Interesting Technical Question

OK , back to your original question

I believe any slight saving on fuel would be insignificant compared with the residual value of a boat with Half as many engine hours on it.

It was for this reason I ran my last boat on one engine when ever I could.

I have followed your post with interest as I dont want to damage my engines with my technique of slow running through Ports and inland waters.
 
I really think we are only talking about SD and D hulls here - certainly not planing.

I was hoping to find some research on the real effects on running on one engine with one locked.
My prop designer chappie is going to do all the theory calcs for me for my next boat and I will publish them here.
 
I simply do not know the answer.
If I had to guess, I reckon it could be as much as 20% and as little as 10% saving on fuel or increased range - this is signifcant over time.

I intend to do some long distance cruising and hence liked the idea of being able to run a bit Nordhavem style - slow and a long way.

As you say there are other gains as well. When my theory figures come back on the savings for my next boat i will publish them so that we can get a better handle on the savings.
 
I have outdrives and on a flat tide on one engine @ 1000 rpm will travel at about 4 knots. Engage the second engine @ 1000 and speed increase to about 7 knots, so makes sense the boat is more efficient on one engine otherwise it would travel at least 8 knots for the same rpm on two engines?

Now I thought you could work out fuel consumption based on bhp output, so if running on one engine @ 1000rpm produces 'n' bhp and running two engines @ 1000 each produces 2 x 'n' bhp, but running 2 x engines a 1000rpm doesn't produce 2 x one engine SOG. Running on one engine must be more efficient.

Simply, running two engines at a given rpm (1000) doesn't give twice the speed as one engine at the same rpm (1000) thus running one is more efficient. Maybe /forums/images/graemlins/confused.gif
 
[ QUOTE ]
its the viscosity that is different. the physics are the same.

[/ QUOTE ]

Hmmm

Imagine for a moment that we have a very coarse-pitch prop made of an indestructible material, running (lubricated) in a medium with little give (ie V high viscosity). Connect a force-measuring thingy to the front and pull it out of the medium.

Next lock the prop and pull it out.

Now I would have thought that the prop that is allowed to rotate would effectively cut a screw-like path through the medium, the force neede would equate to the force required to cut the narrow blade-path on its way out.

Now consider the locked version. Here in order to move the prop forward you have to pull out a circular plug of the material broadly corresponding to the diameter of the blades and the surface area of the blades.

I can envisage the force for the locked prop to be greater than that required to cut a path needed for the blade thickness only.

Now repeat this experiment in a medium with very low viscosity, here we can envisage a situation where the forces become much smaller in both cases to the point when they are zero in both cases in a vacuum (ie 0 viscosity).

So there must be a crossover point for different props in the same medium and for the same prop in two different media.

So I'm with Brendan. It depends.
 
No, I do not think that is right.

I too have noticed that two engines do not produce twice the speed - I think that is commin but it may take more than twice the power to achieve twice the speed - that is dependent on hull and many other factors.

In my case I have twin 700 hp engines.

Top speed with both going is 25 or 26 knots. SD hull.

If i want to cruise at slightly above displacment speed eg say at 11 knots, does it pay me to run on one engine working at say 80% of its power and one prop locked or does it pay me to run both engines at lower percentage and hence not as efficiently?

I am having the rough theory figures produced.


The 42 fotter already mentioned found ir 55 better to run both engines achieving his 7 knots and without prop locking I am sure that in my case that would also be true - but what would it be with prop locking - I am having the calcs done to give me a guide albeit rough.
 
nope sorry, i do not accept that.

We are talking of flowing materials such as water, air, oil etc not semi-solids - then the free to rotate prop has a lot more work to do because it has a lot more of the medium to pass over its blades - irs designed to do that work to generate thrust when driven and so when pulled through will demand an equal amount of energy to achieve that same rpm.

There is no cross over point, no way out of this, the free to rotate prop in any flowing medium such as air or water will give a higher drag than the fixed prop.//// always.... no exceptions.
 
I would guess there may be a point if you are thinking of using one engine a lot when running, slightly coarser props than standard may work to your advantage and also fewer blades to lessen drag?
Of course not as efficient if your going faster and can't have it too coarse so that the engine is overloaded but there may be a point where it balances out.
 
I think as regards resistance of a locked prop its probably down to the surface area of the prop - you are effectively pulling a lump through the water. I imagine that friction differences between one choice of prop and another are small as regards this point.
 
I would think there is a crossover between the less efficient prop and drag, as always a compromise, but if you are going to be running at the lower end of the envelope than maybe worth looking into.
Also worth looking into other areas of drag, but I'm sure the manufacture will already have done this.
 
I would think it makes a difference as to what angle its presented at and with fewer blades probably less surface area and sources of drag, but as you say the difference is probably small.
 
