Prop Pitch on CD36

[Bill Cochrane]

My recently-acquired 1984 CD 36 Cutter, Perkins 4.108, is fitted with a 15", 3-blade max-prop that (according to log) is set at 13.8" pitch. The engine reaches a top speed (no-load) of 4000 rpm; in gear, under load, it tops out at 2300-2400 rpm (the boat will reach hull speed at this setting). Is the prop perhaps pitched too high?

The engine also begins to overheat when pushed much over 5.5 knots (about 1700-1800 rpm). I'm aware that there are a number of things that could cause the overheating...could the prop pitch be contributing to overload? Anyone that could comment on proper pitch with a 3-blade prop?

Thanks, Bill



cochrane@clark.net

[Lee H. Hodsdon]

My recently-acquired 1984 CD 36 Cutter, Perkins 4.108, is fitted with a 15", 3-blade max-prop that (according to log) is set at 13.8" pitch. The engine reaches a top speed (no-load) of 4000 rpm; in gear, under load, it tops out at 2300-2400 rpm (the boat will reach hull speed at this setting). Is the prop perhaps pitched too high?

The engine also begins to overheat when pushed much over 5.5 knots (about 1700-1800 rpm). I'm aware that there are a number of things that could cause the overheating...could the prop pitch be contributing to overload? Anyone that could comment on proper pitch with a 3-blade prop?

Thanks, Bill

Bill,

I have some experience to share, not quite an answer but not really comparing apples to oranges either. In my CD25D I have a Westerbeke 10-2, driving a 3 blade prop. I had some of the same concerns and conditions which you mention. Because the boat was in the water, and I don't know anyting about propellor sizing I looked at what I did understand. First the heat exchanger, I cleaned the heat exchanger tubes to increase cooling water flow. When the boat was purchased a doughnut type zinc was attached to the shaft. With a great deal of help from others on this site I located a source for the Perry Nut (original equipment from CD). Finally I rebuilt the raw water pump, replacing the bearings, impellor and most important the seal.

Now, like I said this is not an answer to your question, but I've eliminated my overheating problem--I can now run the engine at 3000 rpm as necessary when bucking a 4+ kt current, packing box leakage is now down to the occasional drip and the top speed has increased from an indicated (by the "little liar")5.5 kts max to greater than 6.5 kts. I know about 1.33 X the square root of WL length to determine max speed of a displacement hull, thus the "little liar" disclamer.

If you have any of the above conditions some improvement might be possible without changing the prop. Just food for thought.

Regards,

Lee



lhodsdon@nh.ultranet

[Matt Cawthorne]

Bill,
I have been contemplating the pitch of the prop on the 'Patricia Louise' since I bought her. She is a CD-36 built in 1982. I have the same engine, and a Hurth 150-2R with some number stamped on the top like a=1.88, b=1.95. I suspect that the numbers indicate the gear ratios but don't really know. The prop which the boat came with is a 15RH14 fixed three bladed prop. I calibrated the tachometer and then set out on a smooth water speed trial. The boat could reach about 7.0 knots according to my knot meter at 2500 RPM. At 2700 rpm the black smoke was pouring out of the exhaust with no increase in speed.
Since I tend to run the boat typically at 5 knots this would appear to be O.K., but I always worry about having to claw off of a lee shore under power some day. Headed into large waves will dramatically slow the boat, and probably overload the engine at a much slower speed. According to the "Propeller Handbook" by Dave Gerr an engine should be matched to the transmission and prop at just under it's maximum rated rpm. With the perkins turning less than 2500 rpm it would deliver significantly less power and thrust than it is capable of which could mean the difference in a tough situation.
What I did this winter was to send my prop off to a prop shop and have the pitch reduced by 2 inches to a 15RH12. A speed trial this spring yielded 7.2 knots at 3100 rpm (the rpm is from memory it may have been 3200). I aligned the engine (it was not far out) and it seems to run smoother. The prop shop indicates that this prop should not be re-pitched any lower. Will I buy a 15RH10? Maybee.
HERE IS THE DOWNSIDE. A diesel engine should not be run at too low of a load. If you tend to run the engine under loaded as I do then it can carbon up. I have found that the CD-36 burns about .36 galons per hour on average if operated at 4.5 to 5.0 knots. This low fuel consumption is great when offshore and in light air. When operating this way you would like to have higher combustion temperatures which would come with a higher pitch prop. So far I have not had a carbon problem, but who knows? The Ideal situation is to have a variable pitch prop and measure the temperature of the exhaust and adjust the pitch based on exhaust temperature. This is all too much complexity and expense for me. I have and will keep my fixed pitch prop even with it's extra drag.

Do I have a recommendation?
1. Work on your cooling system. It should not be overheating at this rpm. The standard prop for the boat when new was a 15rh14.
2. If you think you might ever get caught on a lee shore, you might want to consider reducing pitch.
3. If you do reduce pitch then run your boat at a little higher speed than you are used to when you know more diesel fuel will be available. This will work the engine harder and help keep the carbon down. When out to sea and fuel is not available then run slow to conserve fuel. Mileage drops significantly when the boat is pushed above 5 knots.

