New diesel-electric hybrid installation - how to size propellers?

What I observe on a modern diesel motor sailor equipped with a controllable prop is that I can over pitch the prop , achieve hull speed at 1000 rpm..near idle... with great fuel range when sail assisted.

Naturally you would want this flexibility when electric motor sailing.

Naturally you would want a controllable pitch propeller so that you could maximize thrust from your electric power plant when motor sailing.

Naturally you would need a monitoring sytem that alerts you when you are overloading the engine. Exhaust temp is what a diesel works on...

 

You are so right about wanting CPP - but especially with two, it's a pity that the budget don't allow it! So back to best compromise for multiple conditions.

I totally agree with your motor-sailing point - in my last boat (51' heavy displacement ketch), I was able to run my old Lehman diesel at just above idle, in the lightest airs, just enough to fill the sails, between 5 - 6 knots, for just a gallon per hour (measured directly from the calibrated day tank). My guests were always astonished by that. I once did a thousand nautical miles nonstop like that, took a bloody week and nearly emptied my fuel tank before the winds came up again, leaving me with just about 30 gallons to go the rest of the 4,000+ miles! I was down to just charging the batteries once a day after that, and little else... that kind of experience, though, leads me to over-propping the boat to take best advantage of that situation.

 

With two props is it possible to motorsail with one prop energized then drag the other prop to generate electriciy and recharge bat banks ?

 

Propeller Design

Ray Thackarey,

There is no mystery over why you are getting 18"-19" diameter of prop for 1100 RPM. It is all down to tip speed.
For sailing boats (and other applications) there are well established max values for tip speed.
People are advising larger props so that you will get best efficiency for your application. But to achieve this you must gear down to maintain tip speed at acceptable levels.
Hope this helps.

Cheers,
Graham

 

Actually, this might help to think things through, especially since we're talking electric motors here, which is a departure from conventional diesels. I'm going to call the manufacturer again and ask them that if I attempt to push the same power (25kW per motor) at lower revs than the recommended 1,100rpm, is it possible? My guess it comes down to windings/wire thickness in the coils and how much current they can therefore take without overheating, BUT if the cooling system is effective enough, maybe monitoring temperature with a preset alarm is enough to protect them.

As I understand it, electric motors are typically not geared, so I need to understand more about how far as can push this particular design. I'll post results of this conversation when I get more information!

 

I would think that the easiest way to size the propeller would be to get information on optimal current draw for given revolution from the motor manufacturer. Then you would try a propeller that you calculate for 25 kw diesel engine. Then all you would need to see if you are overloading the motor is rev. counter on the shaft and amp meter(shunt).

 

Not sure what you’re referring to there?

Slip is defined as the difference between the distance which a screw would advance during one revolution if working in a solid medium and the distance it actually advances in a given medium. It has nothing to do with thrust. You have 3 types: Apparent slip, True slip and Effective slip.

Apparent slip, takes into account the difference between the prop in open water and behind a hull. Since the ships speed Vs usually differs from the local velocity of the water in way of the prop, Va. This is defined

True slip, when dealing with model prop run in open water

Effective slip, being the related to the face pitch of the blades.

This leads you into wake fraction, there being 2 definitions, Froude and Taylor, subtly different.

Froude’s
w(f) = (Vs/Va) -1

and

Taylor’s
w(l) = 1- (Vs/Va)

It is common in the UK to use w(f).

Perhaps what you’re thinking of is the thrust deduction fraction?? This is due to the reduction in the pressure at the aft part of the hull caused by the propeller, since it creates a “suction” effect. It can thus be treated as either an augmented resistance or a deduction from the thrust of the propeller.

Thrust deduction fraction

t = (T – R)/T

Hence (1-t) = R/T

R = resistance of ship and T = thrust of the prop.

Augmented resistance

a = (T – R)/R

Hence (1 + a ) = T/R

Worth noting that T has to overcome not just the augmented resistance R(1 + a) but also that of the rudder and other appendages. So the value of “t” varies with ship lines, prop size, rudder etc, and is usually taken from model experiments. Since R is the resistance of the naked hull i.e. with no appendages

 

Tip speed is to be kept below what it would make it cavitate regardles of whether the boat has a mast and sail or not. That is, unless you are running supercavitating or ventilated propellers.

