Read how UK marine electric propulsion consultant Jamie Marley designed and built his dreamboat ‘Ohm’s Law’, a DIY electric catamaran, powered by twin outboard electric motors using Nissan EV motors and Tesla batteries.
A mission to promote electric boating
Jamie is a Southampton-based marine electrical propulsion specialist who has consulted and worked on dozens of projects for boatbuilders of all description for two decades.
Along with a passion for the sea and sailing, he also has a passion for protecting our marine environments and decided early in his career that replacing ICE technology with electric propulsion would be an essential part of helping achieve that goal.
In 2014 he joined electric propulsion pioneer Torqeedo as Senior Field Service Engineer and worked with them until 2018, as Project Sales Manager specializing in electrification of new and existing passenger ferries worldwide.
With interest in electric boating starting to take off, Jamie realized he wanted to work hands-on with a wide variety of projects and have the freedom to use the most appropriate suppliers. He left Torqeedo to put up his own shingle and start Marine Electrification Solutions, an impartial and independent electric and hybrid marine propulsion consultancy.
His mission is simple and straightforward: To promote safe, environmentally sound, efficient and sustainable zero emission propulsion systems through cooperation.
There are few people with as broad a knowledge of electric propulsion as Jamie, and he has consulted on many major projects including the first RS Electric Boats model, the Spirit 111 which carried Torqeedo’s 100,000th motor and the electrification of Queen Elizabeth’s ceremonial rowboat ‘Gloriana’.
In December 2020 he started work on a project for someone who could be his most demanding client ever. Himself.
Making the plans: what size boat?
Planning an electric boat refit is much like figuring out how to buy any boat. The first step is figuring out the use case:
- What will you be using the boat for?
- Where will you be going?
- For how long?
- At what speed?
“I was looking for something our family (Jamie is married with two boys, 10 and 14) could go out in on a sunny day off the coast of Southern England” says Jamie. “We have a few favourite places where we have a picnic and a swim, maybe go paddleboarding…and the boys love a quick run on a tube or small boogie board.”
So he didn’t need any sleeping berths, a dayboat would be fine, and none of their activities require extreme speed: top speed for safe tubing is about 15 knots (≈ 20mph / 30 kmh). What Jamie and his family need is a steady, reliable hull for the occasionally choppy waters and enough space to hold the equipment, a cooler and a spread-out towel for lying in the sun.
Ex-workboat, 6 metre planing catamaran
Jamie figured something about 6 – 7 metres (20-25 feet) would do nicely. He started scouting around the local marinas and classified listings.
He spotted a 6 metre (20 ft) catamaran hull made by Hunter, a UK brand known for quality and durability. With a centre console and beam of 2.5m wide (8 ft) there was lots of space for the family and the water toys. It had been used as a workboat before, so the insides had taken a bit of a beating, but the structure was all in good shape.
One of the other things that attracted Jamie was the space beneath the deck. “Because it’s really a commercial catamaran hull, the deck was flush, no seams sticking up or anything…completely sealed, and all the cargo and tools would have been on the deck.” This gave him flexibility below on where to place the batteries and the access hatches.
Now it was time to think about the propulsion. From his long experience electrifying boats he knew a planing catamaran was a great hull to work with. The two pontoons mean less water resistance and drag than a monohull, which translates into smaller power requirements and increased range.
The other advantage is that the power requirement can be split between two motors, one on each side. That was good for two reasons. One is redundancy – if there is any issue with one of the motors, the other can propel the boat. Also, he would be able to experiment and tweak each motor separately when he built them, to find the optimum set-up.
Doing the calculations: use case
Let’s look at the power usage for the boat. To do that, we need to begin with calculating the energy usage. And to do that, we need to look at a use case.
Typical use case: It’s a sunny weekend day so the family wants to get out to one of their favourite spots for a nice lunch, some swimming, sunbathing, paddleboarding and maybe some tubing. The tubing and planing speed are what require the most energy in a boat.
Let’s start our energy need calculations with that.
With 10 minutes at planing speed they would be able to travel about 3.5 miles (5.6 km) from port – plenty for what they need. Once there, the boat would use almost zero energy during swimming and paddleboarding, would need energy for the 10 minutes of tubing and another 10 minutes at planing speed to return home.
That is the upwards end of the energy required. There is every possibility that on a sunny day they would be quite happy to enjoy a leisurely cruise out to and back from their swimming spot. And at cruising speed the spots they could visit would be considerably farther from port.
