Prop pitch selection question on a 30hp Johnson

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

yeah 400-500 would put me within range when I am running with a heavy load but I would be SOL when running light still. Or I could swap props for different applications. I want a prop that lands me in between on both so I don't have to do that though. I have heard mixed opinions that the cupping will take away from top speed though?

And yes dropping it 800 seemed unrealistic to me but this guy called himself "the prop god" lol......

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

We're being too picky.You can't prop for two different loads with one prop.Prop for the light load probably the 14".
let the heavy fall where it may.The heavy is secondary and isn't going to bother the motor.
Ask the "prop god what his mods will do for you.holeshot?Speed? handling?mid range?
Also is it cost effective?

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

Yeah I am probably getting greedy lol...... Yeah he quoted me at $20 bucks to cup the prop which seemed really reasonable to me since I have had the prop on CL for a week now and only had one call (wrong spline for the guy). Wife is not going to let me spend another $150-200 on a prop unless I get $140 or so for this brand new one I have. I bought a minn kota ipilot online 2 weeks ago without her knowing and I am still in the dog house for that move lol.... She doesn't understand how good a deal $604 for a new (2011) ipilot 55lb thrust PDV2 was!

I am going to call the prop god again this morning and have another chat with him before I bring the prop to him on Monday. Will update what he tells me.

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

Sure seems like a deal.It will be very interesting to see the results.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

So I take the boat out today just to play with the new trolling motor and I ran the boat with about 50lbs more weight than the light load I posted in the OP and with the 13 pitch on I ran WOT at 5850.....ugh and when I trimmed the motor back to its highest notch I ran at 5910 RPM. So I am now thinking I can turn a 14 pitch aluminum easily and probably a 15 pitch stainless. Or my tach is screwed up and I am not getting anywhere with this lol...... I plan on getting some GPS readings on Monday so maybe that will help.

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

delete

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

Look at it positively.If you throttle back to 5800 you'll be doing about 28 at 12% slip
with plenty of get up and go when all loaded up.
If the "Prop God" can turn the prop into performing like a 14 you should be at about 5700
at 12% around 30 mph.Hope I'm not being too optimistic.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

How exactly do you calculate the prop slip steelspike? And I think your about correct with your calculations. I traveled 3.4 miles in right at 7 minutes today (give or take 20 seconds or so) and in my calculations I was doing 29.14 MPH.

When I first ran the motor on the lake today I was at 5710 WOT and as I ran it more the RPM's started climbing. This motor only has about 5hrs run time after sitting for 3 years so the seafoam may be doing its trick and loosing up the motor and cleaning it out : ) Also I noticed that in a hard turn at WOT the prop would loose grip on the water and slip, not the blowout I had first thought. This may be due to the aluminum prop not having any cupping or very little?

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

Was the distance setup using a gps? Measured mile(s) usually aren't too accurate but can be an aid in determining approximate speed.
I use a prop calculator. It has 5 parameters, rpm,gear ratio,prop pitch,slip and speed. You put in 4 and it will give you the 5th.
In your case we had the prop,rpm and ratio I guessed 12% slip to get the reasonable speed.
Normally we would have had speed and would use that calculation to get an actual indication of slip.
Slip or per sieved slip can help us determine if a setup works.Slip can vary if a tach isn't accurate or if speed is not right
,thus the need for gps speed.Poor setup bad prop match etc So the whole idea is to pinpoint the weakiness.
Props can have high slip, heavy boats usually have higher slip As prop speed goes down slip usually goes up.
A certain amount of judgement may be needed to evaluate calculator results.
some times a defective tach might indicate very low or high rpm.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

The distance was done on a distance calculator on a map that measures down to a hundreth of a mile. I ran the boat at WOT from 2 points on the lake. I could have been off by a tenth of a mile either way on my measurement and like I said I didn't have exact time. It was just a close estimate. 28-30 mph is what I think I am running but Monday (or maybe today?) I am going to get a GPS mph.

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

Does seem pretty close.
I think 28-30 would be pretty typical.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

Ok so here it is...... 28.65 at 5840 RPM with 950lbs and a 10X13 aluminum prop. It would fluctuate up and down depending on the wind, waves and RPM but it ranged from 28.2 at 5780 RPM to 28.7 at 5860 RPM but the avg was 28.6 at 5840

So how do I get her to 30 MPH.......lol

If I did the prop slip calculations correct I believe I am at 14% slip?

