Switched from Aluminum to SS prop

[erictetterton]

i have a 1990 50hp johnson on a 1752 Alweld. I had an aluminum prop on it that was kinda worn and missing a little bit. This prop performed well, got up on plane fast and topped out at 32mph 4400rpm. The only marking i can find on this prop is the stamp 15. I will investigate further when i get it cleaned up.

I found a Stiletto SS prop on craigslist for $50 and it only had a couple small blemishes on it. I installed it and took the boat out on the water yesterday. When taking off, it seemed to struggle a lot compared to the other prop. It took close to 8 seconds to get on plane with the new prop compared to 3 seconds with the old one. At WOT my tach read 4400rpm and 29mph. I was told this prop was a 15 pitch but I haven't confirmed this to be true. I will see if i can find the markings on the prop later today when i get home.

I just installed new spark plugs and set the gap to .030 as recommended by the factory. I am going to install new plug wires next week to hopefully fix my hard to start issue. I installed the new plugs between switching the props, so i dont know if theres an issue with the new plugs or if i just have the wrong prop for my motor.

I will try to find out exact specs of each prop when i get home. I know thats the meat of this question and its left out as of now.

 

[Johny25]

You should be running that engine at 4500-5500rpm and closer to the top end of that is best. I would say 5000 or above for engine health. I cannot remember what the gear ratio on a 50hp was so I can't do the calcs properly right now but it seems like the SS is likely a higher pitch than your aluminum was

 

[Gotem]

The ss prop is most likely a different pitch than the aluminium.

 

[Johny25]

Ok assuming I have the correct info on your gear ratio of 12:29 also known as 2.41:1 then the only way I can get your posted numbers to come out reasonably is with a 19 and 21 pitch prop. This does not add up though. Either I am wrong about your gear ratio or your numbers are off somewhere.

Your RPM reading did not change although your MPH and holeshot did? This does not seem right at all if you changed pitch, your RPM should have changed drastically given the description you gave regarding sluggish out of the hole. Check your tach and find out the pitch on those props

 

[erictetterton]

sorry for the delay in response. I just got home and pulled the prop off and the Stiletto has written on it ST22015. 22 pitch I'm guessing?

The aluminum one I had up there says PropCo J55 15SP

Do these sound right for the numbers I have provided or do I have an issue with my motor?

 

[MDCrappie]

erictetterton » Mon Jun 10, 2013 4:43 pm[/url]"]sorry for the delay in response. I just got home and pulled the prop off and the Stiletto has written on it ST22015. 22 pitch I'm guessing?

The aluminum one I had up there says PropCo J55 15SP

Do these sound right for the numbers I have provided or do I have an issue with my motor?

That Stiletto prop is 15 pitch as is the Propco

Sounds like the Stiletto is designed for a Pontoon boat

https://www.boatownersworld.com/Props/stiletto/star_series.htm

Also - sounds like your tach is off
Your L/U ratio is 2.41 - If you were spinning a 15" prop at 4400rpm thru these gears you would be moving 26mph at 100% efficiency
If you were spinning 5500rpm you would be moving at 32.4 with 100% efficiency.

Check your tach and also are you using GPS?

 

[erictetterton]

OK now I understand. Yea I have a GPS and it says 32-33mph WOT with the aluminum. Not sure how to mess with the tach but I'll check it out tomorrow

 

[erictetterton]

I'm thinking about putting this prop on eBay and getting a new SS one that would perform better for my boat. Any suggestions as to which pitch prop would work good for my setup? I found a 13 spline 11 3/4 X 17 pitch on craigslist that came off a 50 johnson. Would the higher the pitch give me more top end and less at holeshot, or the other way around?

 

[MDCrappie]

erictetterton » Mon Jun 10, 2013 7:33 pm[/url]"]I'm thinking about putting this prop on eBay and getting a new SS one that would perform better for my boat. Any suggestions as to which pitch prop would work good for my setup? I found a 13 spline 11 3/4 X 17 pitch on craigslist that came off a 50 johnson. Would the higher the pitch give me more top end and less at holeshot, or the other way around?

The higher pitch will make your holeshot slower. Think of putting a 10 speed in 10th gear and starting to pedal. The lower the pitch the more power.

Check the back of your tach and see what the pole setting is on. Pretty sure the OMC's are 6.

If you're getting over 32 mph, and you're L/U is the 2.41 you are likely spinning 6000rpm, but this does not coordinate with your slow holeshot.....unless your boat is extremely stern heavy or you are not trimming the motor all the way against the transom when taking off.
Do you have power trim? If not the motor tilt pin could be set too far from the transom, causing the slow holeshot.
If this is the case then you would need a higher pitch.

 

[erictetterton]

Yes I have pow'r tran tilt/trim and I trim it all the way down on takeoff. I am very stern heavy with 3 batteries and 12gallon gas can in the back. So you're saying that a 17 pitch would be better than a 15 for my application?

Thanks for the info on the tach. I'm going to mess with it tomorrow evening

 

[MDCrappie]

erictetterton » Mon Jun 10, 2013 8:16 pm[/url]"]Yes I have pow'r tran tilt/trim and I trim it all the way down on takeoff. I am very stern heavy with 3 batteries and 12gallon gas can in the back. So you're saying that a 17 pitch would be better than a 15 for my application?

Thanks for the info on the tach. I'm going to mess with it tomorrow evening

Can't say for sure 17" would be better without knowing your true numbers. You need an accurate measure on your RPM at WOT. IF you are in deed spinning 6K then the 17 would be better. You would then need something like transom wedges, a hyrdofoil or tabs (or re-arrange weight) to get the bow down on take off. Even drilling the barrel of the prop could help provide some slip on take off. (Don't try doing this on your own though, unless you know what you're doing)

 

[Johny25]

Yes there are incorrect numbers here somewhere.......and without a good tach and mph reading nobody can give good advice on what prop would be best

 

[erictetterton]

I checked my tach and it was indeed set on 6. I took the boat out again yesterday and it was reading 4800rpm WOT in really strong current. I found a stiletto triad 2 17pitch for my motor on craigslist for $120. I may buy it this weekend

 

[Johny25]

4800rpm is still on the low end of your RPM range so going to a 17 pitch would just make it worse. You want that motor at 5000rpm or above otherwise you are just going to be lugging your motor. I would not advise buying any props until you get solid information on RPM and MPH

 

[Quackrstackr]

A SS prop won't flex like an AL will so it runs at a true pitch unlike an AL.

You're under rpm'd as it is if your tach is correct. I would be leary of going to anything in a larger pitch if so. You may need to drop down to a 13p SS.

There are a lot of factors when it comes to props besides simply the pitch. Blade rake, cup and physical blade size all play into it.

 

[erictetterton]

Took the boat out today and still can't get over 4800rpm. I wonder if I should buy a new tach?

 

[erictetterton]

How could I get an accurate rpm reading?

 

[MDCrappie]

erictetterton » Tue Jun 18, 2013 3:34 pm[/url]"]How could I get an accurate rpm reading?

Borrow a shop tach from someone and compare it to your in dash while running.

 

[Johny25]

Tiny tach runs about $50 online