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

I'm about to install a donated Glacier Bay OSSA Powerlite dual-redundant system on a 56' steel sailing trawler about 100,000lb. This includes two generators at 25kW each, powering on dual independent systems two DC drive motors, to which I will be connecting prop shafts either side of the keel via a simple thrust bearing and shaft log arrangement, probably with PSS dry stuffing boxes.

Please let's not discuss whether a total of 50kW is enough for such a medium displacement trawler, I'm well aware that I'm not going to be able to power this vessel at hull speed. This organization has made a very strong "green" commitment, and this actually fits the application of a long-range cruiser very well. 5 knots is likely the maximum speed when not motor-sailing. I'm very happy with over 100 miles a day and am sure it will be very efficient, with the right choice of propellers. And here is the rub!

At a maximum of 1100 rpm from both the DC drives, how do I go about making the propeller calculations? I've read Gerr's Propeller Handbook from cover to cover (twice) and really can't figure out where to start in this application! Further, I've been offered a matching pair opposite rotation 3-bladed propellers of 28" diameter, let's take them as a starting point.

How do I go about calculating best estimate for pitch with this set-up?

 

Propeller sizing calculators

I just used Castle Marine's Propcalc application for 100,000lb, 53' waterline, twin engines inboard, max rpm 1,000 and got the result of 18.63 inch 3-bladed props with a pitch of 15.3 inch, for a surprisingly good maximum speed of 6.48 knots.

Doesn't that sound like a rather small propeller for the job? I'd be grateful for any pointers on where this might be going wrong.

I've also tried Surfbaud's Excel propeller calculator, and it comes up with a similar result. Everyone around here at the Yard says that an 18" or 19" propeller is way too small for a full displacement boat of this size, so I'm rather suspicious of these results.

What looks like the best online calculator I could find is VicProp's http://vicprop.com/calculator.htm, which gave me the result of 18.8 inch diameter and 11.4 inch pitch - so I have quite a large range to choose from... Obviously this isn't a science, but an art.

But I'm happy to get consistent reports that I can attain a cruising speed of over 6 knots with such a big boat using this relatively low power configuration! Of course, I'm under no illusions about close-quarters maneuverability if high power is required, or ability to claw off a lee shore, but in this application the vessel's main use will be offshore cruising and good seamanship will be required.

 

I recently replaced my propellers from twin blade folding in sail-drives powered by 20hp diesels... My propellers are geared to 1406 rpm maximum (at 3600 engine rpm) and I went from 15 x 10 to 15 x 12 three blade fixed propellers http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-121.html#post498124 the boat weighs in around 8,800 pounds (4000kg + 1500kg? in fuel)... Propellers are NOT handed, as there seems to be no point in a cat...

My experience seemed best identified as "suck it and see" which is quite an expensive methodology as 17 spline blades come in at Au$1400 the pair.... and are quite rare here - so nil chance of used/second hand... I can still pull 3600 engine rpm so I am not over propped... I need to do a long run to affirm preliminary consumption results... indications suggest 3 L/hour at 3000 rpm... This gives 6.7 knots on either engine with the other in neutral (propeller free-rotating and the dingy not in the water.). Max is about 10knots... A significant improvement...

 

In the posts I submit, I may sound like a know-it-all. I learn so much from the posts I read in this forumn, I think it's only fair to contribute when I can. My small motorsailer {Albin 25 lenthened to 30ft dbl ended) is not a 50/50. It's a 100/100. Impossible? No. Because she motorsails 100% of the time, except when docking.She could sail or motor, but her best comfort, economy and speed is motorsailing. I have a black inverted triangle on the mainsail. She is after all, a motorsailer.

 

PS: My point is consider more motorsailing in propeller calculations

 

You size the propellers according to the power available, regardless of type. You first need to calculate the power necessary to move the vessel at target speed. Then you calculate the ideal diameter. If you have to use a smaller diameter, adjust pitch.

