Prop position?

[John Beardslee]

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, I think you'll find the new high rake ABC wheels will launch better due to the reduced thrust cone, ie: more water pushing straight back verses lift. Only my 2 cents just thinking about what I've tested so far and seeing how these new props launch.

Thanks, John

 

[dwilfong]

Well lets see if I can explain my self here. Always hard for me to put it down on paper or in this case typing on this thing.

Pitch =What? what pitch are we talking about, the pitch changes on the prop as in progression from leading edge to trailing . also progression changes from hub to tip.

So when you talk different pitched blade what are you really saying?

If the leading edge the part that hits the water first is the same then you are just talking progression not true pitch.

What dose progression do? keeps the water on the blade? So what we are taking is different progression of pitch or a true change in pitch at the leading edge that hits the water first.

So what you are doing is just making half the prop work better. You can add more progression on the power stroke. The blade in the water is not taking load when there is not as much power in the BDC position. so the inertia of rotation is not impeded as much and there is more available when you come back to the firing position.

High rake prop will push the water towards the hub where it hits the highest progression on the blade. so this will make launching harder.

In my testing with no ski or rear sponsons I found the high rake props where harder to launch. but with a small V cut at the hub it made a BIG difference in the launch and how the rake acted on the boats attitude when up at speed. the rake did not suck the rear down as much with this release at the hub when up on it.

Now a single blade prop? helical screw?........HMMMMMMMM

I think thy had a boat swamp ATV thing in the USSR that was made like this????????????????????

 

[Christian Lucas]

Hi ,

there are intresting artikel about propellerposition at Rolla Propellers , http://www.rolla-propellers.ch/Rolla/Papers.htmlSpezial this , http://www.rolla-propellers.ch/Rolla/Papers_files/Surface_piercing_propellers-Propeller_hull_interaction.pdf , and this

, http://www.rolla-propellers.ch/Rolla/Papers_files/Surface_piercing_propellers-Methodical_series_model_test_results.pdf, with inclined propellershaft and the forces . For me i need only flexcable for outbord lowerunits and tunnelhulls in- and offshore . For all other i use straight propshaft with a low motorposition for a low propshaftangle .

 

[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, I think you'll find the new high rake ABC wheels will launch better due to the reduced thrust cone, ie: more water pushing straight back verses lift. Only my 2 cents just thinking about what I've tested so far and seeing how these new props launch.

Thanks, John

Yes, that is what I am using now. They have a very low rooster tail and work really good for SAW.

John

 

[Charles Perdue]

Some food for thought.

On the newer vehicles that we drive, the ones that use an electronic crankshaft pickup for the engine timing instead of a distributor, also use this same pickup to measure how much that the crank accelerates on each power stroke. This measurement when it detects a lack of crankshaft acceleration at the specified time, tells the computer that the engine is misfiring and if it is, on which cylinder.

HOWEVER...if the engine RPM is increased above about 3000 to 3500 RPM's this measurement is no longer accurate. The time of the rate increase is just too small to be measured precisely and the fact is that inertia of all the moving parts will not allow the crank to accelerate and de-accelerate enough to be useful. This is Newtons first law of motion...an object that is in motion tries to stay in motion and an object at rest tries to remain at rest. It takes a lot of energy to change either of these.

A V8 engine has 4 power strokes on each revolution, so at say 3500 RPM's the count is 233 power strokes per second.

A toy boat engine at 30,000 (a very conservative number) has 500 power strokes per second. My thoughts on this is yes, there is a difference in crank speed for a small portion ( at 180* exhaust timing, it is for only 1/4 of one engine revolution) but the difference is so slight as to be undetectable.

Charles

 

[John Finch]

That answers the question for me. Thanks Charles.

John

 

[Christian Lucas]

Hi Charles,

that's right and we can use Newtons first law of motion if we take a dual mass flywheel that smoose out any speeddifference . https://en.m.wikipedia.org/wiki/Dual_mass_flywheel .

BUT it is possible to get a drivesystem that declarate the propspeed when the propellerblade hit the watersurface and accelarate the propellerblade when it leaves the water . Just right as the propellerpitch of the propeller that hit the water is higher and need lower speed and when it leaves the water the pitch is lower and the propeller need higher speed to compensate this if you have a inclined propshaft . For this you need a spring conected propshaft to the motorshaft . Best with adjustable springrate to find the highest powerlevel.

