Wot rpm below spec

[bigman348]

I have a Honda 40 4stroke and wot is 5200 rpm at 25.5 mph on my 18ft crestliner Nordic. I have read that my operating range is between 5,500 and 6,000 rpm. My question is should I change prop to get into the 5800 rpm range? I get on plane pretty well and am satisfied with 25.5 mph but wouldn't want to go any slower. And from what I read if u want to gain rpm I am going to lose speed. So if I leave it will I hurt anything running at a lower rpm? And if I do change prop to put in correct range will I loose speed or will the increase in rpm offset the changes that the pitch will make?

Thanks in advance,
Matt

 

[BonairII]

Re: Wot rpm below spec

Change your prop. Drop a couple inches in pitch. Yes, you'll lose a tiny bit of speed. (maybe 1mph or so)

Currently....you're lugging your motor a bit. Keeping your motor healthy is much more important than trying to save 1-2mph.

 

[jestor68]

Re: Wot rpm below spec

Your WOT rpm range is 5000-6000 rpm. Your motor's 40 HP is rated at 5500 rpm.

Ideally, you want to turn at least 5500 rpm at WOT in order to develop full power throughout the rpm range.

At 5200 rpm, you are within the recommended rpm range, and will not hurt the motor; but the motor would be more responsive with the next size lower pitch prop(available in one inch increments) to get the WOT rpm closer to 5500 rpm. You should lose 1 mph by reducing 1 inch of pitch(and gaining about 200 rpm.

You present WOT rpm and speed indicates that you are running a 11/12 pitch now with your 2.08 gear ratio.

 

[bigman348]

Re: Wot rpm below spec

Thanks for the quick response and great info. I agree that I should run motor at full potential rather than at max speed. My current prop is 11 1/4 x 13. Is a stainless prop worth looking into? And should I drop 1 inch or 2?

Thanks again,
Matt

 

[BonairII]

Re: Wot rpm below spec

SS not worth it IMO. The small performance gain(if any)....isn't worth risking your lower unit. If you strike an underwater object....an aluminum prop will break before damaging your LU gears/prop shaft etc(in theory anyway).

A stainless prop is much tougher and may allow your prop shaft etc to get bent/damaged before failing.

JMO

 

[jestor68]

Re: Wot rpm below spec

To get all the way to your optimal rpm, it appears a 2 inch change is in order; 13 to 11 pitch. I recommend Solas aluminum props for the best bang for the buck; as their cutting edge design and casting process produces props that can actually perform as well as some SS props.

It looks like the Solas Amita 11-3/5 X 11(#331-116-11) would give predictable results, as it's almost the same diameter as your present prop. You should expect to gain 300-400 rpm. Right now your prop slip percentage is a little high(18%); probably due to too low a prop speed. I'm hoping that the better design of the Solas will get that slip percentage down to about the ten percent range, in which case you should still get about 25 mph due to the higher rpm. Your motor will be happier being able to develop it's full rated power.

Mercury engineers have conducted tests between aluminum and SS props and found no significant difference in shock loads to the drive system. I guess that's what the shock absorbing hub is for. Drive units are most often damaged by hitting something hard with the unit itself; not just a prop.

 

[bigman348]

Re: Wot rpm below spec

Thanks again for the info. I am gonna order the prop suggested tomorrow and will post results next time I take the boat out.

Matt

 

[bigman348]

Re: Wot rpm below spec

Got the new prop today. Excited to put it on but can't find a torque spec for prop nut. Google search and an iboats search has given me anything from finger tight to 100 ft lbs. I ordered a service manual but am not demonstrating very good patients and want to install prop and test tomorrow. Any help would be greatly appreciated.

 

[Watermann]

Re: Wot rpm below spec

I've always tightened my prop nut to where I just have to use a wrench to tighten the castle slot inline with the shaft hole after getting it finger tight. Not sure how you would ever get a correct torque having to align a slot with the shaft hole.

 

[steelespike]

Re: Wot rpm below spec

Quotes I've seen speaking generically are 40-50 ft lbs. My feeling is be sure its tight leaning toward slightly less is more.
Don't forget to put a little lube on the prop shaft.

 

[bigman348]

Re: Wot rpm below spec

Thanks again for all the help. Couldn't find a Honda spec anywhere for the prop nut. When I removed the nut it was just snug so that's what I went with. Took out today with new prop and am very happy with the results. WOT is 5800 rpm at 24.5 mph. Pops right up on plane and feel better that motor operating in correct rpm range. 2 inch change was just what I needed.