Small electric boat power data

I've been able to test (and enjoy) my boat and have some useful data. The boat is the 14ft skin-on-teak boat in my gallery and the O/B is Brushless Outboard thread in this forum. Having had enough of lead batteries I bought some lithium ion batteries in the form of 9S (32.4v nominal) 3Ah which are new robot lawnmower batteries for a model no longer made, and therefore sold off cheap on UK eBay. 16 batteries made up into two larger units all wired in parallel so each of the new units is 32.4v 24Ah.

I found that I have steering ability at 40W (on flat water), moves reasonably on 100W, and max power is 1500W (2hp input to motor) which points the bow up and leaves a fair old wake behind a boat that was really designed for rowing. On the UK Norfolk Broads I chose 370W (1/2hp) as my cruising power which gives about 4 hours and 20 miles endurance and range. Even at this low level you can knock 100W off and still barely detect the reduction in speed.

There is a big problem with weed on some of the broads and the highest power consumption I saw was 2.5kW when a huge lump of weed was tangled round the prop! Speed was almost zero. My "efficient" model aeroplane prop cost so much time in de-weeding that journeys were quicker with the original outboard prop (3hp Evinrude).

It was very gratifying that I was able to carry out a mini rescue; a hired dayboat crossed the river in front of me heading for what I thought was a hidden creek - there was no creek and he hit the reeds at cruising speed and got stuck, and reversing his little diesel did not pull him out of the reeds. I passed him a line and was easily able to pull him out stern first. Mrs. hirer said "sorry, I was taking pictures"! Still, no harm done.

Finally, I also solved the problem of charging a 32.4v battery from solar panels. My MPPT charger did not cover this range even with custom settings so after spending ages pondering and considering buying a new and expensive MPPT charger I dispensed with that altogether and connected two flexible panels directly to the battery. The maximum possible voltage from the panels is about 36v which is less than the full charge voltage of 9 Li-ion in series so can't overcharge, and the max current is about 6A so also impossible to charge at too high a current. Even if I connect all three of my panels in series the voltage is tied down to the battery voltage and this will also help to compensate for cloudier days. Job done.

 

A tiny bit more data: the propeller continues to turn (without doing much) at less than 4 watts, and accompanying some fairly sedate rowers on the upper reaches of the river Thames 100 watts was easily enough to keep up or catch up so I have a solid 15 hours endurance at rowing speed.