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bobkart

Well-known member
Joined
Dec 22, 2018
Messages
346
Location
Pacific Northwest
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
 
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.
 
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.
 
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).
 
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.
 
Thanks Leftie, I see it: https://endless-sphere.com/forums/viewtopic.php?f=39&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.
 
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.
 
GetOffYourGas said:
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 than 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."
 
LeftieBiker said:
"Men tend to sell their boats to keep their wives happy because the boat usually costs more to maintain."
The good news for me is that she has been wanting this more than I have. My two reasons are not wanting to burn any more gasoline than I 'have to', and of course the high costs usually associated with boating. Once I tested the waters last year with an approach that avoids both of those downsides, and seeing potential for more, I was much more fully 'onboard' with the idea.

Here's an example of the kind of workups I've gone through. Take the Boston Whaler 110 Tender, their smallest boat, and the only one I see that you can buy (new) without a motor:

https://www.bostonwhaler.com/family-overview/super-sport-boat-models/110-tender/

There's a very affordable 20hp electric motor by Stealth. Never mind that the 110 Tender is only rated for 15hp; their Sport model is the same boat and rated for 25hp. I think the difference is tiller versus remote control. One could buy a 110 Sport with the smallest motor they force you to buy it with, then sell the motor (at a big loss no doubt) and not have to build your own console/steering setup onto the Tender.

The weight capacity on the 110 Tender is 845 pounds. Take off the 400 pounds for motor and battery (per earlier calculations) and you have 445 pounds remaining. There's a four-person capacity, but you're not squeezing four people into 445 pounds. So the batteries have taken up about 1.5 persons-worth of capacity. I.e. after the ~360 pounds for us two, there's less than 100 pounds left over. Dropping to 15hp gives back 100 pounds and thus a third person is then an option. Of course now top speed has suffered. This is the big challenge I'm facing with this project.
 
My boat is powered by a Torqeedo Travel 1003. It is a 1kW motor which produces thrust equivalent to a 3HP gasoline motor. It has an integrated 520Wh lithium battery which I charge at home. You can also charge via solar panels if you wish.

The Travel series is nice, but for any larger boat, I would go with the Cruise series instead.

When I bought the motor in 2011, Torqeedo was the only real game in town. Minn Kota had trolling motors, but as I had mentioned, I tried that. A trolling motor is not good enough for my needs. Today there are certainly more options that I would evaluation. Frankly, Torqeedo has not really improved the technology nor the price of their small outboards; a Travel 1003 costs the same $2k it did back in 2011, and comes with a 532Wh battery over my 520Wh one. I was really hoping to see much more significant improvements over the course of a decade!

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

There's one I haven't heard before :lol:
 
GetOffYourGas said:
My boat is powered by a Torqeedo Travel 1003.
Ah, I should have seen that in your signature . . . sorry!

I find this website to be a good reference for electric outboard motors:

https://plugboats.com/electric-outboards-more-than-5kw/

That's their page for 5kW-and up outboards.

I have the Torqeedo Cruise 10.0 on my list. It has a few downsides: heavy for its power level, and definitely more expensive than other options.

One thing I don't agree with in the descriptions of many outboard motors is the 'horsepower equivalent'. I get that electric motors have more low-end torque, but for my purposes I need to compare the maximum power output, and (for example) Elco rating the 8.8kW motor at '20hp equivalent' seems deceptive. (In the comparison you made to 3hp, it was appropriate, as you compared thrust.)

I really want to like the Pure Watercraft entry: https://www.purewatercraft.com/product/pure-outboard/

Great power-to-weight ratio, but it's not available yet and you have to buy their batteries (at very high prices). I might still be considering that route but I was in discussion with a salesperson there and I discovered that their batteries can't be recharged while in use, and even when not in use, the recharge rate isn't very high until you get more than one of them involved. That killed solar panels and/or a propane generator as range extension options (never mind that I can build the same capacity battery for 1/4 their price).
 
If they made a powerable version of this (no sail), I'd be very interested:

https://iflysail.com/

This seems like the ultimate in terms of decent speed being achievable from not a lot of power.
 
It would need to be completely redesigned to delete the sails, but put a motor in each side along with the batteries, and maybe a long motor shaft passing through part of the pack to get the weight balance right...
 
bobkart said:
One thing I don't agree with in the descriptions of many outboard motors is the 'horsepower equivalent'. I get that electric motors have more low-end torque, but for my purposes I need to compare the maximum power output, and (for example) Elco rating the 8.8kW motor at '20hp equivalent' seems deceptive. (In the comparison you made to 3hp, it was appropriate, as you compared thrust.)

