Speed vs RPM, my math isn't mathin.

[Selidster]

I have scowered the search in Trawerler Forum and read quite a few threads but haven't found a thread that helped me.

I can't seem to get my head around our boat speed at certain RPMs. We have a KK42 with a Lehman 120. We are over 3000 miles into the great loop, so I have a good baseline of how we are running. We are maintaining 1.75 gph and 3.75 miles per gallon, so I'm actually happy with that. But things just don't seem right. I have the power curve from Ford for the engine. Max torque is about 1650 rpms. The fuel range vs speed curve from Krogen says I need less than 30 hp to make 7 knots. According the Ford curve 1200 rpms is about 60 hp. I like running 7 knots but am having to run 1900 rpms in calm currentless conditions. From the Ford curve it looks like 1900 is about 85 horsepower, more than enough to move the boat according to the Krogen curve. Hull speed is about 8.1 knots.

It seems to me that I should be able to run at 1650 rpms to have maximum torque and plenty of horsepower to make my 7 knots. I have done the math many times. I have checked to propeller size and we have what the prop calculators say we should have, 28" x 20". Bottom is clean, no prop damage.

My math. Correct me if I'm wrong please.
7 knots = 8.05 mph
8.05 * 5280 = 42,504 feet/hour
42,504' * 12" = 510,048 inches/hour
510,048"/16" = 31,878 propeller rotations/hour (20" pitch minus 20% slippage)
31,878 /60 minutes = 531.3 prop rotations per minute
531.3 * 2.91 (trans ratio) = 1546 engine rpm

Where have I gone wrong?
Thank you in advance.
Any help is appreciated and I'll sleep better if I can understand this.

30% slippage:
510048"/14" = 36,432 prop rotations per hour
36,432"/60 minutes = 607.2 prop rotations per minute
607.2*2.91 (trans ratio) = 1767 engine rpm


I realize I didn't use nautical miles but I'm not sure that will be a huge difference.

 

[SteveK]

I do not see any allowance for current and wind.

 

[l00smarble]

I am not an expert but it would seem like you may be under-propped. I think the Ford curve would tell you how much the engine CAN make at a given RPM but it won't actually make that power unless there is a load to work against. For example you could raise the RPM in neutral to all points on the curve and make no useable power. Don't the symptoms seem like they add up to being under-propped? What happens at WOT? Rated RPM?

 

[Jeff F]

We are over 3000 miles into the great loop, so I have a good baseline of how we are running. We are maintaining 1.75 gph and 3.75 miles per gallon, so I'm actually happy with that. ...I like running 7 knots but am having to run 1900 rpms in calm currentless conditions.

It seems to me that I should be able to run at 1650 rpms to have maximum torque and plenty of horsepower to make my 7 knots.

You can, with a prop adjustment. It sounds very much like you're under propped.

Your fuel consumption gives confirmation of the 30 hp demand. Ford graphs show maximum available power. As long as demand is well under max available you're good.

There's endless debate here about propping. On a KK42 at 7 knots you've got lots of latitude. I wouldn't lose sleep over it. Talk to a prop shop.

 

[Selidster]

I do not see any allowance for current and wind.

We have been in quite a few locations where wind and current are minimal. Between locks is very low current. It still takes 1900 + rpm to get to 7 knots. I appreciate your point and have tried to monitor the boat in calm winds and minimal current to see what she does.
Thank you

 

[Selidster]

I am not an expert but it would seem like you may be under-propped. I think the Ford curve would tell you how much the engine CAN make at a given RPM but it won't actually make that power unless there is a load to work against. For example you could raise the RPM in neutral to all points on the curve and make no useable power. Don't the symptoms seem like they add up to being under-propped? What happens at WOT? Rated RPM?

I haven’t found wot speed. My engine will spin up to a point I chicken out. It will jump to 2700 almost instantly then I back off for fear of damage. 2500 is supposed to be the wot top end.

I think under -propped as well but I’ve checked several prop calculators and they have had the same results. And that’s the prop I have.

Thank you for responding

 

[rslifkin]

The only way to know for sure if you're underpropped is to do a WOT test. And before starting, confirm your tach is actually accurate. Use a photo tach to get a true RPM reading at a few different RPMs and compare to the tach at the helm. If it's off, there's usually a calibration adjustment on the tach.

