Need Your Thoughts

[David Young]

I messed up my old prop. It has no numbers or markings. My boat would do 35.8mph @ 5,500rpm. The factory repair manual said the standard prop from the factory was a 13 1/4 with a 17 pitch so i ordered a new Solas 13 1/4 17 pitch. My boat now does 34.4mph @ 5,000rpm. My boat 'planes' in 4 seconds with both props. I'm still happy with my top speed but i was wondering what pitch would bring my (WOT) rpm back up to 5,500? Thanks for your help

 

[batman99]

IMO, Solas aluminum props feel 1 pitch higher compared to different brands. Thus, your current Solars 17 feels like an 18 pitch. With this in mind, I bet a Solars aluminum15 pitch (which feels like a 16 pitch) would be the correct replacement fit. re: On average, each pitch change allows engine to run 200 rpms different. If wondering, I have a Solas aluminum 15 pitch 4-blade on my Johnson 115 OB and love it. Lots of hole shot, lots of slow RPM traction around the dock and great mid range control as well. The 4 blade is my main prop and my 3 blade is my spare.

Hope this helps.

 

[David Young]

I thought a higher number pitch gave a higher top speed but less torque. I know 'nothing' about props. I'm happy with my new prop but next time i'll order a 15 pitch. Do you think this wound give me back my 1 or so mph on top speed?

Thanks for your help

 

[steelespike]

Interesting using your old numbers calculating for prop size gets 17.42 pitch which I think with more accurate slip would calculate to a 17" pitch.
​ If we calculate with 5000 and the same numbers we get 31.8. If we calculate with 34.4 with the same numbers we get 5400.
If we calculate for slip using 5000 and the same numbers we get about 4% slip.highly un likely.Is it possible that your tests were aided by or hindered
by current or wind?
Is it possible you knocked the throttle out of adjustment while manipulation the motor during the prop change?

 

[David Young]

The speeds were done with a GPS on my wifes cell phone. Into a slight wind with both props. I'll check the linkage to see if i'm getting 'full' throttle. I think the tach is working right because i can tell by the sound the motor it isn't doing as much rpm with the new prop. Whats strange is i used 4100-4200 rpm as my cruising speed which was 26-27 mph with the old prop. The new prop is doing 29-30 mph at the same rpm.. Does this seem right?

 

[batman99]

I thought a higher number pitch gave a higher top speed but less torque. I know 'nothing' about props. I'm happy with my new prop but next time i'll order a 15 pitch. Do you think this wound give me back my 1 or so mph on top speed?

Thanks for your help

In a perfect linear calculation world, you are correct. Higher pitch prop would make water speed much faster. re: 19' makes boat go xx mph, 21" makes boat go even faster and 23' speed makes boat go even faster. Faster as in straight liner line on a graph paper. In reality, gearing doesn't work that way. Take for example a 10 speed peedle bike. In 5 gear, bike goes xx MPH speed, in 7 gear, ground speed is only a little faster but 10 gear makes it goes slower. Slower because "optimal" peddle RPM to resistance ratio isn't properly matched. Same results with motors as well. Too high of gear doesn't always make ground speed faster. Too high of gear can make ground speed slower. Thus, why boat speed (travel speed) is always set by its MAX WOT (engine RPM speed) first. Then, ground speed results 2nd.

Long mumblings short.... I set my Prop's size based on "average" loaded boat and against its engine max RPM number (based on WOT throttle state). Both at same time. Travel speed as in MPH results value (for my unique boat) is results of its optimal prop size. Your unique boat's prop size will vary...

Hope this helps.

 

[steelespike]

I think we know the prop[ seems to be loading the motor more than the old prop. So at tyne same rpm it may be faster.
But the prop apparently is not allowing the motor to rev as freely.How does the hole shot compare is it easier to keep on plane at reduced throttle just on planing speed.

 

[David Young]

This is getting over my head. I have an I.Q. of a rock :facepalm:. I'm getting full throttle, just checked. The old prop was made out of stainless steel. It weighed a lot more than the aluminum one i now have. I wish the old one had numbers on it so i would know more on where i started from. The boat planes right at 4 seconds with both props. I've never try to see what speed or rpm it holds plane. Most of the time i just let off at 4100-4200, my favorite cruising rpm.


Thank you all for your ideals.

 

[SkiDad]

Why don't you repair the old stainless?

 

[David Young]

Why don't you repair the old stainless?

My new prop just cost $120, i've read a SS prop would cost more than that to repair.

 

[David Young]

I've found the number of 384138 on my old prop. I searched this number only to find it was sold with a few different pitch, so i still don't know what pitch it is

 

[SkiDad]

i paid 160 recently to get mine done (which was the price for a repair) - it wasn't really beat up, so for that price I got it all tuned up and the pitch evened out and some cupping added - I can't say enough about the job that Sante Marine in Ohio did on it.

If you decide to go to a 15 solas that will get your rpm back up to close to what it was but I feel your speed will drop another mph.

 

[David Young]

Thanks to everyone for their help. I'll just stay with the Solas 17 pitch for now.