Boat speed while motoring

Jeff Kimbel · May 28, 2009, 04:40:00 PM

I know this question will beg a dozen different questions - but I would like to get a quick calibration:

I have the original M-25 (1987 model year) on board, and my top speed (open throttle) while motoring in perfectly calm conditions is about 3.8 knots. With any type of current on the nose, that drops precipitously. I also find myself smiling & waving to every like-sized boat as they overtake me while motoring. Under sail, I don't find the same problem. I have a wing-keel, and no equipment that adds significant weight.

Does this sound slow? I'm considering having the compression tested on the engine to see if that is an issue. Also - might this be more of a prop issue?

Thanks - I appreciate any input.

Jeff

Ted Pounds · May 28, 2009, 06:07:55 PM

Jeff,
It does beg a lot of questions: What is your RPM? Is the tach calibrated? What prop do you have? HOw are you measuring speed? (GPS I assume since you say it slows with a current.) What's the condition of the bottom? Any excess smoking? (the engine, not you) I'm sure others will have a few thought. And to answer you question (finally) it does sound way too slow. My XP would do 6.5 knots no problem and that's only an extra couple of HP.

sailingdream · May 28, 2009, 06:24:41 PM

Something is wrong there. I have a 25XP with 3 prop and can hit 6.5 at about 2400-2500rpm. At the begining of the season, I slow down a bit as the season goes and get a little growth on the hull but thats down to the low end of the 6knt scale.

Jeff Kimbel · May 28, 2009, 06:27:24 PM

A little more information:

I'm unable to give an rpm reading, as the diode on my alternator went out last fall and I haven't replaced it yet. I assume that the prop is original - it is a 2 blade, and if my memory serves, measures 7" from tip to tip. The bottom is in good shape - very minor growth. No smoking to speak of (I do enjoy a sundowner though).

Thanks -

Jeff

Roger Blake · May 28, 2009, 06:29:31 PM

I'm newer with a 1998 M35B, but for comparison, at 2300rmps I'm usually around 5+ knots. Suspect something is amiss, if you are using your knot meter as a guage, perhaps it is afoul...try using a GPS to measure SOG.

Jeff Kimbel · May 28, 2009, 06:32:19 PM

Oh - and I am measuring speed with my GPS.

Jeff

Ron Hill · May 28, 2009, 06:56:43 PM

JK24 : Ted asked you all of the right questions. I believe that the first thing you need to check is your prop.
I believe that the standard 2 bladed C34 prop was a 15" diameter and about? a 12"? pitch. It's stamped on the prop hub diameter & pitch!! Look in the Mainsheet tech notes as it (factory specs) are all written up.
Find that out and then come back and ask the question after you get your tach fixed.

RV61 · May 28, 2009, 06:59:21 PM

Jeff,
My 1986 with the M-25 and has three blade and cruising in calm lake no real currents she cruises at 6.2 to 6.5 full throtle 7.6 to 7.8. My RPM guage is not calibrated correctly so I cannot give accurate RPMs. I would say you have an issue. Would not want to try and fight adverse condions with that little of power. At first thought I would check your throttle cable as well and be sure you proper amount of fuel. No blockage from dirty fuel filters or lines etc.
Rick

Footloose · May 29, 2009, 03:41:29 AM

Jeff,
I have the two blade prop and get 6-6.5 kt at 2300-2500 rpm. I am on a lake with no current. You said your prop is 7in tip to tip, if that is correct, your have a prop that is way undersized.

Tom Soko · May 29, 2009, 05:55:13 AM

JK24,
I hate to disagree with Ron, but the original prop that came with the C34 (and also the C36) was either a 15x10x1 two blade, or a 15x9x1 three blade. I have heard that you can go up 1" or so in pitch, but any more than that and you will be overloading the engine.

prh77 · May 29, 2009, 06:01:11 AM

First thing you have to do is verify RPM.... either fix the alternator or buy/borrow an optical tach ( $20-$30 0n the internet ).All other issues, ie prop selection,throttle rigging, clogged filters are verified/discarded by a no load rpm of 3200 and a loaded rpm of 3000-3200.

MarkT · June 01, 2009, 06:37:24 PM

It sure sounds like a prop issue.

I have almost the exact same setup and we get over 6 knots.
The boat has a 3 blade prop with M25 engine and wing keel.

If the engine is fairly easy to start then it is unlikely to be compression that is the issue. If you just got the boat then it is possible that you have bad fuel and blocked filters.

Mark Tamblyn