 

[1080tommy1]

Re: Prop pitch selection question on a 30hp Johnson

This is a great thread and great information. I am just starting to get into attempting to prop my 40hp Yamaha and am kind of intimidated by all the different possibilities. The prop I have on there now is a 12 1/4X9 and just installed a Tiny Tach. I can tell just by the sound of the motor she is over reving and will have it out hopefully this week for a test with the tach so I have an rpm starting point. I was thinking a 12 pitch would do it but seeing your numbers I am now thinking I will be in the 13-14 pitch range. I will be running light 50% and heavy 50% because it is a hunting boat. When I fish I only have 2 people but when hunting I have an additional probably 300 lbs. Thanks for this very informative thread and you have definitely enlightened me a little bit.

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

14%, thats what I came up with.High end of normal.
I know you were half kidding about 30 mph but if we could get your slip down to 6%
,a reasonable target to me, your speed would be about 31.
You might discuss it with a prop shop just a little cup added to the 13 al prop might do the job.
Also did we discuss motor height?

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

We'll be glad to help you out. But you need to start your on thread. You'll need to give us the info needed in "read this first" at the top of the prop main page.Get us the info,we'll get you going.

This is a great thread and great information. I am just starting to get into attempting to prop my 40hp Yamaha and am kind of intimidated by all the different possibilities. The prop I have on there now is a 12 1/4X9 and just installed a Tiny Tach. I can tell just by the sound of the motor she is over reving and will have it out hopefully this week for a test with the tach so I have an rpm starting point. I was thinking a 12 pitch would do it but seeing your numbers I am now thinking I will be in the 13-14 pitch range. I will be running light 50% and heavy 50% because it is a hunting boat. When I fish I only have 2 people but when hunting I have an additional probably 300 lbs. Thanks for this very informative thread and you have definitely enlightened me a little bit.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

Actually I was serious about the 30mph And with some of the prop calcs I did late last night I think I can get to 32mph if I go up in pitch to a 14 pitch stainless and put a little cupping on it to get the RPM down to 5650-5700. I say this because I believe I can turn a 14 stainless at about 5900 RPM so a little cupping for grip and RPM drop

1080tommy, I really don't mind you posting in here since the thread is almost done but you should start your own so you get more response to your needs and questions. I would like to see your RPM's with that 9 pitch, it seems really low pitch for a 40 hp? Unless your pushing a pontoon?

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

Ok steelspike so I was going to just buy a 14 SS prop because form the knowledge I have that would get me highest MPH within RPM range. But I talked to this "prop god" again today and he seems to want to argue with me saying I should cup the 11pitch SS I have and that would give me best performance. Now this does not seem possible to me from what I have learned? Not sure how an 11 could outperform a 14 at the same RPM range? He wants to cup the 11 to get me at about 5600-5700rpm (remember it was 6290 to start with a heavy load 1150lbs) but if I can turn a 14 SS at the same 5600-5700 RPM theoretically the 14 should go much faster than the 11 no matter what right?

This guy told me I didn't know what I was talking about which kinda got under my skin a little. But maybe he is right and I know less than I thought about propellers?

 

[steelespike]

Re: Prop pitch selection question on a 30hp Johnson

Rule of thumb 200 rpm per inch. Keep in mind I've seen about 100 and almost 300.
And then there is "the faster you go the harder it is to go faster".
However I think the most speed light load would probably be the 14ss.
Don't forget as you add cup and and or pitch you slow hole shot.If you have good hole shot
now it probably won't bother.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

Ok new development!! Took the Cablela's stainless 11 pitch to a prop guy and the prop is a 9.767 pitch prop not an 11! This would explain the 6290 RPM reading I got. I am a little peeved at Cabelas right now. I'm going there tonight to get my money back! That prop has caused me a lot of headache.

 

[Johny25]

Re: Prop pitch selection question on a 30hp Johnson

Ok so I got my money back from Cabela's and I ordered a new solas saturn 10X14 stainless last night. Will updated results when I run the prop. This thread is probably to long and over thought out...lol. Thanks for being patient with me through this thread steelspike : )