In that case...

You have to stall the prop for this to be so. From zero kts up to a certain speed (depending on its pitch), a locked prop won't be stalled and will be generating the same drag as one that is rotating. The drag on the shaft will be created by the spin imparted to the medium by the propellor - even if it is not rotating. Once you stall the prop I'll agree drag will fall - but if you are below the stall speed of the prop (which on a 30kt mobo could be very significant), I'd suggest it makes no difference whether locked or not.

Thoughts? Might account for different results in different scenarios. Note that the reduction in drag when stalling is (like stalling a plane wing), a one-time event, drag doesn't continue to decrease with increasing speed so this isn't perpetual motion.
 
"You have to stall the prop for this to be so. From zero kts up to a certain speed (depending on its pitch), a locked prop won't be stalled and will be generating the same drag as one that is rotating. The drag on the shaft will be created by the spin imparted to the medium by the propellor - even if it is not rotating. Once you stall the prop I'll agree drag will fall - but if you are below the stall speed of the prop (which on a 30kt mobo could be very significant), I'd suggest it makes no difference whether locked or not."

i do not understand what you are saying here ... can you please expand the point. What do you mean by stalled in this context. A locked prop is a locked prop - it does not turn and always gives less drag than a prop free to roate when pulled through the water .... I must be missing the point here so please exapnd it a bit so I can understand.
 
"The drag on the shaft will be created by the spin imparted to the medium by the propellor - even if it is not rotating. "

I know this is not true - a locked prop will exert almost nothing on the shaft - maybe a little due to the surface are an angle of the blades but so little it can be ignored.
 
OK.

Driving prop - rotation creates good angle of attack, resulting "lift" from the blades results in thrust on the propshaft. If angle of attack bad (usually due to too many rpm for the fwd speed), the blades are hydrodynamically stalled just as with an aircraft wing and "lift" or thrust up the shaft falls off very suddenly.

Driven prop (ie windmilling). Reverse the forces so the "lift" created by the blades is now drag on the shaft, if the rotational speed of the prop is reduced then the blades will stall and the "lift" created, or drag on the shaft, reduces. Locking the prop obviously stops the rotational speed so the blades should be stalled. However what I am suggesting is that for coarse props stopping the rotation may not be enough to get the blades to their stalling angle of attack - it may also require significant fwd speed of the vessel tyo achieve the stalled/locked/low drag condition.

It may be possible to have a fine-pitch prop that is stalled at any fwd boat speed due to the angle of attack with no rotation allowed from a standstill. My suggestion though is that it may be possible to have a coarse-pitch application where at slow fwd speeds - even with a locked prop - the angle of attack may not be stalled, hence there may be no difference in drag, once up above the stall angle then the drag on the shaft will reduce.

See here Q14

windmilling.gif


[ QUOTE ]
The angle of attack of a fixed pitch propeller, and thus its thrust (lift), depends on the forward speed of the aircraft and the rotational velocity. Following a non catastrophic engine failure the pilot tends to lower the nose so that forward airspeed is maintained while at the same time the rotational velocity of the engine/propeller is winding down. As the forward velocity remains more or less unchanged while the rotational velocity is decreasing the angle of attack must be continually decreasing and at some particular rpm the angle of attack will become negative to the point where the thrust force reverses and the propeller autorotates, driving the engine. This acts as greatly increased aerodynamic drag which seriously affects the L/D ratio and thus glide angles. The drag (reverse thrust) is much greater than that of a stationary propeller, also the engine rotation may cause additional mechanical problems if oil supply is affected.

If the forward speed is increased windmilling will increase, if forward speed is decreased windmilling will decrease, thus the windmilling may be stopped by temporarily reducing airspeed so that the reverse thrust is removed.

In the diagram the upper figure shows the forces associated with a section of a propeller blade operating normally. The lower figure shows the forces and the negative angle of attack (aoa) associated with the propeller now windmilling at the same forward velocity.

[/ QUOTE ]
 
Freewheeling

For what its worth, on the sea trials I have been involved with on a single screw boat if you want to crash stop in the shortest distance with no use of rudder much better to have the engine at dead slow ahead (in gear at idle) than simply stopped.

Exactly as, I think it was Duncan, says.
 
Re: Freewheeling

[ QUOTE ]
For what its worth, on the sea trials I have been involved with on a single screw boat if you want to crash stop in the shortest distance with no use of rudder much better to have the engine at dead slow ahead (in gear at idle) than simply stopped.

Exactly as, I think it was Duncan, says.

[/ QUOTE ]

Have I missed something here, how do you crash stop with the engine at dead slow ahead ???????/
 
I posted that URL earlier, along with the start of the thread from the T&T list. It suggests that letting the prop freewheel on one engine gives a better mpg.

Rick
 
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