 

I was generalizing the concept of "slip" to a paddlewheel, and provided the definition I used which works with the simple formula for maximum efficiency in open water. Nothing to do with wakes or vessel interactions, didn't mention anything about them.

Slip does not completely define efficiency of a paddleweel or propeller, even in open water. The theoretical upper limit of efficiency can be estimated for a given slip however.

 

From a post #8 by Ad Hoc in the What is Propeller Slip? thread:

The definition of slip I gave above is entirely consistent with this, and for a propeller it gives exactly the same result, assuming open water conditions.

 

You’re combining equations to you give the same result. In doing so, you’re mixing up the definition of slip like it is an equation.

My post #8 used the context of “thrust” simply to relate to the poster that it is geometry (some text book definitions use this too, for the same reason), nothing to do with any actual thrust magnitude/calculation. Hence I used a term he was familiar with that being “thrust” of a prop, as is sometimes used for clarity.

The definition of ‘slip’ does not relate to thrust it is about geometry and velocity of water; how much distance the prop can move and how much it does move. Any definition of slip when formulated, has either in its definiton Vs or Va or N or P. There is no thrust element.

 

I understand there is a need to demonstrate that I'm wrong and the exchange could continue indefinately on that basis. But it won't.

 

Err..how so? If you think I am wrong in defining slip, please state so.

The only need is to correct the definition of slip. Makes no odds who actually states it. If thrust is in the formula that defines slip, from the various texts about that describes it in words, then I’d be happy to stand corrected, but it does not. My uni notes do not nor any of my ref books I have on my shelf. Nor when i have designed props for boats in the past.

Incorrect definitions serves no one any good. It leads to confusion or heated debated by each assuming an incorrect definition as their premise.

It does seem there are many posters on here on various threads that do not like being corrected with the use of definitions. I noticed Gonzo the other day got harped at for the same reasons, for daring to suggest a simply efficiency formulae beings stated by another poster, was clearly incorrect. Gave Leo a good chuckle too. ( http://www.boatdesign.net/forums/boat-design/sailing-directly-windward-27000-11.html#post504248 )

Definitions are there for a reason.

 

I'm surprised that the manufacturer hasn't been able to help you about the prop.

BTW that 36 hp @ 1100 rpm is 172 lb ft so I'm not sure a transmission will help:

Looking at the 39 hp Yanmar,continuous rating is 35 hp at 3000 rpm so about 62 lb. ft..and using the yanmar trans. at 3 to 1 you'd be at 180 pound ft.

 

In post #67 above I gave the definition of slip for a paddlewheel which I used in the simple formula in that post. Since a paddlewheel is not helical, it is not possible to use a definition of slip for a paddlewheel which is based on helical pitch. Therefore I thought it was appropriate to state how I was defining slip for a paddlewheel for the purposes of that post. The definition of slip I used was the ratio of the speed of the paddlewheel while producing thrust to the speed of the paddlewheel producing zero thrust. What might be confusing is by "speed of the paddlewheel" I meant rotational/shaft speed of the paddlewheel.

The standard definitions of slip associated with a propeller use a pitch value for the calculations. There are several differing ways to determine pitch.

ITTC - Recommended Proceedures and Guidelines; Model Manufacture, Propeller Models; Terminology and Nomenclature for Propeller Geometry. Effective Date 1999 contains several definitions for pitch including:
Pitch, analysis
Pitch, effective
Pitch, face
Pitch, geometric (nose-tail pitch)
Pitch, hydrodynamic

Pitch, analysis is defined as "Advance per revolution at zero thrust as determined experimentally". Note that the zero thrust condition is used.

For a given propeller the various definitions of pitch will result in different numerical values and different corresponding slip values.

If a propeller is tested under "zero slip" conditions with zero slip determined using one of the pitch values other than the analysis pitch value, the thrust will be non-zero even though the slip is nominally zero.

Back to the definition of slip in my post #67 above. If that definition of slip is applied to a propeller rather than a paddlewheel, it will give the same result for slip as the usual equations for propeller slip which use propeller pitch, provided the analysis pitch is used in those equations.