What pitch prop

[MBH]

Right now I have a 15p ss prop on my merc 40 and the tiny tac said it was 4580 rpms at wot. I would like to gain more top end speed

 

[turbotodd]

What is the max RPM for your particular motor?

If it was 5000 max, a 14" prop would get you real close. If 5500, drop to a 13". If 6000, somewhere between a 12" and a 13" (probably a 13"). Just guessing here, but at least with your RPM data, it should be somewhat educated. I don't know how much top end speed you're going to gain, maybe a couple MPH, but if your max RPM is supposed to be 6000 and you're at 4600 now, you're really lugging the engine. If you dropped down to a 13" prop to get the RPM up, it'd probably pick up a mph or two AND come out of the hole better AND use less fuel at cruising speed. A win-win-win.

https://www.rbbi.com/folders/prop/propcalc.htm

That's a good calculator program that will usually get you real close. I put mine in there and it was spot-on to the tenth of a MPH (2.08 gear, 6080 RPM, 11" pitch and 6% slip). The GPS data showed a constant 28.6 mph with an occasional gust of wind pushing me to a 29.0-29.1. I had good air that day.

 

[chrispy186]

turbotodd » Yesterday, 22:26[/url]"]What is the max RPM for your particular motor?

If it was 5000 max, a 14" prop would get you real close. If 5500, drop to a 13". If 6000, somewhere between a 12" and a 13" (probably a 13"). Just guessing here, but at least with your RPM data, it should be somewhat educated. I don't know how much top end speed you're going to gain, maybe a couple MPH, but if your max RPM is supposed to be 6000 and you're at 4600 now, you're really lugging the engine. If you dropped down to a 13" prop to get the RPM up, it'd probably pick up a mph or two AND come out of the hole better AND use less fuel at cruising speed. A win-win-win.

https://www.rbbi.com/folders/prop/propcalc.htm

That's a good calculator program that will usually get you real close. I put mine in there and it was spot-on to the tenth of a MPH (2.08 gear, 6080 RPM, 11" pitch and 6% slip). The GPS data showed a constant 28.6 mph with an occasional gust of wind pushing me to a 29.0-29.1. I had good air that day.

This always confuses me. I just used the calculator, and everything I put in said if I dropped down in pitch, I would gain RPM but lose speed. Going up in pitch would gain speed but lose RPM. Right now im running around 5125 RPM's, with max being 5500. Im also looking to get more speed, but also dont want to go too low in the RPM range.

 

[chevyrulz]

the higher the pitch, the higher the top speed, at the expense of "holeshot" (acceleration). so a higher pitch prop will get you more top speed IF you don't go so high in pitch that the boat won't get onto plane. the higher the pitch, the less WOT rpms you'll turn, which can overwork your motor if you get outside the recommended WOT range. so if your motor's recommended WOT rpm range is 4500-5500 (which it probably is), then you shouldn't go any higher pitch than your 15 because you say the WOT rpm is already 4500. you can, but you shouldn't for engine longevity

the better idea would be to get that prop "worked" so that it turns more RPMS & gives you more top speed, and/or installing hydraulic tilt/trim will allow you to trim up to gain more RPMS, probably a few hundred, & a few mph. those, along w/ the obvious removing weight, will be the more reliable way to gain top speed without overworking your motor

 

[MBH]

So if I go from ss to alum prop I should stay at a 15p

 

[chevyrulz]

that's what i'd do

 

[MBH]

I mostly run in a river that is known to drop or rise within hours.. floating logs and such. I always heard that a alum prop will break before the lower unit..

 

[chevyrulz]

that's true, your more likely to cause damage to the lower unit internals by smacking a rock w/ a ss prop versus an alum prop

but if you run rocks, you should look into a river runner, it bolts on to protect the prop

or better yet, a jet lower

or just run an alum prop & keep a spare prop on board w/ tools