I did a lot of number crunching and estimating or the performance of my electric drive before I built it. That was mostly based on performance reports from other similar boats and some other prediction formulas and spreadsheets. Once I was up and running I was getting significantly better performance than predicted. I was exceeding predictions by 30 to 50%. I predicted 5 knots would take 3600 Watts. It takes more like 2400 (flat calm). I attribute most of this to the excellent match of my propeller. I am lucky to have a large diameter feathering prop (18") where I can change the pitch (not underway). If you are simply swapping the electric for the diesel with the same fixed prop you are probably giving up some significant performance. I would fit the largest prop that will fit and set up the reduction gear to get it spinning at the slowest practical speed. That's harder with a fixed prop as you are committed to the pitch so it's harder to experiment. But there are big gains in performance to be made by getting a good match with the prop.
http://dan.pfeiffer.net/10m/electric_drive_performance.htm
Having said all that, I would certainly give up motor performance for sailing performance. I have a feathering prop so I get a good bit of both. But sailing comes first. It also depends on your use. I am mostly day sailing these days so 25 miles of electric range at 5 knots is fine (better than expected). I plan to get back to occasional voyaging and for that I feel the need for sustained motoring capacity to make passages up rivers and through canals from time to time. And for now I think the most practical way to achieve that is with an inboard diesel genset. I think I could get over 55 miles range at 3 knots but loafing along like that amidst real ship traffic is not appealing.
Dan Pfeiffer
Hi, DanThe props are probably the real key here...And, actually, the use of the cat.If you have a cat, then consider retractable engines in the middle.Stupid to have a big fast cat, with fixed props hanging there, behind...Many cats are not real sailboats, just double hull motoring boats. No mast.I don't know about John's cat yet.I just think, that 2x 18 KW electric is a way overkill for a cat sailboat.It is supposed to go fast, when sail. Not by battery powered engines.The sails are much more powerful, and can last across an ocean for years.My 8000 lb old mono can sail without an engine, of course, but it helps, with even a small 10 HP outboard, when coming in waters without much sailing possibility...Cheers,CarstenOn Monday, 31 January 2022, 03:06:09 CET, Dan Pfeiffer <dan@pfeiffer.net> wrote:One reason you might choose this particular kit is to get the water-cooled motor. I think if you want to be able to motor continuously at speed you really need more robust cooling than the air cooled units will give you. Also at more modest speeds you will not be pushing the system so hard. But I don't think there is enough battery reserve here.
If I am reading the description correctly you will have a 72V 200AH AGM bank in each hull? That will be 100AH usable capacity (50% depth of discharge). I am sure there are some Catamaran dynamics that make the comparison problematic but for my 13,000 lb monohull 100AH of usable capacity would get me about 10 miles at 5 knots. That would not be good enough for my purposes. Is a 26,000 lb cat like two 13,000 monohulls? Probably not and with the higher hull speed of 40'LWL maybe you would do better? Still seems like too short a range for practical purposes.
Regardless of how you divide up the battery banks you still have a total capacity (with 24 batteries) of 28,800 Whrs. But with AGM you have 50% depth of discharge so it's 14,400Whrs usable which is marginal I think.
I would consider running at 48V and configuring the batteries for 4S3P on each hull for two 48V 300AH banks. Still the same Whrs total - that's the good thing about looking at it in Whrs. The motor would be 12kW at that voltage and 2500RPM max. I thought the ME1616 is 96 V max anyway? So 144V banks won't be an option?
Then again, with a CAT it might be nice to be able to run the port side from the starboard battery and higher voltage would be better for that. But I am not so keen on the high voltage. But it can be done. You still have the same total Whrs and that's what the range is based on.
Prop speed depends on the size of the props. I have an 18" prop with a 3:1 reduction for 800 RPM max. This has been an excellent match for me. Bigger and slower prop will be better. You have 19" so I would target a prop speed of 600 to 900 RPM. This may be another reason to go 48V because max motor RPM will be 2500. At 72 it's 3600 RPM? That means 4:1 to get the prop rotation where you want it. That may be more difficult to come up with regarding reduction gear parts. You can do 3:1 pretty simply though I am not sure what those limits might be with the Thunderstruck reduction units.
But it also depends on pitch. You say 19deg pitch? That is typically expressed in inches of pitch. How many inches will the boat move forward for one rev of the prop. Can you translate that 19deg to inches of pitch? If that's what it was, 19 inches, then it's a pretty big pitch which also means slow RPM. My 18" prop is set to a theoretical pitch of 13.5". It's a feathering prop so the behavior is not exactly the same. But if it was 19" of pitch I would need an even slower RPM. Like 600.
Lots of variables to work with...
Dan Pfeiffer
On 2022-01-30 4:24 pm, Carsten via groups.io wrote:
Is it a sail cat or just engine powered, John ??I'm a bit confused, why you need 2x18KW for a catOn Saturday, 29 January 2022, 14:26:19 CET, john winterrowd via groups.io <sailorboy55577=yahoo.com@groups.io> wrote:Greetings my fellow Electrosailors, the day has come after nearly a year of waiting for chips, motors, pumps, fans and now the final details need to be identified, she is getting repowered with Thunderstruck 18 KW kits.
Facts and figures for SV/Retreat Time Catamaran
LOA 44 '
Water line 40'
Beam 20
Draft 3.5
Propeller 19 " Diameter 3 blade
Pitch 19 degree
Displacement 26,000 lbs.
Battery bank per hull consists of 12 group 31 AGM Duracell 100AH
Before I commit to keeping the 144-volt banks, I am asking the group to consider a few thoughts,
Sacrificing a few amps by dividing each battery bank in half operating at 72 volts, this way I may be able to parallel them together for higher total amp hours? Or have two separate banks for redundancy charging would be done on 110 volts instead of 220 volts.
Prop speed remains unknown to achieve at best half of hull speed, this is where I could use your help, without the variables of current and wind, I'm looking for a laboratory answer, this is to determine what gear reductions would best suit this vessels propulsion and efficiency. 
https://vicprop.com/displacement_size_new.php
Any help or advice will greatly be appreciated as Thunderstruck is planning to program and tune and ship next week.
Thanks John
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