Hobie 20 Electric Motor Sizing

In the location I sail my Hobie 20, I need to mount a motor to reduce the probability of damage and enable me to sail solo more often.



I'm planning to build a mount on the rear cross beam and then mount an electric motor.

Struggling with the sizing of the motor.

The two options I'm looking at are:
* 6 kW MRP, 3 KW Rated, and weighing 3 KG.
and
* 3.7 kW MRP, 2 KW rated, and weighing 1.62 KG.

The motor diameters are the same and I estimate that the drag differences will be largely negligible but the larger motor is more expensive, and is significantly longer and heavier.

I'm not sure what the point of diminishing returns is for motor power for a boat like my Hobie 20. I'm assuming it'll carry a max of 800 pounds of people and cargo, with a 450 pound boat weight.

 

2 kW (2.7 HP) is adequate. I assume you only need it to get in and out of a marina or boat ramp. A 100 Ah AGM battery weighs about 65 lbs, plus wiring and mounts. The difference in weight between the motors is not much.

 

In and out of the marina/boat ramp and I'd like to use it when the wind dies completely. Might also use it for cruising up creeks etc for camping and such.

I plan to use it with a Lifepo4 battery and custom mount to save some weight. Might try getting hydrogeneration to work as well.

 

Depends on how much speed you expect for no wind situations. Myself not a fan of immersed Motorhead Troll Motor type designs, compared to lower drag Asian type trailing shaft designs. Without the motor being immersed, the prop doesn't have to work in the Disturbed water created by the motor brick and less worries about seals, Motorhead collisions, and corrosion. Trailing shaft also allows more flexibility and access to an infinite variety of much lower cost Motors at the mounting platform sitting above the water. Plus it can be instantly pulled out of water to get up faster on a single hull and may not require a rudder. Also much better and instantly adjusts propeller depth for shallow creeks for running in mud or running in surface piercing mode, if that's necessary for the shallowness or obstructions. Kits used to be available without the motor at a very reasonable price, I still have one in storage bought many years ago.

Hydrogeneration using a tow propeller to recharge batteries may require switching out to a different prop between power and generation modes , or special variable pitch propeller to get the best charging efficiency, would be my guess.

 

Here's the general idea of the training shaft on a much smaller scale,

 

It would be helpful if you better defined your needs and how you see yourself using the motor with respect to range/runtime, if you need to fight current and or strong headwinds, swell, etc..

You don’t need much power at all to push an efficient pair of semi-planing hulls like your beach cat.

Weight isn’t nearly as big a factor as windage, current and swell, so if those pose a challenge then having extra power and a bigger more aggressive prop with better bite will come in handy.

If however you just want a motor to putt along when you want to give sailing a break, or as an emergency to reach the shore or get back to the dock, you might be able to make due with a more simple, small and lightweight setup.

 

Looking for 4 mile range at a reasonable clip from a full charge. Maybe 4-5 kts would be good. With no wind, 1-2 kts current, no waves.

I don't plan on using it when it's windy every to get out of the ramp. In that case it's about 1/3rd of a mile to somewhere I can tie up and rig. Assume a max of 18 kts wind speed head on, flat water.

 

So it sounds like you intend to leave the motor submerged at all times based on your comments on physical motor size and regen?

And I assume you would leave the motor fixed and use your tiller and rudder setup to steer?

The biggest challenge you’ll have (especially if in salt/brackish water) will be your interconnects and protecting your electronics like the ESC given it’s an open wet boat that can flip over.

A “waterproof” motor is only one part of the equation.

Electronics typically need airflow, heat sinks or water cooling which make them vulnerable and prone to water damage.

Portacruise above isn’t a big fan of having the electronics in the motor housing for understandable reasons, but there are a few important benefits, primarily cooling and keeping the interconnects and wiring runs short and enclosed in a sealed housing…. well, at least until the seal fails.

Keep in mind those Flipsky motors need a good amount of voltage to perform well, which means your battery and its management and charge system will be more complicated.

If you could be happy with a slower cruising speed (4-5MPH) it would simplify the motor system as you probably only need about 500W to get going that fast allowing for a lower operating voltage and the need for less battery capacity to achieve your desired range.

If you are not in a hurry, you might wait until the next “wave” of products hit the market.

Something like an ELite by EPropulsion would be perfect for your needs if it had the option to extend range via an external battery, and I have a feeling that feature will be something they may incorporate into the next version of that style of motor design which is highly portable, lightweight, low voltage, low drag, simple, waterproof and completely self contained.

 

A speed of 4 mph might be achievable for shorter distances than 4 miles (for maybe that 1/3 mile?) with a human pedal powered motor with a fit individual under calm conditions? Pedal Power has been used for dead calm conditions to make progress during sailing yacht races. That is, if you be interested in that possibility, which would be much simpler than an electric system with batteries and electronics? That's depending on how much drag there is with those long cat hulls, which look to be pretty fast. Your cat looks to be planing on one hull under sail, even though it's not a conventional motor planing Hull design. I didn't quite follow about the 18 knots wind speed, is that what you're making forward progress against with sail, while tacking into the wind? Or is it about some expectations as regarding the motor power?

There's also progress being made with HPB / Electric hybrids but we might have to await further improvements, to see if it develops as a viable future option:

Seacycler Combines Pedal and Electric Power - Professional BoatBuilder: An IBEX Technical Journal https://www.proboat.com/2023/04/seacycler/

Hope this helps.

 

18 kts head wind is the maximum I intend to take this boat off of a boat ramp with bare poles. So I'd like enough power to make it out about a thousand feet to where I can tie up and put up sails.

Id planned on making a flip up motor in case I run into something and to reduce drag. I've attached an image of how I envision it looking though I haven't figured out where to put the ESC yet. (Cross sectional)

The comment on motor size had more to do with the space it'll take up when in the stowed position. I'm not really interested in using something commercial off the shelf since I don't think it'll be as low profile as I want. Same issue with the pedal drives.

Just really not sure how much power I'd need. Not sure on battery sizing either though that might come down to the sizes I can find prepackaged with the right form factor and voltage.

 

Since you seem to be primarily hung up on how powerful a motor and big a battery to choose, I will repeat what I said about those Flipsky motors with regard to voltage.

In the specs you’ll see they provide a voltage range of operation.

Most run of the mill lithium batteries and their BMS’s are designed for 1C continuous discharge rates.

So in order to get a 3kW motor to run full blast reliably, it’s best to pair it with a 36V 100ah pack, and the 6kW a 72V 100ah, both of which would be quite large and heavy.

This would roughly provide the industry standard 1hr of runtime at full throttle.

Now the minimum you can run those two motors on is about 24V, and if you did that, the 3kW would need a 125A max discharge and the 6kW a 250A.

You can discharge a smaller capacity battery at higher rates, but you need to make sure the BMS can deliver the needed continuous current and that it has a good balancing circuit since there’s a greater chance of cell deviation occurring at the higher discharge rates.

As I mentioned before, you probably only need about 500W to get that hull to it non-semi planing hull speed (4-5MPH), so a 1kW motor would be a good match allowing you to cruise at around half throttle leaving a little extra power on tap when you need to get through conditions or just want to make a short blast.

Trying to get it to go much faster will require more power and a more complex battery and battery management and charge system.

You might want to look back over Flipskys site and identify some smaller less powerful 12v capable motors in the 1-2kW range to use instead, or start identifying your battery options and work backwards.