RPM

[rappareems]

I just finished a two week cruise on Lake Ontario ... lots of motor sailing.  I ran at 2000 RPMs at about 5+ knots.  My '86 is the 25 horse engine but in reality I think it is more like a 21.  Ron/Stu what is your take on running at 2000?  It seems smooth to me but that can't be hull speed.  One good thing is I burn app. a liter per hour.

Mark

[Tom Soko]

Mark,
The 1986 Universal M25 IS a 21 hp engine!  The M25XP (introduced in 1987) is a 23 hp.  Many ignorant brokers list them otherwise.

[rappareems]

Tom,

What about hull speed and RPM?

Mark

[Stu Jackson]

I run at anywhere from 2450 to 2650 at 6 (+/-).  2000 is kinda lugging it.  No reason to to use a tad more, between 75% and 80% of WOT which is 2000.  We average 0.493 gallons per hour.  Just find a sweet spot and use it, doesn't matter what anyone else does.

[Footloose]

I, like Stu, run at about 2500 when I am trying to get somewhere.  When I get to our mooring field, which is a no wake zone, I reduce engine speed to 2000 rpm, which slows me down to 5 knots, right where you were.

[Les Luzar]

I have an 87 with an M25XP with a three blade prop, 15x10,  and I cruise at 2,500 to 2,600 RPM and run between 6.3 mph and 7.2 mph depending on sea conditions, wind direction, and current. My maximum indicated RPM at full throttle is about 3,100 RPM, which used to be 3,400 RPM before I repitched my prop from 9 to 10 inches. I also use about half gallon of fuel per hour.

[Stu Jackson]

mph or knots?

[Les Luzar]

ooops.....knots (I stand corrected) Thanks!

[Bobg]

at 2500 I can't get more than 5.6 knots out of mine, 3 blade prop, not sure of the dia

[Ron Hill]

Mark : First of all there is NO pat answer to your question - as there are a number of variables that enter in.

If you haven't read my Mainsheet tech note article on "Props, Pitch, Engine rpm and Boat speed" please do.

As a starter let me ask you how your boats' engine instrument tach measures against a laser tach measuring actual engine rpm??? 

Now do you see what I mean??  A few thoughts

[rappareems]

Thanks to all... and I agree there are lots of variables.  My engine seems to be very happy at 2000 ... purrs sweetly, very little vibration and usually I am motor sailing which will put me over 6k ... and as I said I am only burning 1 liter per hour at that rpm.  I tend to fuel in Canada hence my use of liters.  Sue has logged that over the years.  So, a little over a quart.  If I am bucking a head wind and need to make harbor I will push to 2500...

[Indian Falls]

Here's my 2 cents:

I run at about 2200 rpm and maintain about 6 to 6.5 kts.

Seems to me it's just not worth the extra noise and fuel consumption to run over 2500 rpm.

I have a 3 blade 10x15 prop.

We recently crossed to Toronto and back on a club cruise.... no wind... motored both ways.

found no reason to run at anything close to 3000 rpm. 

2700 rpm only gained .5 kt and all this was verified with a gps.

Boy do I need to check my rpm with a gauge or what?