Prop Shaft

**gn7** · 04-29-2014, 10:47 AM

Originally Posted by WESTERNAERO

I think this is what Bob is saying.
Attachment 39625

Pretty much. Its not a lot different when the prop is submerged, except the top down is adding thrust. But its easy to see the effects of the lower force on the up swing.
If it was equal, there would no to very little lift because it would simply counter the down swing that creates the lift. And the prop wouldn't "swim" and there would no benefit to running a circle boat off the snout, it would be neutral.

**SnoC653** · 04-29-2014, 11:54 AM

Originally Posted by WESTERNAERO

Yes, yes, yes. Again, this is why I like the idea of use using modern machines to make these props.
And just so you digitizing haters know. I can draw a prop from nothing on CAD, I don't need a prop to copy. So if I ever decide to make one for myself, who should I send the royalty check to?

Since nobody has a patent on props, if you design your own, why would you want to send someone a royalty check? As for hydros running shorter shafts that is true of the modern boats. I know the old conventional hydroplanes ran one inch shafts from the engine back to the prop, right between the driver's feet with no guard. Our 280 had a 1" shaft and every boat up to the seven liters I knew of used one inch shafts. I'm not sure what the unlimited ran back in the day, but even at 1 1/8" that is a lot of risk running between the driver's legs. I don't recall ever hearing of a shaft on one failing anywhere other than the prop or the coupler. Shaft whip wasn't the cause of failure.

Bob, how many shafts do you know of that have failed in the middle?

**gn7** · 04-29-2014, 12:44 PM

Originally Posted by SnoC653

Since nobody has a patent on props, if you design your own, why would you want to send someone a royalty check? As for hydros running shorter shafts that is true of the modern boats. I know the old conventional hydroplanes ran one inch shafts from the engine back to the prop, right between the driver's feet with no guard. Our 280 had a 1" shaft and every boat up to the seven liters I knew of used one inch shafts. I'm not sure what the unlimited ran back in the day, but even at 1 1/8" that is a lot of risk running between the driver's legs. I don't recall ever hearing of a shaft on one failing anywhere other than the prop or the coupler. Shaft whip wasn't the cause of failure.

Bob, how many shafts do you know of that have failed in the middle?

NONE. I said that repeatedly. It just does not happen. I seen a TFH kick both short shafts out the back on shut down when the twin props were in infancy. But that was a mechanical failure at the couplers.
Never seen a single prop shaft fail between the back of front coupler and the back of the strut..

Pat Hoban GN30 had a shaft fail dead center in the log. But it was a brand new shaft of "commercial over the counter" 316 that broke the first time out, and broke on an EXTREME angle due to a flaw in the material. Looked like the mill "spliced" the run with a high angle scarf type joint you would use joining to end grain planks. Shit material that had no business being used a boat shaft.

**Propless** · 04-29-2014, 01:05 PM

Originally Posted by gn7

Pretty much. Its not a lot different when the prop is submerged, except the top down is adding thrust. But its easy to see the effects of the lower force on the up swing.
If it was equal, there would no to very little lift because it would simply counter the down swing that creates the lift. And the prop wouldn't "swim" and there would no benefit to running a circle boat off the snout, it would be neutral.

I've always had a hard time understanding why this is true. I know its true, but I just cant wrap my head around why its true when a prop is fully submerged.

I think my little brain came up with a theory. Lets see if I'm on the right track here. It has to do with the props angle (shaft angle), right ? Sense the prop is angled under the hull, lets say 8 deg. And the hull is angled bow up slightly, lets say a few deg. The prop is running through the water at maybe 10 deg. Now think of the angle of the props ears (pitch) compared to its true direction of travel through the water, when both ears are straight out to the sides. The ear that's on the downward travel would have a more aggressive (higher pitch) compared to its true direction of travel through the water, creating lift, right ?

Now as far as the paddle wheel effect, ears having more bite when there down compared to up (fully submerged). They should have the same angle/pitch to direction of travel at that point. So could it be a rake issue ? The upper ear would be raked back more than the lower ear at that point (again compared to true direction of travel). Or is it just disruption of water to the upper ear by the hardware ? Maybe a combination of both ?

**Sharp shooter** · 04-29-2014, 01:09 PM

Originally Posted by HotWater

990hp is pretty good! I think a regular casale case can take about 37's with water jackets.

