Getting Into Electric Power Catamarans

Looking to head out on Saturday to a nearby small lake (nearby river is running too fast for comfort for a maiden voyage).


I'm going to start with the smaller motor though (that's the 2.2kW motor).


Deck comes to 29 pounds so far, but I'm going to add another couple of crosspieces as the flex is too much when standing with all weight on one deck plank.
It turns out I didn't need the extra crosspieces, because when the deck is installed, the hull crosspieces are enough to prevent the excess deck plank deflection I was concerned about. So I'm at 117 pounds before motor/battery/seat/pilot.


I turned the deck around so the transom is towards the bow (to the left in the picture), as I'm using the smaller motor (~550W) for this first time out. I turned the tiller handle on that around 180 degrees a while back, and want to try that configuration before turning it back. I thought about it a bit more (after turning it around), and having the motor in front seems impractical for beaching and clearing underwater obstacles.


About 60 pounds were added after the 117, so call it 180 pounds total before pilot. Maximum total weight is specified at 525 pounds. This will matter more when I try the 2.2kW motor, because the 48V-nominal pack I have for that weighs around 150 pounds (200Ah). Obviously I need a smaller amp-hour-capacity pack for it to comfortably fit, say 50-100Ah. I'll likely end up around 10 pounds over capacity for that experiment, so I'll need to be careful.
Today went pretty close to how I expected it would. There were a couple surprises:

- the boat sat lower in the water that I was hoping (basically to the black tape line)
- the front-steer approach worked poorly

Less surprising: the bow was heavy (as I was concerned it might have been). Control in the forward direction was difficult. Facing the other way and using the motor in reverse worked well control-wise, but I don't believe all power is available in reverse, and of course the prop isn't as efficient in that direction. I managed to touch 9km/h now and then (by sitting practically on the battery), so close to 6mph, about what the much larger motor was able to push the 10-foot inflatable to (but with two people aboard).

I added up weights and I think I was right at 400 pounds total. The Hydrobike (complete) weighs 125 pounds and is supposed to have a 400-pound capacity on top of that. I have trouble seeing how I could have added another 125 pounds and been 'safe' . . . I feel like the hulls would have been nearly completely submerged. So trying the larger motor with the 150-pound battery pack looks to be out of the question, unless/until more hulls are involved. I'm thinking that a four-hull model could work out for that and another passenger, as I put the total weight of that setup at 800 pounds. But I have many more things to try before heading in that direction. One thing on that list is a 50Ah 48V-nominal battery pack. I got a line on one of those a few weeks back, and will likely touch base there again. Such a pack would come to somewhere in the low 40.x-pound range. That and the heavier 2.2kW motor (low 30.x-pound range) would come to just 10-15 pounds more than I was at today.

Changes since the previous run include:

- motor back to original rear-facing thrust
- aluminum crossbars and knobs
- quick-release deck pins
- better seat

I also tried some 40Ah LTO cells (5S). Seemed like less voltage than 4S LFP, so may try 6S next.

Speed was once again just at 9km/h peak (approaching 6mph). This is probably related to the propeller RPM/pitch limits typical of trolling motors. And it looks like I could stand to have a bit more weight towards the bow.

I have a real seat on the way, and I have a swivel to put between that and the base you see here. Also, a composite deck is coming. And I'm working towards trying the 2.2kW motor, but have to put together a much smaller 48V-nominal pack.
bobkart said:
Speed was once again just at 9km/h peak (approaching 6mph). This is probably related to the propeller RPM/pitch limits typical of trolling motors. And it looks like I could stand to have a bit more weight towards the bow.

Great video's and a nice way to enjoy the water.
As for speed the pitch of the trolling motor props are indeed not as efficient for low weight boats to get to higher speeds. This video is talks about it:

Thanks for the link, SlowCo. I've seen that brand mentioned before, and Kipawa. I contacted Kipawa and they don't make a model for my motor. I may reach out to Small Town Bassin at some point, but now I'm lined up to try the 2.2kW motor. I have another half-dozen of the 40Ah cells coming, and can combine that with half of my LiFePO4 pack, to get around 48V nominal and come in under the 525-pound total weight limit.

