Prop for Yamaha F25 on Lund SSV-14

[stever07]

Trying to solve two issues.

First with an average load the outboard is going into overrev protection, this is in every trim position. Slight blowout in sharp turns.

Second with a light load, just myself and the essentials, the boat porpoises for a bit once coming on plane, then stops and I'm guessing cavitates or ventilates because I can't get an accurate rpm reading, have to back off the throttle to 5800rpm to make the issue subside, this is with the outboard trimmed in all the way. Have tried all trim positions. Huge blowout in turns.

The details:

Propeller: stock Yamaha aluminium 10ish" diameter with 11" pitch
WOT with average load: 6230rpm then overrev protection at 28mph
Outboard: 2015 Yamaha F25 25hp 20" shaft
Boat: 2015 Lund SSV-14 tiller 14'2? length 295lbs 20" transom

Looking to replace with stainless steel, open to 4 or 3 blade suggestions.

Thanks for the help!

Steve

 

[steelespike]

When changing props there is always some unknown results. When you go to a SS prop you introduce another unknown result.
When going to a SS prop it is usually suggested you go down one inch in pitch.But you need to lose some rpm so I think perhaps an 11 or 12"
SS prop.If you boat where you are likely to strike rocks the SS is costly to repair.If your likely to strike mud or sand the SS will hold up better.
The SS should resist venting. In an aluminum I think a 12" would be a good choice.The Solas amita has a good reputation for predictable results.

 

[stever07]

Thank you for the response. Pretty much confirms my train of thought.

I'm dead set on stainless for the longevity and there is little chance of striking rocks. Go in with electric to scout out new waters.

My biggest issue is I don't exactly know my WOT rpm because of overrev protection. Would it max out at 6230 or 6500, etc.

Is the following true to maintain current WOT rpm? Switching to stainless, drop 1" pitch. Switching to cupped prop, drop 1" pitch. Switching to 4 blade drop, 1" pitch.

I feel like I could benefit in the venting, possible cavitation and blowout issues from a cupped 4 blade. Make sense to anyone?

Thanks again!

 

[steelespike]

If you look up F25 performance reports you will see that an 11" pitch on aluminum boats about 600 dry weight run about 6000
and about 25 mph.It appears your over 6200. You need to reduce rpm So I don't see any reason to reduce pitch.
According to the chart you need to be at 6000 just going to a SS prop should drop 150-200.And a 12" drop about as much again.
The SS should solve any ventilation issues.A SS 4 blade should also solve the rpm numbers.and grip on the water.
By the way what we used to call "cavitation" Is now ventilation.(prop fouled with air). You don't feel cavitation. Cavitation occurs in
" Low pressure" vacuum areas usually on the underside of the blades water boils in the low pressure eventually eroding the metal.Usually an indication the prop is working well.

 

[stever07]

Some good information there steelespike, thank you. Unfortunately being in Canada I'm unable to view Yamaha's performance reports.

Good to know the difference between cavitation and ventilation, kind of new at this. The problem I'm having I can definitely feel, so ventilation.

So if I wanted to go 4 blade cupped stainless I should go 12" pitch? Would this effectively lower WOT by 800rpm? Seems like possibly too much.

Was looking at some offerings from powertech. Specifically the SRA4 and SRA3.

 

[steelespike]

No need to go to a 12" with the 4 blade SS prop.Being SS and a 4 blade I think an 11" 4 blade SS should work.
Typical rule of thumb is 150 to 200 rpm per inch.Your 25 would tend to be on the high side of those numbers.
The 4 blade could drop rpm as much as 200 being SS could knock off another 200.
Keep in mind these numbers are not carved in stone and numbers could surprise.
Why is it that you can't view the Yamaha Performance Bulletins? They are out there is cyber space.
Are you also not able to view the Evinrude Etec site?

 

[stever07]

Thank you very much for the help, I now have a good understanding of the direction I should go in.

Will go with your recommendation and go with the high side.

Being in Canada I can can view the US site fill out all the information, motor, boat type etc but when I go to view the performance number it redirects me to the Canadian home page. Free health care though, haha.