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bobkart
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Electric Boating

Sun Jan 03, 2021 11:52 am

This year I'll be diving deeper into electric boating. Last year I dipped my toe in, with a trolling motor on a ten-foot inflatable boat. We live near a river with a 5mph speed limit, so this combination was perfect. But getting that small taste gave me an appetite for more. As with EVs, speed and range are the big challenge, along with cost. I have some ideas that may firm up in a month or two, and will likely add details here as that happens. It's more of a journey than a destination.

I suspect we have a few members who are into electric boating. I'm hoping they'll be willing to share their electric boating experiences here.

As a start, here's a video from last year, from one of the first outings we took up the river.

https://youtu.be/j1RHdICoh7g
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LeftieBiker
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Re: Electric Boating

Sun Jan 03, 2021 1:35 pm

I dunno - that seems pretty fast already. ;) We used to take a trolling motor and small deep cycle battery to the local lake parks and rent a rowboat. I'd carry the rig on a folding lightweight handcart. (I still have the Minn-kota (sp?) motor, and for that matter the dead battery, but haven't used it in many years.) Years ago, though, they stopped allowing trolling motors on the rental boats. When I was in better health, I toyed with the idea of an EV conversion on a small covered boat, and with getting a folding boat, but it never came to pass.
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bobkart
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Re: Electric Boating

Sun Jan 03, 2021 3:48 pm

The videos are definitely more watchable when sped up!

I get 4-5mph top speed, for 4-5 hours, out of a 200Ah 12.8V LiFePO4 battery (~37 pounds). So call it 20 miles of range. And it's easy enough to bring a second battery. This little setup actually goes further per dollar than a Leaf: the latter costs around 2.5-3 cents per mile (at ~$0.10/kWh), with this boat costing more like one cent per mile. Way slower of course.

Late in the year I picked up a 2.2kW motor (4x the power), but was only able to increase the speed by ~50% (4mph to 6mph). Something about 'hull speed limit' is involved here it seems. But I did make one interesting observation during that experiment: running at the lower speed (4mph) with the larger motor did not use nearly as much power as the smaller motor did. I put that down to the larger propeller on the larger motor, which can turn more slowly to achieve the same 4 mph, thus losing a fair bit of the drag that the faster-turning propeller experiences.
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Re: Electric Boating

Mon Jan 04, 2021 1:37 pm

I've settled on the following requirements:

- boat length in the 3-5-meter range
- reasonably seaworthy as I won't be confined to just rivers and lakes (Puget Sound is calling)
- minimum two persons capacity, four is more likely what we want
- around 20mph top speed would be the target
- I'd like to get at least one hour of battery runtime at that top speed

That comes to 20 miles of range at full power. Of course 'cruising speed' is more efficient, so typical runtime and range will be longer.

20hp looks to be the sweet spot for both that size boat and what is practical battery-weight-wise. Certainly less power is acceptable if it roughly achieves the stated goals. I've left some wiggle room in the requirements, as tradeoffs are likely to come into play.

In explorations I've made so far, I see that a typical weight-to-power ratio for electric outboards is 3-5 pounds per horsepower (at the propeller shaft). For LiFePO4, I'm seeing ~16 pounds per kWh, so one hour of runtime comes to ~12 pounds per horsepower-hour. This disregards two things: inefficiencies between battery output and what fraction of that power makes it to the propeller shaft, and not wanting to discharge the batteries all the way to empty. Each of those could be roughly characterized at 90%, so together they add ~20% to the battery size requirement. So that consideration takes the 12 pounds number up to 14.4 pounds, giving a total of 17.4-19.4 pounds per horsepower (for motor and batteries for one hour of full-power runtime). For simplicity, let's round up to 20 to consider cabling, battery boxes, etc.

Picking some typical horsepower values and looking at the resulting motor-plus-battery weights gives us (very roughly):

5hp: 100 pounds
10hp: 200 pounds
15hp: 300 pounds
20hp: 400 pounds
25hp: 500 pounds
30hp: 600 pounds
35hp: 700 pounds
40hp: 800 pounds

Finding a boat with enough weight capacity for both this weight and the passengers, that can still achieve a reasonable speed with the chosen power level with that much weight on board, is a big part of this challenge.

I'm considering some approaches to extending range:

- solar panels
- propane generator
- fuels cells (hydrogen, propane, methanol)

Fuel Cells are looking *expensive*, so that's probably not an option. I think I can fit between 1-2 kWp of solar panels over the boat, depending on its size. So far generators are weighing as much as 1.5 hours of battery weight for full-power runtime, (~24 pounds per 'horsepower' delivered to the battery), so unless I target over three hours of full-power runtime, just adding more battery will beat the generator option weight-wise (but probably not cost-wise). There's also the noise and pollution consideration, although propane burns *much* cleaner than gasoline.

