Engine RPM vs hull speed

[kss1220]

Being new to a Catalina 34 w/ a Universal 25 I am unsure as to what the speed of the boat should be at the usual 2000-2200 rpms I cruise with while under power.  As it is right now I am achieving about 5.8 knots.  To me this seems a bit on the slow side.  Should'nt the engine be capable of achieving hull speed without maxing the RPM out which is not desirable for any extended time frame thus causing serious damage to the engine.  Is there any advantage to three blade props vs two?

[c34member]

I am not a diesel engineer, but to my knowledge there is no harm to be done in pushing a marine diesel to the firewall.  Parts may wear quicker, but not for as long.  Diesels run better hotter and under load (my VW Jetta TDI cooks along at 190 degrees for hours on end on the highway).  I have the M35-BC and max out at about 3,100 rpm.  I usually motor at around 2,400 to 2,900 rpm; 5.0 to 6.8 knots.

I am surprised the U-25 moves you as well as it does at 2,200 rpm.  I must have too much weight aboard.

A three bladed prop will improve your motoring and cripple your sailing.  Life is a compromise.

[dave davis]

This is always a touchy subject. Very few of us have a calibrated knot meter or Tach. All we can do is tell you what we expect.
  With a clean prop and a clean bottom, I expect to get 6 knots at 2000 RPM on smooth water with my 2 bladed Martec folding prop. As soon as I hit waves and wind, she drops down to about 5.5 Knots depending on how much of either. The same goes for a dirty prop or bottom. See what I mean, touchy subject!!!
To tell you the truth, my Knot Meter agrees with most others when going side by side, but my tach seems to be running a little low. At full RPM,  (about 2800-2900) which I never use, I recall getting about 7 Kns

[Jim Price]

I get about 6.8 - 7.0 kts according to knot meter at 2600 RPM with a Universal 25XP.  However, my GPS sometimes indicates about .2 -.3 kts slower, but sometimes right on.  So I do not really know as the GPS takes time to update and the paddel wheel may not be sparkling clean.  Also I have a 3-blade fixed.  Anyway, she gets where she needs to go fast enough.

[john daley]

I'm no engineer but  understand your biggest danger is in running your motor at a constant 2000 - 2200 rpm. Diesels  must be driven hard on a reasonably regular basis, even if its occasional bursts. What they hate is constant gentle motoring. I run my Yanmar from 2200 up to 2600 and in heavy conditions over 3000rpm. I always give it a bit of a gentler idle when I come back in to ease it off a bit before closing it down.

[captran]

Holy Moly!  When I read Charlies reply above.  Am I, too, doing my boat a disservice?  I have a 97 with the M-35 and usually cruise at @2000, maybe ocassionally pushing it to 2300.  The water temp in the Bahamas is typically 84 and I preferred keeping my engine temp lower than 180.  The most I've ever pushed it is maybe 2500prm for a short period of time.  I also found I was burning substantially more fuel at 2200 than at 19 or 20.  Less than 1/2 GPH at the lower rpm and closer to 1 GPH at 2300.  what effects do the rest of you see??????  Thanks

Randy

[Roc]

On my M35B, I cruise at 2500.  If I put the throttle all the way, I can get the rpm's up to 3100.  I believe Universal recommends cruising speed to be 2500 on this engine according to what I've read in the owner's manual.  I have a 3 blade fixed prop.

Roc-

[kenkillian]

The variables are many as Ron pointed out, but just for comparison sake, I like to run about 2500-2600 RPM's. With a clean bottom, and a fixed three blade prop and a heavy boat, I will usually be at approx6.0 - 6.2 knts per hour.  I have spent much time callibrating my knot meter with GPS, so am quite confident of this speed.  I will use slightly over 1/2 gallon per hour.  I run out of throttle at about 3100 RPM's which is about where it should, so feel pretty comfortable that the tach is reasonably close.

[Stu Jackson]

In addition to Ron's suggestions and others, you also may choose to consider the interaction of the following three measurements:

1.  Boat speed
2.  RPM
3.  Engine temperature

Our balance is at 2000 rpm, 5.8 to 6.0 kts, 160 to 182 temp.

The variable temp is due to the time lag between me cleaning out the old 2 inch HX every six months, since I've been too busy sailing this season to install the new 3 inch HX I bought two years ago!!! :)

The percieved "low" rpm is because I haven't bothered to determine what the rpm is exactly, nor calibrate it perfectly.  I figure it's most likely simply proportional to the max rpm I can get full out when motoring and in neutral.  The "real" rpm is around 2500.

Disclaimer:  this approach is brought to you by the guy who also hasn't used his fuel gauge, and reckons fuel use by engine hours.  Now, if the engine hour meter went south, I'd have to rethink that, wouldn't I?

My fuel use is a tad less than 1/2 gal per hour, measured "painstakingly" over the course of four and a half years.

[This message was edited by Stu Jackson #224 1986 "Aquavite" on September 18, 2002 at 03:49 PM.]