I was out for the first sail of the season and it was great. On the way back I had to motor for lack of wind. I was playing with my GPS and the speed disagreed with my knotmeter, reading.04 kts lower in no current. The bottom is clean and I was doing 2000rpm and I was not up to hull speed. What is the "right rpm" for the 36'CD with a Perkins 4-108? I've looked at various sources and have not come up with a good answer. I settled on 2000-2100rpm years ago because the engine sounded "right", not very scientific but today with not much to do,I started wondering what is correct.
Jerry Axler
cutter36@erols.com
Cruising Rpm for Perkins 4-108
Moderator: Jim Walsh
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Mike Thorpe
Re: Cruising Rpm for Perkins 4-108
Jerry,
There are a lot of variables to consider and I'm no expert but for what it is worth. The Perkins owners manual that came with Journey's End says 2500RPM for cruising and 2900RPM Max. I think this assumes a 3 blade 15x14 prop and no additional engine loads. A larger than stock alternator (60Amps) or engine driven refrigeration will change that.
A .04 difference between you knowmeter and GPS sounds well with in reason to me and could be due to current even when you think there isn't any.
A sugestion...try graphing changes in RPM and boat speed thru the water to find out at what point additinal RPM's don't provide an increase in boat speed. If you could also calculate fuel usage then you would know the best RPM for cruising and the best for fuel economey.
One final thought, diesel engines like to work. Running them at lower RPM's is not necessarily a good thing. Hope this helps.
Mike Thorpe
CD-36 Journy's End
Hyannis, MA
mthorpe@capecod.net
There are a lot of variables to consider and I'm no expert but for what it is worth. The Perkins owners manual that came with Journey's End says 2500RPM for cruising and 2900RPM Max. I think this assumes a 3 blade 15x14 prop and no additional engine loads. A larger than stock alternator (60Amps) or engine driven refrigeration will change that.
A .04 difference between you knowmeter and GPS sounds well with in reason to me and could be due to current even when you think there isn't any.
A sugestion...try graphing changes in RPM and boat speed thru the water to find out at what point additinal RPM's don't provide an increase in boat speed. If you could also calculate fuel usage then you would know the best RPM for cruising and the best for fuel economey.
One final thought, diesel engines like to work. Running them at lower RPM's is not necessarily a good thing. Hope this helps.
Mike Thorpe
CD-36 Journy's End
Hyannis, MA
Jerry Axler wrote: I was out for the first sail of the season and it was great. On the way back I had to motor for lack of wind. I was playing with my GPS and the speed disagreed with my knotmeter, reading.04 kts lower in no current. The bottom is clean and I was doing 2000rpm and I was not up to hull speed. What is the "right rpm" for the 36'CD with a Perkins 4-108? I've looked at various sources and have not come up with a good answer. I settled on 2000-2100rpm years ago because the engine sounded "right", not very scientific but today with not much to do,I started wondering what is correct.
Jerry Axler
mthorpe@capecod.net
-
matt cawthorne
Re: Cruising Rpm for Perkins 4-108
Jerry,
I do not think that there is a right RPM. The manual for my 4.108M states 4000 rpm as maximum, and 3000 as maximum cruising RPM. I crusied s/v Patricia Louise at 1500 rpm for years with no apparent ill effects. I used to run it slow because it was really smooth there. Two notes of caution about running the boat fast.
Firstly, it is unlikely that your tachometer is correct. Calibrations of my tach against a known standard yielded an error of over 300 rpm at 3K. When I changed the alternator the calibration changed as well.
Secondly, sometimes fuel consumptions is important. Some years ago I did a fuel consumption study, integrating data from speed trials that I had performed, data from Dave Gerr's 'Propeller Handbook', and a couple of reasonable assumptions. Fuel mileage is about 2 mpg if you push the boat along at hull speed. It peaks at about 12.5 mpg if you run the boat at 3.5 knots. Now, if you are going to Bermuda and run out of wind, or are motoring back to port and the tank is near empty the best thing to do is run the boat slow to extend your range. You can carbon the engine up if you idle it alot, but that does not mean that you have to push it at hull speed all of the time.
