RPMS - No replies?

[Mike Thompson]

No one replied... Maybe my question was TOO specific.
How fast do you run your engine? Is 3000 rpm TOO FAST?

Mike
CD28 HAVEN

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Full message was:

I have a CD28 with a Universal M3-20 diesel.
The manual says cruising RPM are 2600 to 3300.
I have a 2 blade 13x8 propellor. Now for some
observations...
(Don't take the absolute value of the knots too
seriously, there was a current against me)
RPM KNOTS SOUND
2000 3.5 Like a sewing machine
2500 4.0 Like a tractor (diesel clack-clack)
2800 4.3 Slight booming sound in deck.
3000 4.5 Very noisy and vibration felt in cockpit seats.

What to do?
a) I would like to motor at 2000 especially if I am
also using the sails. Will this harm the engine?
b) I usually motor at 2500 because the noise isn't too
bad but this is below the recommended range. Is this bad?
c) 3300 is quite unpleasant but faster. Is it safe?

Mike
CD28 "Haven"



tmike@ma.ultranet.com

[Ken Cave]

It sounds like someone re-powered your Cape Dory 28 with a "new" engine!

Most that I have seen are still running with a Volvo MD7a or b model-and they run a lot slower.

Top speed is around 2600 rpm on these babies-and I have installed a three bladed prop and run mine at 1800 and cruise around 5.2-5.7 knots.

Sorry I can't be of any further help!

Ken Cave



bcavd@whidbey.net

[Tom]

Mike, I think we're confused by your engine description. I'm unfamiliar with an M3-20 Universal engine. Volvo engines have the prefix "M" and these are higher rpm engines where 3000 RPM might be right. I wonder if you have a 3 cylinder Universal engine? These were usually called model 25 and had 21 horsepower. The top of the power curve on these engines is 1800 RPM. One of the advantages of the Universal engines is that they are low rpm and thus run quieter and with less vibration than Yanmars and Perkins and higher speed engines. 3000 would be way too fast. With the model 25 most of us seem to run it at around 2000-2200 rpm give or take with a top end at 2600 for cruising although it's not very efficient at that rpm. Universal did make a 2 cylinder engine, but I'm not sure what the number is. Since you said you had the manual it sounded like you had some other engine than we're familiar with. My manuals are on the boat a long way away so I couldn't check to see if Universal made a model they called M3-20. I thought you might have a Volvo manual and a Universal engine so I was confused and thought it best to stay out of it and let someone with an M3-20 engine respond. Since you haven't received any responses I'm guessing that there is some confusion over engine designation. Universal might have made a higher rev engine in which case these comments are irrelevent, but that may be why you didn't get any comeback on your inquiry. Hope this helps.

No one replied... Maybe my question was TOO specific.
How fast do you run your engine? Is 3000 rpm TOO FAST?

Mike
CD28 HAVEN

----------------------------------------------
Full message was:

I have a CD28 with a Universal M3-20 diesel.
The manual says cruising RPM are 2600 to 3300.
I have a 2 blade 13x8 propellor. Now for some
observations...
(Don't take the absolute value of the knots too
seriously, there was a current against me)
RPM KNOTS SOUND
2000 3.5 Like a sewing machine
2500 4.0 Like a tractor (diesel clack-clack)
2800 4.3 Slight booming sound in deck.
3000 4.5 Very noisy and vibration felt in cockpit seats.

What to do?
a) I would like to motor at 2000 especially if I am
also using the sails. Will this harm the engine?
b) I usually motor at 2500 because the noise isn't too
bad but this is below the recommended range. Is this bad?
c) 3300 is quite unpleasant but faster. Is it safe?

Mike
CD28 "Haven"

TomCambria@mindspring.com

[Mark Yashinsky]

Still dont know what your engine is, but how many cylinders does it have (1, 2, 3)??? An even number while run a little smoother than an odd number of cylinders, but you will have vibrations that get other parts of the boat buzzing some. How much buzzing is too much and can cause damage is hard to describe here. One item, a small marine diesel will never feel (or not feel) like a modern automotive V8 in a highly insulated car. Other guidelines, are not to continuously run the engine at or near redline. The rest is of your questions is how it feels and what speed your get for the fuel consumption. At some point you will get diminishing returns (fuel usage will greatly increase) for your increase in speed (probably not much of an increase). Have to try it. FYI, friend's CD27 that has a Yanmar GM20 and a 2 bladed prop, generally does not like cruising (not motorsailing) above 3000. The lazerette's start buzzing above that and the engine noise does too and really dont get an increase in speed (about at hull speed anyway). Motorsailing generally has the engine helping to push the boat, not causing drag by spinning in the flow of the water. Should need that much wind to push the boat, exclusively. If you need to motorsail, maybe there is not enough wind to warrent having the sails up and possibly causing drag. The boat is not that big and heavy!!!

[Bill Goldsmith]

Hmmmmmmmmmmm.

There are many variables that could affect this situation. For instance if you had a 1.5 knot current against you and you got your knot readings from a gps or loran (speed over ground), then your performance isn't too bad at all. If those are accurate knotmeter impeller readings (speed over water) then you could probably get better performance. If so have an expert at a prop shop check your prop specs, you might have the wrong prop. There are tables that assist prop shops in finding the ideal prop based on displacement, horsepower, LWL, etc. If it's the right prop, at next haulout have it checked. They can get out of tune, and can be tuned up.

Bill Goldsmith
CD27
Second Chance

No one replied... Maybe my question was TOO specific.
How fast do you run your engine? Is 3000 rpm TOO FAST?

