Hydraulic drive problem

Naaah, LL, you are wrong there, a freewheeling prop causes less drag than a prop that is producing shaft torque.

 

Well done LL,--- I knew I would find one, there always one.

 

If a prop were actually able to free wheel it might be less drag than a stalled locked prop.

Sadly the stern bearing , transmission and stuffing box all slow down the rotation enough so the rotation is efficient , at producing drag.

Folks with shaft mounted alternators seem only to use them when at hull speed, with excess wind.

Think of the spinning prop as a drag bucket , about 50% larger than the prop diameter.

Think of the locked prop as a drag bucket 75% of the prop diameter .Depends on the BAR.

There IS one "cure" for this hassle , a 2 blade locked just right behind deadwood.

This has the advantage of a more efficient prop underway , although the diameter will need to be larger.

With the proper prop brake , one that locks the prop where YOU want it , (not just locks it) the prop drag can be reduces to almost nothing.

There is a "sweet spot" (that changes with speed) where the prop behind the deadwood will not attempt to spin in either direction.

You find it with two pipe wrenches and great care , the point where the prop does not "want" to spin.

Mark it and adjust the prop lock for that spot.

Then you just pay for the drag of the surface area of the prop blades, a bit more drag than the folding prop folks get , but with $2000 to $5000 lower cost and perhaps 1000% higher reliability.

FF

 

Dear Fast Fred,

Following the "efficient / correct installation" instructions the propeller should have a distance from the deadwood equal to the point - where the flow field astern to this - has none turbulence to be appeared at the max value Speed of advance - Va.
This instruction "protects" from known cavitation issues of the propellers blades.
In most cases, when there is no space to move the propeller's position more astern, the manufacturers are modified the shape of the deadwood to be near of NACA section's trailing edge regarding to avoid serious turbulence before the flow field is entered to meet the propeller's blades suction side. ( or math-speaking to avoid increase the Curl. on Vector Va / Speed of advance)
So if the propeller is installed correctly there is no angular defined position of its blades - related to deadwood - where the trailing torque could be reduced or disappeared.

Based to that the most efficient way to reduce drag of a fixed pitch propeller is to let it free runs with lowest possible friction and avoid any applied brakes on its shaft and specially to take care for all the related mechanical moving parts, to be well lubricated.

An alternative more efficient option is to use patent propellers special to be used on Sailing Yachts that have the ability to fold up the blades.

 

All:

Rather than debate theory versus reality any more, why don't we all just stop?

In real world versus theory discussions, real world wins every time. I think the true solution here is situational - every situation is different, and each one needs to be measured individually to determine the correct situational solution.

Fast Fred has brought up great points - and I agree that his practical solution is right under the circumstances he outlines. I also think the theory discussed is correct as well. With a well designed boat, deadwood and foil sections I'm certain a freewheeling prop would have less drag. Since both parties are right in their circumstances, further debate is pointless.

It would be interesting to measure drag via load cell under tow for each configuration - freewheeling, optimally located two blade prop behind deadwood and three (or more) bladed prop stopped without positional optimization. With measurements it would be possible to see if the differences are statistically significant, using the same boat as a datum reference.

--
CutOnce

 

Documents

Dear Cutonce,

Please see the attached file regarding the points that you are mentioned.

 

Information

Dear All,

There is also a more extended research on this matter admits the same results and conclusions. Please see the attached file.

 

Thanks. It's nice to see clean research!

--
CutOnce

 

"Furthermore, for the freewheeling case, the
magnitude of the hydrodynamic resistance is significantly
affected by the amount of frictional torque on the shaft,
low torque being accompanied by low drag."

This is stated at the end of the article , and the Current issue of Pro boat builder #130 has a note , "Pay as You Go"

That references a cruiser with bike chain to drive an alternator.

From Hugh Wellborn,

"We had a controllable pitch Hundestad prop that could be declutched and clutched for alternator power. but the speed kill was significant until we had 8+K on the go.

Otherwise it would take off 2K upwind so we would not use it."

FF

 

A point of view.

Dear Fast Fred,

The matter which we have to respect here is simply "the Energy".
As far we have the ability to absorb a quantity of energy from the free-wheeling Propeller Shaft this amount ("will be paid") and should be subtracted from the Total Energy potential amount of the ship's Hull and will be added to Hull drag quantity.

Expressed by Other words:

When a mechanism is driven definitely is observed to spend energy
and when it works to drive another, then produces.

In our case the mechanism (Sailing Yacht) produces kinetic Energy and the driven mechanism of Propeller + Shaft spends an amount of that.
If it is free-wheeling the amount is less than this when is braking it.
Always when the brakes are engaged the mechanism absorbs and spends more power than during a free run operation.

 

Not necessarily - this is the point a forum member made to me earlier. A two bladed prop sitting "braked" in the "shadow" of deadwood may have less drag than one freewheeling. Even when freewheeling the energy is consumed, and the turning of the shaft generates friction. As I said before, each situation may be different, and only by measuring drag will you know conclusively what works best for a given set of conditions on a particular boat.

Over time, everyone gets to "know" what works best for them - by watching boat speed and trying different configurations at the same power levels.

--
CutOnce

 

Some Notes

Dear Cutonce,

I'm sorry for my delay to respond ; I was abroad.

The base of the discussed issue is that we should avoid the propeller to "stall".
This could be done only if the propeller continue to turn.
Regarding the consumed "friction work" is known that is about of 5% of the applied torque for each journal bearing and more or less the same for a Hydraulic engaged Reduction Gear (Total 14% to the propeller's applied torque [ two (2) bearings + the Red.Gear]).

Now consider about on a stationary propeller. The Pressure side (astern) "stalls" and the applied torque on suction side ( fore) totally reacts as drag.

In this case the Total Resistance is an integrated sum of "stalling" react from the Propellers Shape.

As conclusion we can say that always the Propeller should be turn to avoid stalling. Note that the drag is depended to the water velocity squared.

So, there is only one way to hide the propeller if you be able to avoid the blades' touch to the water flow.