under propping

05bill

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i know that over propping is harmful to most modern diesel engines, but other than not getting an engines full potential efficiency, is under propping harmful ?
and i am assuming that under propping is far more diserable than over propping ?


thanks ,

05bill
 

djmarchand

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Take the case of a boat that is perfectly propped to hit its rated rpm at wot. Then drop down several hundred rpm to a comfortable cruise speed, say 2,000 rpm and 15 kts. It will take x hp to move the boat, say 200 hp for argument's sake. It will also take y ft lbs of torque as well, say 150 ft lbs (yes I could calculate the exact number but I am lazy, so bear with me).

Then take the same boat and engine and take out 3" of pitch. To get to the same cruise speed as before is going to require reving higher, say 2,300 rpm. The hp will be approximately the same- 200 hp. The only difference will be prop efficiency and it might be better or might be worse. The torque will be proportionally lower, about 130 ft lbs.

So which is easier on the engine: 2,000 rpm and 150 ft lbs of torque or 2,300 rpm and 130 ft lbs both at the same hp. The latter. Rpms at these levels doesn't hurt an engine much. Torque does because it comes with more heat and increased forces on internal parts.

Of course propping is a compromise and at extremes: 4,000 rpm and 75 ft lbs probably won't be so good on the engine either.

David
 

BillD

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05bill,

Under propping is one of three of your marine diesel engine's BEST friends.
The other two best friends are a properly designed exhaust and clean fuel to the engine.

By under propping you leave some hp you paid for on the table but if you are happy with the performance so what?

Remember, rpms gets the oil and coolant pumping and flowing through the engine. These "fluids" get rid of heat.

dj explained it nicely with number and math.

Another way is like this.

What do you think was "better" for your old standard shift car's or truck's engine?

"Lugging" the engine around town in 4th gear listening to it "ping" OR running around town in 2nd or 3rd gear listening to it "sing" ?

Bottom line........prop down...use the rpms to increase your desired speed all will be good.

FWIW, Bill D
 
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05bill

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Thanks fellows that was well explained in both instances. I am under propped but I only cruise at 13.5 - 15 knots depending on current, 2600 rpm's on an engine , yanmar 370, rated at 3350 rpm's, so I am pleased to hear what you both have said.


Its hard to imagine why these boat manufacturer's over prop other than for a higher speed at wot. They have to know that this is not a good thing for the engine.

Thanks again,

05bill
 

Powderpro

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If an engine is truly over-propped, the top speed will not be faster than if it were correctly propped. The correct prop that allows an engine full RPMs and the load is 99-100% will give the optimum top speed. More pitch does equal more speed, but if the engine is not able to reach full RPMs (too much pitch), then the shaft speed will be slower and thus the top speed can/will suffer.

I've built new boats that were slightly over propped at sea trial and were not quite making full RPMs. I reduced the pitch by 1", the engine was then able to hit full RPMs, my top speed was the same or better and I was burning less fuel. It makes absolutely no sense when boat builders or boat owners over pitch their props. I've heard it argued that they overpitch because they "want to increase cruise speed". Or "I never run it wide open", insinuating it doesn't matter if the engine can reach full RPMs or not. In fact they are not increasing cruise speed, but they are increasing fuel burn, which is just stupid. Prop the boat correctly and it will perform the best regardless of RPM. Being underpropped is fine and easier on the engine, but top speed will obviously suffer.
 
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05bill

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over propped

as i indicated above at launch full fuel, some water,etc.at about 95 % of cruising weight the boat with a 22 x 22 prop, photo tacked at 3300 rpms. just to be safe went to a 22 x 20 prop. did not photo tack but added some more "stuff" , about another 150 lbs. So with the decreased pitch I picked up approximately 150 -200 rpms so i feel comfortable with the propping. I burn about 4 gph at 2600 rpm's,and the 13-15 knots are just fine with me.

Again guys thank you. this is a great forum to get knowledgeable responses .

Regards,

05bill
 

BillD

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05bill,

I read and follow your posts over on boatdiesel.
Take care of the Yanmar as suggested by Tony Athens, keep up with "marine age" of the engine and the way you run the 370 it will last as long as you own the boat and you'll leave a nice used engine in the boat for the next owner.:D

FWIW, Bill D
 

eyschulman

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putting on weight

One of the big factors in over propped new boats is the fact that perfect propping on launch often = over propped 6 months latter after boat gains considerable weight as most new boats do and builders do not take that into account.
 

WC1966

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One of the big factors in over propped new boats is the fact that perfect propping on launch often = over propped 6 months latter after boat gains considerable weight as most new boats do and builders do not take that into account.
Sort of like the aging process, my weight has increased about a pound a year over the last 30 years. Guess my physical activity vs fuel burn at 61 needs to be addressed.
 

Sailorgp

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If you had cut out just one saltine cracker each day for the last 30 years you wouldn't have gained that annual pound
 
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