Westerbeke 10/2 cruise RPM

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M

Mickstr

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Jun 1, 2007
272
O'Day 322 Mt.Sinai
I've been limited to a cruising RPM for the last few years by a sluggish raw water flow through the boat (272LE with a Westerbeke 10/2). I rarely exceeded 25-2800 rpm to keep water temperature at 160-170. This last winter I finally replaced a few more hoses, while removing and cleaning the heat exchanger (turned out to have some impeller pieces and grass inside it).

So now I've got great water flow, and am wondering what rpm is recommended for cruising, and/or what is the redline, and/or at what rpm will the governor kick in? I made a run back and forth across Bellport Bay (NY) Sunday, maybe 2 1/2 nm roundtrip, and at 3,000rpm stayed at about 160-170 degrees.
 
jibes138

jibes138

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Jan 27, 2008
3,086
ODay 35 Beaufort, NC
rather than worry about RPM you should figure out the hull speed of your vessel. Approximately= 1.34 x (LWL)^1/2

Then go for a ride and see what RPM gets you cruising at hull speed. Any more RPM's will just be stressing things and not making you go any faster. Another consideration is natural frequencies in your motor system. Sometimes you can get a lot of vibration at certain RPM's and a slight change in speed makes a huge difference.

After hull speed you will be trying to force the boat to ride up the bow wave and start planning which you will observe as the stern squatting since the boat is too heavy to force up the bow wave.
 
M

Mickstr

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Jun 1, 2007
272
O'Day 322 Mt.Sinai
Agree. Unfortunately, I started the season with a broken knotmeter, and the antenna is not getting to my chart plotter, so I'm flying blind! Westerbeke answered my email and said to keep it under 3700 rpm. So, I'll probably max out at 3,000 this season.
If I get a speed indicator (knot meter or GPS) working, I'll do as you suggest.

Thanks jibes138.
 
RAD

RAD

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Jun 3, 2004
2,330
Catalina 30 Bay Shore, N.Y.
My rule of thumb for max engine RPM has always been 20% of red line which is about 3000 in your case and to measure speed start the clock at a buoy and when you get to the next stop the clock and then break out the charts,rulers,calculators,etc.......
Do you know about our 10th annual North East rOnDAYvous being held at Bay Shore this weekend?
 
RAD

RAD

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Jun 3, 2004
2,330
Catalina 30 Bay Shore, N.Y.
Oh....I just looked over at the other forum and I see that you are coming....great if you need any info call me
516 592 7780 any time or hail me on VHF 09 (SV Free Spirit) Friday eve through Sunday
let me know how may your bringing for a head count ( Saturday dinner is catered)
 
M

Mickstr

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Jun 1, 2007
272
O'Day 322 Mt.Sinai
So far just me. Brother joining late Saturday (by car) after dinner for sail home.
 
rajhnsn

rajhnsn

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Oct 7, 2008
379
Oday Oday 35 Chesapeake Bay
My tachometer has not worked for years. I basically adjust my RPM's by three factors for cruising speed; the sound of the motor, speed on my GPS, and how low my stern is riding in the water. The GPS speed is not actually giving me speed through the water which I would think is needed to determine hull speed. Since I can actually see the difference of the stern squatting down in the water by a few inches directly related to the throttle, I know I'm at or above hull speed and I back off until it sits up a little higher. This is by no means the best solution but it has been working so I have put repair efforts into other things. I suspect that the tach problem is related to the higher powered alternator installed by a previous owner. Any ideas to repair this would be welcome.
 
SailingCal21

SailingCal21

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May 30, 2006
351
Oday 34 Chesapeake Bay
Stern squat, I've wondered why Karma's scupper discharges sometimes are way low in the water whilst motoring. Gotta check that out this weekend.

Trouble is I'm pretty sure that her hull speed is somewhere around 7.2 knts. With a clean bottom, highest motoring speeds I've seen at cruising RPM is 6.2 knts. I'm running the original 2 blade prop.
 
rajhnsn

rajhnsn

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Oct 7, 2008
379
Oday Oday 35 Chesapeake Bay
I'm not sure if it is that high. I'll bet jibes138 has done the math for our boats. jibes?
 
jibes138

jibes138

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Jan 27, 2008
3,086
ODay 35 Beaufort, NC
LWL is listed as 28 feet 9 inches for the 35. Square root is about 5.3. Times 1.34 = 7.1 . This formula is a rough approximation so Sailing Cal is pretty much spot on with his numbers. I always felt like cruising up in the 6's was pushing the motor, I typically cruise at 5.6 or so but it seems like it is a sweet spot in the engine RPM for minimum vibration and noise. These boats are a little underpowered at 21 HP. All the replacement engines and the newer M25 models are up around 25 HP. There have been times when powering into steep chop I've wanted more HP. You might say why not sail in those conditions but sometimes the channels don't give you that option where you are nose to the wind in a narrow channel and straight into a steep chop.
 
rajhnsn

rajhnsn

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Oct 7, 2008
379
Oday Oday 35 Chesapeake Bay
I stand corrected. I agree that we are underpowered. I've timed the C & D canal wrong and had ground hogs move along the shore faster than me. If I were to repower, I would go for a 30 hp. I'm glad to know there would still be some hull speed available.
 
SailingCal21

SailingCal21

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May 30, 2006
351
Oday 34 Chesapeake Bay
I always wondered about the powering of the 34/35s, thought that it was just Karma. Someone gave me a 3 bladed prop, figured that I'd try that when I haul her the next time.

I use this site to calculate boat performance and do comparisons: http://www.image-ination.com/sailcalc.html There is also a prop/speed calculator at the bottom of the page.
 
rajhnsn

rajhnsn

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Oct 7, 2008
379
Oday Oday 35 Chesapeake Bay
Great site. Very interesting to compare similar boats. I'd be interested to know if a 3 blade prop would get us closer to hull speed under motor alone.
 
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