Overpropping

markpierce said:

Idle (one-third WOT) for me equals 3 knots. Normal cruise (one knot less than hull speed) three-fourths WOT. Maximum (hull) speed a 92% WOT. See no need for overpropping here. Especially don't want a higher minimum speed.

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With my 26X26 I did 3.8 ktns at idle. The 25X27 now does 4.2 at Idle. At 1550 RPM the 26X26 di 7.5 knots. The 25X27 does 8 Knots. I am now over propped and can not reach my 2650 design WOT for the 6BT 5.9 Cummins. WOT is 2590. I am pretty sure I am ok. BUT, I don't believe I gain anything based on the laws of physics. It still cost the same to move the 30,000 lbs of boat through the water.

fryedaze said:

With my 26X26 I did 3.8 ktns at idle. The 25X27 now does 4.2 at Idle. At 1550 RPM the 26X26 di 7.5 knots. The 25X27 does 8 Knots. I am now over propped and can not reach my 2650 design WOT for the 6BT 5.9 Cummins. WOT is 2590. I am pretty sure I am ok. BUT, I don't believe I gain anything based on the laws of physics. It still cost the same to move the 30,000 lbs of boat through the water.

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BTW, I did change prop sizes on purpose. I tore up my props running aground. My spare props just happened to be a different size. I assume they were the original design and the props on the boat were from a previous engagement with the bottom by the previous owner.

fryedaze said:

With my 26X26 I did 3.8 ktns at idle. The 25X27 now does 4.2 at Idle. At 1550 RPM the 26X26 di 7.5 knots. The 25X27 does 8 Knots. I am now over propped and can not reach my 2650 design WOT for the 6BT 5.9 Cummins. WOT is 2590. I am pretty sure I am ok. BUT, I don't believe I gain anything based on the laws of physics. It still cost the same to move the 30,000 lbs of boat through the water.

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You are correct about that it takes the same amount of force to move the boat through the water. However, there are things you can change on your boat to generate that force with less fuel. Engines have a range where they are most efficient (generate most HP per gallon of fuel). This is generally near peake torque. There is a range of optimum propeller sizes for a given boat at a given speed relative to the drive train. As an example, if your propeller was optimal for your boat, and you swapped it with one that was 2"smaller in diameter, it would yield less MPG. Not suggesting you start swapping props, it's just that the object of the exercise is to get the most MPG out of your boat and maybe reduce engine noise without any detrimental effect.

Ted

O C Diver said:

. Not suggesting you start swapping props, it's just that the object of the exercise is to get the most MPG out of your boat and maybe reduce engine noise without any detrimental effect. Ted

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Or prop it by the book and ignore internet wives tales. Unless a problem exists, chasing the mythical most MPG by swapping out props is a money losing proposition.

Today, the quest on larger high hour yachts is prop selection to minimize noise and vibration while not damaging the engine (by operating it out of its sweet spot) and associated systems.

But as said already on this thread, the few hours per year most TF vessels see invites over propping discussion with little measurable difference one way or the other. The rub for many, me included, is when sea trialing a vessel if rated full RPM cannot be reached, wondering what is wrong?

sunchaser said:

Or prop it by the book and ignore internet wives tales. Unless a problem exists, chasing the mythical most MPG by swapping out props is a money losing proposition.

Today, the quest on larger high hour yachts is prop selection to minimize noise and vibration while not damaging the engine (by operating it out of its sweet spot) and associated systems.

But as said already on this thread, the few hours per year most TF vessels see invites over propping discussion with little measurable difference one way or the other. The rub for many, me included, is when sea trialing a vessel if rated full RPM cannot be reached, wondering what is wrong?

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Tom, to OC Diver's point, diesels run best when loaded a bit. If you run a diesel at 1/5th of its rated horsepower - which is all it takes to move Delfin at 7.5 knots, yet she takes 80% of that hp to move her at hull speed in a bit of seaway - then doesn't a bit of overprop at minimal rpm but peak torque load the engine a mite? If so, isn't this a benefit regardless of its impact on fuel consumption? I suppose it could be harmful if you needed to run at WOT for more than a few minutes, but that never seems to happen, to us at least.

