Black art of prop selection

[Sean]

My "new to me" 4.3 V6 has an Alpha gen 1 drive.
On it there is a Quicksilver Stainless Steel 13 1/2 inch 23" 3 blade prop.

I want to move to a Highfive SS prop. (better pulling & midrange... less top end). Is there a conversion chart somewhere to say what prop is equivilant to which other prop?

I could just move to the 13 1/4, 23 highfive... but there's two more blades. Should I decrease diameter? change pitch? both...???

The Mercury selector says the 23" or a 25" highfive and reccomends a host of other options! at $500+ a crack I don't want to guess too much.

does anyone have some experience with this?
All opinions appreciated.

 

[john lamon]

You really need to put the boat in the water and try the prop you have to see what RPM's your at, hard to say just rolling the dice, how much does the boat weigh? how long is it? hard to guess without knowing, if I had to guess I'd say about a 21" is what your going to be after, a 25 seems high, especially if your after bottom end, is there a dealer near you that would be willing to lend props to try? my dealer will loan his used props out to try, I always try before I buy.

 

[Sean]

Yes, I realize the best way is to test, test, test...

But I'm a trailer type boater (not a cottager) and the local marinas only warm up so much to you if you're not local. My local dryland marine is no longer on the Merc demo prop program as it cost him too much to carry all those props.

The current 23* was horrible out of the hole on the 17' donor boat...but the motor was only 175 HP then too. My boat is 18' and lighter than the donor. I will start with the current prop when she goes in the water next spring and adjust from there.

BUT, what I'm really looking for a conversion chart (or some comparison of charactoristics) for Quicksilver Stainless 3 blade props vs Stainless five blade designs.

 

[john lamon]

BUT, what I'm really looking for a conversion chart (or some comparison of charactoristics) for Quicksilver Stainless 3 blade props vs Stainless five blade designs.

As you already know a 3 blade prop will be faster but also less stable at speed, the hi-five is better out of the hole and holds better in the corners but it is also less efficient cruising and you will burn more gas, personally I am a big fan of four blade props, best of both worlds, as for a conversion your 23p 3 blade would be similar to a 21p hi-five.

I have a 19p hi-five you could try but the shipping would probably be to high to make it worth while.

You can also play around with the PVS plugs to fine tune the hole shot, not sure if your familiar with those, the hi-five has them, here is the link explaining that.

http://www.mercurymarine.com/media/mercury/documents/PVS.pdf

 

[Sean]

Yes, I like the PVS feature on the High Five.
It is definately the prop I want to go to.... Still, just not sure which pitch.

Once the motor is finished I plan on having it Dyno'd so I can make a more precise guess.

 

[Sean]

For the sake of finishing this thread...(future readers)
My engine dyno'd just under 240HP. So, I chose a 1:47 ratio for my stern drive (over the 1:84 most 4.3s run). This made a huge difference in prop selection.

I went with a 19p High Five and got a great holeshot where the boat jumps out and on plane very quickly. It holds great in the corners, has good throttle response and a strong midrange but, I can only pull 4,400 RPM (max 5,000) and top speed is currently 78kph (48.5 MPH).

The correction here may be a 19p 4 blade prop that pulls less water, buys me some rpm and still has enough low end to get on plane and tow for watersports.

 

[Sean]

Looks like this thread ain't over...
I just bought a Michigan Wheel XHS 4 20p prop (14 1/8 dia.) MW do not make a 19p 4 blade application. My hope is to gain around 400 rpm with the loss of a blade and then give back 200 of it by going up 1" in pitch.

The net result on my home made computer model is a 4.8mph gain at 4,600 rpm to run at 53.3 mph. (up from 48.5 mph).

I'll be testing Saturday in 20* sunny weather over Canadian Thanksgiving long weekend! If this works then, next summer I will get a 21p 3 blade (tempest) and see if I can get over 58.5 mph (as shown on my model)

At that point I will gave covered the gamut. 5 blade power prop, 4 blade cruising prop, 3 blade speed prop.... Woo hoo!!! At least...that's the goal.

 

[Sean]

Okay, so I had the MW XHS analysed (without getting it wet).

Each blade measured 23.4 sqin so, x4 = 93.75 sqin of blade surface.
My High Five was 17.9 sqin x5 = 89.68 sqin of blade surface.

Given that the MW was 1" greater pitch (cost = 150-200 rpm) AND greater surface area of 4 sqin ... the MW looks like it should yeild 49.5 mph at 4,200 rpm. Not much different than the High Five and farther away from using the engine potential of 5,200 rpm. So, I took it back.

I have borrowed an old Mercury Laser 14 3/8 x 21, 3 blade prop for another baseline.
It's more conventional than the "5" and people seem more comfortable discussing performance -up vs. performance - down.

FWIW, the Laser has 23.6 sqin x 3 blades for a surface area of 70.87... It is up 2'' in pitch so that'll cost 300-400 rpms but, the reduced drag of 3 blades combined with 18.8 sqin LESS of surface area should help this prop spin up. I estimate Speeds in the mid to high 50's at engine rpms of around 4,700 to 5,100 rpm. We'll see how the hole shot and midrange are....

At the end of the day I think I'll wind up back with a 4 blade 14" prop like the Powertech SCE4 but, I will not get there untill next spring as this is my last weekend out for 2013.

 

[Sean]

The ol' Merc Laser was a total POS!!!!
IT was heavy, and unbalanced plus it sent vibrations throughout the entire boat. because of this I just couldn't run it for long. I tried to test what I could and found the hole shot sucked...followed by a less than responsive midrange. The top end speed....once you got there, (@ 4,200) was okay, but not a lot more than my current High five 19p. The boat was NO fun to drive. I shouls also mention that fuel consumption increased dramatically also.

On Saturday I went back to my High Five and was immediately rewarded with a great day on the water. maybe I don't need a new prop.

 

[Sean]

But, I do need a spare...
Found a brand new Solas 4 blade 17p for $190... Just gunna get it and we'll see next spring how it works.

My calculator says if I can spin it to 5,100 it should do 52.5mph.