Planing: 2kW of power per 50 kilograms of boat
A rule of thumb is that it takes about 2kW of power per 50 kilograms of boat to get it planing. It depends on the shape of the hull, and a catamaran is going to be more efficient than a monohull, but this is a rule of thumb. (This equation roughly translates as 2.5HP for 100 pounds of boat).
|Power required for planing speed||2kW per 50kg (2.5HP per 100 lbs)|
Jamie figured the hull and two outboard motors would weigh somewhere around 1,500kg (≈3,300 lbs). That’s 30 ‘parcels’ of 50 kilograms, so multiplying that by 2kW of power means 60kW for planing speed.
|Boat Weight = 1,500kg ÷ 50kg = 30||30 X 2kW = 60kW|
This turns out to be a fairly easy number to work with, because if getting the boat on plane takes 60 kiloWatts of power, in an hour of planing it will require 60 kiloWatthours of energy – which works out to 1 kWh per minute.
|60 kiloWatts in 1 hour||1 kiloWatt in 1 minute|
So the thirty minutes of planing Jamie might need is going to require 30 kiloWatt hours of energy. He will need batteries with at least that much capacity. To make sure he always has enough energy, let’s double that and say Ohm’s Law is going to want a battery – or batteries – with 60kWh of energy storage.
Next: Battery weight
Part two of the calculation is the weight of those batteries. For that we need to understand energy density – how much energy can be stored in what weight of battery. A good rule of thumb for a lithium ion battery is that it takes 5 kilograms of lithium ion battery to store 1 kilowatt hour of energy. So 60 kWh of energy is going to be stored in (roughly) 300 kilograms of battery.
|5kg li-ion stores 1 kWh||60kWh = 5 kg X 60 = 300kg|
We’d better circle back here, because our calculations were based on the boat weighing 1,500 kilograms. Now we’re at 1,800 kilograms and we haven’t got Jamie and Vicky and the kids and paddleboards and the lunch on board yet!
Let’s say the people and their paraphernalia add another 300 kilograms. Now we’re at 2,200 kilograms and we need to revisit our planing power.
2200 kgs is 44 ‘parcels’ of 50 kilograms, so we are going to need 88 kilowatts of power…but then we are going to need more battery…you can see how this can be a bit like chasing one’s tale, but it is a calculation that needs to be done.
In the end, when Jamie worked out his use cases and built in some extra capacity for more planing or unexpected emergencies, he decided to go with 120 kilowatt hours of battery storage – 600 kilograms of battery weight – to make a total boat weight of 2,500 kilograms.
Total weight: 2,500 kg / 5,500 lbs
You’ll notice that we haven’t talked about the power of the motors yet. The reason is that we needed to know the total boat weight before we could figure out the power requirement and we couldn’t figure out the boat weight until we figured out the energy requirement.
We now know we’ve got 2,500 kg of total boat weight, which is 50 parcels of 50 kgs. When we multiply that by 2kW we need 100 kw of power for planing speed.
|Final Boat Weight = 2,500kg ÷ 50kg = 50||50 X 2kW = 100kW|
Finding the motors
Even though Jamie was not going to be doing a lot of planing, there are lots of reasons, you don’t want any drive train – electric or fossil fuel, on a car or a boat – working at maximum output all the time. Jamie knew he needed extra power available above 100 kw, and figured that 120kW of power would give him plenty to do any planing and easily handle the cruising speeds.
One of the nice things about a catamaran is that it naturally has two motors – one on each pontoon – so the full weight of the boat doesn’t always have to be borne by one motor. Two 60 kW outboards would fit the bill for Ohm’s Law. And for redundancy, if he did happen to have any problems out on the water with one motor, he could easily get to shore running with just one.
Creating the electric outboards
The power of a Nissan Leaf motor matches up nicely with what Jamie needed. One EV motor for each outboard. Time to start looking in detail at that.
Fossil fuel outboards have a combustion motor – the powerhead – under the cowling at the top of the setup. That motor turns a vertical shaft in the lower leg – or lower unit as it is called in North America – which in turn uses a right angle gear connection to turn a horizontal shaft with the propeller on the other end.
To convert it to an electric motor, the ICE powerhead will be replaced with an electric motor.
Simple concept, but in practice it is more complicated, because it involves precise engineering to link the existing spline on the lower leg’s shaft with the one on the electric motor to match up exactly as the original combustion motor did. It also involves calculations about the RPMs of the original motor and the new electric one. Finally, the torque properties of an electric motor are different from ICE, so that will need to be looked at.
Jamie had the Nissan motors, the hunt now was to find the casing and lower units from a petrol /gas motor. He found what he wanted at a sale of marine equipment at the UK Ministry of Defense, and picked up a pair of used Mariner Optimax outboards.
He wasn’t concerned what shape the powerheads were in, of course, but he knew that since the motors had been used by the Ministry they would have been well maintained with gear lubrication and all of the other things that would assure the lower shafts and gears were in good shape.
Getting ready for Part II – putting it all together
We’ve covered a lot in this article. To recap: Jamie has found his boat, done his energy/power calculations, and sourced a couple of outboard casings. Basically he has taken an assortment of ‘shell’ components for a boat that ran on fossil fuel and is now going to clean it all up with electric propulsion.
We will pick up the next stage of the Ohm’s Law journey in a Part II Plugboats article. It will cover the details of finding the Nissan motors and Tesla batteries, converting the outboards to electric, placing the batteries in the hull, testing, re-testing and getting Ohm’s Law ready for her maiden voyage.
If you found Part 1 of this article through social media, Part II will also be posted there. Keep your eye out.
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