Once you have a confirmed accurate tach, do the WOT test. If rated RPM is 2500, I'd expect to see that or a little more at WOT with the boat loaded heavily. If you get right up to the governor or are above 2600 at WOT, it's likely underpropped and a bit more pitch will bring your cruising speed down. If you're around 2500 - 2600, it's fine as is. If you only get 2400 at WOT, then you're getting into overpropped territory and you wouldn't want to add any more pitch. Throttle up gradually as you do the test, there's no need to slam the throttle open and hope nothing breaks.

As far as the power curve, there's often 2 curves published. One is the maximum output curve, the other is the intended prop curve. That second curve is the power they expect you to output at any given RPM in real world use, as prop power demand increases exponentially as you increase RPM. A higher pitched prop would lower your cruising RPM, but you also wouldn't have use of full power (as you'd never reach full RPM) and while it depends on the engine a bit, loading the engine more heavily than the manufacturer intended at a given RPM can also shorten engine lifespan (and will likely increase exhaust smoke and therefore transom soot).

 

[PierreR]

You are reading the graphs wrong. It sounds like you are right where you need to be. 1.75 gph translates to around 30 hp. That is all your engine is actually producing at the speed you are recording.
1.75 gph/.057 gphp= 30.7hp
Fully loaded your engine is capable of 85 hp but your propeller is actually absorbing 30.7 hp in propulsion. If you were producing 85 hp your fuel burn would be 85 * .055 = 4.675 gph. The answer is you are not producing that power and are not paying for it either.

You will notice that I used a different specific fuel consumption for the two calculations. That is because the specific fuel consumption curve at the bottom of your first graph is curved.

 

[luna]

I don't think you did anything wrong in your calculations. You adjusted for Statute Miles with your velocity value so within reason, no worries there.

What you are up against is the realization that a heavy displacement boat with not a lot of power, travelling at 7 knots just have to live with the fact that the propeller has about 35% slip.

Changing propellers to run at your desired 7 Knots could be an option provided you don't overload the engine. Don't expect the slip to change much.

You also would be wise to warm up to this kind of realization. To move your boat at 7 knots takes about 30 HP.
It is easier on the engine to have it develop this 30 HP at 1,900 RPM instead of 1,600 RPM as each firing of the cylinders does less work, more air is drawn through the engine and more oil and coolant is circulated. Your engine has the ability to make more HP at 1,900 RPM than at 1,600RPM, but because the load is fixed at 30 HP you are using less of the engine's capacity at 1,900 RPM than at 1,600 RPM. In short RPMs do not wear out engines, heavy loading does. It may take a tiny bit more fuel but that is almost unmeasurable.

As long as your engine makes its operating temperature I think you are likely in a better place now than if you changed props and added load.

 

[Jeff F]

I think under -propped as well but I’ve checked several prop calculators and they have had the same results. And that’s the prop I have.

The other possibility is that your tach is way off. Check that first.

 

[DDW]

In short RPMs do not wear out engines, heavy loading does.

I don't agree with that statement, at least not unqualified. Overloading an engine will wear it out prematurely, loading it heavily will wear it out sooner than not, but more rpm means more piston and bearing travel. Engines don't last forever even run without load, and not much longer than if they had a reasonable load. At 30 hp he is well within reasonable load at the lower rpm.

The standard advice here makes sense: Check the tach with something reliable. The run at WOT and see if you reach max rpm. If you reach max rpm with lower than WOT then probably you can increase pitch.

Most trawler type yachts are underpropped at the very low cruising rpms we use, but it is a necessary compromise if you ever are going to use the engine in the higher rpm ranges. A consequence of the "one pitch fits all speeds" assumption.

 

[Selidster]

The only way to know for sure if you're underpropped is to do a WOT test. And before starting, confirm your tach is actually accurate. Use a photo tach to get a true RPM reading at a few different RPMs and compare to the tach at the helm. If it's off, there's usually a calibration adjustment on the tach.

Once you have a confirmed accurate tach, do the WOT test. If rated RPM is 2500, I'd expect to see that or a little more at WOT with the boat loaded heavily. If you get right up to the governor or are above 2600 at WOT, it's likely underpropped and a bit more pitch will bring your cruising speed down. If you're around 2500 - 2600, it's fine as is. If you only get 2400 at WOT, then you're getting into overpropped territory and you wouldn't want to add any more pitch. Throttle up gradually as you do the test, there's no need to slam the throttle open and hope nothing breaks.

