Why is that ?

[HPLou]

1984 Thundercraft Magnum 230
Engine: Merc 260 (5.7L) serial 6846787 RPM 4200-4800
Stern Drive Alpha One serial 6847830

I just bought a new Black Max new design prop with a Plastic Hub Kit to fit my Stern. I do not have to P/N since it is not on my bill but;

4.25'' dia x 21 pitch (new blade shape)
= Top speed 38 MPH at max 4300 RPM (speedometer)

I replaced the prop for a other New Black Max
4.5'' dia x 19 pitch
= Top Speed 32 MPH at max 4100 RPM (speedometer same day))

I am trying to reach max RPM and I can't. The store that sold me the props told me I will get 500 RPM more with the 19 pitch prop. And I didn't. He will replace the prop again, but I'm starting to dought about his competences.

Why I'm I loosing speed and RPM with the 19 pitch prop?

 

[hwsiii]

Re: Why is that ?

I don't have the blade geometry specs on the NEW Black Max, but I do have the specifications on the older Black Max Stainless.
I will post some differences so you can see why your dealer is mystified by why it didn't do what he thought it should do. And I will tell you why EVERYONE is mystified at times by what should happen and doesn't. There are other props on the market that show much more dramatic differences between one pitch and another in the same make and model of prop. The Bravo II changes from a progressive pitch of 3" and a parabolic rake of 8 degrees from its lowest pitch to a progressive pitch of 4" and a parabolic rake of 15 degrees to its highest pitch props, and that is in just one make and model of prop.
We will find this more and more common the future in my opinion, and that is why an Enertia in the lower pitches will push a good sized boat very well but in the higher pitches it is useless for larger boats.

Black Max Prop Specs

In the old days you would NEVER see this, only a change in Diameter and Pitches. But as they scientifically design the newer props they are absolutely taking advantage of all their new found knowledge of how props work and what happens when you change pitches to try and keep them doing what they want it to do, but sometimes they make errors. And it sure does make for some unexpected surprises for somebody who needs to change pitch in the same make and model of prop.
Lou you may find a new 19" pitch will only lose 1 or 2 MPH. and that is the way it should have worked, your dealer was right in his numbers. If it doesn't you will need to change the model of prop you are going to use. Those props are hand finished and the prop just may been messed up in the process of the pour and the finishing.

H

 

[HPLou]

Re: Why is that ?

thank you hwsiii for the great explanation. I forgot to mentionned that I purchased an aluminium prop. I have taken my boat for a ride this morning and I reinstall the prop that was on the boat when I purchased it in Oct 2008. It is a 15'' x 17 pitch p\n 48.78118A40L 17P. I can reach 4800 max RPM with that prop and my speedometer indicated 37 MPH. I think it is a very acceptable result for my type of boat. And I want to get close as possible to that performance with my spare prop that I am trying right now. In my first message I was referring to an Quicksilver Black Max #1916P.19 (14.5 x 19P).
With that prop my max RPM is 4200 an 32 MPH.
H, I dought the store will replace the Quicksilver prop for an other brand as you suggested. I will give him those specs tomorrow and see what alternative prop he comes up with. Or else, I will ask to give me back the Quicksilver 14.25 x 21P. For the same performance (19P vs 21P) I can reach 40 MPH with the 21P. I will get back with my final decision shortly.
Thank You very much

 

[hwsiii]

Re: Why is that ?

I would only buy a prop that keeps your RPM up to maximum, running at 4,100 RPM is detrimental to the engine, in my opinion.


H

 

[HPLou]

Re: Why is that ?

thanks hwsiii for the precision. This is what I will do for sure.

 

[hwsiii]

Re: Why is that ?

You are very welcome.


H