It seems like there is a large, untapped market for higher-pitch trolling motor propellers. The trolling motor brands themselves seem clueless. I really doubt it would be too much trouble for them to make alternate-pitch propellers.
bobkart said:
It seems like there is a large, untapped market for higher-pitch trolling motor propellers. The trolling motor brands themselves seem clueless. I really doubt it would be too much trouble for them to make alternate-pitch propellers.

I agree. Being able to easily buy different pitch props for higher and lower weight/speed boats would be great. Also a real PWM controller to adjust the speed instead of just burning of power with heat at lower settings would increase efficiency a lot:

As would be a series with brushless motors instead of the brushed motors in lower end trolling motors without the Torqeedo prices...
I'm top-balancing a dozen 40Ah LTO cells. The plan is to make an 11S battery from them, to approximate the voltage profile of an 8S LiFePO4 battery. Then I can combine this with half of my 48V-nominal LiFePO4 battery, and come in under the weight limit for two hulls (200 pounds per hull). I know it's not advisable, mixing battery chemistries like that, but this is just to see what the 2.2kW motor will do on this boat (without having to buy another 10-12 LTO cells).
Hey, cool project.
Check out APC model airplane props. They have some models that are appropriate for use with trolling motors. I used an APC 11x6 to replace the stock 3 blade props on a 12V Haswing Protruar 1.0. 55lb thrust, I think. This was pushing a 6m sailing proa. With the stock prop, the motor would rev out and draw about 30A.
I was able to modify the APC prop to fit the motor's shaft using a drill and a file (to machine a slot to fit the pin that went through the shaft). Since I used a tractor prop (pulling, not pushing), I also fitted a spinner (nose cone) around it, and reoriented the motor's control head the other way.
With the APC prop, I found a 10-20% increase in efficiency over the stock prop. I could also go faster with the APC prop, to the limits of my DIY LiFePO4 pack (40A load breaker). Top speed was over 6km/h. The 100AH pack gave over 20km range, but I never tested the limit. It was a sailboat after all.
I was concerned about the durability of the APC props, since they are much finer than the stock prop, which was almost bulletproof. After a season around Vancouver in wood infested waters, it never failed, but looked a bit worn. One downside was that it was much easier for the APC prop to cavitate in chop, but it was manageable using the throttle.
Thanks for the advice. I bought one after first hearing about that option (3-blade 8" x 6" pusher), but haven't tried installing it yet. The drilling-out part seems easy enough, but making the groove I'm less sure about. A file sounds like a good idea for that. I'd be interested to see a picture of the spinner you added.

The stock prop on my trolling motor has about a 10" diameter and I estimate the pitch at 4.4". Based on how well the one above works, I may want to try different diameter/pitch numbers next. Fortunately, these props are inexpensive (until you get to the carbon fiber models).

Either my cellphone speedometer is off or the speed increase due to the more powerful motor was underwhelming. I only saw 10km/h, but it felt faster. Definitely too much weight at the rear, made worse by the high thrust below the waterline tipping the bow up. Would likely pick up a bit of speed with more weight up front. But it does seem like I'm pushing into hull-speed-limit territory. Was hoping for more there.

I had difficulties controlling the motor, as the throttle is very touchy and the tiller arm is very short. If I were to use this motor I'd go with a PWM remote throttle and normally use more like 1/4 throttle (along with some form of remote steering). I did notice that I could push the 10-foot inflatable with this motor the same speed as the top speed of the 12V trolling motor, but with easily less power consumption (larger propeller at lower RPM, so better efficiency). A foot throttle with very light spring return to the 1/4-throttle point, then an aggressive spring for the rest of the pedal travel, would make it easy to cruise at the right throttle setting while still allowing bursts of higher speed.