As far as the power curve, there's often 2 curves published. One is the maximum output curve, the other is the intended prop curve. That second curve is the power they expect you to output at any given RPM in real world use, as prop power demand increases exponentially as you increase RPM. A higher pitched prop would lower your cruising RPM, but you also wouldn't have use of full power (as you'd never reach full RPM) and while it depends on the engine a bit, loading the engine more heavily than the manufacturer intended at a given RPM can also shorten engine lifespan (and will likely increase exhaust smoke and therefore transom soot).

I have checked the tach and adjusted. I believe it’s correct. We are fuelin up and filling water tomorrow. That is about 8000 lbs so the boat will be fully loaded. I’ll do the wot test as you describe. We’ll see where we end up.
Thank you

 

[Selidster]

You are reading the graphs wrong. It sounds like you are right where you need to be. 1.75 gph translates to around 30 hp. That is all your engine is actually producing at the speed you are recording.
1.75 gph/.057 gphp= 30.7hp
Fully loaded your engine is capable of 85 hp but your propeller is actually absorbing 30.7 hp in propulsion. If you were producing 85 hp your fuel burn would be 85 * .055 = 4.675 gph. The answer is you are not producing that power and are not paying for it either.

You will notice that I used a different specific fuel consumption for the two calculations. That is because the specific fuel consumption curve at the bottom of your first graph is curved.

That is great. I didn’t know the fuel use to hp formula. That helps tremendously.
Thank you

 

[Selidster]

The other possibility is that your tach is way off. Check that first.

I’ve checked and adjusted to a strobe. Accurate I believe.

 

[O C Diver]

You have nothing to worry about, the boat is running at normal efficiency.

Let's start out with the obvious:

Krogen didn't measure fuel consumption with full fuel tanks, full water tanks, and literally a ton or two of stuff onboard. They did it with a new boat and nothing onboard. They did it with a new motor, not decades old. The propeller was new, not worn down a bit. They likely had zero electrical consumption (the refrigerator and freezer weren't on). Lastly, they tend to round in their favor. Remember, 40 years ago, nobody was going to challenge their numbers with digital flow instruments. I can see marketing saying, "Let's say it does 7 knots at 30 HP (not 28, 29, 31, or 32) and burns a 1.5 GPH (the Lehman probably produces 18 or 19 HP per gallon at peak efficiency)".

If you want better fuel economy, slow down to 6 knots and likely be over 5 MPG.

Assuming your Loop trip is around 6,000 nautical miles, you will likey consume an additional 225 gallons of diesel. Considering the cost of Looping, that's a rounding error.

Don't worry, be happy on your trip!

Ted

 

[Jeff F]

I think under -propped as well but I’ve checked several prop calculators and they have had the same results. And that’s the prop I have.

I'd point out that the prop could have been altered since it was labeled. Also prop calculators use generic assumptions that may not line up exactly with your boat. It's an imprecise science. I'm far from a prop expert, but wouldn't expect anything close to 30% slip in your application.

I wouldn't make any changes in the interest of fuel economy or engine longevity, but there are some decent arguments for "right sizing" the prop, which would be a) lower cruise rpm may be less intrusive, and b) if you want maximum power from the engine it's available. If neither is important to you then carry on.

 

[Jeff F]

... Remember, 40 years ago, nobody was going to challenge their numbers with digital flow instruments.

I'm admiring the hand-drawn graphs. They're worth careful study, and perfectly illustrate your point about slowing down.

Some of it is marketing, but James Krogen made a very good study of requirements. I always liked his work.

ETA: IIRC Kady brought the marketing. A successful outcome...

 

[O C Diver]

I'm admiring the hand-drawn graphs. They're worth careful study, and perfectly illustrate your point about slowing down.

Some of it is marketing, but James Krogen made a very good study of requirements. I always liked his work.

ETA: IIRC Kady brought the marketing. A successful outcome...

In 2022 USA diesel was $6 to $8 per gallon. In Canada it was over $9. That year I did the Rideau and the Trent Severn. Slowing down from 7 knots to 6 knots increased fuel economy from 3.5 MPG to 5 MPG. Slowing 1 knot improved efficiency by 43%. With the exception of cruising in seas where the boat handled waves better, I never went back to 7 knots. It's also much quieter.

Ted

 

[PierreR]

The bottom line is where economics hits head on with theoretical math. If you do anything to try to improve the situation, you will not recover the costs and your overall ownership costs will go up.

