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

His power is 66 hp and displacement has varied between 55 000 and 100 000 lbs, thus 25-45 tonne and 1.5-2.5 hp/tonne. All the hp/tonne values I gave for my boats and Swans were for light displacement.

 

Many of the old Swans seem to originally have 2-3 hp/tonne. Is that enough for safety? http://www.classicswan.org/swan_by_ss.php

The current models seem to have 5-6 hp/tonne. Is this for better safety or just for faster cruising speed requirements?

ALL the newer boats are large , perhaps the cruising generator , 200A++ alt , air cond compressor , refrigeration compressor , water maker and scuba compressor take some power?

FF

 

Yes, but it is a clear trend not just limited to Swans. 30' 4 tonne sailboats have now 18-27 hp while they used to have 7-15 hp. Also I don't think the extra ~50 hp on Swan 53 is needed for auxiliaries. The 7 kW shore power maybe gives an idea of the auxiliary need.

 

How has the displacement varied between 55-100k lbs? This is not a cargo ship where the light and loaded displacement varies enormously.

 

My understanding is that one of the planned uses of the vessel is to carry relief supplies.

Also I would not be surprised if the builders do not have a good estimate of what the weight of the vessel will be when launched.

 

Sorry if there's a confusion, I think I may have mistakenly typed in the displacement previously.

We cannot find the builder or designer, this was acquired as an empty hull. We have been able to put together an accurate Free!Ship model since I started this thread, it looks like with a lot of estimates that she will be a 50 ton vessel. We will load up to 10 tons in the cargo bay for rescue missions. I'll see if I can post our latest Free!Ship model as an attachment here:

View attachment T2_RevB_Nov23.fbm

 

At the OPs request, posts regarding the more general concept / feasibility / timeframe requirements of a post-disaster rescue ship have been split to http://www.boatdesign.net/forums/open-discussion/concept-post-disaster-rescue-ship-40757.html so that discussion can proceed while leaving this thread about the more specific question.

 

I'll take the risk of displaying my lack of knowledge regarding propeller design.

A basic difference between electric motors and combustion motors is that the electrical motor can provide maximum torque from zero rpm, whereas cumbustion motors start from zero torque at zero rpm and have a rather limited rpm range where it will produce high torque. A consequence of low torque at low rpm for combustion engines is that you want the propeller to slip, or it would be like starting a car on high gear without a clutch. An electric car doesn't need a clutch and an electric drive in a boat shouldn´t need propeller slip.

As I understand it you should go for the maximum practical propeller diameter when you have an electric drive, to minimize slip. The pitch is easily calculated from the maximum design speed at maximum rpm and low slip (say 5 - 10%).

"Safe" horsepowers to keep clear of a lee shore has been discussed. In my understanding less power is needed with electric drives because of low propeller slip and high motor torque at low rpm. Also, if 2 - 3 hp/ tonne is adequate for a 25' boat, 1 - 2 hp/tonne should be adequate for a 50' because the front area scales with the square and displacement scales with the cube. For an electric drive with large propellers the necessary "safe" power should be even less.

Erik

 

The power needed to rotate a propeller in a bollard pull (or high slip) situation depends on the cubic of rpm and for a moving displacement vessel the power demand is something like ~rpm^2.5. For modern combustion engines the torque curve is almost flat, thus maximum power available depends about linearly on rpm. Thus there is no problems at low rpm and the engine is working hard only very close to the maximum rpm it can reach. You can compare propeller curve to full power from e.g. Yanmar bulletins: http://www.yanmarmarine.com/index.cfm/go/Sailboat-engines/

For both electric drive and diesel you choose a propeller that gives best efficiency at desired operating speeds and doesn't overload the engine. For both overload can only happen near the maximum rpm possible with the chosen propeller.

For a displacement vessel to have only 5-10% slip at maximum speed, a very big diameter is needed and/or a very low power. A very big diameter needs a very low rpm and a lot of space. Such low slips are used only for pedal power or very low electric power vessels (and for high speed planning vessels). For this boat a 10% slip at full power would need ~1 m diameter for two propellers and 33 hp/propeller.

 

With a diesel , you monitor exhaust temp to judge maximium prop size and load. How do you monitor the loading of an electric motor ? Logic says that a controlable pitch prop would be the choice for a hybrid powered vessel.

 

Propellers always "slip". The amount of slip at zero vessel speed doesn't need to be taken into account with an internal combustion engine unless thrust at zero speed, also known as bollard pull, is important.

 

I understand from the responses that too little propeller slip is never a problem.

"How do you monitor the loading of an electric motor ?"

My experience is with brushless permanent magnet DC motors, but all motors are similar except that the magnetic field is generated with different means. The torque for an electrical motor is proportional to the winding current, until saturation starts to occur. Output power equals angular rate times torque. If you control the winding current you control the torque. The winding current give off heat because of the resistance, which is the major reason for losses in a DC motor. You can overheat a DC motor, so the motor temperature should be monitored and the controller should automatically limit the winding current when it gets too hot. Apart from overheating I don't see how you can overload a DC motor, so it shouldn't be possible if you have a reasonably modern controller. Angular rate is also easy to control, and is limited automatically by the input voltage.

If you have sized your electrical motors so the maximum power is limited by the power supply and not by the motor torque at max load speed, it's possible to go up in torque when reducing rpm so the shaft output power remains the same (minus increased winding losses). As I see it, it's so easy to control an electrical motor so a controllable pitch propeller is not a justified complication.

Erik

 

In the system we're installing, the DC drives are raw water cooled, and the wheelhouse displays actually monitor the drive temperature, so that will certainly be one of the criteria I'll monitor and record during sea trials. I guess that would be analogous to monitoring diesel exhaust temperature. It's going to be a fascinating exercise, and for the benefit of this forum I'll record everything including sea state, wind/direction, knots, power in kW, propeller rpm etc...

Ppropeller diameter and pitch are going to have a very big effect on the DC motor temperatures, which will probably dictate the maximum diameter/slip/pitch to get to the motors' maximum revs. Fortunately we have a very friendly yard here with easy haulage and a large selection of used props we can experiment with. Taguchi's Design of Experiments should be the perfect methodology to calculate efficiencies with the fewest iterations.

 

Voltage and amperage are excellent electric motor monitoring parameters. Especially when two motors are involved, for comparison. No need for two sets of gauges, just put an ON/ON switch in for each motor. (i.e. port/strd motor selection for monitoring.)

-Tom

 

I've looked for but can't find an article I read claiming a 9.9 hp gas outboard developed 85 lbs thrust. Some larger trolling motors develope 85 lbs or even more thrust, yet they only draw about 2 hp electricity from the battery to accomplish this. I also read somewhere that combustion motors are rated at their max horsepower and electric motors at their continuous horsepower. Max horsepower of an electric motor can be as much as 10 X it's rated horsepower. So? How can you compare electric and combustion motors? I use a rule of thumb 1 electric horsepower equals 5 combustion engine horsepower. Works fr me in the real world.