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I have some data to donate after all your kind help in various subjects and
another question again.

I bought a 1980 Sport 17 which was transformed into a Montauk. It's powered by a 115 Mariner 2 stroke 2+2 inline, 1994 installed at the right height.

The prop is a 20" pitch 5 blade Mercury HighFive with medium PVS plugs.

Data with medium load = total 1400lbs.

Unbelievable hole shot about 3 seconds on plane!

Cruising still on plane even on 1900 RPMs!

Great performance (planning and control) on choppy water even with 6bft weather!!!B)
BUT!!!
Annoying vibration at 1200 to 1600 RPMs.

Almost zero response to reverse.

Awfull WOT 4200Rpms (max 5250 fro 115) and a top speed of 37 mph with my setup.:@

So here's my question:

I've decided to move to four blade prop in order not to lose the benefits of the five blade and to raise my RPMs and performance.

With this setup I must go to a 17 pitch (most likely Mercury Trophy Plus).

How the one blade less will affect the RPMs? Does anyone know?

Thank you all for one more time.Soon I'll update my personal page with my new Whaler (17') project.:D

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It is NOT necessarily the number of blades that will affect you engine speed. If you are only able to hit 4200 RPM with the 20" HighFive, something else is very wrong, probably with your motor. What other propellers have you used on this boat/motor combination?

I have read several anecdotal reports of 17 foot Whalers running Mercury 90s with the 20" HighFive, so your boat with 115 HP should not have any trouble turning it up to speeds in the upper 40s.

Looking at your data, I find 37 MPH at 4200 RPM difficult to believe. Something else is odd here. That would mean your 20" prop has more than 8 percent NEGATIVE propeller slip, which is not possible.

Is your tachometer accurate? Are you sure you do not have a 24" pitch HighFive?

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Dear Tom,
I also believe that something is odd with all this but it's a fact!
With factory's 19' pitch aluminum i reach 4900.
What else to do? I believe that tach and speedometer are ok.
Unfortunatelly i don't have any option of test props here and
i don't want to add some hundred bucks on the bookshelf.:(

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Whoa! You are only able to hit 4900 RPM with an aluminum 19" pitch propeller? Now I know something is wrong with your motor. You should be getting another 1000 RPM out of it.

What top speed did you get with the 19" aluminum propeller?

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Whoa!Relax...
I remind you that the engine is 1994 115 2+2 meaning
WOT 4750-5250 RPMs.
So now you have to recalculate and give the right answer!:D
By the way, i asked mercury about it and they say that 1 blade
less and larger diameter eliminates each other, so with 17 pitch
i'll be getting almost 5000 with medium load!!!:o
Let me know of your thoughts...

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OK, 4750-5250 WOT RPM range.

Can you confrm the gear ratio for your motor?

What top speed did you get with the 19" aluminum propeller?

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That annoying vibration is probably the transitioning period of that 2+2 feature.

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The gear ratio is 2.07 and the speed with 19'' is 44 mph
By the way the vibration is not from 2+2 because the
bottom 2 carbs work at 1800 RPMs and up.
I thing i'm going nuts with this thing.

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Just as a frame of reference, I can squeeze 5200 RPM out of 90 yamie on a 17 ft hull. I use a 19 inch stainless series yamaha prop. Rear ratio 2:0. a 115 should blow the socks off this.