new 4 blade-rpm's too low

[sabastianunf]

Posted before when deciding on a prop - now I have it but I am not turning as many rpm's as I would like to. I was running a 17p 3 blade at 4200 rpms at wot on my 120 merc. sterndrive. I purchased a vortex 4 blade 16p that is cupped. Now I am only hitting 3900 rpm's. Merc reccomends 3900-4300 rpms at wot. My biggest dissapointment is that when launching I only get @3200 rpms. It still pulls better than the 3 blade but I think it can perform better if it turned a little faster. I am using the prop only for wakeboarding and want the best hole-shot possible and I am not concerned about top end at all - as long as I can hit 23 mph - currently hitting 29 mph. I was wondering if I should get the prop pitched down to 14 which would bring rpms up to the top of the range or if I should drill holes to ventalate the prop to improve low speed rpm's. Which would get me the best hole shot with the least wear/stress on the engine? Thanks for any and all good advice.

 

[walleyehed]

Re: new 4 blade-rpm's too low

One of the problems may have been that your 17 was un-cupped and less efficient, plus maybe some wear. The cupped 16 is going to drop the R's in this case. I would be careful of the 14P unless you are willing to send it in if it provides too many RPM when you get into the powerband. It could be "fine-tuned" by doing so. This is one of the problems we face trying to hit an exact match on application.

 

[sabastianunf]

Re: new 4 blade-rpm's too low

so you think it would be better to ajust the pitch rather than drill the holes?

 

[WillyBWright]

Re: new 4 blade-rpm's too low

Ventilated props don't do well on sterndrives. Have it repitched. In general, 4-blades don't do well on four-bangers. But Mercury's High-Five props do (you'd have to get plugs for the vents). That's because the blades are rather small. The typical 4-blade blades are usually about the same size as blades on a 3-blade. That fourth blade adds a lot of additional drag.

 

[sabastianunf]

Re: new 4 blade-rpm's too low

Thanks for the help y'all. I guess the next question is should I do down 1 or 2 inches. I think 2 inches would put me 100 rpms over the rec wot range. How bad would this be for the motor? If I use throttle to keep rpm's within range would I be damaging anything? Also I think I read on this forum that merc may have raised the rec. wot rpm range on some of thier motors. Mine says 3900-4300 rpms on the valve cover - is this correct for a 1978 120hp merc sterndrive?

 

[walleyehed]

Re: new 4 blade-rpm's too low

4400RPM will cause no issues as long as the tach is reasonably accurate (within a hundred RPM)

 

[sabastianunf]

Re: new 4 blade-rpm's too low

I just have one more question for y'all. If I multiply the rpm's by the pitch of the prop I can - in theory without slippage - calculate how many of inches per minute I will travel. When running these #'s at launch i come up with:
3200x16=51,200
3400x15=51,000
3600x14=50,400
and at wot
3900x16=62,400
4100x15=61,500
4300x14=60,200
I know these are theoretical #'s that do not account for slippage but looking at these #'s it seems like I am getting the best performance with the prop at a 16 pitch. Is that correct or by going to a 15 pitch and being in the middle of my power band would I get better performance or less engine wear not shown by these #'s?