Yet Another Prop Question

[solbes]

-25 mornings have me dreaming of warm weather. Looks like great advice on the previous prop question, so I thought I'd add my own. Lund 1625 XL SS with 75 hp Yamaha EFI that we bought last August. The stock prop is a Yamaha 13.25 x 17 aluminum. I get about 40 mph GPS unloaded at 6000 rpm, the recommended max. Good hole shot for a F75. I'm not sure that a bow lifting prop would be a good idea since the boat is already a little stern heavy from the 369 lb 4S.

Anyway, I'm pretty satisfied with the current setup. I would spring for a polished stainless prop if I can get a couple more mph top end without killing the hole shot. Here's what I came up with in the order I'd try:

1) Yamaha Performance SS, 13 1/4 x 16 ($360)

2) Quicksilver Silverado SS, 13 1/8 x 16 ($320)

3) Stilletto Advantage SS, 13 1/4 x 17 ($340)

4) Quicksilver Silverado SS, 13 x 18 ($320)

5) Yamaha Performance SS, 13 1/4 x 18 ($360)

6) Keep current prop, no improvement likely

Opinions?

[marine_man]

Typically, when you switch from Aluminum to Stainless you drop pitch by 1 and you're set...

That said, I'd go with what you've laid out... Yamaha stainless props are pretty good, and I'd be surprised if you get much more performance out of the merc vs the yamaha.

You'll get 1-3 mph more out of the stainless, and you will get better holeshot as well making the change.

You may be able to swing the 17 and keep your rpm's up (and gain a bit more speed, but your holeshot will stay about the same as it is now with the aluminum), but I think you'll have trouble staying close to the recommended 6k rpm area with a 18.

Good Luck!

marine_man

[fasternu]

Just curious, why is it with everything being equal, that a stainless prop outperforms aluminum? Not disagreeing, just wondering the science behind it, so to speak. Thanks!

[marine_man]

It's kind of a 1-2 punch... Stainless is quite a bit stronger than aluminum and on holeshot, the stainless prop's blades won't bend / twist the way aluminum will, getting you better holeshot.

With respect to higher top end, Stainless gives prop designers more opportunity to cup the prop blades and do different things with the blades, leading and trailing edges.

The tradeoff for all this stiffness / strength is if you hit something with a aluminum prop the blades bend more, absorbing more of the impact and sending less energy into the lower unit. If you hit something with a stainless prop, you'll dent the prop, but take out most of the gear train in the lower unit, depending on speed at the time of impact.

marine_man

[hydro]

Another reason SS props perform better is that for the same blade strength, the cross section of the blade can be thinner. Less horsepower is spent slicing the blade through the water and that power can be transferred to forward motion.

[LMITOUT]

I have a tiller, but a lot of people like the Mercury Vengeance prop for the Yamaha 90. Might be another to add to the list to try.

[solbes]

I think that I read the quicksilver silverado is the same design as a merc vengeance. Had heard good things about them too. Thanks for all the replies. I'll probably start out with either the Yamaha or Merc Ven/Silverado 16 pitch and see where I land.

[prosport17]

In addition to being stronger then Alum. the SS prop is also more efficient. Here's what mean. With a SS prop you get a smoother/truer leading and trailing edge on the blade. Having this allows the prop to move through the water with out creating drag (air bubbles, i believe the technical term is cavitation) without this "drag" the prop/motor develops more thurst, pushiong the boat forward better.

The other item of difference is what I believe is termed "cupping" of the blade. On a Alum prop the trailing edge is fairly flat if you will. Since steel is stronger and machined smoother manufactures are able to "cup" the trailing edge of the blade a little bit (fold it inwards to grab more water). Since steel is stronger, the blade resists flex with the added cup and the smoother finish resists the cavitation also, as opposed to the Alum. I'm not a prop guy, just a retired airline pilot that spent a lot of time in turbo-prop aircraft. So any prop guys. let me know if I'm off base here.