explanation gives me something to think about.
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Old 07-02-2019, 07:44 PM   #33
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Quote:
Originally Posted by Nomad Willy View Post

I wonder how accurately a good prop man can remove just the right amount of metal to up 100rpm?
I would not want to reduce a props diameter. With repitching, you can undue the change if the new pitch is unsatisfactory.

On my repitch from 22 to 20, the recommendation was to reduce diameter instead of repitching. More tip clearance to hull was a consideration but the tip clearance was already more than adequate and I wanted the undoing option, which I ended up needing. I also did not want to reduce prop walk with a smaller wheel. And, would'nt the tips be thicker after shortening the blades?
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Old 07-03-2019, 06:37 AM   #34
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"I remember in Nigel Calder's book he recommended a gauge on each cylinder, eg. 6 in your engine room, to truly see the health of each individual cylinder.

Honestly, would you think that's overkill or very important?"


Perhaps on a racing lobster boat ,or super powered sport fish,, but most cruisers seem to operate in displacement mode most of the time , where even an EGT would be a fun luxury.
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Old 07-03-2019, 07:24 AM   #35
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I would not want to reduce a props diameter. With repitching, you can undue the change if the new pitch is unsatisfactory.

On my repitch from 22 to 20, the recommendation was to reduce diameter instead of repitching. More tip clearance to hull was a consideration but the tip clearance was already more than adequate and I wanted the undoing option, which I ended up needing. I also did not want to reduce prop walk with a smaller wheel. And, would'nt the tips be thicker after shortening the blades?
Generally a 2" pitch change on props that sized are not recommended.
It tends to be too much of a deviation from the props core and the results are not typically welcome.
Pitch changes for those extremes are nest suited to a prop change.
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Old 07-03-2019, 07:38 AM   #36
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Quote:
Originally Posted by ghost View Post
A clarification on running over-propped. It's not just an overload at high rpm's, its technically an overload at all rpm's on a boat with a fixed transmission ratio. The difference is that at low rpm's your engine likely has some extra cooling capacity that you can take advantage of. Being overloaded on most boats near cruise to wide open is a good recipe for a heat related failure. One of the most non intuitive methods of cooling an engine is not the raw water/antifreeze circuit, in many ways that's a backup. The primary way an engine cools itself is in the very combustion process in which the engine exhausts its spent exhaust and inhales a fresh and cool supply of air/fuel. The water cooling system simply cools the secondary heat remaining in the block. If you run the same HP and fewer rpm's, then you are operating at a higher heat load per cycle. Which is why "load" on an engine is so critical. The engine may be more "efficient" at this higher load, but it is less forgiving. A well running engine has some built in margin to deal with this. But keep in mind that over time, your heat exchanger gets a bit more gummed up, your oil loses some of its lubricating potential at the end of the season, your valve train may drift a little out of spec, and your fuel may lose a bit of its cetane rating, your old impeller moves just a bit less coolant. All these things taken together, put you a little closer towards the margins as well. I've had my current boat for over a dozen years now.
IMO, you're confusing planing speed HP requirement with displacement speed HP requirements. With planing hulls the HP requirements more closely match HP production from the engine. With displacement speed operations the HP requirement curve is much steeper as you approach and pass theoretical hull speed. With my boat at 1,500 RPM cruising at 7 knots, the vessel is using less than half of the available HP for that RPM. Increasing the speed by 1 knot requires an 80% increase in HP. Increasing the speed to 9 knots doubles the HP requirements over 8 knots. A planing hull on plane with that motor and the correct prop pitch will likely be using 2/3 to 3/4 of the available HP at 1,500 RPM. The HP requirement as you increase RPM on plane is a more gradual curve. If I were to prop my boat to be able to turn maximum RPM, my 7 knot cruise speed would require 2,100 RPM. At that RPM, the engine would be using about a third of the available HP. As I mentioned previously, one of the advantages of an electronically controlled engine is the ability to know the real time power consumption and its percentage of available HP for a specific RPM. This takes some of the guess work out of over propping and allows you to confirm the engine is operating well within safe parameters.

Ted
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Old 07-03-2019, 09:06 AM   #37
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Hey Ted, I was trying to think through where we fundamentally disagree, but the mechanic of your description met pretty closely with what I was trying to represent. Namely, you were comparatively normalizing by rpm, in your case horsepower for the same rpm. I see I used the term “overload” which is biased as a conclusion on whether things are good/bad. I should just say “load”, since without direct measurement there is still opinion as to whether it’s under or over. So maybe if I restate to increasing load at all rpms, that would be a better representation.
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Old 07-03-2019, 09:17 AM   #38
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Hey Ted, I was trying to think through where we fundamentally disagree, but the mechanic of your description met pretty closely with what I was trying to represent. Namely, you were comparatively normalizing by rpm, in your case horsepower for the same rpm. I see I used the term “overload” which is biased as a conclusion on whether things are good/bad. I should just say “load”, since without direct measurement there is still opinion as to whether it’s under or over. So maybe if I restate to increasing load at all rpms, that would be a better representation.
Yes, I can agree with that.
As with so many things in boating, over propping requires knowing all the parameters and limitations, not just the simple theroy.

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Old 07-03-2019, 09:18 AM   #39
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Probably most that are overpropped are because the boat came that way.

Then when they haul out and perhaps think about prop load they notice many here on TF are overpropped and most seem experienced or even old salts. Then they recall hearing one will burn less fuel and of course the’ve had the experience of going to the fuel float (of barge) and getting shocked at the cost. And that’s it. They just keep on keep’in on thereafter.

Then a few come along every now and then w a bunch of numbers and theories that are very overwhelming and it’s rejected because even the experts on TF continue to argue about whether it’s good or bad. Then they think of fuel again. So the motive to change is very small so they chime in w the others claiming it’s the smart thing to do. So if it’s the smart thing to do it would be dumb to do any different and they get to talk about how smart it is to do w the big boys on TF too. And it goes on and on.

But another thing goes on and on. That’s the fact that no engine manufacturer recommends overpropping.
Then there’s hull speed. All the old salts talk about hull speed. So many attempt to or do run their boat at hull speed. May be the perfect speed for SD boats not close to FD but for those close to hull speed it’s not the speed to go. And for FD it’s the speed to never ever go unless the boat is overpowered (most are) and you’re doing a spring check on systems engine wise.

But if you’re at all concerned about fuel consumption most SD and all FD boats should be run 1/2 to one knot below hull speed. But the average TF guy is confused again and hears many run at a “sweet spot” and then we have license to find a speed he feels is nice. Almost always the speed that is run at the rpm where there is the least vibration and hence noise as well and almost absolutely everybody like that. Hard not to like smooth and relatively quiet. And the old salts on TF are frequently talk’in about it so it must be the thing to do.

But re overpropping is not recommended by any engine manufacturer.
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Old 07-03-2019, 09:34 AM   #40
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But re overpropping is not recommended by any engine manufacturer.
So you follow all your engine manufacturer's recommendations including his brand of engine oil and only OEM parts?

Ted
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