Bold claim for new propellors

Thanks baekmo, it is gratifying to read explanations laden technical reasons. It is not easy to reach deeper understanding, because my knowledge is scarce, but i'll study them trying to assimilate properly the concepts.
Thanks again.

 

If the claimed improvement is coming from bow lift, then maybe shifting weight backward toward the transom will do the same job ? I guess when you trim a motor out to get bow lift and more speed, you introduce a vertical component into the line of thrust, are we to understand that the motor now won't have to be trimmed so much, if at all, and all that thrust can be directed forward, not slanting slightly downward ? What happens when you have to slow down in the rough, will you need more in-trim to keep the nose down? Doesn't the characteristic of the prop to pull the stern down have to be paid for in reduced revs ? All a bit confusing really.

 

When a boat starts to move forward water is displaced from under the boat and accelerated back from the boat, (the boat transom drops into the hole caused by the displaced water) water moving rearwards is reaction which is the driving force like any propulsion system.
The boat is said to be "climbing over the hump" which is really just a higher area of the water level and a pressure wave formed by the forward movement of the boat. As the boat speed increases it is able to "climb over the hump" (if enough power is available) and to start planing (preferably flat and level for comfort and safety) and dynamic pressures of water (and the air) have come into effect.
Many methods are employed to alter the planing angle of the boat such as" riding on the prop only" (for less drag) which means the bow will be where the boat will ride on the water most of the time. All of the differing opinions of what and how a propeller works is understandable as it is a very complex subject. Everything we do requires that friction and drag must play their part otherwise things will not work there has to be trade off`s there is no 100 percent efficiency. It is very likely someone has a Patent on this more efficient design propeller or design.

 

I agree that a non-uniform inlet flow can cause a propeller to generate a transverse force, but the effect of rake on that transverse force is not obvious to me. I'll need to draw a free-body diagram or two.

It is entirely possible to have an accelerating flow at the surface with the pressure at the surface equal to atmospheric. The local surface elevation will change proportional to the change in the dynamic pressure of the flow, which is proportional to the difference of the squares of the velocity.

Consider a stream line along the surface from some point upstream (location 0) to just ahead of the propeller (location 1):

density / 2 * V0^2 + density * g * h0 +p0 = density / 2 * V1^2 + density * g * h1 +p1

V0, V1 are the velocity magnitudes; h0, h1 are the surface elevation; po, p1 are the pressures.

p0 = p1 = atmospheric pressure

The equation can then be simplified to:

1/2 * V0^2 + g * h0 = 1/2 * V1^2 + g * h1

The surface elevation difference between locations 1 and 2 is given by:

h0 - h1 = (V0^2 - V1^2? / (2 * g)​

The inflow into a propeller beneath a free surface will not be uniform, but the non-uniformity will be a bit more complex than assuming the flow at the top of the propeller is not accelerated.

 

Well, it remains to be seen how this propellor actually performs in comparison to others, if in fact it does get 10% extra mileage, I would amazed. If its gets 10% better only if we're too lazy to use the trim switch, that would be a bit disappointing.

 

Per definition, a propeller "close to the surface" means that (h0-h1) is close to zero, thus no velocity difference possible!

 

Do you disagree with the analysis provided? If not then what keeps the free surface of the water from deforming if the propeller is close to the surface?

Edit: For the analysis given above in post 49:

a stream line along the surface from some point upstream (location 0) to just ahead of the propeller (location 1)​

There is nothing by definition which says that (h0 - h1) is close to zero.

 

Baeckmo is right, David. In the above analysis you have used the Bernoulli's equation. The term "h" in that equation is not the surface height but the depth below the reference zero-hydrostatic pressure isosurface.
Hence, for a prop operating close to the surface the difference h0-h1 is indeed close to zero. It is precisely equal to zero for points which are on the water surface - which is the zero-hydrostatic pressure isosurface, even when it is not flat. Hence the difference h0-h1 is actually by definition equal to zero for all points which belong to the streamline considered by you.
Cheers

 

Perhaps it would be more sense when discussing boat propellers to refer to the ability of a propeller to expel x number of tons of water per minute at RPM using 85 HP. ie a 9-1/2 x 9 inch prop expels 4.5 tons of water per minute.

 

Sorry daiquiri, but you and baeckmo are both incorrect. Yes, it is Bernoulli's equation including the gravitational term; density * g * h, applied to a steady incompressible flow along a streamline. "h" is the height above an arbitrary fixed reference elevation. For analysis involving a free surface a common (but not required) choice for "h" is the height of the undisturbed free surface such as the free surface height far upstream.

Also note that p0 and p1 are the pressure, not the hydrostatic pressure. The condition at the free surface is the pressure, not the hydrostatic pressure, equals atmospheric pressure.

With the interpretation which you and baeckmo are using the velocity at the surface of free surface waves would be constant, not oscillatory. Also the flow over a weir would have constant velocity along it's surface for the entire length of the flow, Observation of the flow over a weir reveals that the elevation of the free surface drops and the surface velocity increases as the flow approaches the weir.

Anyone interested in more detail might consult a good reference on free surface flows such as water waves or on what civil engineers call "hydraulics".

Textbooks on aerodynamics generally ignore the gravitational term since the effects of gravity are small compared to the other terms due to the combination of the density of air, and the magnitudes of the lengths and velocities for most aerodynamic problems. Free surface flows are a different situation.

 

The flow approaching a propeller below a free surface will have a vertical velocity gradient due to the presence of the free surface, but not for the reason baeckmo incorrectly identified above. Rather the vertical gradient results from the distortion of the free surface due to the acceleration of the flow and the corresponding distortion of the streamlines approaching the propeller which are no longer axially symmetric.

 

What is the value of the hydrostatic pressure at the reference elevation?

 

If the reference point is far enough upstream that the flow and free surface is essentially undisturbed then the hydrostatic pressure equals atmospheric pressure. The choice of the reference elevation and reference pressure are entire arbitrary give the assumption of incompressible flow.

 

Mercury says that rake helps with ventilation. They don't explain why it causes bow lift, but my guess is that with the ability to tolerate more ventilation, the less ventilated bottom and rising side of the blade path do relatively more work. More force deeper and on the rising side = higher bow.
http://www.mercuryracing.com/blog/prop-school-part-3-blade-rake/

 

Yes, you are right. Good arguments. I evidently need a brush up on the free-surface flow equations. Haven't used them for ages (because I didn't need to - but that's not a good excuse for ignorance). Cheers