Cruising RPM

[dprice]

I have a 1998 with a M35B.

Full throttle gets me to 2400 RPM.
Engine runs at 180F.

I thought I had read that rated RPM for this engine was 3000 and that 70-80% of this was recommended cruising speed and that this should be able to be achieved at around 80% throttle.

Should 3000 RPM be obtainable or is a max RPM of 2400 "normal".

Engine has a stock 51A alternator and no other loads other than propulsion.

Thanks.

[rbeecham]

Dave,

I have hull #1430 and can attain 3000 rpm.  I also run at 165 degrees.  Is this boat new to you or is this lack of rpm something new?  It sounds like you may have a fouled prop and/or shaft.  I had the same type of problem on a prior boat, the prop looked like a block of cement when the boat was pulled.  Once cleaned the boat ran normal.

Dick Beecham                  rbeecham@jersey.net
"Refuge"
C34 MK II  Hull # 1430
Rock Hall, MD

[dprice]

Boat is new to me this season.  Prop and shaft was clean as of 2 months ago.  It is my impression that it is running the same as it was when first launched.  Though I don't believe I tried to run it at full throttle until now.

I have an Electronic Dwell/Tach from my "messing around with cars days", though I am not sure how I would hook it up.  

I will take a look at the tech articles and check the dip switch settings.

[Ken Juul]

Harder to measure rpm on a diesel.  Don't have the spark plugs to use as a trigger.  If you have a marine engine repair shop near your boat, may want to ask how they measure rpm, and if you can borrow/rent their device.  If the rpm checks out you may have too much pitch on your prop.

[Roc]

My engine RPM at full throttle is 3100.  I have a 3 blade fixed prop.  I run it at 2500 cruising speed and the temp is at 170 degrees.  My boat has been in the water since the end of April and I haven't dove under the boat yet to clean the prop and just last weekend, I did throttle all the way up and got it to 3100.

Roc-

[tsoko]

David,
After the dip switches set correctly, there is an easy way to adjust the tach using a fluorescent droplight (60hz, 120V).  Put a piece of tape (contrasting color) on the outer edge of the main pulley on the front of the engine.  Start the engine and set it at a fast idle.  With the fluorescent light plugged into shore power, shine the light on the pulley (being careful not to catch any cords, hair, etc.).  Adjust the engine speed until there seems to be four pieces of tape “frozen” in place at four equidistant places on the pulley (such as 1,4,7, and 10 o’clock positions).  That will be exactly 1800 RPM.  Adjust the tach accordingly using a small screwdriver or allen wrench in the back of the tach.  Make sure you see the tape freeze in four places, as it will also freeze in place at 1200 and 2400 RPM, but you will see a different number of pieces of tape.  Hope this helps.

[Ken Juul]

What a fantastic bit of information.  I am constantly amazed at what I learn on this board.

[Roc]

FYI, my main pulley has a piece of reflective white tape.  Apparently left on there from the person that set the RPM when my boat was commissioned.

Roc-

[dprice]

I learned 2 new things this eve.

1.  My tach has a 4 position dipswitch not the 14 discussed in the tech article and on the teleflex web site.  Only position B is "on" what ever that means i'm not sure yet.  I emailed teleflex to see if they can tell me.

2.  The flourescent light trick does not work if your drop light uses PLC type compact flourescent lamps.  I believe they use electronic ballsts which run at significantly higher freq. than 60hz.  I backed into that tibit after seeing nothing but a blur.

So for this eve.  It has been 2 steps forward and 2 steps back.

[Norris Johnson]

On a diesel engine, I use a vibrating reed tach. On top end, I read 2400 on my dial tach. And 2800 on my reed tach. I haven't corrected the dial tach, yet.