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

There is scattered data available (just had a quick check):

Volpich J., Bridge I.C., “Paddle-wheels-part, I, II & III” IESS, 1954/1955, 1955/1956 & 1056/1057 (The most respected)

Some German papers:
Krappinger O “Schaufelradberechnug (Paddle wheel calculation) Schiffstechnik 1954.
Gerebers F, “Das Schaufelrad im Modellversuch: Zwei Berichte der Schiffbautechnischen Versuchsanstalt, Wein (The paddle wheel in model test: 2 reports of the Vienna model basin), Spinger 1952.

The slip has been measured for various vessels and type of paddle wheels. They seem to range from 0.14 to 0.22. Since the slip is dependent upon the tangential velocity of the circle. But it appeas 0.16 is taken as the norm. When the paddles are feathered, the overall PC can approach or be the same as a screw propeller. Since feathering to suit different wake condition can yield a high efficiency.

That is about the extent of my knowledge on paddle wheels…not much, don’t design with them. Beyond that, I’d have to check ref books/papers.

 

Im still not clear on the proper procedure for propping an electric motor ? How do you determine when the motor is being overloaded ?

 

The design you posted do not show an easily driven sailing trawler. It will no sail upwind, according to you, so it needs adequate power. Unless the rescue missions are always downwind I suppose

 

Ad Hoc, thanks for the paddlewheel references.

I assume paddlewheels are similar to propellers in terms of slip sets an upper, theoretical limit for efficiency, and actual efficiency would be lower.

 

All propulsion systems that use the surrounding fluid for propulsion use the same formula for the theoretical limit for efficiency, which is 100% for 0% slip. This is based on the fact that all the velocity you create in the fluid is wasted, Thus using an infinite area and thus zero velocity change results to 0% slip and theoretically 100% efficiency.

But all practical propulsion systems have their own efficiency of producing the velocity needed for propulsion and this efficiency approaches 0% at 0% slip, since at 0% slip you have no thrust you only have the friction from the blades or whatever you use for propulsion.

When you multiply these two efficiencies you end up to an efficiency curve which has a maximum (say around 10% slip) and is 0% both at 0% and 100% slip.

 

Main issues with paddlewheels, and the main reasons for which they were set aside in favor of propellers, are:
. 1) big sensitivity to immersion of the paddles - which is very detrimental for both propulsive efficiency and for directional stability in a seaway;
. 2) higher cost and constructive complexity, when compared to propellers;
. 3) vulnerability to impact with floating objects.
. 4) low-speed steering sluggishness for single-wheeled boats.
Several authors (ex. Carlton, Ghose and Gokarn) claim that the efficiency of paddlewheels can be made comparable (I remember reading somewhere up to 65%) to the efficiency of propellers by adopting very large diameters and feathering paddles (which maintain an optimum angle relative to the waterflow during the submerged, pushing phase). But large diameter wheels and feathering paddles both mean cost and weight.

However, this guy has not abbandoned the paddlewheel concept, and has even found an original application for it: http://www.marinepropulsors.com/smp/files/downloads/smp11/Paper/FA3-1_Harte.pdf
An interesting thing he has discovered through sea trials, and which confirms the previous point n.1, is that a change of 5 mm (0.2 in) in paddle depth has caused his maximum boat speed to vary by +/- 6 kts (page5, table n.3).

I am somewhat skeptical of his claims that paddlewheels can be competitive for propulsion of high-speed craft because weight reduction is everything for the performance of high-speed boats, and a paddle-wheel mechanism imho cannot compete with a prop when it comes to weight. Besides that, a large-diameter wheel also mean a higher aerodynamic resistance at high speeds.

Cheers

 

I was referring to the theortical maximum efficiency for a given slip, and the actual efficiency at the same slip.

Theoretical maximum efficiency = 1 / (1 + Slip Ratio) So for a slip of 10% the theoretical maximum efficiency is 1 / (1 + 0.1) = 90.9% Actual efficiency will be less.