 

Gonzo, in theory I agree with you, but in this case the brief is different: the power available is two 33hp DC motors. What I'm trying to do is determine the most efficient propellers to move this boat in the most effective way.

 

"What I'm trying to do is determine the most efficient propellers to move this boat in the most effective way."


Then you'll want the larger diameter, two-bladed ones with less pitch.

-Tom

 

Agree with Tom here. That is the ideal.

 

"What I'm trying to do is determine the most efficient propellers to move this boat in the most effective way."

Then you'll want the larger diameter, two-bladed ones with less pitch.


Your problem is you are attempting to find a "solution" with a computer.

The computer will "optimize" for the manner that the programmer decided.

Useless for you.

I suggest you get a copy of Skines , and start with your OWN assumptions of what "efficiency " is.

I would look at the largest diameter prop that could reasonably be mounted and work on a 2 blade solution.

If you could keep the rpm under 300 at cruise the efficiency will follow.

Your pitch will probably be higher than ideal for a motor boat to account for some sail help.

FF

 

The organization may be "green" but they don't have a clue about power requirements. A ship that size in any kind of weather will loose control with that power.

 

Funny - in over 21,000 nm of cruising, I've never been in a position where raw power was of any help whatsoever in weather. Makes me wonder what people did in the age of sail. I think making statements like "...not having a clue" are a bit strong; people need to consider how they'd phrase such a thing if talking face-to-face.

I think you're making the mistaken assumption that I'm putting together a boat to meet the usual market demand of a first boat buyer alpha-type rich lawyer who wants to know that if he shoves the throttle forward hard, he'll make it back to the yacht club in time to change into his blazer for evening martini and canapes...

On a more serious note, for short bursts a lot of people don't understand that electric motors can be over-run considerably for much more than their specified power rating, as long as you watch the temperature. Mind you, in this application I'm certainly concerned that with a bad propeller selection, increasing the revs may not actually develop much more thrust, increase cavitation etc.

Everything's a compromise, there are a lot of variables (even down to the budget and skill levels I have to work with), and it seems that even the professionals have as much difficulty as the amateurs in working this conundrum through.

 

2 x 25 kW, displacement 100,000 lb. That's 1500 lb/hp.

At that weight/power ratio, this system is basically just a docking thruster. It will get you in and out of a marina slip in calm weather. Gonzo is right; it is not nearly enough power to maintain steering control, let alone headway, in any significant weather. If that's what you want, OK- just be aware that you will not be able to use the engines to get this boat off a lee shore if the wind picks up. The insurer might have something to say about that.

The burst power of electrics will be of no benefit here; they're limited by how much the generator side of the system can put out.

But let's say you want to go ahead anyway. Pull out Gerr's propeller book, go to chapter 6. Use chart 6-1 to find Wf. Then use formula 6-3 and chart 2-1 to get Va. Now, formula 6-7 will give you Bp. Go to the Bp-delta charts for the propeller style you want to use, and drop a vertical ruler on your Bp value. Find the point on that vertical line that corresponds to maximum efficiency (look at chart 6-3 for how to read these) and read off the value of the closest Delta curve. Adjust delta according to table 6-2, then formula 6-8 will give you the propeller diameter. Go back to the Bp-delta chart with your Bp and new delta, and pick off the pitch ratio Pm/Dp (= pitch / diameter). Do the cavitation check calculation. Iterate as necessary until you find something that fits. That's your starting point, from which you adapt accordingly once the boat's in the water and her performance has been measured.

 

Using simple Rules of Thumb your boat is 45tons. (2240 per ton)

2 hp per ton will slow cruise with nothing for wind or waves. 90hp required at prop.

3hp per ton will give a faster but less "efficient" cruise.

Plan on a cruise speed of .9 times the sq rt of the lwl.

FF

 

In the Age of Sail, harpooning whales was acceptable. So was losing a large percentage of vessels and crews to bad weather and groundings. I doubt your "green" customers are willing to accepts those conditions.