CMB 21VAC

[BB Mods's]

Ill be taking video today of our 21 VAC in action, going to start cutting the pipe down today, prop is a 1614, pipe is the cmb 21 nitro pipe.

 

[DON MAHER]

Aaron,

Your basic math of 1.875 pitch X 39000rpm does= 70mph (give or take a little) at 0% slip! The average slip is somewere about 30%. Now as Stu said, Jim is turning 33500rpm X 3.2 cup at 30% slip= 70mph (give or take a little)

Don

 

[Ian Inverarity]

Aaron,

Your basic math of 1.875 pitch X 39000rpm does= 70mph (give or take a little) at 0% slip! The average slip is somewere about 30%. Now as Stu said, Jim is turning 33500rpm X 3.2 cup at 30% slip= 70mph (give or take a little)

Don

Don,

Aaron referred to the leading edge pitch of the ABC1614, which he has measured as 1.875 inches. This is measuring the pitch of a small sweep of the blade from the leading edge rather than the whole blade or the small section of cup near the trailing edge. The leading edge pitch is generally the lowest pitch on the blade, and as stated by boaters such as Andy Brown and Marty Davis the boat will not go faster than leading edge pitch. If the prop is not working efficiently it wont even go as fast as the leading edge pitch. This is why Aaron said the least RPM the engine could be turning to run 70mph is 39000rpm with that prop, and that applies whatever the cup may be.

Ian.

 

[LarryConrad]

How do these engines compare to the Valvolas in a hydro? Can they pull as much prop as a Nova?

 

[DON MAHER]

Ian,

If Andy or Marty said that a prop can not go faster than it's leading edge ( I don.t think so) then why do we have progressive pitch props?? Also if that were true, adding pitch or cup to the trailing edge would do nothing!!!!

39000rpm at 1.875" of pitch is 70mph at 100% efficiency. The problem is there is no such thing as 100% efficiency in boating period! Like I said before the avgerage is about 70%,so 39000rpm X 1.875 pitch at 70% would be 49mph.

33500rpm X 3.2 pitch at 70% will be 70mph. The numbers do not lie.

Don

 

[Ian Inverarity]

Ian,

If Andy or Marty said that a prop can not go faster than it's leading edge ( I don.t think so) then why do we have progressive pitch props?? Also if that were true, adding pitch or cup to the trailing edge would do nothing!!!!

39000rpm at 1.875" of pitch is 70mph at 100% efficiency. The problem is there is no such thing as 100% efficiency in boating period! Like I said before the avgerage is about 70%,so 39000rpm X 1.875 pitch at 70% would be 49mph.

33500rpm X 3.2 pitch at 70% will be 70mph. The numbers do not lie.

Don

Don,

The pitch progression on a propeller is to accelerate the water backwards to create thrust. Adding pitch or cup can increase the prop efficiency up to the point where it is running at effectively the leading edge pitch but can go no further. Adding further cup or pitch just puts more load on the engine. The prop isnt running at 100% efficiency when it is running at the leading edge pitch, just at it's limit. And both I and Aaron said the least RPM because often the prop doesnt even run at the LE pitch. Thus the advance of the prop per revolution is actually even less then the LE pitch in many cases. So 39000 RPM is the absolute minimum for 70mph with a 1614, most likely even more RPM would be needed.

If a prop were to go faster than it's leading edge pitch then that would mean the leading part of the blade would start to push water ahead of it, not good at all!

Ian.

 

[Al Hobbs]

Ian and Aaron have done a good job of explaining why the leading edge limits the top speed. The leading edge is biting into the water at a certain rate, based on its pitch. So, the boat can not go faster without the leading edge pushing water forward, or acting as a brake.

That is why some people cut back the leading edge to create a faster prop.

So, to go faster with the same pitch, your only alternative is to turn greater rpm. And, who knows what the limits are for more rpm?

I don't pretend to be a prop guru, but this is what I have been told by people that I respect.

Al Hobbs

 

[Luca Gray]

Aaron, Ian and Al i'm with you guy's on this,

can not go faster then leading edge pitch unless you turn more rpm's,

cheers Luca

 

[shoboat]

Guys,

This is not a stock 1614/3 Chopper propeller, it has quite a bit of work in it.

Jim is straight up on the performance of his set up.

Leading Edge Pitch is not everything in a good propeller, there are quite a

few other things you can do do make these propellers faster and more

efficient.

Keep Testing,

Mark Sholund

 

[Luca Gray]

Mark

your right L.P is not everything,

but you can not got faster then L.P x your rpm's,

if this was not the case. why is everyone running running such high L.P props on there SAW boats...

cheers Luca

 

[Ian Inverarity]

Guys,

This is not a stock 1614/3 Chopper propeller, it has quite a bit of work in it.

Jim is straight up on the performance of his set up.

Leading Edge Pitch is not everything in a good propeller, there are quite a

few other things you can do do make these propellers faster and more

efficient.

Keep Testing,

Mark Sholund

Mark,

Did you modify the leading edge pitch on that 1614? If so, what was the leading edge pitch? :unsure:

Ian.

 

[shoboat]

Ian,

The whole propeller has been modified.

The propeller from ABC is just a blank that we started with and we go from there.

I don't remember the particulars on that propeller since I do so many every

week. I can tell you that we have found a couple of different ways to make

them more efficent in the water. RPM = SPEED

I can tell you Jim is making a ton of HP and RPM with his set up.

Thanks,

Mark Sholund

 

[Ron Zaker Jr]

Mr. Jim Wilson from California said I could quote him on this. He just ran the new 21VAC in stock form. This is in a SD3 mono hull set up in oval trim. Boat speed on the straight is 70mph and 66mph in the turns with 33,500 max rpm. Great clear through the entire throttle range. Head clearance is set at .006" and remove the shield on the front bearing. Shield did cause some binding. He believes this motor in a 4+ pound hydro will go 85+. Maybe I should start running 21 hydros if I could find a boat to handle this speed.

JAE .21 $79.95

 

[Ian Inverarity]

Ian,

The whole propeller has been modified.

The propeller from ABC is just a blank that we started with and we go from there.

I don't remember the particulars on that propeller since I do so many every

week. I can tell you that we have found a couple of different ways to make

them more efficent in the water. RPM = SPEED

I can tell you Jim is making a ton of HP and RPM with his set up.

Thanks,

Mark Sholund

Mark,

Thanks for replying, you must be happy to see such fantastic results with your props!

According to Stuart, Jim is running 33500rpm max, certainly a ton of RPM from a stock engine!

I am a little surprised when you say you dont remember the specifics of the prop tho, I would have thought you would have written it down, what happens if Jim loses or damages that prop, can he just order a 1614 with 3.2" cup from you and get the same prop?

In general terms for 1614 Chopper props, rather than the specifics relating to the prop that you did for Jim, what sort of mods do you do to the leading edge pitch? (if you care to share of course! B) )

Ian.

 

[Stuart Barr]

Ron,

I have not seen a JAE go that fast yet

Stu

 

[Speed Jr.]

Ron,

I have not seen a JAE go that fast yet

Stu

........yet

 

[Luca Gray]

Stu

here is a link to Aaron Alberico's JAE 21 going low 80's, yes i know it is not 85mph, :lol:



cheers Luca