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I got the boating bug later on in life and so I bought an inflatable kayak. I quickly found that the idea was nice but paddling a canoe for longer than 20 minutes was not how I wanted to spend my time – I needed a motorized inflatable kayak with an electric trolling motor!
Designing and building an inflatable boat with trolling motor is the most fun I’ve had in years.
- Can you put a motor on an inflatable kayak?
- DIY Solar Powered Kayak Parts List
- DIY Solar Powered Boat – Step By Step Instructions
- Step 1 – Choose Your Inflatable Kayak – Which Is Best For Motorizing?
- Step 2 – Measure And Fit The Plywood Floor Into The Kayak
- Step 3 – Choosing The Best Trolling Motor For An Inflatable Boat
- Step 4 – Fitting A DIY Remote Control Trolling Motor Variable Speed Controller
- Step 5 – What size battery for trolling motor?
- Step 6 – How To Choose The Solar Panels For Your Trolling Motor
- Step 7 – Do I Need A Solar Charge Controller?
- Step 8 – Mounting The Solar Panels And connecting the solar panels to the battery
- Related Questions
Can you put a motor on an inflatable kayak?
Well, I had no idea, but so I started to plan how I could do it. This post includes details of my DIY solar kayak design journey, from choosing the best inflatable kayak for the job to adding refinements such as fitting a wireless PWM for trolling motor variable speed control.
Designing A Solar Powered Boat From Scratch
Originally I simply wanted to fit an electric trolling motor because I didn’t want to paddle everywhere. Learning as I went, I found that a trolling motor battery size of about 90Ah is required to give a reasonable amount of time on the water.
On top of that, a trolling motor battery has to be deep cycle, which means it can be discharged up to 80% of it’s capacity without damage. A normal car battery can’t do this, being designed for 5% to 10% discharge.
The big downside is the weight of such a battery. I purchased a Varta brand 90 Ah ‘beast’ of a battery weighing 30kg (66 lbs). Lifting this battery in and out of an inflatable kayak became very tiring – I needed another solution.
Having previous experience of installing off-grid solar panel systems including batteries, I realised that solar would be the perfect way to reduce the battery size needed and radically increase the range.
There were many questions that I didn’t have the answers to, from the size and type of trolling motor, the size of the battery needed, to the area of solar panels require.
I’ve always been an organized guy and so I approached this like any other construction or engineering project – what do we need to do the job?
DIY Solar Powered Kayak Parts List
The gear needed changed as the design changed, but the list of materials below represents the final parts I used to build a DIY solar kayak.
- Itiwit 3-man inflatable kayak (other canoes are suitable – see table in Step 1 of the build process below)
- 2 x 150 watt flexible solar panels, 1120 mm x 540 mm
- Aluminum angle for constructing frames for flexible solar panels – 8 meters x 15 mm x 15 mm
- MPPT Solar Charge Controller (essential for maximum efficiency for small solar panel area)
- Marine ply-wood – 1200 mm long x 700 wide x 12 mm thick
- Hardwood for transom cross-piece – 1000 mm long x 100 wide x 30 mm thick
- 30 Ah lithium phosphate battery
- Anderson battery connectors
- 16 mm flexible cable – 2 meters
- Assorted screws and fixings
- Marine varnish – small tin
- Tubular upright supports for solar panel mountings ( I used metal detachable broom handles!)
- PWM (Pulse Width Modulation) variable speed controller
- Tool case – to adapt and house all electronic components
How Much Does A Motorized Kayak Cost?
Commercially available inflatable motorized kayaks are expensive so the DIY option is a great way to go. Cost breakdown is as follows:
- Inflatable kayak – $800
- Solar panels + control – $400
- Plywood, hardwood uprights, tubular solar panel supports, varnish and fixings – $50
- Lithium phosphate battery 30 Ah – $250
- Anderson battery connectors – $20
- Minn Kota 30lbs thrust trolling motor – $200
- Equipment case, PWM variable speed controller and wiring – $70
- Aluminum angle pieces – £30
Grand total cost = $1820
Time to build = 1 to 2 days
DIY Solar Powered Boat – Step By Step Instructions
The best inflatable kayaks need to be assed from the points of view of safety, stability, toughness and carrying capacity. For the most part, the well known brands like Sevylor and Sea Eagle are extremely well-made, being constructed from layered polyester-type material.
