lost 900 rpm going from aluminum to stainless..... help!

[morgandelong]

Hey guys, new to the forums here, looking to reach out to some experienced prop guys. I have a 2009 alumacraft dominator sport 175 with a 115 yamaha F115 fourstroke. I was running a aluminum 13x19K seeing 5900 rpm on the tach at 37.5 mph with 2 adults and 2 kids, trolling motor on bow and 2 mid mounted batteries. Motor is set at second hole with cavitation plate level or maybe a touch higher than bottom of hull. I didn't like the way the boat cavitated easily in corners and didnt seem to trim out well with spray at about the console from side of boat. I bought a 13.25x17 turning point stainless express mach 3 with more rake and cupping for bow lift. I ran it yesterday and was only able to pull 5100 rpm at 38 mph with same load. What is going on! I am looking for a stainless prop to put me in the 5800 to 6000 rpm range and would be nice to see 39 to 40 mph with better handling. What are my options now?? Would a solas or other type of prop in the same size and pitch as the mach 3 with less rake and cupping help me out?

 

[ondarvr]

There is no real way to gauge how two different props will work until they've been tried on the exact boat/motor combo, so can you return the prop and get the next lower pitch? If not you can have the current one re-pitched to help dial it in.

​Just so everyone is on the same page when it comes to using the terms, what you are experiencing with the other prop is ventilation, cavitation is something very different, but the terms are often confused.

 

[morgandelong]

thanks for the reply. I am struggling with location, we have no prop shop near us and the dealership here is quite useless. The prop i bought i can return and can try another one, just takes time. The next lower pitch in that brand of prop is the 13.8 x 15. I feel that isn't going to help me with top end at all... but i could be wrong. Seems to me that plain 13x17 stainless should do me well but i just dont know what brand or what type of rake/cup i may need. I found it crazy that going down two pitches and went with that style of prop brought my RPM down to an unuseable level.... crazy

 

[ondarvr]

If your numbers are correct the 15" may work well for you.

​Not all manufactures measure pitch in the same way, then you change material and design, this opens the door to very different results than what the listed numbers may lead you to believe.

 

[jimmbo]

Was the Aluminum prop cupped? As ondarvr said switching prop brands or even prop lines within the brand makes for apples to oranges comparisons

 

[morgandelong]

thanks guys, lots of good info. The prop i ran before was a standard yamaha 13x19K aluminum prop. Had no cupping to it at all. is it possible a 13.8x15 prop could produce top end i'm looking for? Its a fishing boat so i'm not expecting crazy performance out of a f115 but would like to see good solid numbers with proper RPM numbers. if its possible to see a little higher MPH with a 13.8x15 i would try it. whats the advantage of the larger diameter prop vs the smaller diameter and more pitch

 

[Sea Rider]

If with 2 kids and 2 adults up assuming it's your current fixed load and OB achieving 5900 wot rpm from max 6000 wot range, you're already there. If wanting slight more top end, repitch + 1/2 pitch to 19.5 at a prop shop. OB will lose slight hole shot but will gain slight more top end speed. If it were my combo would saty with current OEM prop. OEM props works fantastic well.....

As many have found out it's not all aboutt sitting an OB on transom and voil?, some will need to be transon height fine tunned for combo to work as expected. OB would benefit going per an OB/transom height optimization. An OB well height seated on transom should not have prop aereation issues at close turns.

If possible post a pic of OB shot sideways showing both upper and lower horizontal plates.


Happy Boating

 

[morgandelong]

I'm out of town right now so unable to get pictures. I am told that this hull design is prone to blow out with aluminum props. I am in the 2nd hole right now... do you think if i went to the 3rd hole with this 13.25 x 17 mach 3 I will see a big enough increase in rpm from 5100? I do understand I'm at the best rpm right now with my aluminum but I would like to go stainless for the all around better performance they provide.

 

[flyingscott]

I am curious why are you not looking at yamaha stainless props for your boat. They make very good props and might take some of the guess work out of it.

 

[morgandelong]

I was looking at them but they are 750 cdn plus tax. I can get a solas or another turning point for 400. Does anyone know if the solas new saturn is similar to the yamaha reliance Prop? Do the yamaha props have much rake or cupping? I can only assume that's what brought my rpm down so much with the mach 3 express??

 

[Sea Rider]

What can do for the moment is go on a wot spin as usually loaded on flat calm, no wind watr cond, when combo is on plane, trim OB for OB to sit at 90?, once there hull, prop shaft, AV plate will ride parallel to water surface. Perform close turns at speed, if prop aereates, OB sits too high on transom.

If in need to raise OB up, cut transom down to cancel areation issues it's another story.. This test will determine if OB sits at the sweet OB/transom height. Any standard Al , SS prop will achieve aeration issues if OB sits too high, due to prop spinning with low water bed level at lower leg. It's like swimming with both hands while having fingers wide open...

Happy Boating

 

[ondarvr]

Dropping the motor (if possible) will reduce ventilation on turns with AL prop, but may also reduce top speed and increase bow rise, a SS cupped prop can typically be mounted higher and not ventilate. The SS prop may also increase performance a bit, nothing extreme, but a bit, so it just depends on how much you want to spend to get that last bit of performance, both ways work.

 

[morgandelong]

Ok thanks guys, gave me some things to try.

 

[gm280]

I realize that everybody strives for that very last MPH figure. I know, I did the very same thing with my Bass Boat. But in all honesty, rarely did I ever use that WOT capability after getting that last drop out of the boat/prop combination. I raised the engine with a jack plate, changed props and achieved that 63 MPH speed at WOT. But 99% of the time I ran in the 30 to 40 MPH range. Better gas mileage and easier to drive situation. But to each their own. However, I wouldn't spend tons of money for one mile per hour difference. JMHO

 

[morgandelong]

I agree 100%, I figure the better it runs at WOT the better it cruises as well, could be wrong though

 

[SkiDad]

i didn't read the whole thread but I had a 13.75x15 turning point on my last boat and it ran about 5300 and 38 mph if i re-call correctly. It acted like a 17p rpm wise. It might be the ticket for you b/c of the rpms you are telling us. I don't think it's gonna get you to 40 mph though. A better prop for you would be a Powertech - i would contact them for the pitch and style you need.