7 to 8 pitch change, big difference?

[huntin dawg]

Gents,
I have a 7 pitch prop on my 7.5 HP Merc right now and I just bought a spare that is 8 pitch. Will there be a noticeable difference in performance between the two?

Thanks,
HD

 

[Barnacle_Bill]

Re: 7 to 8 pitch change, big difference?

I doubt you will notice much difference. Motor should turn about 200 RPM slower at WOT.

 

[huntin dawg]

Re: 7 to 8 pitch change, big difference?

How will it effect boat speed?

Thanks,
HD

 

[CaptainHook]

Re: 7 to 8 pitch change, big difference?

Common rule is about a 200-250 rpm change per inch of pitch (as long as your staying the same with diameter).

Performance will be affected depending on the operating rpm of the motor. If it is spinning 5500 rpm now and drops to say 5300 and that is still in the manufacturers spec'd WOT range then you most likely won't really notice. But if your low in the range now say 5000 rpm then you will see the boat being slower if it drops out of the range where it makes peak hp.

 

[steelespike]

Re: 7 to 8 pitch change, big difference?

What type boat/load?
On small motors a 1" pitch change is signifigant.As suggested if you are at the upper end of the rpm range now it may be ok.It will reduce your ability to push a load.
Speed depends on the ability to push the boat..It is possible speed may be up with a light load I would try it out. As a spare it may be ok just may be slower.If the 7" struggles the 8 will really struggle.

 

[huntin dawg]

Re: 7 to 8 pitch change, big difference?

What type boat/load?
On small motors a 1" pitch change is signifigant.As suggested if you are at the upper end of the rpm range now it may be ok.It will reduce your ability to push a load.
Speed depends on the ability to push the boat..It is possible speed may be up with a light load I would try it out. As a spare it may be ok just may be slower.If the 7" struggles the 8 will really struggle.

It will be pushing a small 11' jon boat. During fishing season it will be light but duck season I'll be loaded down pretty good.

HD

 

[Mischief Managed]

Re: 7 to 8 pitch change, big difference?

Common rule is about a 200-250 rpm change per inch of pitch (as long as your staying the same with diameter).

I suspect you'll see a bigger difference than 200-250 RPM. The percentage of the difference in pitch is huge. For example: Going from 27" to 28" in pitch will only change RPM 3.7% Going from 7" to 8" in pitch will change RPM 14.3% If you were running 5000 RPM with 7", you will only run 4285 RPM with 8", assuming flat engine torque, the same load, and the same prop slip.

 

[steelespike]

Re: 7 to 8 pitch change, big difference?

I would suggest trying both props and run the one that suits each purpose best
and keep the other as a spare.