Am I over-prop'ed

[lr172]

My new to me H34 has a 2 blade Maxi-prop. My research indicated the factory recommend pitch (it is adjustable) was a bit aggressive. I chose not to change it before putting it in the water this spring. After some time in the water, I have observed that I hit 6.5 -6.7 kts at around 2800. If I push it to 3000, I get to 7.2, but I start getting some light smoking of the exhaust (enough leave carbon staining on the transom).

I am guessing that I am over-prop'ed and it is too much load for the RPM range. I am still learning diesels. The engine is supposed to make max torque around 3400.

Looking for some other opinions about my situation. I am thinking to reduce the pitch next fall and keep my RPM's below 2800 this year.

Larry

 

[kloudie1]

Sounds like you are correct, Larry.. but before you make a decision, get a hand held optical tachometer and measure the RPM and compare to the Yanmar tachometer. The original tachometers are notoriously inaccurate .. Set 2000 on the cockpit tacho and read the optical and write it down .. do that for 2500 and 3000 so that you know what the engine is really turning.. Yes you'd be fine operating as you say for this season ..

 

[seadaddler]

Hand Tack

Where can you get one those hand held tack's.
Nick

 

[kloudie1]

Hey, Nick.. these aren't very expensive and they work.. They go on sale occasionally and you can find 20% off coupons ..
http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html
or
http://www.harborfreight.com/digital-contact-tachometer-66400.html
The "contact" one has to touch the center of the crankshaft pulley to work.. the optical one can "look" at the crank pulley and gives a number without touching.. It helps to put a radial white stripe at the outer edge of the pulley to get a better reading.. They are handy to set generator rpm as well..
Probably some on E-bay as well

 

[Roger Long]

Back before computers, the proof of a proper prop match for commercial boats was considered to be seeing black smoke just starting to appear in the exhaust in the last 50 - 100 rpm before full throttle.

For best engine life and least maintenance, you want to be running at about 90% rpm which will be 80% of horsepower due to typical fixed prop characteristics. That would be 2700 rpm. If you are not seeing smoke there, then your prop is probably just right.

A lot of pleasure boat engines have a maximum horsepower for which they are only rated for one hour of operation of 10% of total operating hours and a continuous rating at which they can run all day. The 90%/80% rule applies to the continuous rating so you should check your manual.

If you are over propped, your engine will stop turning up to the rpm the manual says it is capable of and just make black smoke. Open your engine right up. It won't hurt it for a couple minutes. If it goes up to 3400 (and you are sure your tach is correct) your pitch is probably OK. Most diesels make a little smoke and stain the transom. It should be light grey in color at 90% rpm and not like a heavy truck pulling away from a stop light.

3400 rpm sounds like a 1 hour max rating. 3000 would then be about the maximum continuous RMP for a typical engine. Check your manual or with your engine dealer. I think you'll find that 2700 is about where you want to run your engine if that's the case.

 

[kloudie1]

Not to step on your toes, Roger..
Quick check of the 3GMF manual ... 3400 is continuous duty and 3600 is one hour rating..
Sounds like his prop is about right to me as well, given that the tacho is most likely reading low a little..

 

[Roger Long]

Not to step on your toes..

Not at all. I'm sure the OP appreciates the additional info as well.

Looking back, I see that my post isn't the clearest writing I ever did.

He wants to cruise 2800 - 3000 as much as he can with this engine. If he has to slow down below that often for spray, comfort, or other reasons, he should try to get up to 3000 as often as he can. He should also open her right up for about 10 minutes a day. The engine should turn right up to the max and showing some smoke.

My engine went up to the 1 hour rating with very little smoke and was clearly being limited by the governor and not the prop. I had a couple inches of pitch put in so smoke should start showing up earlier and she may not reach max rpm. I wouldn't recommend this for everyone but I almost never run over continuous RPM except to exercise the engine and be sure the exhaust elbow is blown out. I'll probably paint a red line on the tach so I don't have to worry about crew or guests going past max continuous RPM.

 

[lr172]

For this engine max continuous is 3400 and max intermittent is 3600. I see grey smoke at 3000 and I have taken it to at least 3200, but got heavier black smoke like when you hit a good bit of throttle at lower speeds.

I can't remember if I got it up to 3400 yet. I will do a WOT run and see how many RPM's I get.

It sounds like as long as I get to 3400 or so, I am at the correct pitch. I would like to be able to motor at hull speed, which gives me grey smoke. Am I ok running like this continuously?

Thanks for all of the assistance here.

Larry

 

[Roger Long]

I have taken it to at least 3200, but got heavier black smoke like when you hit a good bit of throttle at lower speeds.

That sounds a bit heavy on the pitch. I'd aim for light grey smoke just starting at Max Continuous and able to turn up to Intermittent Maximum with although with some black. If the prop prevents it from turning up to at least Intermitent Maximum, you will hurt the engine running it that way for long, even at reduced throttle. It's like always being in too high a gear in your car.

BTW make sure it isn't the throttle linkage adjustment that's limiting the RPM. If the RPM stops climbing, the throttle still has travel, and adding throttle just makes the smoke blacker without the engine speeding up, you have too much pitch.

If you think you want to operate often at Intermittent Maximum, say to do a quick trip against current 1/2 an hour or so out and back each day, I would reduce the pitch so that the light grey smoke is just starting at 3600 RPM.

Try to avoid any kind of operation that makes black smoke, Nice, slow power changes are what a diesel likes. Except for maneuvering I add about 200 rpm, wait for the speed to come up, add another 200 etc. Same thing in reverse slowing down. Your engine, like mine, may have a rough rpm range that you want to move through more quickly though.

 

[Dan Johnson]

Larry...

My H34 has the original prop, a two blade 15x12 right hander. 3,400 is about all I get at WOT. I regularly cruise at 2,800 RPM at about 6.5 knots--all day long.

I got my laser tach through an Amazon.com vendor for a reasonable amount.

 

[Allan12210]

I have the original 15 x 15 two bladed prop. I usually cruise at 2800-3000 with no smoking and about 6.4 knots. Larry, your prop sounds over pitched to me as yes you get good speed but are overworking the engine at an equivalent rpm. I think an important factor is throttle position...to get the same rpm as others you have to firewall the throttle more resulting in a bigger burden on the motor. For me to go the same speed as you means I would push the motor to 3600+ rpm and shorten its life. Maybe with the bigger bite I suspect your prop is giving you then you should run at 2600-2800 rpm putting the engine in its most efficient load range. By the way, what is your fuel consumption at 2800?..mine is just under 0.6 gal/hour. So, less pitch will put less burden on the engine at a fixed rpm, but the tradeoff is that you will go slower at that rpm. Its all about optimum performance on the engine (fuel economy and length of service).

Allan

 

[lr172]

Thanks for all the help guys. For this season, I'll keep it at 2800 and below (no smoke level) and adjust the pitch in the fall.

On another note, I had a decent amount of black soot in the air cleaner canister when I took it apart to clean the filter is this a normal sign? I bought shims to adjust my timing, but didn't get enough time to do it before my launch.

Dan, when you got your laser tach,did you find the Yanmar tach to be accurate?

Thanks,


Larry

 

[Dan Johnson]

Larry...

look for laser tachs on Amazon.com. They vary in price by vendor for the exact same models, so do some searching. I think mine was around $30. It's a Neiko. I've used it to check everything from boat motors to ceiling fans. It's a neat toy!

No, the Yanmar tach's reading was different from the laser tach by about 200-300 RPM. It's a bit on the low side.