Universal Atomic Diesel - How Fast can she go?

[zeida]

I have a 1981 24 hp Universal Atomic Diesel Model # 5425 on my CD-33 sailboat. I have been told that the ideal cruising speed while motoring is around 22 to 25 rpm's. The engine manual indicates 2800 as its maximum rpm. I have also been told that if you push the throttle all the way up for a few seconds and look at your tachometer reading while at full speed, (mine read 40 rpm's) you can then run your engine at 80% of that full speed, and that as a rule, all diesel engines will tolerate that speed. in my case, 80% of 40 would be 32, and I tried pushing my engine to the 32 rpm's, and it seemed to go with no problem. While motoring up the intercoastal, and trying to make a run for the bridges, sometimes I needed the extra speed, and I even pushed the boat up to 35 rpm's... although I did not sustain that speed for long. However, the engine seemed not to be affected by running at the 32 rpm's instead of the usual 25 rpm's. I wonder if anybody else has knowledge of this or similar experiences? Any comments appreciated.
Zeida



zcecil@attglobal.net

[Al Levesque]

I have a 1981 24 hp Universal Atomic Diesel Model # 5425 on my CD-33 sailboat. I have been told that the ideal cruising speed while motoring is around 22 to 25 rpm's.

We have the same engine on our CD33. Our prop is 3 blades at 14X9. We found that the maximum speed was about 2250 rpm. Taking 80%, we thought that 1800 was to be our cruising speed. Since then, we have found that if we slow the engine to about 1750, the engine runs slightly cooler and we get about the same boat speed. We use about 0.6 gallons per hour and have no visible smoke, although we get some smudge on the transom.

I have thought we should be running the engine faster (like maybe 80% of 2800) but I suppose we would have to reduce the prop to do that. The markings on the prop suggest that it was already reduced once from 14X10.

I am anxious to see the other responses you get.

Al



albertlevesque@cove.com

[Rick Robinson]

I have the same engine, but only a two blade prop. I've run about 800 miles from St Paul MN to Kentucky Lake most of it at about 2200 - 2400 RPM. The manual does say 2800 for max RPM and I have pushed it past that to make a couple of locks, but only for short periods of time. When I did push to a higher RPM, the engine temp started to climb. I'm not a diesel gear-head, but I would think the main concern isn't necessarily the actual RPM but the hotter engine temp that results from the higher RPM.

When the RPMs are kept below 2500 the engine temp stays around 170 - 175 f. If I push it above 2500 the temp climbs quickly. When I push the engine I keep a close eye on the temp and have given up making a lock or two to save the engine. In the long run I'd rather have my engine last a few hundred hours longer then save a couple of hours on one leg of a trip.




Rick_R@NTR.NET

[Matt Cawthorne]

Zieda,
I do not have your engine, but I am a structural analyst who works on rotating machinery for a living. In fact fatigue analysis consumes much of my working life. If your manual indicates 2800 rpm as your maximum you should follow it's recommendations. Most times when you exceed an engines maximum recommended RPM you will not notice anything immediately. In fact some engines may not incur any fatigue damage with moderate exceedances of the maximum RPM. Some engines (depending on minute manufacturing and material differences) will incur some fatigue damage. When the engine has been run long enough above it's allowable RPM a small fatigue crack will start. From here on out the fatigue crack will continue to grow. It will grow slowly if you run your engine slowly and it will grow quickly if you continue to exceed the recommended RPM. At this point you still will not be able to notice a problem, but the fateful day approaches. When the crack hits critical length the affected part will suddenly snap in two. My guess is that it will happen just when you need it the most. The process can take years or happen in hours depending on many variables.

My recommendation:

Find a photo tachometer from somewhere and borrow it. Check your tachometer to see if it is accurate. If it is not then make a little table of true RPM versus what your tachometer indicates.

If you are, in fact, exceeding the engine manufacturers recommendations then get your prop re-pitched. If the pitch change required is more than about 2 inches you will need to buy a new prop. With the throttle wide open you should not be able to exceed the recommended RPM. I suspect that your engine will drive the boat at hull speed without exceeding the recommended rpm provided the correct prop is used. Some people use a little less pitch than is ideal allowing for the extra torque required when the prop is covered with barnacles. Be careful not to over pitch the blades or you can get into a smoking engine problem and not be able to achieve the engines capability. It can be a bit of a trial and error process.

This bulletin board is a great place to survey other owners and determine what pitch (for a given diameter, number of blades and transmission ratio) allows your engine to just achieve maximum rpm with the throttle wide open.

Good luck,
Matt

I have a 1981 24 hp Universal Atomic Diesel Model # 5425 on my CD-33 sailboat. I have been told that the ideal cruising speed while motoring is around 22 to 25 rpm's. The engine manual indicates 2800 as its maximum rpm. I have also been told that if you push the throttle all the way up for a few seconds and look at your tachometer reading while at full speed, (mine read 40 rpm's) you can then run your engine at 80% of that full speed, and that as a rule, all diesel engines will tolerate that speed. in my case, 80% of 40 would be 32, and I tried pushing my engine to the 32 rpm's, and it seemed to go with no problem. While motoring up the intercoastal, and trying to make a run for the bridges, sometimes I needed the extra speed, and I even pushed the boat up to 35 rpm's... although I did not sustain that speed for long. However, the engine seemed not to be affected by running at the 32 rpm's instead of the usual 25 rpm's. I wonder if anybody else has knowledge of this or similar experiences? Any comments appreciated.
Zeida

mcawthor@bellatlantic.net

[M.R. Bober]

Whoa hoss!! I doubt that you are spinning that Universal at 4000rpm. Have you replaced the alternator? Your tachometer is most likely driven by the alternator and replacement units can deceive.
I am curious as to the speed of the boat.
Mitchell Bober
RESPITE
CD330



thebobers@erol.com

[Ryan Turner]

</LI>
I've got the same engine and boat and recently went through an extensive process of determining which prop to use for optimal engine performance, etc. The result -- I've now got a 3-blade 13-11 prop. Based on the performance I'm getting 3-blade 13-10 or 14-8 would be better. I expect that your tachometer is not reading correctly because the rpm's that you report are way too high for this engine. The critical thing is engine temperature -- it should never exceed 195 degrees F. I operate mine between 175 and 190 depending on whether I want to go fast or slow.

Ryan Turner
s/v Zenobia
CD33 hull no. 100
Oriental, NC



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