When comparing the two motor types, you really should look at thrust. That is what moves your boat forward, and ultimately what matters.

Strictly in terms of power, 1kW = 1.34HP

HOWEVER, propellers are more efficient at low speeds. In order to drive a prop at low speeds with lots of power, you need a that low-end torque. That can be done with a gas motor to a degree with gearing, but an electric motor naturally shines at low speeds. So most (all?) electric outboards have a large prop driven at slow speeds whereas all gas outboards have a small prop driven at high speeds. This is why you can get more thrust from less power (again, better efficiency).

One downside that I have is my lake is particularly shallow and weedy. A large, slow-moving prop does an excellent job of collecting weeds. I end up cleaning the prop often, especially if I am fishing (i.e. intentionally in the weeds).
 
GetOffYourGas said:
When comparing the two motor types, you really should look at thrust. That is what moves your boat forward, and ultimately what matters.
I mostly agree, as you're using 'thrust' as opposed to 'static thrust'. But Elco (for example) is quoting the static thrust of their '20hp' motor at 390 pounds, but the input power is only 8.88kW:

https://www.elcomotoryachts.com/wp-content/uploads/2019/01/2020-Elco-SPEC-EP-20.pdf

I get that 390 pounds of static thrust is probably comparable to what a 20hp gas engine can produce (for the reasons you outline). So that part is fair, and if you're pushing a barge with this motor, that will be the primary consideration. But at cruising speeds, the Elco won't compare to a 20hp gas outboard. Assume 8kW of that 8.88kW input power makes it to the propeller shaft, that's only 10.7hp. If we again assume that the propeller on the Elco is comparable to that of a 20hp gas outboard (appearances would suggest that), there's no way to get the 'high-speed thrust' (as opposed to 'static thrust') of the gas engine from not much more than half the power to that same propeller.

Thrust ('propulsive force') will be a factor of prop shaft RPM and torque, times the efficiency of the propeller *at that RPM and boat speed*. A propeller's efficiency will be different for a static thrust situation (100% slip) versus at cruising speed (10%-20% slip). Where it gets more interesting is when the electric outboard has abandoned traditional 'gas outboard' propeller parameters (RPM range, diameter, pitch, rake, ...), and instead tried better to maximize efficiency for an electric motor's torque/power curve. Torqeedo and ePropulsion (for example) do this, and achieve more propulsive force ('thrust') for the same prop-shaft power as a result. Lower propeller RPM is a big part of it (as you mention).

I'm not trying to be disagreeable, more trying to peel back a technical layer on the subject. Also not trying to be an authority on this subject; I'm by no means a propeller or propulsion expert. But I am a student of physics, and have built the above understanding up as part of my exploration into this area over the last year. So I could easily be off on some parts of that understanding. Feel free to correct/clarify.
 
A power catamaran looks to be a good choice for practical-yet-efficient hull configuration:

https://electrek.co/2017/10/18/electric-boat-record-pure-watercraft-outboard/

boat_in_water1-e1508286965929.jpg
 
Your post triggered a memory for me. A few years ago, I met a gentleman who had built his own solar electric trimaran. He had a bank of batteries charged exclusively by on-board solar panels, driving a Torqeedo outboard. He named his boat "Ra" after the Egyption sun god.

He took the boat around the "great loop" - basically up the ICW (intracoastal waterway) to NY, then up the Hudson/Erie Canal to the Great Lakes, and finally back down the Mississippi river to the Gulf of Mexico. I live close to the Erie Canal and met him when he was waiting for one of the locks.

https://www.passagemaker.com/destinations/all-electric-boat-great-loop
 
GetOffYourGas said:
Your post triggered a memory for me. A few years ago, I met a gentleman who had built his own solar electric trimaran. He had a bank of batteries charged exclusively by on-board solar panels, driving a Torqeedo outboard. He named his boat "Ra" after the Egyption sun god.

He took the boat around the "great loop" - basically up the ICW (intracoastal waterway) to NY, then up the Hudson/Erie Canal to the Great Lakes, and finally back down the Mississippi river to the Gulf of Mexico. I live close to the Erie Canal and met him when he was waiting for one of the locks.

https://www.passagemaker.com/destinations/all-electric-boat-great-loop
Interesting read, thanks I had no idea, he must have passed through my city(Minneapolis) within 5 miles from me :)
They didn't say but using their numbers it would have taken 240 days and then they said they did stop for a while, so maybe close to a year in total? impressive though to do it all with the power of the sun :cool:
 
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