Diameter vs pitch

[zodiac pro9]

My pro9 is powered by a yamaha f60. I'm trying to get more top end. I'm running an 11 1/8 x 13 aluminum 3 blade prop. 32 mph at 5400 wot. I changed the prop to an 11 1/4 x 14 and picked up 3 mph while rpm stayed at 5400 rpm. I want to get more rpm and mph without lugging the motor. I'm trying for 5800 rpm and 40 mph. By going up one more inch I should pick up a few more mph and dropping an inch in diameter gain some rpm. I'm thinking a 10 x 15 prop. Any thoughts, opinions or experience would be appreciated

[Nos4r2]

Welcome to Ribnet.

Stick with standard diameter. You'll increase the RPM, but also increase slip if you drop the diameter.
Whats the max WOT rpm of your motor? I have a feeling you're not far off the max speed the boat will do at 35mph.

[zodiac pro9]

The maximum wot the motor is rated for is 6000. That's why I think there is some speed to be had

[zodiac pro9]

Two different boat dealers said I'd lose rpm by putting the bigger pitch on but it stayed the same AND picked up speed. .???

[Markyboyo]

As Nos said the general rule of thumb is to loose about 200 rpm per inch of pitch gained.
Had you increased your pitch but stuck with the same diameter then you may have seen an increase in speed but would definitely seen the WOT revs drop.
Smaller diameter props are easier to spin than larger diameter props of the same pitch but as nos pointed out this is because they slip more.

You might find that when very lightly loaded you could eek out a couple of extra mph from where you are now by dropping the dia and increasing the pitch but you will also notice the boat will take longer to plane and the performance will be diminished much more than is was when you run with a heavy load as the smaller diameter prop will slip much more when loaded up.

Providing your RPM doesn't drop too low with a heavy load I'd stick with the prop you have now. You are already doing very well if you are getting 35mph with a 60.

[9D280]

FWIW I had the standard merc 10" x 14 on and had to keep the throttle back so as not to over -rev & destroy the engine. (I was getting 6.2K instead of the 5.8K It's meant to top out at)

I went to a Yam 11.25 x 14 (yes, it's a Yam prop & I have had to modify the locking arrangement slightly, but it does fit!) - by keeping pitch the same & upping the diameter by the bet part of an inch I lost about 1000 rpm by going up a massive amout of dameter. On the plus side it now doesn't "let go" at the slightest provocation....

I guess you lucked out and got a tad more grip to offset the extra drag. Just shows how prop selection can't be done 100% by theory!


Now, I am still c. 200RPM short of my top RPM. Be you interested in punting the 13" prop? PM me if so.

[Nos4r2]

Originally Posted by zodiac pro9

The maximum wot the motor is rated for is 6000. That's why I think there is some speed to be had

You're highly unlikely to gain any RPM by going up in pitch if you keep the diameter the same. You may find if it's a different type of prop (stainless/ally/number of blades/make) or in better condition it'll stay the same (as you have) but in order to gain RPM and go up in pitch it'd have to be a very drastic change-eg from a 5 blade stainless 21" pitch prop to a 23" 3 blade ally. That increase in rpm will only be slip.

[boilermaker1]

I have a Yamaha f60 on a 4.5 M Tornado and I get 35 mph max. similar figures to yours. I think wot for these engine is 5000-5500 rpm so you are near max.
I do not you will gain much by changing prop