This assumes "slip" is defined as the ratio of the average speed of the thrust producing element to the speed it would be traveling if it was not producing thrust.

 

Found it here: http://doku.b.tu-harburg.de/volltex...rechnungsunterlagen_fuer_Schaufelraeder_n.pdf

 

daiquiri's link doesn't work for me but this one appears to: http://doku.b.tu-harburg.de/volltexte/2009/667/ Date is 1959 though, not 1954.

 

Ok, that's the same one. Now... Who speaks German here?

 

Michael,
An electric motor is overloaded when it overheats, so that's not a good criteria for selecting propeller. I would start by first choosing the largest practical diameter, which according to Joakims estimates is unlikely to be too large for optimal efficiency. Then I would calculate the pitch for my estimated maximum speed with my estimated slip. So far I believe the procedure is the same regardless of power source.

If the motor is designed for a certain maximum power at a certain rpm, I would simply check that I achieve that power at that rpm. Rpm is easy to measure, and most likely a signal is available from the control electronics. Input power to the motor is easily measured by the voltage and the current drawn from the batteries. If the power is too low at the target rpm, the pitch is too low.

This procedure is aimed at assuring that the motor operates as designed. If the cooling of the motor isn't the limiting factor, it is possible to get more power from the motor at the cost of more heating and therefore reduced efficiency. A reduction in power will also increase efficiency.

I have no practical experience in selecting propellers, so this procedure is only based on general engineering knowledge, and an understanding of electrical motors.

Erik

 

The problem is motor sailing. With a diesel and a controllable pitch prop, a vessel motor sailing is very efficient....you can over pitch the prop , gain more speed...because the sails are removing load from the drive train. How do you do this with an electric motor sailer equipped with a controllable pitch prop ? do you measure load...amperes or heat from the motor ?

 

It probably depends on the motor control system. I would expect newer systems to have built in overload protection as well as overspeed protection which would reduce power to the motor to prevent overloading and/or overspeeding.

Motor speed could be an issue though if there isn't a clutch/disconnect between the motor and the prop shaft and the boat starts sailing at high speeds. With a controllable pitch prop the pitch would need to be controlled to keep the motor from over-speeding at high boat speeds. A fixed pitch prop would need to be selected to prevent overspeeding under most or all sailing conditions. The crew would need to moniter motor speed to make sure the motor isn't over-speeding, and be able to lock the shaft if needed.

 

With a CPP I would control pitch so I get the design motor rpm at which maximum motor power is specified. Then I would set the desired power to the motors (there are different ways for the controller to effectuate this command) to get the extra push I want. It will be necessary to adjust pitch to maintain design rpm as motor power is adjusted and as boat speed changes.

Erik

 

Erik, thank you for the comments, after a number of people recommending larger propellers I'm working seriously at putting on the biggest ones I can practically fit without too much down-angle on the shafts. If after pitching them optimally, the DC motors can't attain their maximum manufacturer's revs of 1,100 then clearly reducing diameter would be needed. Fortunately I have access to multiple props to try but can't justify the cost of variable pitch. Though after reading Gerr's book twice and on the third read, and lots of tips so far, I still remain mystified as to why the algorithms in three different popular PC or internet propeller calculators all seem to agree that this particular application requires 18 - 19" props! But I'm leaning towards testing larger diameters largely because of the preponderence of advice and also the fact that I anticipate lift from motor-sailing much of the time, which I hope would justify the larger diameters even if they are marginal in power-only maximum rev situations.

Michael, the control system for the drives actually has digital monitoring of revs, power in kW and temperature. I guess I could also add a voltmeter to therefore calculate current as in amps=watts/volts, though just a kW reading will probably suffice. I assume that the manufacturer has given the revs of 1,100 rpm as the right number to match the current the coils can take without overheating - BUT I guess I could play with that as long as I'm monitoring the temperature... Looking through the manuals, this system is pretty sophisticated - we will be able to set revs, and the generator/DC drive combo even provides for emergency power with automatic slow-down when reaching over-temp conditions.