Prop calculators - is there a difference between electric and diesel drives?

Yes 2000 N at 6 kn is the right order of magnitude. I checked Swan 65 and it was 1700 N at 6 kn, with 15.2 m LWL, 35 000 kg displacement and 72 m2 wetted area.

2000 N is not much for two 18" propellers and you will get OK efficiency with that. 18" (e.g. 18x15) is a quite typical size for a 30-50 hp sailboat propeller that runs at 1200-1400 rpm. At 2*1000 N you will need less than 10 hp/engine.

Actually originally the Swan 65 had only 89 hp engine, thus not much more than what you are planing to have: http://www.classicswan.org/swan_65.php

 

25kW (33HP) per motor at 1,100rpm.

Don't understand or know the answer, still researching. Just the manufacturer's rep said the system isn't designed to be geared down and that as-is should work adequately in this application.

I also have a lot of other things on my plate without an extra project of putting in a gearbox or belt transmission right now, so this is as always boiling down to priorities, budget, time available, skills available and compromises. If I can get away reasonably well with direct driving the shafts from these DC motors at 1,100rpm then it helps a lot.

 

Thank you, this is confirming similar conclusions from others. A question for you: I re-ran VicProp with the new maximum recommended revs of 1,100rpm and it's now recommending an 18.8" diameter 3-blade prop pitched at 11.5" for a speed of 6.6 knots. Do you have a particular reason for suggesting 15" pitch beyond the Swan comparable? My trawler is going to displace somewhat more, with cargo possibly up to 70 tons.

Another thought is that she'll be motorsailing a great deal of the time when under power - would that compromise for a greater diameter and pitch to take best advantage of the lift from the sails? My logic is probably getting very leaky at this point...

Right now, the plan is direct drive and I'd like to be capable of delivering close to full horsepower under bollard pull type conditions. What bollard pull do you think I could get with the two 18" props at 1100rpm or if the motor torque will constrain it?

 

2000 N thrust at 5 kts gives 5.1 kW (6.9 HP) power. That's where the initial incomprehentions with Ad Hoc and Mike Jones were created. You have initially asked for help in dimensioning a prop for 33 HP @ 5 knots @ 1400 rpm, and now the design data is completely different. Ad Hoc might have been a bit harsh on you, but it turns out now that your initial question was ill-posed.

With your new data and assuming a wake factor of 15% a 18" prop will likely give you some 45% efficiency - which is not exceptional, considering that imho at least 50% more could be obtained with a proper rpm reduction.

Good luck.

 

I have no idea how VicProp works, but a quick test didn't really make me believe in it. It suggested that my boat would have a top speed of 6.75 knots while it does 7.4 knots. And I should have a 14.2x8.9 3 blade propeller while I have had 16x11 and 15x10.5 and both were good fits.

I guess by 70 tons you ment 70 000 lbs as you said earlier?

I did not suggest 18x15, I just said those are used at similar power and rpm sailboat diesels. More information is needed to really know the correct propeller, but anyway 18x15 is not far off, if the resistance really is 2000 N at 6 knots (then you should get much more than 6.6 knots at full power). Maybe pitch is not that important to be accurate, since you can get 1400 rpm as well. But you should know more about the engines you have (power curves, recommended rpm use etc.)

The same applies to bollard pull, but the order of magnitude should be around 6000 N.

 

It was 2000 N at 6 knots. At that conditions two 18" propellers can have clearly over 50% efficiency at say 700 rpm, which would be OK for ~1100 rpm at full power. What size and rpm would give 50% better efficiency?

 

I wouldn't say the design data is completely different, but I am learning as I go along. I did post the 1400rpm figure in good faith from the manual, but after a meeting with the manufacturer last Friday I got the new max rev figure of 1,100rpm. But I don't think that's what caused the rift in discussion with Ad Hoc. Does posting 1,100 rpm versus 1,400 rpm suddenly make my original question "ill-posed"? I think not.

 

Sorry, I can see how that was confusing. I mentioned 70 tons as a possible fully laden figure with cargo. I'm only over the last few days getting credible displacement figures for this hull through measurements and running Free!Ship. It's now coming in at between 45 and 50 tons without cargo.

Agreed. Actually, your estimate of 18x15 may well prove to be correct!

 

And now you want to motor sail efficiently too at higher speeds?

Ask your manufacturer if you can mount the two DC motors in series (in-line)

Punch that into a reduction gearbox (preferably a Hydraulic CPP version) with a single CPP prop. This can deal with whatever Torque curve issues your supplier might have. Possibly save you money sooner than you think too.

Then you will have a drive line worth talking about.

If your manufacturer can't see this as an advantage he's in the wrong business.

As you mentioned this is a hybrid drive are you going to be motoring on batteries too. What happens when the battery pack gets tired and low on voltage?

 

Ignore the battery comment, don't want to get into that.

 

Are you going to use fixed propellers? If yes, you must carefully think how important is sailing performance vs. motor performance to you. Already 2*18" 3 blade propellers are going to add about 700 N to resistance at 6 knots, if they are fixed and locked. If you let them rotate freely, you may get ~50% lower drag. Drag increases in the square of diameter, thus the resistance really will be an issue, if you increase the diameter much.

 

Right, I take note that you were talking about 6 kts.
However, (at this point just for the sake of discussion since the input data is continually changing), for 700 rpm and 6 kts I could calculate that the max efficiency is obtained from a 22"x13" prop (around 57%).
Cheers

 

Yes, it's not much point to make very accurate calculations until there is more accurate data. But, if we continue with this 2000 N at 6 knots assumption, 22x13 and 18x15 are equal in efficiency (1 kN/propeller, 6 knots boat speed, 15% wake fraction, 3 blades, 40-60% EAR, 58-59% open water efficiency). With higher thrust 22x13 will be more efficient (e.g. 3 kN/propeller at 8 knots 55% vs. 50%), but it is also heavier load and maximum rpm would only be ~1000 rpm while 18x15 would be around 1100 rpm. 22x11.5 would be closer to 1100 rpm at maximum power, but it would have 55% and 52% efficiencies at those two points. Thus better at full load, but worse at cruising speed compared to 18x15.

It may be possible to go to even lower rpm without a gearbox, if you can live without the maximum power. You just need to know the power curve of the engines and also check that there is enough cooling and no other issues against running them at low rpm and high torque. E.g. a 24x18 propeller would give you 64 and 59% efficiencies at those two points, but only ~700 rpm at ~30 hp. Due to better efficiency you may get about the same maximum thrust depending on the power curve.

 

What is needed now is an accurate description of the electronic speed controller so that the dynamic response of the system to a seaway can be taken into account. If it is amp controlled, the bigger, flatter prop can be used for efficiency gains without risking a loss in versatility. If it is a shaft speed control, the amp draw will cycle greatly in a large seaway and the motor efficiency drop will cancel the prop efficiency gain and the total average power will be less because the controller throttles back to a greater degree. In that case, you need a smaller, higher slip prop that doesn't cause as much amp fluctuation.

I like the idea of stacking the motors on a single shaft.

 

Dynamic considerations to improve efficiency in large seaways don't sound like first priority to me at this time. Tradeoffs may be needed between efficiency in calm water, ablility to motorsail at higher speeds, and sufficient thrust to maintain desired speed into wind and waves. After a suitable candidate propeller has been found considering those tradeoffs then dynamic considerations and interactions with the motor control system can be taken into account as the propeller selection is tweaked.

Based on where Ray appears to be with the project his highest priority should probably be to decide what the largest diameter propellers he should provide room for. It sounds like he is a little ways away from ordering propellers.