So a little test to exhaustion today. My extremely light 50 amp hour lithium $270 and Watersnake drives my inflatable 2.3 tender with me alone on it at 3.3 knots average for 4.3 nautical miles with not much chop and not a lot of wind. Tender is high drag flat plastic bottom. See attached pics. Get the short 36 inch shaft for a tender. Would have got the 65lbs thrust if I wasn't in such a hurry but was not in stock at Airlie Beach BCF.

The voltage of the battery at the test end was 12.74 which is pretty much exhausted (16%?) although it had not given up yet and was still pushing the tender at 2.9 knots. The motor was on top speed out of five possible 5 forward speeds and obviously may have gone a lot further on the lesser settings. Initially the top level 5 gave 3.3 knots whereas level 4 gave 3.0 knots and the third level down gave 2.2 knots. I'm not one for hanging around unnecessarily. The battery in a box didn't get overly warm nor did the leads despite it being a hot sunny day.

This is fine for use as a day sail tender+relatively short lunch excursion and or even 2 or three such outings in a week but the problem comes when you have to get that 40 amps back in from the solar on your boat when the fridge instruments lights etc are also draining the boats main battery/ies and require most of the available solar. No problem when you return home or to marina each night.

Still reasonably pleased tho as I feared it might have been slower and more restricted range. Can't remember what the 2 stroke 3.3 got from its little onboard tank but probably not much more distance wise.

Outboard on the trailer sailor is four stroke and yacht diesel so this avoids messing with two stroke plus the petrol smells in the car when it is laid on its side.

Ideal for that use . Have the same motor (54lb/24in )in a well,pushing a 25ft Contessa ,at 3 knots in still water,on setting3 Can get 4+knots on setting5 .
3 adequate,as just trying to get out of the moorings,and into the Bay ,and could go 5/10k patiently !
100amp gel battery -30 kg + so static,charged by 40w panel .
Tried 18lb,with 25amp Lithium, on 10 tinny,but hardly worth the effort - easier to row !
But Bestest -No Fuel/Oil = No fumes/smell + so easy to carry with no placement issues .

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PLanter4 said..
Ideal for that use . Have the same motor (54lb/24in )in a well,pushing a 25ft Contessa ,at 3 knots in still water,on setting3 Can get 4+knots on setting5 .
3 adequate,as just trying to get out of the moorings,and into the Bay ,and could go 5/10k patiently !
100amp gel battery -30 kg + so static,charged by 40w panel .
Tried 18lb,with 25amp Lithium, on 10 tinny,but hardly worth the effort - easier to row !
But Bestest -No Fuel/Oil = No fumes/smell + so easy to carry with no placement issues .

Wow my tender must have even more drag than I thought. Sounds great for your use. Did you get two props with yours? Just wondering if they're different pitches. I thought one was just a spare.

The 50ah lithium I have is only 10 kilograms which I think is ideal on such a small tender although obviously in a full size yacht you can go a lot heavier. Given however that a lithium can be taken right down to zero without damage it is equal to a 100 amp gel because they shouldn't be taken below 50%. All good until the lithium catches fire initiating a violent reaction that you can't put out :(.

The motor is so simple with no water cooling except water passing over the engine housing - plus so cheap. Even the 65lb thrust one is only $339 and I saw it somewhere for 299. I put a 40 amp fuse in mine despite them recommending a 50 and it didn't blow. The motor is supposed to draw about 40 amps flat out so one hour and 18 minutes is pretty good for a 50 amp hour battery with 15% left. Will have to put a clamp meter on it next time to see what it is actually drawing. It is possible that the BMS is limiting the output of the battery and hence the thrust of the motor.

Yes - got mine from Davos - seamless - was going to go the next size up , but 54lb/24in fits/ works well as Contessa a very slippery shape /clean hull .
Also,can pull up,and store in well with aperture plugged .
Have seen a few inflatable yacht tenders here ,that have the 18lb unit/25amp lithium,as easier than rowing + better use of space ?

Actually correction yes the 26 inch shaft is best for a tender not 36. I actually got a 40 something shaft and cut it down although not enough yet and I will do it again.

+ rinse with fresh water,and spray with WD40 or Stihl Superclean,to control corrosion ,especially if kept on the boat .