If there ever was a situation for trying it's my boat. Basically a displacement hull, the same weight and waterline as your KK 42. Major differences though are your boat has a lower prismatic coefficient and lacks fin stabilization I an not quite double your fuel burn for the same speed. 3.1 gph
The active fins knock the efficiency of the hull down a bunch. It takes about half a knot off the speed for the same power setting. So basically I would be going 7.5knots instead of 7 at the same power setting. At those speeds that is a lot of fuel burn.
The second issue is that I have twin engine Detroit 453's at a compression ratio of 17:1. My specific fuel consumption is closer to 0.62 gallons per hp. but I am running them at only 22% power so my specific fuel consumption is closer to 0.065 gallons per hour per hp.
Now if I repowered with tier 4 diesels of slightly lower hp say 94hp each instead of 112hp each I could probably drop the fuel burn down closer to 2 gph for significant fuel saving. Running close to 5,000 miles a year I could recover my investment some time after I am dead.
I did remove my props and have the serviced and re-pitched to factory spec for $900. The result was an improvement of about 50 rpm and smoother running. Not enough to justify the cost but I felt like I tried.
The Detroit's run beautiful with no smoking on startup or even at full power. They purr with no complaints and very little maintenance for as complex as they are. They will last forever at 1,300 lbs for a 3.0 liter 4 cylinder engine. Hatteras recommends running them at 55% power or 8.5-8.8 knots at 6.6-7.2 gph.
Do I like it? No I want to improve it but I hear the words of my father in the back of my head. "You just hada mess with it didn't you. That"ll cost yah". Then I remind myself that I am retired and tell my self Shut The Hell Up and Just Enjoy The Boat! Keep the costs down. How am I doing on this boat project so far? No way in hell will I recover the costs of what I have done to the boat but, the next guy will reap the benefits. I feel like a dumbars but the boat is less than 5% of my net worth and I can't take it with me.

 

[Jeff F]

Most trawler type yachts are underpropped at the very low cruising rpms we use, but it is a necessary compromise if you ever are going to use the engine in the higher rpm ranges. A consequence of the "one pitch fits all speeds" assumption.

I agree with what you're trying to say here...

I think however we should be careful with using the term 'underpropped' this way.

We have a well accepted and established process for establishing whether a trawler is correctly propped, which basically tests the intersection of power and load at full power.

This has nothing to do with the fact that at lower power demand will be a small percent of available power with a fixed pitch. I wouldn't want anyone conflating the two. You could be over propped and this would still be true.

It's a terminology thing. I think we have really good discussions here, and want to nudge the bar a bit higher.

 

[Jeff F]

I did remove my props and have the serviced and re-pitched to factory spec for $900.

I think for long distance travellers like the OP this is money well spent, after having gathered the appropriate data. The prop drives the boat. It should be close to perfect.

 

[Jeff F]

...A higher pitched prop would lower your cruising RPM, but you also wouldn't have use of full power (as you'd never reach full RPM) and while it depends on the engine a bit, loading the engine more heavily than the manufacturer intended at a given RPM can also shorten engine lifespan (and will likely increase exhaust smoke and therefore transom soot).

I'd correct that to say "over propping would lower..."

If the OP is under propped your warnings don't have any applicability until they're over propped. Adding pitch to get to correct propping avoid the dangers you cite, and allows access to full power.

Adding pitch in this case does not mean over propping.

 

[O C Diver]

While we're rolling the dice for the OP, my question is, how many hours are on the Lehman 120?

The second thing I would like to point out is that it's a 120, not the 135 that was used in the later boats.

Finally, asking an engine to produce more HP at a lower RPM increases heat. From recollection, Lehmans can suffer low coolant flow around number 6 cylinder (closest to the transmission) from gathered deposits in the coolant loop around #6.

I could see a case for taking a freshly rebuilt engine and pitching the propeller to lower RPM, a little. The trade off being to reduce the top end to 7.5 knots. In my mind, there's a reason why Krogen switched from 120 to 135. Until the OP comes back with the number of hours on the engine or the rebuild, I would run it at 1,800 RPM, accept whatever speed you get, and realize the difference is showing up a half hour later on the average cruising day.

Ted

 

[MYTraveler]

I would ignore power (by definition, you are making enough), and instead begin by calculating actual slippage at 7 knots = 1- (rpm necessary to make 7 knots / theoretical rpm necessary to produce 7 knots with no slippage). If you confirm more than 30 percent, you have something producing lots of drag below the waterline (unless you are traveling into a headwind -- eliminate that variable by running twice, once into the wind, once with it, and averaging). Dirty bottom is most likely explanation, but it could be misaligned rudders, etc.