Matt
mcawthor@bellatlantic.net
I do not think that there is a right RPM. The manual for my 4.108M states 4000 rpm as maximum, and 3000 as maximum cruising RPM. I crusied s/v Patricia Louise at 1500 rpm for years with no apparent ill effects. I used to run it slow because it was really smooth there. Two notes of caution about running the boat fast.
Firstly, it is unlikely that your tachometer is correct. Calibrations of my tach against a known standard yielded an error of over 300 rpm at 3K. When I changed the alternator the calibration changed as well.
Secondly, sometimes fuel consumptions is important. Some years ago I did a fuel consumption study, integrating data from speed trials that I had performed, data from Dave Gerr's 'Propeller Handbook', and a couple of reasonable assumptions. Fuel mileage is about 2 mpg if you push the boat along at hull speed. It peaks at about 12.5 mpg if you run the boat at 3.5 knots. Now, if you are going to Bermuda and run out of wind, or are motoring back to port and the tank is near empty the best thing to do is run the boat slow to extend your range. You can carbon the engine up if you idle it alot, but that does not mean that you have to push it at hull speed all of the time.
Matt
mcawthor@bellatlantic.net
-
Jerry Axler
Re: Cruising Rpm for Perkins 4-108
Wow are we on a different planet boatwise. At 2100rpm I do about 7kts. give or take a bit and I consume approx 1/2-3/4 a gal per hour. That figures out to be around 10-14 nm per gallon. I've motored from Cape May to Vineyardhavenand used under 40 gallons, so I'm a little confused about the mileage you are getting. Although I do agree that the tach readings could be suspect, the mileage covered and the fuel used are actual measurements.matt cawthorne wrote: Jerry,
I do not think that there is a right RPM. The manual for my 4.108M states 4000 rpm as maximum, and 3000 as maximum cruising RPM. I crusied s/v Patricia Louise at 1500 rpm for years with no apparent ill effects. I used to run it slow because it was really smooth there. Two notes of caution about running the boat fast.
Firstly, it is unlikely that your tachometer is correct. Calibrations of my tach against a known standard yielded an error of over 300 rpm at 3K. When I changed the alternator the calibration changed as well.
Secondly, sometimes fuel consumptions is important. Some years ago I did a fuel consumption study, integrating data from speed trials that I had performed, data from Dave Gerr's 'Propeller Handbook', and a couple of reasonable assumptions. Fuel mileage is about 2 mpg if you push the boat along at hull speed. It peaks at about 12.5 mpg if you run the boat at 3.5 knots. Now, if you are going to Bermuda and run out of wind, or are motoring back to port and the tank is near empty the best thing to do is run the boat slow to extend your range. You can carbon the engine up if you idle it alot, but that does not mean that you have to push it at hull speed all of the time.
Matt
My ear tells me that the engine does not seem to be straining at 2100rpm, but it starts to sound a little strained above it. Not scientific, but I don't have the availability or knowledge to measure it more accurately.
I will be measuring fuel consumption at the higher rpms to see what difference a few hundred rpm will do to the results.
Jerry Axler
cutter36@erols.com
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Jerry Axler
Re: Cruising Rpm for Perkins 4-108
Thanks for the response. I did an average to get the speed by going 1/2 mile with the current and 1/2 mile in the opposite direction, so the current can be factored out. As you suggested,I will try the different rpms to see how they affect mileage and speed.Mike Thorpe wrote: Jerry,
There are a lot of variables to consider and I'm no expert but for what it is worth. The Perkins owners manual that came with Journey's End says 2500RPM for cruising and 2900RPM Max. I think this assumes a 3 blade 15x14 prop and no additional engine loads. A larger than stock alternator (60Amps) or engine driven refrigeration will change that.
A .04 difference between you knowmeter and GPS sounds well with in reason to me and could be due to current even when you think there isn't any.