Mike
CD28 HAVEN

----------------------------------------------
Full message was:

I have a CD28 with a Universal M3-20 diesel.
The manual says cruising RPM are 2600 to 3300.
I have a 2 blade 13x8 propellor. Now for some
observations...
(Don't take the absolute value of the knots too
seriously, there was a current against me)
RPM KNOTS SOUND
2000 3.5 Like a sewing machine
2500 4.0 Like a tractor (diesel clack-clack)
2800 4.3 Slight booming sound in deck.
3000 4.5 Very noisy and vibration felt in cockpit seats.

What to do?
a) I would like to motor at 2000 especially if I am
also using the sails. Will this harm the engine?
b) I usually motor at 2500 because the noise isn't too
bad but this is below the recommended range. Is this bad?
c) 3300 is quite unpleasant but faster. Is it safe?

Mike
CD28 "Haven"

goldy@bestweb.net

[Mike Thompson]

Thanks for your replies.
- I usually motorsail to get the boat to point higher. When there
is no wind I motor with the sails down.
- The M3-20 is a 3 cylinder engine with 18 HP.
- In the data below, the adverse current was no more than a 1/2 knot.
- My hull speed is 6.3 knots but at tolerable noise levels
the engine does not drive the boat anywhere near that speed.
Well water is slippery I suppose,
Mike
CD28 HAVEN

No one replied... Maybe my question was TOO specific.
How fast do you run your engine? Is 3000 rpm TOO FAST?

Mike
CD28 HAVEN

----------------------------------------------
Full message was:

I have a CD28 with a Universal M3-20 diesel.
The manual says cruising RPM are 2600 to 3300.
I have a 2 blade 13x8 propellor. Now for some
observations...
(Don't take the absolute value of the knots too
seriously, there was a current against me)
RPM KNOTS SOUND
2000 3.5 Like a sewing machine
2500 4.0 Like a tractor (diesel clack-clack)
2800 4.3 Slight booming sound in deck.
3000 4.5 Very noisy and vibration felt in cockpit seats.

What to do?
a) I would like to motor at 2000 especially if I am
also using the sails. Will this harm the engine?
b) I usually motor at 2500 because the noise isn't too
bad but this is below the recommended range. Is this bad?
c) 3300 is quite unpleasant but faster. Is it safe?

Mike
CD28 "Haven"

tmike@ma.ultranet.com

[Ken Coit]

Mike,

I have zero knowledge of your specific engine and only one guideline I read recently but don't seem to be able to find. Maybe it was on this board. The guideline was this: normal cruising RPM should be about 70% of the maximium RPM. Thus, an engine capable of 3000 RPM continuous ought to be run about 2100 RPM while cruising. I'm sorry, but I can't come up with the reference.

Nigel Calder has at least one chapter in his "Marine Diesel Engines" on engine and prop selection that might be of help. Take a look.

Good luck,

Ken Coit
S/V Parfait



parfait@nc.rr.com

[Scott]

At the risk of mentioning the obvious, make sure your prop is not fouled (e.g. marine growth). My last big suspected engine problem turned out to be a prop covered by barnacles.

No one replied... Maybe my question was TOO specific.
How fast do you run your engine? Is 3000 rpm TOO FAST?

Mike
CD28 HAVEN

----------------------------------------------
Full message was:

I have a CD28 with a Universal M3-20 diesel.
The manual says cruising RPM are 2600 to 3300.
I have a 2 blade 13x8 propellor. Now for some
observations...
(Don't take the absolute value of the knots too
seriously, there was a current against me)
RPM KNOTS SOUND
2000 3.5 Like a sewing machine
2500 4.0 Like a tractor (diesel clack-clack)
2800 4.3 Slight booming sound in deck.
3000 4.5 Very noisy and vibration felt in cockpit seats.

What to do?
a) I would like to motor at 2000 especially if I am
also using the sails. Will this harm the engine?
b) I usually motor at 2500 because the noise isn't too
bad but this is below the recommended range. Is this bad?
c) 3300 is quite unpleasant but faster. Is it safe?

Mike
CD28 "Haven"

[Mark Yashinsky]

Here is another question. At what point w/ the throttle, do you reach maximum rpm and what is it??? If you reach max rpm at only 1/2 throttle, then you need more prop, your not pushing enough water and will never get up to speed. If you have the throttle at 100% and are, say, 500 rpm below max (redline), then you are over proped (unlikely). Did you buy the boat w/ the 3 cylinder engine in, or did you do the repower??? It's very possible that she had a 1 cylinder in from when she was built, and whoever, found that the boat did not have enough power for the conditions and put in a larger engine. But the question is, was the prop changed to use the extra ponies now available.

[Mike Thompson]

Interesting, I'll look into this one when I next visit the boat.
Both prop and engine are fairly new.
Mike
CD28 HAVEN

Here is another question. At what point w/ the throttle, do you reach maximum rpm and what is it??? If you reach max rpm at only 1/2 throttle, then you need more prop, your not pushing enough water and will never get up to speed. If you have the throttle at 100% and are, say, 500 rpm below max (redline), then you are over proped (unlikely). Did you buy the boat w/ the 3 cylinder engine in, or did you do the repower??? It's very possible that she had a 1 cylinder in from when she was built, and whoever, found that the boat did not have enough power for the conditions and put in a larger engine. But the question is, was the prop changed to use the extra ponies now available.

tmike@ultranet.com