My other half is not that keen on me pursuing this project much further, with the 'real' boat (16-foot power cat) likely coming next month. We'll see. I know I'm learning things with this project, some of which might be applicable to the larger boat
I am a long time wannabe boat designer and frequent where you might get decent advice. It can be bit cranky place at times but there is tremendous amount if knowledge there.

Anyway, there have been good descriptions of hull design resistance parameters here and how around hull speed the waves start to just add steeper and steeper uphill for the boat to climb. Ways to over come this are planing with dynamic lift (need decent chunk of power and light weight and flat areas to create the lift - with correct shape. Or narrow enough shape that it doesn’t make much waves in the 1st place and it can poke through them rather than trying to climb on top. This is where the narrow catamaran hulls come to play.

For speed there are to parameters that are imperative, weight and length. I am afraid that 20 mph with those short hulls is going to be really tough with any powerplant. The coach cats are efficient and you should get pretty good idea of their performance from existing setups.
I have toyed with this kind of ideas for a while but in more conservative speed ranges and bit different goals. I think stabilized monohull ie. Trimaran with minimal side floats are probably the ultimate in efficiency. Still 20mph is high without planing is tough call.

Planing boats can be quite efficient too if small enough. This compares 2 very similar hulls, one is planing hull and reaches 20mph range with relatively modest power.

I think that accepting but more modest performance goals might make sense. 15 knots ie near 20mph range will also require structure that can handle fair amount more forces.
A good thread about similar power cats.
Thanks for joining the conversation, kerosene.

I was hoping for more like 10mph from these hulls, but it looks like 6mph is it. So I'm back to the 12V trolling motor and will be trying the remote steering idea next (swivel seat to turn). Then possibly go to a three-hull version for two-person capacity.

The 16-foot cat is delayed due to lack of shipping containers from UK to US. This is the Rowing Solutions RS16. It's based on the same model of sailing cat by RS Sailing. Their electric version is claimed to hit 25km/h (15mph) with a 6.5kW motor (close to 10hp). With a 15hp motor they claim 32km/h (20mph). I'll be happy if I can hit those numbers. It's rated for a maximum of 20hp, so could see close to 24mph at that power level.
It took six months to ship (because of the pandemic), but the boat should arrive this weekend.
Order placed for ePropulsion Navy 6.0 Evo:

I should see about 14mph top speed. With the 10kWh battery I have, should get 1.5 hours at that speed, with 10% battery capacity to spare. So 21 miles of range.

At half power, I should see close to 10mph. So three hours of runtime and 30 miles of range.
Got a smaller motor I had laying around installed (2.2kW Hangkai). Came as a tiller model so had to fabricate steering linkage and wire throttle/etc. for remote use.

Seeing just over 7mph top speed, and would run for 4.5 hours at full power, for a range of over 30 miles.
SlowCo said:
Great result :thumb:
Now build a solar roof on top of it for extra range and charging during a trip.
Yep, that's on my list. I estimate there's room for between 1kWp and 2kWp worth of panels. Using flexible panels, the added weight could be around 60 pounds (for 16x TP Solar 100Wp panels). That's before cables/connectors/controllers, and of course the additional structure to hold them. That could double my run time at a 3kW power level (on a good sun day).
More power :thumb:
As these motors are meant for slower, heavier boats you should look for a more agressive prop. Search for people using trolling motors with different propellors on light crafts like canoes and kayaks. They gain a lot of speed with only a slight increase in amp draw.
The good news is that these motors come with two propeller sizes (low and high pitch).

At maximum propeller RPM, theoretical speeds for the two pitches involved come to 12.5 mph and nearly 16 mph. I understand that I'll likely not achieve that RPM, and that there will be some slip percentage. 10.x mph and 13.x mph are more reasonable estimations considering those factors.
Any day now on the 6kW motor. In the meantime, we got out on Lake Sammamish this last weekend.