Let me know what eventually solves your overheating problem.

Good luck
Matt

My recently-acquired 1984 CD 36 Cutter, Perkins 4.108, is fitted with a 15", 3-blade max-prop that (according to log) is set at 13.8" pitch. The engine reaches a top speed (no-load) of 4000 rpm; in gear, under load, it tops out at 2300-2400 rpm (the boat will reach hull speed at this setting). Is the prop perhaps pitched too high?

The engine also begins to overheat when pushed much over 5.5 knots (about 1700-1800 rpm). I'm aware that there are a number of things that could cause the overheating...could the prop pitch be contributing to overload? Anyone that could comment on proper pitch with a 3-blade prop?

Thanks, Bill

mcawthor@bellatlantic.net

[Bill Cochrane]

Matt...

Thanks for sharing your experience. I asked the same question on Boatdiesel, (www.boatdiesel.com, and follow links to Forum) and Peter Compton ran a pitch calculation for me that suggested 15x11 for a 3-blade fixed prop and 15x12 for the MaxProp, assuming that the maxprop was slightly less efficient than a fixed one . Those recommendations seem to track very well with your experience. I gather, though, that if you ran the boat (with the 15x14 pitch) at, for example, 6.5 knots, it would run at that speed for some time and not overheat?

Thanks again, Bill

Bill,
I have been contemplating the pitch of the prop on the 'Patricia Louise' since I bought her. She is a CD-36 built in 1982. I have the same engine, and a Hurth 150-2R with some number stamped on the top like a=1.88, b=1.95. I suspect that the numbers indicate the gear ratios but don't really know. The prop which the boat came with is a 15RH14 fixed three bladed prop. I calibrated the tachometer and then set out on a smooth water speed trial. The boat could reach about 7.0 knots according to my knot meter at 2500 RPM. At 2700 rpm the black smoke was pouring out of the exhaust with no increase in speed.
Since I tend to run the boat typically at 5 knots this would appear to be O.K., but I always worry about having to claw off of a lee shore under power some day. Headed into large waves will dramatically slow the boat, and probably overload the engine at a much slower speed. According to the "Propeller Handbook" by Dave Gerr an engine should be matched to the transmission and prop at just under it's maximum rated rpm. With the perkins turning less than 2500 rpm it would deliver significantly less power and thrust than it is capable of which could mean the difference in a tough situation.
What I did this winter was to send my prop off to a prop shop and have the pitch reduced by 2 inches to a 15RH12. A speed trial this spring yielded 7.2 knots at 3100 rpm (the rpm is from memory it may have been 3200). I aligned the engine (it was not far out) and it seems to run smoother. The prop shop indicates that this prop should not be re-pitched any lower. Will I buy a 15RH10? Maybee.
HERE IS THE DOWNSIDE. A diesel engine should not be run at too low of a load. If you tend to run the engine under loaded as I do then it can carbon up. I have found that the CD-36 burns about .36 galons per hour on average if operated at 4.5 to 5.0 knots. This low fuel consumption is great when offshore and in light air. When operating this way you would like to have higher combustion temperatures which would come with a higher pitch prop. So far I have not had a carbon problem, but who knows? The Ideal situation is to have a variable pitch prop and measure the temperature of the exhaust and adjust the pitch based on exhaust temperature. This is all too much complexity and expense for me. I have and will keep my fixed pitch prop even with it's extra drag.

Do I have a recommendation?
1. Work on your cooling system. It should not be overheating at this rpm. The standard prop for the boat when new was a 15rh14.
2. If you think you might ever get caught on a lee shore, you might want to consider reducing pitch.
3. If you do reduce pitch then run your boat at a little higher speed than you are used to when you know more diesel fuel will be available. This will work the engine harder and help keep the carbon down. When out to sea and fuel is not available then run slow to conserve fuel. Mileage drops significantly when the boat is pushed above 5 knots.

Let me know what eventually solves your overheating problem.

Good luck
Matt

My recently-acquired 1984 CD 36 Cutter, Perkins 4.108, is fitted with a 15", 3-blade max-prop that (according to log) is set at 13.8" pitch. The engine reaches a top speed (no-load) of 4000 rpm; in gear, under load, it tops out at 2300-2400 rpm (the boat will reach hull speed at this setting). Is the prop perhaps pitched too high?

The engine also begins to overheat when pushed much over 5.5 knots (about 1700-1800 rpm). I'm aware that there are a number of things that could cause the overheating...could the prop pitch be contributing to overload? Anyone that could comment on proper pitch with a 3-blade prop?

Thanks, Bill

cochrane@clark.net

[Duane Yoslov]

My recently-acquired 1984 CD 36 Cutter, Perkins 4.108, is fitted with a 15", 3-blade max-prop that (according to log) is set at 13.8" pitch. The engine reaches a top speed (no-load) of 4000 rpm; in gear, under load, it tops out at 2300-2400 rpm (the boat will reach hull speed at this setting). Is the prop perhaps pitched too high?