O C Diver said:

Probably immaterial if you see no change in the exhaust gas. At 1450 I'm guessing you are pretty close to peak torque. Most diesels are operating at there best efficiency at peak torque provided they have reasonable load. Something to consider: many large generators run there entire life at reduced rpm. My 9 KW Onan has a 2400 rpm Kabota Diesel running at 1800 rpm. Clearly it doesn't have a problem as long as it's load falls somewhere between minimum and maximum. That's kind of like running a slightly over propped propulsion engine at reduced rpm.

Ted

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Yes, peak torque looks to be right at 1375 or so on this engine. As per my question to Sunchaser, isn't a bit of load from minor overpropping potentially beneficial in that it provides a more "reasonable load"?

Delfin said:

Tom, to OC Diver's point, diesels run best when loaded a bit. If you run a diesel at 1/5th of its rated horsepower - which is all it takes to move Delfin at 7.5 knots, yet she takes 80% of that hp to move her at hull speed in a bit of seaway - then doesn't a bit of overprop at minimal rpm but peak torque load the engine a mite? If so, isn't this a benefit regardless of its impact on fuel consumption? I suppose it could be harmful if you needed to run at WOT for more than a few minutes, but that never seems to happen, to us at least.

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Like most with larger trawlers, I am over powered, but in in my case not over propped. In our cases our existing engine and prop set ups will do the engines no harm so long as oil temperatures are in the 185 to 200 F range and no excessive coolant heat exists.

I just read today on boatdiesel the old axiom, "under propping creates no issues, whereas over propping does." I'm too lazy to refute these statements and ignore my half century of large to small diesel use.

My doubts on the over prop question stem from no verifiable data is available but lots of arm waving takes place. In today's diesels precise fuel mapping exists with new vessels, whether a slow Nordhavn or a fast Grand Banks, using the fuel mapping precise GPM and load to assist in prop selection.

sunchaser said:

Or prop it by the book and ignore internet wives tales. Unless a problem exists, chasing the mythical most MPG by swapping out props is a money losing proposition.

Today, the quest on larger high hour yachts is prop selection to minimize noise and vibration while not damaging the engine (by operating it out of its sweet spot) and associated systems.

But as said already on this thread, the few hours per year most TF vessels see invites over propping discussion with little measurable difference one way or the other. The rub for many, me included, is when sea trialing a vessel if rated full RPM cannot be reached, wondering what is wrong?

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Thus the point of the post.

Maybe it's not really internet wives tales....maybe there is data out there that proves something...maybe not everything...but something.

A very knowledgeable poster and I have traded PMs over the subject and yes the broad stroke comments may be as flawed as the arguments against overpropping. Each tidbit and data point has to be put in perspective. Comments that allude to "science" and that it takes the same energy to drive the boat at a given speed are often correct...but fail to address the key point of efficiency or as you point out the trouble that it's worth.

The quote I picked addressed that there were dangers involved but even a published "expert" didn't see much danger in saying it...just that those risks could be "mitigated" to a degree that in some instances...overpropping might be worth the effort.

Thus again the point of the OP.

I knew I wouldn't change many or any minds...but totally unfamiliar people might deserve all the info out there....as it trickles in.

Scott, are you referring to our comparing fuel use, etc?
The way I look at it is if propped "reasonably" one would be ok unless the enigne is run "on the edge" and then it would be necessary to have it dialed in.

By the way I think your boat rocks more than mine...... LOL

jleonard said:

Scott, are you referring to our comparing fuel use, etc?
The way I look at it is if propped "reasonably" one would be ok unless the enigne is run "on the edge" and then it would be necessary to have it dialed in.

By the way I think your boat rocks more than mine...... LOL

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Have you been following the 1988 Albin interior thread?

Another couple dollars and there's a few marine stores that could lay claim to her any day....there's not enough left to prove it's the boat I bought except for the HIN...