The better canoes have an extra layer of very tough material underneath that is very difficult to tear, although it can be punctured. It’s the same material used by truckers to tie down their loads.
In general, look for mid-range priced inflatables with strong compartments housing the inflatable bladders. The kayak I used is made by Itiwit, a European company. It has 3 compartments, one for the floor and one each side. even if one punctures (it has never happened to me) you can limp home.
It’s probably wise to choose a kayak for 3 passengers. Even if there are just the two of you, the extra space for mounting the motor and solar panels is very useful. The Itiwit does the job admirably, but if I did it again I would choose the Sea Eagle.
All of the craft in table below have a wide beam of around 1 meter – they are extremely difficult to capsize. This is a feature of most of the medium to top-end inflatable kayaks and canoes.
The last thing to consider before making you choice is the carrying capacity in kg. Flexible solar panels are light but the weight of all the components add up. This is one of the reasons why I chose a 3-man canoe. The extra weight carrying capacity is useful for the extra gear needed to run it.
Best Inflatable Kayaks For Motorizing
The chart below gives some basic specs for 5 of the top popular inflatable kayaks:
Max. Weight Can Carry (kg)
Itiwit 3 man
Sevylor 2 man Canadian
Intex Excursion Pro
My recommendation is the Sea Eagle for it’s sheer toughness – all of their kayak models are ideal kayaks for motorizing. In the video below the team are running a vehicle over an inflated 380X model.
Step 2 – Measure And Fit The Plywood Floor Into The Kayak
Essential Inflatable Kayak Mods Pre-Motorization
Fitting a solar panel on a kayak and also attaching a trolling motor means that a stable base is essential. The sides of inflatable boats are tubular, they can’t be drilled and fixing anything with glue is fraught with difficulties.
In any case, fitting anything to one side of a kayak would cause it to list at an uncomfortable angle. Any structure needs to spread the weight across the beam and be as sturdy as possible. The solution is to fit a marine ply-wood hard floor over the top of the inflated floor compartment.
Inflatable Kayak Seat Upgrade
As you can from the photo above I decided to take advantage of this secure base to use it as a seat and attach a back-rest for improved comfort. You can just seen an extra piece of wood to the front, the result of an early attempt at mounting the solar panels – the design changed as I moved forward, as you’ll see later.
This arrangement also increases the overall stability of the arrangement as the weight of the person sitting in the kayak acts as a counter-weight to the trolling motor.
The procedure for measuring and cutting the ply-wood floor piece is quite straight-forward. The ply-wood base is measured with the sides deflated. When cut it is inserted before inflating the sides.
- Lay the kayak out flat and leave uninflated
- Measure across the floor panel (see photo below) and cut the plywood
- Position centrally over the floor
- Inflate the sides first and then the floor
- Check for stability
(Notice how the inflated tubular sides expand to hold the base-plate in place.)
Making sure that the floor fits OK, remove it by deflating the kayak as much as needed to free it up, and fasten the upright supports as shown.
Position these close to, and in line with, the contour of the kayak sides. They rise above the top of the sides so that the transom (cross member) can be bolted to them.
The trolling motor transom also acts as a back-rest, so position the back-rest hinge at a comfortable distance from the uprights. A thin foam cushion place here makes it quite comfortable.
When that’s done, the parts are fixed together ready to fit the trolling motor.
NOTE: The photo at the top of the page shows the first design I tried with the trolling motor at the rear. It didn’t work out so the design presented here step-by-step reflects the final configuration, which is a side-mounted motor.
Step 3 – Choosing The Best Trolling Motor For An Inflatable Boat
What size trolling motor do I need for a kayak?