It takes a steady hand with the grinder, but 48's are possible while retaining the water jackets on standard Casale cases.

**gn7** · 04-29-2014, 01:17 PM

Originally Posted by Sharp shooter

It takes a steady hand with the grinder, but 48's are possible while retaining the water jackets on standard Casale cases.

With HD gears

**gn7** · 04-29-2014, 01:28 PM

Originally Posted by Propless

I've always had a hard time understanding why this is true. I know its true, but I just cant wrap my head around why its true when a prop is fully submerged.

I think my little brain came up with a theory. Lets see if I'm on the right track here. It has to do with the props angle (shaft angle), right ? Sense the prop is angled under the hull, lets say 8 deg. And the hull is angled bow up slightly, lets say a few deg. The prop is running through the water at maybe 10 deg. Now think of the angle of the props ears (pitch) compared to its true direction of travel through the water, when both ears are straight out to the sides. The ear that's on the downward travel would have a more aggressive (higher pitch) compared to its true direction of travel through the water, creating lift, right ?

Now as far as the paddle wheel effect, ears having more bite when there down compared to up (fully submerged). They should have the same angle/pitch to direction of travel at that point. So could it be a rake issue ? The upper ear would be raked back more than the lower ear at that point (again compared to true direction of travel). Or is it just disruption of water to the upper ear by the hardware ? Maybe a combination of both ?

Its more like the prop is too close to the surface. It moves the water (creating the rooster tail) as opposed to moving the prop thru the water that happens on the down side. To get anything approaching equal force off the prop would require something like this. Not unlike a Jet Ski prop or fan shroud on a car engine.

Take a prop on a 8* angle on 4 ft of water, with the top ear 1" below the surface, and spin it 10,000 RPM and tell me how much water flies in the air. That water creates some lift, but its countered by the ear trying to drive itself to the bottom of the lake. That force on the strut is countered by the upward force of the opposite side to a much greater degree.
The water thrown on the bottom of the boat does jack shit to propel the boat. It just destroys blast and cav plates.

**Propless** · 04-29-2014, 04:18 PM

Originally Posted by gn7

Its more like the prop is too close to the surface. It moves the water (creating the rooster tail) as opposed to moving the prop thru the water that happens on the down side. To get anything approaching equal force off the prop would require something like this. Not unlike a Jet Ski prop or fan shroud on a car engine.

Take a prop on a 8* angle on 4 ft of water, and spin it 10,000 RPM and tell me how much water flies in the air. That water creates some but its countered by the ear trying to drive itself to the bottom of the lake. That force on the strut is countered by the upward force of the opposite side to a much greater degree.
The water thrown on the bottom of the boat does jack shit to propel the boat. It just destroys blast and cav plates.

OK, but lets picture this, same example you used, 8 deg shaft angle to the boats motion. Now lets use a prop that has an 8 deg angle on the blades. The downward moving blade will be creating no forward motion, just lift like a paddle wheel. But the upward moving blade would have a 16 deg angle and create forward motion as well as some downward thrust. Less downward thrust than the upward thrust of the paddle wheel on the downward motion side, Right ? We would end up with lots of lift and a little forward motion, Right ? So now we put the shaft at a 4 deg angle, same 8 deg blade angle, the downward moving blade will be at a 4 deg angle creating less lift but more forward motion, Right ? And the upward moving blade will be at 12 deg angle creating less forward motion than the downward moving blade that's at a 4 deg angle, Right ? The downward blade creates less lift, and the upward blade creates more downward trust, Right ? The down ward moving blade will have a much higher inch of movement per rotation (higher true pitch) than the upward moving blade. So we end up with less lift and more forward motion, Right ? Now put the shaft at 0 deg. Now both sides are creating the same forces. Same 8 deg blade angle, so same true pitch, same forward thrust, and equal up/down force. No lift, just forward motion, as long as the prop is fully submerged. So shaft angle as well as blade angle, compared to true forward motion angle will have a HUGE effect on lift, Right ? Not just how much water the prop throws in the air.

And I still think the shaft angle basically causing the upper blade to be raked back more than the lower blade (compared to true forward motion) has something to do with the "paddlewheel" effect (pushing the back of the boat one way or the other). I just cant figure it out in my head.