Raised my motor, but questions...

[McGillicuddy]

Big's experience propping the 18 and Kens advice are concurrent.

My question would be can you get the lower planing speed with the smaller diameter, & significant cup & rake of NRS3 prop relative to the 2+2 midrange rpm. Might ask Ken if the NRS4 blade option would reduce the planing speed and sill get the nose up as Bigshrimpin suggested..

[erebus]

Boats now on her mooring in Cape Cod Bay, so after work this evening in the dead flat calm I took her on another sea trial with a known prop.
Took off the unknown mystery prop and threw on a Michigan Wheel Vortex 13.25X17P we had kicking around the shop.
Top end RPMS were about ~5600 at 29kts.
The original mystery prop topped out at ~5000.

So from this I can infer that the mystery prop is probably a 19" or a 20"?
200 rpm's per inch of pitch, right? so if the 17p runs 5600 and the mystery runs 5000 then its got somewhere around 3 inches more pitch?
I'm guessing its a 19p.
So, the merc prop calculator with the input from today's tests tells me I should run a 19p prop on my boat/motor combo (actually it said an 18.38").
It recommends the 4 blade spitfire 12.5"x19"p. Or a 13x19 black max.
4 blade gives more stern lift right?
Going to stick with aluminum for now, as sometimes I need to do a little dredging to make it back to my mooring if I spend too long at the beach.
No point ruining a nice stainless. At least not yet.

[Bushwacker]

Yes, the extra blade alone on a 4B will add extra stern lift, although blade rake and how it's cupped also affect stern lift. If your Vortex is a 4B version, you might get more than 200 rpm/inch of pitch with it. The Vortex is an excellent prop; it's comparable SS version is the Apollo, which would probably slightly larger in diameter for the same pitch.

[erebus]

I did some more reading online regarding 4 blade props, and it seems it is recommended that you reduce the pitch by 1 inch or so when switching to a 4 blade from a 3 blade.
With my 17p 3 blade test prop I was turning ~5600 RPMS on a motor that should max out at 4750-5250 (2 stroke merc 115).
For optimal RPMS that prop should be a 19.
So if I get a 4 blade spitfire should I get the 17p? Also the mercury prop selector says I should get a 12.5" diameter 4 blade spitfire.
12.5" seems awfully wee...
Quicksilver Nemesis which is the cheapy version of the 4 blade spitfire comes in a 14x17 and a 14x19.
14 Seems awfully large...

[erebus]

Originally Posted by Bushwacker

If your Vortex is a 4B version...

It was just a regular 3 blade.
Have yet to try a four blade. Getting there tho!

[erebus]

Had a quick run around Cape Cod Bay yesterday with a brand new 4 blade Quicksilver Nemesis 4 blade 12.8" (they call it 13")x17p prop.

Right off the bat she seems to want to plane off faster.
Not sure if the bow rises more initially before the stern comes up but that's what I thought first time up on plane. Just an observation, maybe I imagined it.
Planes good and at a much lower rpm/speed. Can hold plane till about 13 knots on smooth seas at about 3200 RPMS.
Max full throttle was 5000 RPM's on the nose and 31 knots via GPS.
Can cruise happily at 18-20 knots at about 3600 RPM's.
4 blader also grips good in a tight turn, alleviating some of the sliding sensation from moving the motor up two holes I noticed with the old three blade.

So now the question is do I change pitch, or is that pretty good?
I ordered a 4 blade nemesis 13x19p I can try too, but Ill have to pay for it. Or I just run the 17 and call it good. If I go 19 that will lower my RPMS by about 400, correct?
Maybe I should try a 15P?
115 merc likes 4750-5250. So the 17p is smack dab in the middle.
They don't make a 16 which would probably let me get the full 5250.

17P is probably good enough for my needs...

[McGillicuddy]

I don't think the 19p offers any benefit. i would cancel that order. Its good to hear you're able to plane at 13 knots. Sounds like you've come down quite a bit in that attribute. I would even consider a 15 pitch. Especially if it has a little more diameter. Or are the nemesis props all the same diameter?

Were you able to borrow the nemesis? If so, the shop probably has the 3 blade. You might prefer the hole shot as it will pop up on plane and level very fast.

You might try the three blade black max/black diamond 13-3/4 x 15, or the 17p variant. I think Beaver is running one of these on his 18 w a tohatsu. Maybe he'll chime in.

Good to hear your making progress.

[Bushwacker]

All the performance changes you've reported are exactly what I would expect from a good 4B prop. Being able to plane at 13 kts indicates boat is fairly well balanced. If the load you were carrying was about what you normally carry, I'd say you've found the right prop. If you normally run heavier, a 16 pitch prop would allow motor to rev a little higher. If it's an aluminum prop, a good shop could probably repitch it.

FYI, as I've posted on here before, if I were running an inboard or a 4-stroke outboard, the first thing I'd do would be add a gage to measure intake manifold vacuum. If you observe a minimum vacuum of 5-7" at cruise, your exhaust valves will live a lot longer! (The exhaust valve is the hottest and typically most life limiting part in any 4-stroke engine.) Basic requirement is for prop to reach max recommended rpm at WOT with normal load and still be within recommended range at max load, but beyond that, I think I'd be inclined to pick whatever prop gave me the max speed at about 7" vacuum, provided that min planing speed was acceptable and it didn't ventilate in hard turns!

[erebus]

Originally Posted by McGillicuddy

I would even consider a 15 pitch. Especially if it has a little more diameter. Or are the nemesis props all the same diameter?

The 15p is a little larger in diameter. The 17p is 13 inch and the 15p is 13.4 inch.

I was considering the 15" but going down 2 inches in pitch, wouldn't that raise my rpm's above mercs recommended max for my 115 2 stroke?
Not that I ever run at full throttle...

I did buy the 17p, but I suppose I could cancel the 19p I ordered and buy a 15p instead and see what happens.
About $90 a prop.

[Old'sCool]

Sounds to me like you got it