Efficiency
Hi,
In recent posts there has been a lot of talk about 'efficiency' of various types. For the sake of discussion I'd like to issue a challenge.
The constraints
- 2 identical displacement sailing yachts of 6,500 lbs as built displacement with no EP system or prop fitted. You can now choose any EP system from those described below.
- I contend (within certain constraints) that two identical boats, which have propellers no more than 12 inches in diameter, with the first boat being fitted with a 2 blade direct drive using a Lynch/AGNI/Saietta type motor - it is going to be the most 'efficient'.
- This is when compared to a 3 blade geared driven boat, using a Motoenergy, Lynch, AGNI, Saietta, Perm, PMG, Heinzmann, Etek or Thoosa branded motor.
- No 'system' to have a rated continuous power output of more than 5kW measured at the controller output. You can have whatever rated motor you choose from those above.
- The direct drive 2 blade boat to be 24V system voltage and the 3 blade 48V or more. The type of propeller can be whatever you want as long as the 24V boat is 2 blade and the 48V boat is 3.
- Both boats to use monobloc batteries, the same make and the same individual Ah capacity, wired in series. Batteries as many as you want for the 48V boat. Any chemistry you want as long as each boat uses the same.
Please don't let this raise your hackles, I merely want to prove a point. I accept that I might be wrong and would like to be proved so. I only choose these motors/systems in principal because these are the types that have come up in conversation.
The Challenge
Set a direct course for 4 nautical miles to an agreed landmark from a marker buoy, in identical calm conditions, motoring and not using sails. Set each boat to use 750 Watts measured at the battery shunt with a battery monitor. Do the same again at 1,500 Watts. Once up to power and on a steady course, lock the rudder when passing the marker buoy.
I contend that at 750 Watts the direct drive system will be faster than the 3 blade system. At 1,500 Watts I'm not so sure and I think there is a reasonable chance the 3 blade geared boat may match the direct drive boat. Speed is to be measured as VMG (Velocity Made Good) toward the landmark, as that is what matters.
Now let's look at that dreaded word efficiency in its various forms.
Prop efficiency
How to compare/measure that? As prop efficiency is a measure of power input at the shaft compared to thrust out from the propeller, then I think when comparing the VMG of each boat the direct drive 2 blade boat described will in part indicate the most efficient prop scenario and therefore the fastest boat.
I appreciate we would need to measure and have equal power output at the shaft (torque & rpm) which will then be different for the two systems described when measured at the battery shunt. If needed we could fit each boat with identical direct drives, using 750 Watts power at the battery shunt but one with a 2 blade and one with a 3 blade, which would be a comparison of prop efficiency, albeit at different rpm due to choices of P/D and resultant slip. Even then I think the 2 blade will have a better VMG than the 3 and a lower kWh consumed per nautical mile if equal VMG speeds, which I suppose is like hypermiling in the car EV world.
If we also compared the 2 blade boat to the 3 as initially described - or even with 750 Watts measured at the shaft - then the 2 blade boat would likely have a lower kWh consumed per nautical mile than the 3, shown at the battery shunt.
Weight 'efficiency'
The boat with the 2 blade direct drive described will be lighter than the 3 blade boat. As Force = Mass x Acceleration, this is always good.
Boat cost 'efficiency'
The 2 blade driven boat described will be less expensive overall than the 3 blade boat.
Battery cost 'efficiency'
The 2 blade driven boat described will cost less in battery outlay. Less batteries, less cost.
Battery cost per nautical mile
I also contend if AGM is chosen over LiFePO4 the cost per nautical mile per battery over the battery lifetimes will be greater by around a factor of 1.5 times. I've done some calculations to back this up.
Conclusion
The 2 blade driven boat as initially described, will have a faster VMG than the 3 blade boat. Isn't that what counts?
The 'efficiency' that interests me most is the prop efficiency and I believe the method described can isolate and compare the 2 blade to the 3 blade with VMG, with the 2 blade boat having the best VMG at 750 Watts power measured at the battery shunt. When it gets to 1,500 Watts I'm not so sure. Do others agree and if not can you say why? I think I know why but would like to hear from others too.
Lastly it seems to me that efficiency comes in many forms, but when it comes to my boat what I care about most is the cost per nautical mile for a low power EP system at an acceptable cruise speed with some reserve of power. In that sense I cannot see how a 3 blade equipped boat with gearing can beat a 2 blade direct drive system as described. In my case 4 knots cruise is enough, with 5 knots top speed (my reserve if you like) and 20 miles range at my cruise speed. Maybe that is what it comes down to - how much thrust and range is enough for you and at what weight and price?
John R.
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