 

[dwilfong]

There is still a inherent frequency involved in the eng even if you take into consideration Newton's law.

If the prop is in tune with this I feel it will make a difference.

If frequency did not matter would a tune pipe work at 500 times a second?

Just because you can not measure it dose not mean it is not there.

So think of it this way if it makes a slight difference 500 times a second that adds up 500 times a second.

Remember we are talking very small engs that compound effects not seen in larger engs.

Remember scaling up a .21 that makes 3.2HP when scaled up to say just 1.01 it dose not make the same power per CID.

Scale it up to 500cc its even less......................

 

[Jim Allen]

Charles & Christian,

Thanks for two very good post explaining what takes place.

JA

 

[Andy Brown]

Charles & Christian,

Thanks for two very good post explaining what takes place.

JA

Yes, Christian and Charles hit the nail on the head.

The inertia of the rotating elements impart a relatively smooth and even power flow to the prop and keep the crankshaft speed relatively constant in one revolution.

It is correct that crankshaft speed is not constant in one revolution. In a single cylinder engine the rotating mass effects that ever changing crankshaft speed.

More mass = more constant.

Marty's theory would hold more water (no pun intended) if the rotating mass was extremely small.

And to that degree I do believe there is merit in his theory, if the rotating mass was very small.

Sounds like my friend David Wilfong has been talking to Andy Brown on the phone.

Ah yes, I remember now! We had this complete conversation on the phone about a year ago and my conversation

to David included Marty's theory, variable crankshaft speed, very light rotating mass

and how I think a single bladed prop could take advantage of Marty's theory.

I do have experience with single bladed props.

 

[Terry Keeley]

I do have experience with single bladed props.

I have experience with single bladed props too, but not by choice!

 

[Marty Davis]

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.

Barr Backcut.

 

[Marty Davis]

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

Posts like this further show how much you think about the hobby in VERY technical terms....

 

[Marty Davis]

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

ALSO, if you solder the part of the shaft that is clamped in the collet, the shaft has 2 windings that offset each other and make it fairly stiff. Many don't solder though.... I have always soldered mine.

 

[Marty Davis]

Charles & Christian,

Thanks for two very good post explaining what takes place.

JA

Yes, Christian and Charles hit the nail on the head.

The inertia of the rotating elements impart a relatively smooth and even power flow to the prop and keep the crankshaft speed relatively constant in one revolution.

It is correct that crankshaft speed is not constant in one revolution. In a single cylinder engine the rotating mass effects that ever changing crankshaft speed.

More mass = more constant.

Marty's theory would hold more water (no pun intended) if the rotating mass was extremely small.

And to that degree I do believe there is merit in his theory, if the rotating mass was very small.

Sounds like my friend David Wilfong has been talking to Andy Brown on the phone.

Ah yes, I remember now! We had this complete conversation on the phone about a year ago and my conversation

to David included Marty's theory, variable crankshaft speed, very light rotating mass

and how I think a single bladed prop could take advantage of Marty's theory.

I do have experience with single bladed props.

A side point.....

When I ran SAW back in the 70's and 80's we ALWAYS used a heavy flywheel. Don's question about how to help launch is also solved to a large extent by a heavy flywheel. Probably would not be the greatest for a small pond and quick acceleration needed.

 

[John Finch]

Charles & Christian,

Thanks for two very good post explaining what takes place.

JA

Yes, Christian and Charles hit the nail on the head.

The inertia of the rotating elements impart a relatively smooth and even power flow to the prop and keep the crankshaft speed relatively constant in one revolution.

It is correct that crankshaft speed is not constant in one revolution. In a single cylinder engine the rotating mass effects that ever changing crankshaft speed.

More mass = more constant.

Marty's theory would hold more water (no pun intended) if the rotating mass was extremely small.

And to that degree I do believe there is merit in his theory, if the rotating mass was very small.

Sounds like my friend David Wilfong has been talking to Andy Brown on the phone.

Ah yes, I remember now! We had this complete conversation on the phone about a year ago and my conversation

to David included Marty's theory, variable crankshaft speed, very light rotating mass

and how I think a single bladed prop could take advantage of Marty's theory.

I do have experience with single bladed props.

A side point.....

When I ran SAW back in the 70's and 80's we ALWAYS used a heavy flywheel. Don's question about how to help launch is also solved to a large extent by a heavy flywheel. Probably would not be the greatest for a small pond and quick acceleration needed.

