Prop to top, bottom or middle of rpm range?

[Finnerty]

My Mk3 7.4L is spec'd max 3800-4200 WOT
It powers a heavy (10000 lb) single shaft v-drive planing hull
Current 19X18 3b prop gives 22.5 knots WOT at 3850.

It was suggested to me that I may be lugging the engine running it at a 3200 cruise(16 knots) with this prop .

I'm considering a reprop to 4000 or should I try to get closer to 4200 for best engine life, best low and mid-range
cruise?

 

[steelespike]

While not lugging much if any your surely at the bottom edge of your rpm.
I think shoot for 4000 wot.I think the boat will feel lighter,may stay on plane easier.

 

[jerryjerry05]

The motor is made to run 4200 with an average load. A ​load of 40-50% fuel and 1-2 people and minimum gear.
Then you should be maxing out at 4200.
If it won't reach 4200 then it's not gonna perform as well at 3800.

Now if your "bare bones" and only getting 3800 then reprop.
Is the boat slipped?
Dirty bottom? (My neighbor was searching for a performance problem, he swore the bottom was CLEAN!!!. Turns out the scum was just out of visual range and THICK. Dropped the rpm's by 2000.)
Bent prop?

Too much "stuff" knocks the rpm's down too. Unload the extra.

 

[Texasmark]

I don't think 4 strokers have similar problems with needing the rpms to perform as do (usually) low cubed 2 strokers, but dropping the pitch to up the rpms surely won't hurt, will improve your mpg if carbureted (just like in a car when you get on level ground and back off the gas to maintain speed) and as steel said improve cruise performance and boost hole shot response. Depending on the rig, you may be able to drop the pitch and maintain your current WOT speed at the newly increased rpms.

 

[flyingscott]

Rule of thumb is light load you want to be at the top of your rpm range that way when you load up the boat you stay in the recommended range. I don't like it when people say lugging new motors are built to take it. If you watch a runabout with an outboard or I/O pull a wake board that boat is plowing to make the biggest wake. That motor is technically lugging but to prop it to run that way is not feasible motors can take it because manufacturers are building them better than ever. One other observation you cannot run a boat flat out all the time so any other speed is fine and not lugging.

 

[Silvertip]

Rule of thumb is light load you want to be at the top of your rpm range that way when you load up the boat you stay in the recommended range. I don't like it when people say lugging new motors are built to take it. If you watch a runabout with an outboard or I/O pull a wake board that boat is plowing to make the biggest wake. That motor is technically lugging but to prop it to run that way is not feasible motors can take it because manufacturers are building them better than ever. One other observation you cannot run a boat flat out all the time so any other speed is fine and not lugging.

Install a vacuum gauge on the engine and you will quickly see when an engine is lugging and when it is not. The lower the gauge reads the more the engine is lugging.​

 

[fhhuber]

I would prop such that full throttle CAN make it to red line, max throttle on plane straight line smooth water, then adjust the throttle linkage to prevent it.

 

[Texasmark]

Install a vacuum gauge on the engine and you will quickly see when an engine is lugging and when it is not. The lower the gauge reads the more the engine is lugging.​

Sir, I'll bet you drove a car with vacuum wipers that would stop every time you stomped the gas to pass somebody......so you had to do that then part way out in the other lane let off the gas for a couple of seconds, and get 2 or 3 wipes and hammer down again. Oh and you had to roll down your window (rain or shine, sleet or snow) to stick your arm out signaling a left turn out. Those were the days.

 

[Sea Rider]

Used to install vacuum gauges on car's carb or carb manifold for fuel economy, gauge reading controlled the amount of gas being sucked by engine when steeping on accelerator, the more you accelerate, the more it sucks. Very usefull to tune carb idle, check overall engine conditions along spotting hidden issues too.

Happy Boating

 

[Texasmark]

Used to install vacuum gauges on car's carb or carb manifold for fuel economy, gauge reading controlled the amount of gas being sucked by engine when steeping on accelerator, the more you accelerate, the more it sucks. Very usefull to tune carb idle, check overall engine conditions along spotting hidden issues too.

Happy Boating

That's what we used them for.

 

[flyingscott]

If you prop towards the top of the rpm range you may not lose speed as you will be turning the prop faster and you will gain holeshot.