Once you are satisfied with your slippage, calculate the power actually being demanded of the engine at your preferred rpm. That will be a function of fuel burn at that rpm, although you will have to make an assumption about fuel efficiency -- 30% is a good starting place (most of the fuel goes up in smoke / heat, and does not produce HP. Note that fuel curves typically assume 100% load, but you will certainly have a lower load, and a lower fuel burn than the factory max. Compare that number to the factory's claim about how efficiently their hulls can be pushed (ie, how much power is required). In that regard, factory numbers reflect perfect everything, including NO additional weight and a perfectly clean bottom, etc. Your mileage WILL vary.

Edit: One more thought -- your propellers, if they are the wrong size or damaged, can contribute significantly to slip and excessive power requirements. But I would still begin by calculating actual slippage, and that will point you in the right direction.

It has been a while since I have done these numbers, but as I recall, I but about 20% more fuel (and therefore need about 20% more hp, than the factory numbers.

Good luck.

 

[bglad]

I looked at few KK42 surveys I have on file and 28 x 20 3-blade 2.91 gear ratio is what others had. Is your propeller clean? All it takes is a few barnacle feet to ruin its efficiency.

 

[PierreR]

I think for long distance travellers like the OP this is money well spent, after having gathered the appropriate data. The prop drives the boat. It should be close to perfect.

The OP has his numbers and power graphs for his engine and his boat is well known. According to my calculations he is already close of optimum. In fact to get those numbers he has to have a relatively clean bottom as well. He is not going to get drastic improvement by tweaking the propeller.
Power train calculations are not as simple as they seem. You dealing with Stokes equations and fluid dynamics. When it comes to propellers and efficiency, slip is not a linear relationship.
The slower the boat and larger the propeller the higher the slip but better efficiency.
If we take Geer's guestimates from his slip formula is, Slip=1.4/Knot^0.57. If we take 7 knots as the magic number we get Slip=1.4/7^0.57 = 1.4/3.03= 0.46 or 46% slip can be generally expected. That puts the engine speed right smack dab on top of 1,900 rpm where his is only producing and utilizing 31hp I do not calculate a big saving anywhere. Give me his efficiency on my boat.
Where are you guys getting the 20-30% slip figures from?

 

[DDW]

We have a well accepted and established process for establishing whether a trawler is correctly propped, which basically tests the intersection of power and load at full power.

Well, the term Correctly Propped can be defined as you do, but it is a bit of a misnomer. That means it is correctly propped at max rpm, and perhaps ONLY at max rpm. This is the compromise that a fixed pitch prop enforces. If this were also correct at every other speed, no market for a variable pitch prop would exist. In fact if they were cheap rather than expensive, I would expect they would be fitted to practically every boat (just as they are fitted to practically every airplane). Fixed pitch is a lot like a car with no gearbox. Cheaper but not the greatest solution.

I think it is a very valid question - certainly on SD trawlers - whether Correctly Propped is the best compromise. It is a solution optimized for a condition rarely if ever used.

 

[Jeff F]

I think it is a very valid question - certainly on SD trawlers - whether Correctly Propped is the best compromise. It is a solution optimized for a condition rarely if ever used.

For sure. That leads many (including me) to slightly over prop to optimize for operation in the lower power range. They're still over propped, but by design operate only in the lower power range so it's optimized for that lower power delivery and works.

To be clear, I'm not arguing in favour of over propping here. But I am arguing against under propping as a deliberate strategy in this case. Adding pitch to get to Correct Propping according to conventional wisdom described here has no downside for the OP if they are in fact currently under propped. Doing so should bring the drivetrain to OEM specs.

Again, I think we agree here. I just want to make sure our use of the terminology aligns. We have to live with fixed props, which means recognizing the limitations and constraints and making the best available compromise, and having the language to accurately describe the situation.

 

[rslifkin]

Adding pitch to get to Correct Propping according to conventional wisdom described here has no downside for the OP if they are in fact currently under propped. Doing so should bring the drivetrain to OEM specs.

Agreed

 

[Jeff F]

The OP has his numbers and power graphs for his engine and his boat is well known. According to my calculations he is already close of optimum. In fact to get those numbers he has to have a relatively clean bottom as well. He is not going to get drastic improvement by tweaking the propeller.

I agree 100%

The only real issue at hand (I think) is whether the fixed gear drive train is at OEM spec (right propped) or over propped or under propped. I think we can all agree that the OP is not over propped.

In practical terms the only impact of changes for the OP would be to cruise RPM. If bringing things to OEM spec lowered cruise RPM I'd take that as an unqualified benefit.