Marty,

I noticed that as well. I had a few ops 67s that had brass flywheels. With the brass my ops 65 seemed to keep on trucking at speed where the aluminum flywheel did not seem to be as smooth. I experimented with all kinds flywheel weights by turning down the brass flywheel on a lathe. I wish we had radar guns back then.

John

 

[John Beardslee]

Charles & Christian,

Thanks for two very good post explaining what takes place.

JA

Yes, Christian and Charles hit the nail on the head.

The inertia of the rotating elements impart a relatively smooth and even power flow to the prop and keep the crankshaft speed relatively constant in one revolution.

It is correct that crankshaft speed is not constant in one revolution. In a single cylinder engine the rotating mass effects that ever changing crankshaft speed.

More mass = more constant.

Marty's theory would hold more water (no pun intended) if the rotating mass was extremely small.

And to that degree I do believe there is merit in his theory, if the rotating mass was very small.

Sounds like my friend David Wilfong has been talking to Andy Brown on the phone.

Ah yes, I remember now! We had this complete conversation on the phone about a year ago and my conversation

to David included Marty's theory, variable crankshaft speed, very light rotating mass

and how I think a single bladed prop could take advantage of Marty's theory.

I do have experience with single bladed props.

Hey Andy,

I remember sometime in the 70's Gary Pruesse show up at the Mini Gold Cup at Belle Isle (Detroit) with a 20 hydro that had only a single bladed prop. It ran well if I remember correctly but I'm not sure if it was all that better than anything else. If it was knowing Gary he would have continued to pursue it far there.

Thanks, John

 

[Andy Brown]

Charles & Christian,

Thanks for two very good post explaining what takes place.

JA

Yes, Christian and Charles hit the nail on the head.

The inertia of the rotating elements impart a relatively smooth and even power flow to the prop and keep the crankshaft speed relatively constant in one revolution.

It is correct that crankshaft speed is not constant in one revolution. In a single cylinder engine the rotating mass effects that ever changing crankshaft speed.

More mass = more constant.

Marty's theory would hold more water (no pun intended) if the rotating mass was extremely small.

And to that degree I do believe there is merit in his theory, if the rotating mass was very small.

Sounds like my friend David Wilfong has been talking to Andy Brown on the phone.

Ah yes, I remember now! We had this complete conversation on the phone about a year ago and my conversation

to David included Marty's theory, variable crankshaft speed, very light rotating mass

and how I think a single bladed prop could take advantage of Marty's theory.

I do have experience with single bladed props.

A side point.....

When I ran SAW back in the 70's and 80's we ALWAYS used a heavy flywheel. Don's question about how to help launch is also solved to a large extent by a heavy flywheel. Probably would not be the greatest for a small pond and quick acceleration needed.

Marty,

I noticed that as well. I had a few ops 67s that had brass flywheels. With the brass my ops 65 seemed to keep on trucking at speed where the aluminum flywheel did not seem to be as smooth. I experimented with all kinds flywheel weights by turning down the brass flywheel on a lathe. I wish we had radar guns back then.

John

I compared aluminum to steel back in the late 70's in a Pinckert Gator. Steel carried much more corner speed while running tight on the buoys. The aluminum would drop a lot of speed in the corner and sometimes fall off pipe near the exit buoy.

The effects of gyroscopic precession on hull handling is also a consideration.

 

[John Finch]

Gyroscopic precession ! Smart guy! You must fly airplanes too. Isn't that why we offset the engine on a model plane so it will fly straight?

John

 

[Christian Lucas]

Hi John ,

yes that's the question . For oval racing a light flywheel can be the ticket as acceleration plays the major rule and that make me think about a modelsize dual disk flywheel that is light and has the performance to rise torque at lower rev. But that is not easy to calculate and to do it right . For SAW i think the flywheel can be moore havy spezial when teh boat comes out a corner and accelerate not from stand still like the electric driven boats do.

I have never tryed a singelblade propeller but two friends one of them was Hans Lehner 3 times World Champion F1 -1 kg class and recordsetter he has tryed it for full submerged 1 kg boats class Naviga rules. For full submerged it has possible advance , but there are so many other important things so we could not see a better performance over a two blade propeller . All was difficult to balance and to build and succses was done allways by a twoblade propeller .

Happy Amps Christian