The engine also begins to overheat when pushed much over 5.5 knots (about 1700-1800 rpm). I'm aware that there are a number of things that could cause the overheating...could the prop pitch be contributing to overload? Anyone that could comment on proper pitch with a 3-blade prop?

Thanks, Bill

Bill, we own a CD330 and are contemplating a Maxprop. Can you elaborate on how they managed to fit one on your boat?

[Bill Cochrane]

Duane...

I just acquired the boat; the MaxProp was installed by (or for) the former owner. There was some correspondence onboard indicating that perhaps the installation was not straightforward, but unfortunately there were very few details; the manufacturer indicated that the prop had previously been fitted to CD36's without problem but that changes in production boats did occur; they offered to take back the prop if (unspecified) modifications were unacceptable. You might call the MaxProp folks and see what they know about fitting their prop to Cape Dory boats.

Bill

Bill, we own a CD330 and are contemplating a Maxprop. Can you elaborate on how they managed to fit one on your boat?

cochrane@clark.net

[Matt Cawthorne]

Bill
I never ran the boat for an extended period with the 15RH14 at speeds above 6.0 knots. The temperature gage never read above 160 F
for those speeds.

Matt

Matt...

Thanks for sharing your experience. I asked the same question on Boatdiesel, (www.boatdiesel.com, and follow links to Forum) and Peter Compton ran a pitch calculation for me that suggested 15x11 for a 3-blade fixed prop and 15x12 for the MaxProp, assuming that the maxprop was slightly less efficient than a fixed one . Those recommendations seem to track very well with your experience. I gather, though, that if you ran the boat (with the 15x14 pitch) at, for example, 6.5 knots, it would run at that speed for some time and not overheat?

Thanks again, Bill

Bill,
I have been contemplating the pitch of the prop on the 'Patricia Louise' since I bought her. She is a CD-36 built in 1982. I have the same engine, and a Hurth 150-2R with some number stamped on the top like a=1.88, b=1.95. I suspect that the numbers indicate the gear ratios but don't really know. The prop which the boat came with is a 15RH14 fixed three bladed prop. I calibrated the tachometer and then set out on a smooth water speed trial. The boat could reach about 7.0 knots according to my knot meter at 2500 RPM. At 2700 rpm the black smoke was pouring out of the exhaust with no increase in speed.
Since I tend to run the boat typically at 5 knots this would appear to be O.K., but I always worry about having to claw off of a lee shore under power some day. Headed into large waves will dramatically slow the boat, and probably overload the engine at a much slower speed. According to the "Propeller Handbook" by Dave Gerr an engine should be matched to the transmission and prop at just under it's maximum rated rpm. With the perkins turning less than 2500 rpm it would deliver significantly less power and thrust than it is capable of which could mean the difference in a tough situation.
What I did this winter was to send my prop off to a prop shop and have the pitch reduced by 2 inches to a 15RH12. A speed trial this spring yielded 7.2 knots at 3100 rpm (the rpm is from memory it may have been 3200). I aligned the engine (it was not far out) and it seems to run smoother. The prop shop indicates that this prop should not be re-pitched any lower. Will I buy a 15RH10? Maybee.
HERE IS THE DOWNSIDE. A diesel engine should not be run at too low of a load. If you tend to run the engine under loaded as I do then it can carbon up. I have found that the CD-36 burns about .36 galons per hour on average if operated at 4.5 to 5.0 knots. This low fuel consumption is great when offshore and in light air. When operating this way you would like to have higher combustion temperatures which would come with a higher pitch prop. So far I have not had a carbon problem, but who knows? The Ideal situation is to have a variable pitch prop and measure the temperature of the exhaust and adjust the pitch based on exhaust temperature. This is all too much complexity and expense for me. I have and will keep my fixed pitch prop even with it's extra drag.

Do I have a recommendation?
1. Work on your cooling system. It should not be overheating at this rpm. The standard prop for the boat when new was a 15rh14.
2. If you think you might ever get caught on a lee shore, you might want to consider reducing pitch.
3. If you do reduce pitch then run your boat at a little higher speed than you are used to when you know more diesel fuel will be available. This will work the engine harder and help keep the carbon down. When out to sea and fuel is not available then run slow to conserve fuel. Mileage drops significantly when the boat is pushed above 5 knots.

Let me know what eventually solves your overheating problem.

Good luck
Matt

My recently-acquired 1984 CD 36 Cutter, Perkins 4.108, is fitted with a 15", 3-blade max-prop that (according to log) is set at 13.8" pitch. The engine reaches a top speed (no-load) of 4000 rpm; in gear, under load, it tops out at 2300-2400 rpm (the boat will reach hull speed at this setting). Is the prop perhaps pitched too high?

The engine also begins to overheat when pushed much over 5.5 knots (about 1700-1800 rpm). I'm aware that there are a number of things that could cause the overheating...could the prop pitch be contributing to overload? Anyone that could comment on proper pitch with a 3-blade prop?

Thanks, Bill

mcawthor@bellatlantic.net