Prop position?

[DON MAHER]

Marty,

IMHO yes it does.

 

[John Beardslee]

Hummm, with only one blade at a time in the water with a 2 bladed prop, I'd want at least one to be driven by the power stroke.

John

 

[Marty Davis]

OK, think about this. With a SAW boat it is difficult to launch so it would be nice to have a prop that wasn't a big load at launch. With one blade less pitch that is accomplished. Once the boat is up on the prop, with a SAW boat, it is REALLY up on the prop. Like on the tips only. Now, how much does the lower pitch blade affect the limits of the prop? Also, how much does the lower part of the prop affect the limits of the prop?

My contention is that the power point of the engine drives the highest pitch blade and the prop is really up on the tips making the lower pitch blade not much of a factor.

Thoughts....

 

[dwilfong]

Interesting for sure...........so have you taken it to consideration the back side of the prop........................

The back side is the part on the lower pitch blade that will be the brake..................

I have seen some videos of a surface prop and the water gets pushed aside that is not processed by the blade.

So dragging the lower pitch blade may work with the right back side profile.

 

[Marty Davis]

Interesting for sure...........so have you taken it to consideration the back side of the prop........................

The back side is the part on the lower pitch blade that will be the brake..................

I have seen some videos of a surface prop and the water gets pushed aside that is not processed by the blade.

So dragging the lower pitch blade may work with the right back side profile.

I guess the thing that I found worked so well for me was the ease of launch because of the lower pitch blade and the other blade being high pitch for speed IF it was indexed. This is the conclusion that I reached after running and finding the difference in the 180 degree rotation of the prop. I couldn't figure out anything else that would allow the low pitch blade to not handicap the high pitch blade. The fact that for a SAW boat it runs really right on the tips. One thing that i didn't do was measure just the tips of the 2 blades for pitch. That prop is missing someplace so I can't re-visit that unless that prop surfaces. Maybe Norm has it someplace at the shop. I do know that indexing worked if the high pitch blade was at the point where it was entering the water at the firing point near TDC for SAW. Probably the effect isn't as pronounced with heat race setups because the prop is deeper in the water. I still index all my props even with my heat race boats, but have not concluded the amount of good.....Maybe I will re-visit this sometime. Just to many things to do and learn.

 

[dwilfong]

You know that when you say different pitch that can mean many things.

 

[stan simpson iv]

Ok...not being a smart azz about this question. Say my boat is racing at 70mph and doing well during a race,I grease the shaft in between heats just throw her back in and do not put back same as it was I could lose 5-10 mph.(or gain if put in correct on power stroke)

I run the Jae,she has a ski...where is my prop hitting the water. On launch the sides and bottom are in water. What if I offset my strut to the left of my ski. Would it bite harder... Offset to the right bite less... I have A LOT OF QUESTIONS for you guy's.

 

[Marty Davis]

Ok...not being a smart azz about this question. Say my boat is racing at 70mph and doing well during a race,I grease the shaft in between heats just throw her back in and do not put back same as it was I could lose 5-10 mph.(or gain if put in correct on power stroke)

I run the Jae,she has a ski...where is my prop hitting the water. On launch the sides and bottom are in water. What if I offset my strut to the left of my ski. Would it bite harder... Offset to the right bite less... I have A LOT OF QUESTIONS for you guy's.

You are one of the most inquisitive guys racing and I would never think that.

I would suggest that you try that. It might be even more important for a boat like yours that REALLY rides up on the tips of the prop. Even for heat racing. My boats keep the prop in the water more, I THINK so the effect might not be as much as for yours. I don't think that the ski being offset would make much difference, but who knows.... That is what makes this hobby so interesting - ALL THE VARIABLES.

You would have to use a prop that you could not easily launch and then make one blade less pitch and see if it launches easier and runs really fast.

 

[dwilfong]

OK so lets say that there is nothing on the back of the boat that will touch the water but a prop and a strut?

 

[DalePowellJr]

This is all very interesting to me as my 20 hydro prop is pitched with one more than the other and I always index my props on all my boats too. I had done this just as an experiment on my 20 hydro because as Marty mentioned I needed ease of launch cause my boat is almost 4.5 pounds but still needed top end. I guess still more testing needed.

 

[stan simpson iv]

Hello...let's keep this ball rolling.

 

[Marty Davis]

It sure would be nice to have some other comments, etc. NEVER take what you read as Gospel. As I indicated there are some parts of this that are NOT proven.

If you prove me wrong, I can then stop indexing my props....

