Prop Pitch Help

[98SeaArk]

My '98 150 Johnson turns about 4700-4800 RPM at WOT with a 14.5x21 prop. I used a 14.5x19 and it turned a few more RPM but the MPH slowed down. I was wondering if this was bad for the engine not being able to turn more RPM's, but the prop that is on it (14.5x21) is the prop that makes the boat run the fastest. Any suggestions or comments would be appreciated.

 

[Silvertip]

Re: Prop Pitch Help

You are lugging the engine with the 21. Going to a 19P is like downshifting in your car, engine revs go up but top speed in that gear will be less. Boats have no transmission so the prop needs to be selected carefully. Make sure the engine is in proper state of tune for optimum performance. With the engine down, check the position of the av plate (the plate just above the prop) in relation to the bottom of the boat. If it is below the hull, you need to raise the engine so it sits flush or an inch above the transom. Less lower unit in the water means less drag.

 

[98SeaArk]

Re: Prop Pitch Help

Ok i get what your saying.....next time i get the boat in the water i'll check it out. I'm more of a car guy and i understand now what you mean by downshifting like a car to make more rpm but less mph, that helps.....can you explain "Lugging the engine"?? does this mean its not running at optimum performance??

 

[walleyehed]

Re: Prop Pitch Help

Do we have a carbed 150 or a ficht?

 

[98SeaArk]

Re: Prop Pitch Help

carbed 1998 Johnson 150 Fast Strike

 

[walleyehed]

Re: Prop Pitch Help

OK, 4500, actually, anything under 5500 isn't good, and the 19 won't fix that..we need to see 5800.
The tach needs to be verified and maybe a compression check would be in order.
What kinda boat we got this 150 mounted on?
Size, model, etc...

 

[98SeaArk]

Re: Prop Pitch Help

The Tach has been varified by a local J/E dealer. The 150 is mounted on a 20' SeaArk ZX200 aluminum bass boat no jack plate. The engine has had this prop(14.5x21) on it since day one and has run flawlessly, even thought its never ran at WOT for more than a few hundred yards at a time and it is turning between 47-4800 RPM at WOT.

 

[walleyehed]

Re: Prop Pitch Help

well, I hate to see anything under 5800 as a set-up point on that engine....maybe 5500 with a heavier than normal load. You may have to drop to a 17P on that size of boat and see what happens.
Another issue would be how they checked the tach for accuracy...it has to be checked at WOT..it may be only 50 off at Idle but 500 off at Top-end.
At 47-4800, that engine really isn't even in the power-band range...about 5500-up is when it produces the best...we just don't want to stay at 4800 or the engine will receive extra wear from higher temps...that's working the snot out of it...

 

[98SeaArk]

Re: Prop Pitch Help

thanks walleyehed... the tach was verified at WOT within 100RPM their tach read i think almost 4900 at WOT and the tach on the boat read 4800 at WOT.....Eventhough i do not run the boat at WOT for any period of more than a few hundred yards at a time. I usually cruise at about 3500-3800 RPM at about 35-39MPH. The hull weight on my boat is only 1100lbs and total weight w/ everything in it and me in it is almost 2000lbs. The engine is extremely efficient and get good gas mileage as is. Would it be better to replace the prop eventhough i do not run WOT hardly ever and is it hurting it to run this prop at my normal cruise speed?

 

[Dhadley]

Re: Prop Pitch Help

If we knew your accurate top speed we could calculate the % of slip. That will tell a lot.

Running the motor wide open for a second or a whole day makes no difference. Lugging is lugging. If you're lugging the motor at full throttle, it's being lugged at 1/2 throttle.

If your top rpm is 4800 we're looking for 1000 rpm. That would mean dropping pitch by 5" to a 16" and that doesn't make sense. What altitude are you at? How did they verify your tach? On a dyno or in the water?

 

[98SeaArk]

Re: Prop Pitch Help

tach was verified in their test tank. My top Speed by GPS is 54MPH at 4800RPM. I'm in Alabama so i do not believe i'm at that high of altitude. I mean the engine has been running flawlessly for almost 10 years and runs like a top i'm just trying to get to the bottom of this and decide on a new prop to get maximum performance eventhough i will loose speed.

 

[Silvertip]

Re: Prop Pitch Help

You asked what lugging was. Taking off from a stop sign in high gear in a car is lugging the engine. Losing speed on a hill even though the gas pedal is on the floor is also lugging the engine. Automatic transmissions obviously don't allow that to happen as they are smart enough to know when to downshift.