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Freestar05
Last seen: 3 months 1 week ago
Joined: 11/24/18
Posts: 3
Universal M35 sweet spot rpm

Just launched our (new to us) '92 C36 #1177 with M35 / 30 hp / 539 hrs.. (Prior boat was C42 w/ Yanmar 4JH2E). We motored about 12 miles to a diferent marina and I'm wondering what the optimum rpms are for decent boat speed with decent fuel consumption.  We ran at 2500 tach rpm and the engine ran very quietly and smooth with minimal vibration. Any thoughts / advice?  Thanks. Lee Gardner on Chesapeake Bay

Pgutierrez's picture
Pgutierrez
Last seen: 4 days 9 hours ago
Joined: 5/14/12
Posts: 119

Lee, 2200-2500 rpm is good, much higher you might experience > 0.80 gallons per hour and possibly soot on the transom.

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peter g

2000 C36, MK2, Hull. #1897
wonderful, wonderful, wonderful ! ! !.   5 th Catalina

 

 

Catboat Willy's picture
Catboat Willy
Last seen: 3 weeks 2 days ago
Joined: 12/22/14
Posts: 231

Lee;
We cruise at 2400 RPM (measured) and consume .6 Gallons per hour at 5 kts. A Recently rebuilt engine
Much of this is going to depend on what prop you are using, the additional load on the the engine (frig/AC ?) and how clean your bottom is. Take a look at your engine while cruising and see at what RPM there seems to be minimal vibration. This could be your best RPM. Keep some records and you might want to put an hour meter on the engine.The hour meter on your tachometer only reflects what the total time is when it is running at full RPM.
Good luck with your new boat.

 

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Bill Dolan 1990 Catalina C-36 MKI - Hull #1041  'Williwaw'
Std. Rig, Walk Through, Wing Keel
M35, Oberdorfer Conversion,
Home Waters; Port Charlotte, FL
'You are never out of work if you own a boat'

 
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KevinLenard
Last seen: 1 week 1 hour ago
Joined: 1/28/15
Posts: 102

I use a blasphemous method after years of experimentation with our '91.  To ensure we're not 'running hot' I run at about 2,250 RPM and monitor the watertemp gauge (which may be off by 200 degrees depending upon many factors -- see other threads).  Depending upon wind direction, strength, waves, current and load (pulling dinghy or not), I push the RPM up slightly once she's warmed up until I see the temp creep up a bit over 180 degrees F, then back it off until it comes back dow to 180 (opitimal operating temp).  What I have noticed is that anything over 2,500 RPM puts the open transom under water (not good as it indicates the nose of the vessel is being driven up) and at 2,000 RPM she runs cooler than 180 degrees.  This 'method' gets us an average speed of 7 knots in average conditions, over 7 downwind or with the mainsail up to steady her, and less directly into the wind/waves.

I welcome any criticism/points of caution to this approach!

--

Kevin Lenard
"Firefly"
'91 C-36 Tall Rig, Fin Keel, Hull #1120, Universal M-35 original (not "A" or "B")
CBYC, Scarborough, Ontario, Canada

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alfricke
Last seen: 1 month 1 week ago
Joined: 4/14/09
Posts: 35

Lee,

I'd experiment with more and less RPM and compare it to gains in speed. Making a graph of RPM vs speed through the water will give you an idea of the "sweet spot" for your boat. On ours, 2400 s a great compromise spot, just loping along with optimal fuel consumption. When I want to get somewhere (like to get to the salmon grounds for the "hot" bite,) I bump her up to 2600. This will get Jubilee going about 6.8-7.0 kits.

Al

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Al Fricke
S/V Jubilee
Catalina 36' MkII  #1867
Freestar05
Last seen: 3 months 1 week ago
Joined: 11/24/18
Posts: 3

Thanks - Peter ,Bill, Kevin, and Al - for your comments and great advice. Your experience and methods will help in getting to our optimum rpm's.  Hadn't gotten my new GPS dialed in to get my speed-over-bottom, and the speedo was showing 4.4-4.6, which seemed too slow vs our point-to-point time. Then the First Mate checked the tide table and we were going with an outgoing tide. Look forward to learning more about the 'new' boat as we plan to spend most of May-July cruising the Bay.  Then take her home to Oriental, NC.

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