 

[David Murany]

How can the wind up in the shaft be accurately measured to ensure the blade is where you want it in the engines stroke? With a hard shaft it's a no brainer.

 

[Ray Sametz]

How can the wind up in the shaft be accurately measured to ensure the blade is where you want it in the engines stroke? With a hard shaft it's a no brainer.

My thoughts exactly in post #2

 

[David Murany]

How can the wind up in the shaft be accurately measured to ensure the blade is where you want it in the engines stroke? With a hard shaft it's a no brainer.

My thoughts exactly in post #2

Haven't read back in a couple days. Didn't mean to repeat Ray.

Actually been thinking of trying a wire drive in my 21 since it's closest to a hard shaft. Then this could be experimented with.

 

[DON MAHER]

Marty,

Have you or anyone that has run a uneven pitch prop with success, repitch the low to match the high to see if there is a noticeable difference?

For a number of years I have been running a SAW rigger that is a 10 percent overdrive so indexing is not an option. Anyone have any ideas for a better launch? Details are, cmb 80, 7-1/2lb flat bottom strut, 3/4" center shoe, prop is octura 2170 cut to 63mm, 7.5cup. Has run a high of 122mph. Right now it is about 60/40 on the launch. Have tried a wider shoe, but upset the top end.

 

[shoboat]

Don,

Jim may have something new for you to try from ABC?(2324-23-38%)

A 2323-19-38% or if you want to cut something down a 2821-19-38%.

Both of these have more pitch than you are currently running.

Remember when you reduce the diameter of a propeller you have to

redo the pitch ratio number times the new diameter. So your 2170 at 63 mm

is really a 63 mm x 132.30 mm and also the pitch ratio grows a little bit when

you cut down the original propeller diameter that much. Jim should have the

2324-23-38% done pretty soon and that one should really rock and roll for you.

I believe Martin is testing some new ABC blades with his 80 boat and it is really

strolling along I heard.

I heard your gas boats are running REALLY WELL

with some of the new blades?

Thanks for Reading,

Mark Sholund

 

[John Finch]

Just found this subject and have wondered about it for at least 20 years. The first thing that comes to mind is that 40 years ago a lot of us ran hard shafts. The prop entered the water at about a 5 degree angle, so the blade entering the water on the left side of rotation as viewed from behind had more pitch than the blade coming out of the water on the right side of rotation in relation to the forward movement of the hull. That was an awesome amount of unequal pitch and the boats tracked right really bad but the boats were smooth. I experimented at SAW and adjusted my strut kicked back to the opposite of the angled drive using flex cable of course and actually got my SAW 67 rigger to veer left instead of right. Got some minds thinking now?

Marty sent me a prop with unequal pitch years ago when I was trying for the 100 mph mark. Nice gesture, but I could not quantify the results using cable drive. Recently I was testing some props with different pitches on the blades and then bent the blades so they were equal as I could get them. The boat went faster after making the blades equal, but that may be because I added about ten thousandths more pitch to the lesser blade. Does that prove anything? I don't know. This is a hard thing to prove out.

As far as launching. I have done a bunch of things to help with that. With a rigger if you throw immediate right rudder the boat will most likely get up on the water where a straight launch will not. I have used the pad in front of the strut in the center of the hull and gone as far as 2 inches wide to aerate the prop. I have used a deep vee bottom to aerate the prop and even boogered up the leading edge of the prop close to the hub to cause turbulance. The boogered area gets out of the water for the SAW run. Also drilled holes in the prop at the leading edge. My best solution is a really a light boat. Just some of my crazy experiments but still no definative answer to the subject of different blade pitches. I found the electric boats really prefer both blades to be equal. LOL

 

[John Finch]

I know I am getting long winded, but you guys got me stirred up. About prop depth on a hydro.........I found that getting up on the tips too high will slow your boat down because there is less prop from leading edge to trailing edge. Sort of like high on the tips is a fine threat screw and lower in the water is a course threaded screw. The course threaded screw goes further per each revolution. So you need to find the right height the hub runs out of the water for the best speed. These things I think are more important than the blade pitch differences. Just adding a bit to the conversation, but I would still like to know the answer on prop position.

 

[John Beardslee]

How can the wind up in the shaft be accurately measured to ensure the blade is where you want it in the engines stroke? With a hard shaft it's a no brainer.

If you do some testing you may be surprised at how little a flex shaft actually twists, most of the coil up is due to the difference in size between the shaft and the log/stuffing box. If you support it with a smaller diameter tube and use a torque wrench, the flex shaft is quite ridge.

Thanks, John