(There's also the full-propane solution involving a propane-powered outboard motor. Not electric boating anymore though, but could be an option for when I want to travel much longer distances.)

In hindsight I probably should have kept everything in SI units, but horsepower and pounds are usually what you see in boat specifications (in the US).
Last edited by bobkart on Mon Jan 04, 2021 4:36 pm, edited 1 time in total.
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Re: Electric Boating

Mon Jan 04, 2021 2:42 pm

There is a thread over at endlessphere.com about a guy converting a diesel boat to electric propulsion, by essentially using a bicycle/scooter hubmotor on the output shaft. The advantage to that is there is a huge weight savings from removing the diesel engine, and that is now available for batteries. There is also weight savings in not having to use an outboard motor. Instead of making a light boat heavy, he is making a slightly heavy boat no more so.
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bobkart
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Re: Electric Boating

Mon Jan 04, 2021 4:13 pm

Thanks Leftie, I see it: https://endless-sphere.com/forums/viewt ... 9&t=108411. . . very interesting thread. Also I see they have an entire Electric Watercraft sub-forum, so I may be creating an account there soon. In the meantime I'll read up on anything there that looks relevant to my quest.

Here's a lake video from last year:

https://youtu.be/_r2Xqgwshv4

River videos seem a bit more interesting as there's scenery on both sides.
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Re: Electric Boating

Tue Jan 05, 2021 11:47 am

I have been into electric boating since 2011. I use a Torqeedo outboard to power my 22', 1.5 ton sailboat. It can push the boat over 5 knots on still water at full throttle. But the battery will only last 30 minutes or so = 2.5 mile range. I can still go half that speed at 1/8th throttle (due to the nature of displacement boats). So 4 hours at 2.5 knots is 10 miles. This application is a bit different from yours, though.

My goal is to clear the channel and get to open water. At that point I raise the sails and shut off the motor. I tried a Minn Kota trolling motor before the Torqeedo, and it just didn't give me enough power to overcome a headwind. The windy days that I really wanted to sail, I couldn't even get onto the lake! Torqeedo is pricey, but has been well worth it. Before the Minn Kota, I had a Mercury 4. That thing gave me trouble every spring, and cost a small fortune every year to winterize and then again prep in the spring. The Torqeedo has required only one repair (after a hard grounding) but is otherwise maintenance-free.

Like any good boat owner, I have been toying with upgrading a few feet to a 25' sailboat. If I did that, I'd also invest in a more powerful outboard and battery bank for some decent range. My lake connects to the Erie Canal, and I've always wanted to lower the mast and explore.
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Re: Electric Boating

Tue Jan 05, 2021 1:31 pm

Boat stands for bust out another thousand (dollars).
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bobkart
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Re: Electric Boating

Tue Jan 05, 2021 3:16 pm

GetOffYourGas wrote:
Tue Jan 05, 2021 11:47 am
I have been into electric boating since 2011. I use a Torqeedo outboard to power my 22', 1.5 ton sailboat. It can push the boat over 5 knots on still water at full throttle. But the battery will only last 30 minutes or so = 2.5 mile range. I can still go half that speed at 1/8th throttle (due to the nature of displacement boats). So 4 hours at 2.5 knots is 10 miles. This application is a bit different from yours, though.
...
Thanks for sharing, Gerry. Which Torqeedo model do you have? I tried an ePropulsion Navy 6.0 a few months back, but it wouldn't quite plane the 10-foot inflatable.

I have to add to my requirements:
- unsinkability in some form (a'la Boston Whaler)
- steering wheel
- windshield

As with all my requirements, a certain amount of trade-off is acceptable.

The 'hull speed' issue is an interesting one. There are lots of examples of certain hull configurations exceeding what should be the 'hull speed' limit for their length. Usually these are multi-hull configurations (catamaran is typical), with very long/narrow hulls. Think Hobie-Cat. Short of hydrofoil boats, which don't seem practical for my application, this approach would seem to be the most effective in allowing decent speed with less power that a typical monohull configuration would require.

Regarding boating costs, yes they can be high. I was able to get into last year ('starter boat') for under $2K, and even including additions along the way (like the 2.2kW motor), I stayed under $3K. (This doesn't include the batteries, as I use those for other things.) A lot of the cost of boating can come from fuel costs, and electric boating sidesteps much of that. For this next level of boating, I'm budgeting about as much as my Leaf cost ($15K)

I'll just get these out of the way now:

- "A boat is a hole in the water into which you pour money."
- "The two happiest days in a boat owner’s life: the day you buy the boat, and the day you sell the boat."
Last edited by bobkart on Tue Jan 05, 2021 6:00 pm, edited 1 time in total.
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Re: Electric Boating

Tue Jan 05, 2021 4:26 pm

"Men tend to sell their boats to keep their wives happy because the boat usually costs more to maintain."
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