No Jay....I have no idea whether my bigger prop than yours helps or not...the only way to really know for sure on any boat is testing props on that single boat. Sure there are charts that can point us in the right direction but a whole lot of other factors can affect speed and fuel consumption on 2 different boats so coming close only can make us guess why.

Based on the generic whole of all boats reporting in...I feel that my fuel numbers are close to what they should be as are yours. Some reports of 8 knots and 1.5 gal/hr on a Lehman 120 in an old Taiwan trawler make me cringe when it comes to reality....unless it's per engine on a twin.

A verifiable real world test for cruising prop selection benefits can be done several ways, here is one:

--Prop vessel so it can achieve full rated RPM without incident
--Measure and tabulate fuel burn and vessel speed vs 100 RPM increments
--Repeat with larger cruising prop
--Compare the results
--Do this in flat water with no current and at 180 degree reversal to compensate for wind and latent currents
--Insure tachometers, GPS and fuel measuring equipment are accurate


This testing method requires money, patience and good measuring equipment. Without such the differences are relegated to arm waving and citing fuel burn charts from the manufacturer as proof one way or the other.

Delfin said:

Yes, peak torque looks to be right at 1375 or so on this engine. As per my question to Sunchaser, isn't a bit of load from minor overpropping potentially beneficial in that it provides a more "reasonable load"?

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The simple answer is yes. Don't know if you can obtain the Hp, torque, and fuel consumption graphs for your engine (maybe Boatdiesel). I would look at the graphs for you cruise RPM range and see what HP and fuel consumption the graphs show. I would want to be well below the maximum, but probably at or above 1/3 rd for adequate loading and engine heat generation. This could be comparable to a generator in that you certainly don't want to run it under loaded for extended periods of time for the same reasons. That's how I see it anyway.

Ted

sunchaser said:

A verifiable real world test for cruising prop selection benefits can be done several ways, here is one:

--Prop vessel so it can achieve full rated RPM without incident
--Measure and tabulate fuel burn and vessel speed vs 100 RPM increments
--Repeat with larger cruising prop
--Compare the results
--Do this in flat water with no current and at 180 degree reversal to compensate for wind and latent currents
--Insure tachometers, GPS and fuel measuring equipment are accurate


This testing method requires money, patience and good measuring equipment. Without such the differences are relegated to arm waving and citing fuel burn charts from the manufacturer as proof one way or the other.

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The process you describe is very similar to the results turned into one graph in the article...thus my post....

Someone actually did it and came up with results that made Calder make the opening statement.

psneeld said:

The process you describe is very similar to the results turned into one graph in the article...thus my post....

Someone actually did it and came up with results that made Calder make the opening statement.

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It would seem Calder is citing the very good report done by Norwegian O Gulbrandsen in 2012. The report points to bigger props turning slower, increasing the gear ratio and derating the engine by the use of lower RPM fuel stops to prevent damage.

The report is well worth reading and spends much time on hull design and saving fuel by easing back on the throttles. The report also cites the need to install smaller engines to best take advantage of slower speeds. Way to go Coot.

Now if only I were doing a new build and could do it perfect, kinda like Steve Dashew has already done in his FPBs. Calder and Dashew are two very smart cookies, with Dashew putting his money on the line.

O C Diver said:

The simple answer is yes. Don't know if you can obtain the Hp, torque, and fuel consumption graphs for your engine (maybe Boatdiesel). I would look at the graphs for you cruise RPM range and see what HP and fuel consumption the graphs show. I would want to be well below the maximum, but probably at or above 1/3 rd for adequate loading and engine heat generation. This could be comparable to a generator in that you certainly don't want to run it under loaded for extended periods of time for the same reasons. That's how I see it anyway.

Ted

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Thanks Ted. Can't really hit the 1/3rd target since my hull drives pretty well at very low horsepower levels. Think big sailboat hull design. However, to get 9+ knots hull speed I need a lot of horsepower because I am pushing a lot of water at those speeds. However, I am told that the 3306 is good for a trawler because it can run at low loads for a very long time. Be that as it may, thanks again for helping me with your advice.