A sugestion...try graphing changes in RPM and boat speed thru the water to find out at what point additinal RPM's don't provide an increase in boat speed. If you could also calculate fuel usage then you would know the best RPM for cruising and the best for fuel economey.
One final thought, diesel engines like to work. Running them at lower RPM's is not necessarily a good thing. Hope this helps.
Mike Thorpe
CD-36 Journy's End
Hyannis, MA
Jerry Axler wrote: I was out for the first sail of the season and it was great. On the way back I had to motor for lack of wind. I was playing with my GPS and the speed disagreed with my knotmeter, reading.04 kts lower in no current. The bottom is clean and I was doing 2000rpm and I was not up to hull speed. What is the "right rpm" for the 36'CD with a Perkins 4-108? I've looked at various sources and have not come up with a good answer. I settled on 2000-2100rpm years ago because the engine sounded "right", not very scientific but today with not much to do,I started wondering what is correct.
Jerry Axler
Thanks for the suggestion.
Jerry Axler
cutter36@erols.com
-
matt cawthorne
Sailing in thinner water
Jerry,
It does seem like we are driving different boats. I feel very confident in my numbers. I should be clear about the 2 mpg number, though. The conditions that gave me that were 7.6 knots. I took some time to review my calculations and at 7.0 knots the mileage would be about 5.5 nautical miles per gallon. At 7.6 knots the curve of speed versus rpm had begun to flatten out. Is there any chance that you were motor sailing or had a current with you on the way to Vinyardhaven? Any chance that you were motoring slower than 7 knots on that trip to keep the noise down? My motor runs clean and my prop coating keeps the prop clean. I checked some measurements of fuel usage the first 3 seasons as owner. The fuel usage was about .36 gph at 4.5 knots. This included some 36 hour runs. This checks with the calculations mentioned above. The power required to drive a boat in displacement mode is related to boat speed to the 2.7 power. From that I would expect your fuel consumption to be about 1.2 gph. Your engine will max out at over 2 gph (I can not find the literature right now but I think it is 2.6 gph) at max power at 4000 rpm. You can find the rpm where you are engine limited (that would be a few hundred rpm less than where you can see the black smoke). Find the fuel consumption there by ratioing linearly with rpm (from 2.6 gph at 4000 rpm). You can then follow the 2.7 power curve to 2100 rpm to get a relatively good estimate of fuel consumption. This is in flat water with everything clean. It all goes down from there.
Beyond that I can not explain the difference.
Matt
mcawthor@bellatlantic.net
It does seem like we are driving different boats. I feel very confident in my numbers. I should be clear about the 2 mpg number, though. The conditions that gave me that were 7.6 knots. I took some time to review my calculations and at 7.0 knots the mileage would be about 5.5 nautical miles per gallon. At 7.6 knots the curve of speed versus rpm had begun to flatten out. Is there any chance that you were motor sailing or had a current with you on the way to Vinyardhaven? Any chance that you were motoring slower than 7 knots on that trip to keep the noise down? My motor runs clean and my prop coating keeps the prop clean. I checked some measurements of fuel usage the first 3 seasons as owner. The fuel usage was about .36 gph at 4.5 knots. This included some 36 hour runs. This checks with the calculations mentioned above. The power required to drive a boat in displacement mode is related to boat speed to the 2.7 power. From that I would expect your fuel consumption to be about 1.2 gph. Your engine will max out at over 2 gph (I can not find the literature right now but I think it is 2.6 gph) at max power at 4000 rpm. You can find the rpm where you are engine limited (that would be a few hundred rpm less than where you can see the black smoke). Find the fuel consumption there by ratioing linearly with rpm (from 2.6 gph at 4000 rpm). You can then follow the 2.7 power curve to 2100 rpm to get a relatively good estimate of fuel consumption. This is in flat water with everything clean. It all goes down from there.
Beyond that I can not explain the difference.
Matt
mcawthor@bellatlantic.net