Expected speed under motor?

[Admiral_Swellson]

I feel like my boat has slowed down since initial purchase back in October 2020. It's a 2003 MKII with M35B and three blade prop.

I can't seem to get it going more than 5 kts even with wind behind me. I keep hearing other owners claiming "hull speed" with theirs?

My bottom is clean, so what could be the issue? Can something be slipping in the drive train? I checked transmission fluid and it's pink and at proper lever.

[Jim Hardesty]

I have a similar boat and get near hull speed.  I would start trouble shooting by checking speed with GPS and that you are getting full motor RPM's to make sure the boat is really slow. 
That's all I have.
Jim

[Ron Hill]

Robert : How clean is the prop? No oysters on it?

A thought

[PaulJacobs]

Hi Robert,
Initially I was hesitant to reply because every boat's bottom is slightly different, engines may be different, propellers may be different, sea state is hard to quantify and currents effect the result.  However, on the flip side if your numbers are seriously different, something may be wrong.  So, with all those caveats out of the way, I will add my input.

We just went out last week to bring Pleiades up from Wickford to Barrington to have new interior cushions installed. It is about 13 NM on both the west passage and east passage of Narragansett Bay.  In perfect accord with O'Houlihans "law of the wind" - "there is a 97% probability that when you are sailing the wind will be directly coming from wherever it is you are trying to go" (O'Houlihan was Murphy' s brother-in-law).  So of course the trip from Wickford to Barrington, which is mostly NE with a few jogs to get around Patience and Prudence islands, found us with the wind out of the NE.  Later in the afternoon, the course from Barrington back to Wickford was mostly SW, and thus, with O'Houlihan laughing mightily, the wind had veered to where ... the SW!   Thus both trips were directly into the wind!  We were power sailing with a single reef in the main, primarily to minimize any rolling, since with few exceptions when jogging around islands even a very flat first reefed mainsail could not provide any significant driving force.  The leg up to Barrington included a modest (0.3 - 0.4 knot) mostly adverse am ebb tide induced current.  The return leg to Wickford conversely included  a similarly adverse flood tide, in perfect accord with O'Toole's "law of the current" - "whenever sailing 98% of the time the current will be adverse in your little part of the world"   

Pleiades has a very smooth bottom  - not a "perfect" burnished racing bottom, but recently fine sanded and bottom painted with ZERO "potato chips".  I would rank her bottom as a 9 out of 10.  However, she has been in the water for three weeks without a diver, and likely already has a coat of thin slime, thus dropping her bottom to an 8.

Pleiades has a 7 year old Yanmar 3YM30 engine (LOVE that engine  ) which puts out 29 HP at 3200 RPM.  She also has a FlexOFold 2-blade folding propeller that is nicely balanced with no nasty sounds at any RPM .  I have NOT had the tachometer calibrated, but the readings all seem reasonable.  Finally, Pleiades does NOT have a knotmeter, but we do have a Garmin GPS that reports speed over the ground.  I have checked this by transiting a measured mile in both directions in relatively flat water (viz. <0.5 ft chop), and near dead calm winds, taking the average to eliminate the effects of current, and found the GPS indicated speed to agree within about 3% with distance divided by elapsed time.

Here are  my results.  At 1800 RPM speed = 4.8 +/- 0.3 knots.  At 2000 RPM speed = 5.4 +/- 0.4 knots.  At 2400 RPM speed = 6.0 +/- 0.4 knots.  At 2800 RPM speed = 6.5 +/- 0.5 knots.  And finally at 3000 RPM (WOT) = 6.8 +/- 0.5 knots.    Notice that even at wide open throttle Pleiades is close to but will not quite reach hull speed (7.3 knots).

I hope this was helpful.

[Oldlaxer1]

Stuffing box adjusted properly?  Getting a drip while underway? 

[scgunner]

Robert,

When posting with a problem the more information you can provide the better the odds of one of our board members identifying your problem.

Right off the bat I've got a number of questions. What speed were you making initially? Are you still making the same RPM? How fast was the decline in speed? What condition is your prop in, you'd be amazed at how a nick or two can seriously degrade performance? How long has it been since your injectors were serviced? Any other problems noticeable with the drivetrain? Any big changes done to the boat?

Off the top of my head, with the information you've provided, I'd be willing to bet if you had the injectors and prop serviced you'd get your missing knots back.

[Admiral_Swellson]

Thanks everyone, you've given me plenty of ideas to look into!

[Indian Falls]

I can make 5.5 kts going into a 10kt headwind at 2200 rpm with a 3 blade 15x10 prop. 
I feel that my boat would only make hull speed under power downwind at 3000 rpm.  I don't like running the engine like that.  My sailing buddy's boat can out run me by 1knt when motoring to wind and he has a 2 blade fixed prop.  We did 2, two week vacations in a row the past 2 summers and had to motor most of it.  I just put it at 2200-2300 rpm and suck it up that I'm just over 5kts.  Motoring with the sails up helps a lot unless of course the wind is directly on the nose.