Trolling motors are measure in thrust rather than current draw. Naturally, I initially went for a more powerful model as I thought it would be faster for an inflatable kayak. This isn’t true.
Trolling motors push a boat along at around 5miles/hour, whatever the thrust rating. On a bigger boat a lower thrust motor will take longer to reach it’s speed and any wind or current will slow the kayak down very quickly.
A general rule of thumb is to allow 2 lbs of thrust for every 100lbs of weight, including boat, passengers and equipment. I’m going to mix imperial measurement with metric and use 2 lbs thrust per 45.3 kg as kayak specs are often given in metric weight. (It also makes it easier for me to use the numbers for my European kayak!)
Let’s crunch the numbers:
- My kayak weight 17 kg
- Passengers 160 kg
- Motor, battery, controls and panels 30kg
- Picnic and extras 10kg
- Grand total weight to push along is 217 kg (max capacity for my canoe is 250 kg)
Calculation for trolling motor thrust 217/45.3 x 2 = 9.53 lbs thrust
You can see the smallest trolling motors will work for these tiny craft, even a lower-end 18 lbs thrust model. I opted for a Minn Kota 30lbs, just because it was quickly available in my neighbourhood.
Now we need to know how much current this motor draws. The table below shows the maximum amps pulled in the worst conditions and the associated thrust for a 30lbs trolling motor:
Minn Kota 30lbs Trolling Motor Speed, Amps and Thrust Table
It’s interesting to note that Speed 4 gives plenty of thrust for an inflatable kayak and just 66% of the maximum current, which represents considerable battery savings. I generally use Speed 4 setting and experience very little speed reduction.
A ‘soft-starter’ or electronic speed controller (E.S.C.) would also help to reduce battery draw surge when starting or changing speeds – the 5-speed control handle makes abrupt changes and the current surges each time.
Step 4 – Fitting A DIY Remote Control Trolling Motor Variable Speed Controller
How does trolling motor speed control work?
Conventional low-thrust trolling motors run at 12 volt and have a 5-speed step-change speed control system It can be a little choppy when speeding up or down. It can also put a strain on the battery if used a lot. With a solar powered kayak we need to conserve battery power as much as possible.
My solution is to fit a PWM Electronic Speed Controller (ESC) to smooth out the current surges associated with this kind of control. It also allows a much finer range of speeds – a variable speed trolling motor is great for fishing!
The drawing below shows the basic principle of operation – a later step covers the actual schematic diagram for the whole solar circuit.
An ESC chops up the d.c. current into pulses of 0% volts or 100% volts and adjusts the timing of each pulse. For example, if the pulses are at 100% voltage for 50% of the time, the trolling motor will run at half speed.
The speed controller I use has a remote control, which is a great improvement on manual control, particularly better than the old way of manual switching.
Special Note: Make sure to buy a PWM speed controller that operates at a frequency significantly higher than 12,000 cycles per second (Hz).
The device operation can be heard as an annoying ‘clicking’ below this value. Cheaper ESCs are often found to operate below this frequency threshold.
Step 5 – What size battery for trolling motor?
In the battery world everything is not as it seems! Logically, you would expect a 100 Ah battery to deliver 1 amp for 100 hours or 100 amp for 1 hour, but this is not the case.
There is a law that states the higher battery discharge current, the less capacity in Ah is available. At higher discharge rates this can be considerable – yet another reason to keep the current draw down as much as possible.
What Is The Best Battery For A Minn Kota Trolling Motor?
Later I’ll show a chart for motor running time with and without solar support, but for now suffice to say I want the smallest and lightest battery I can get away with, which is a 12V 30Ah lithium phosphate model. They are more expensive than lead acid but much lighter and with a longer life.
A deep-cycle lead-acid battery is good for perhaps 500 charge/discharge cycles, while lithium phosphate (LiFePo4) can reach 2000 cycles if treated properly, even if fully discharged – a good quality LiFePo4 battery is a purchase for a lifetime. The size I bought weighs 4 kg instead of 10kg for the lead-acid equivalent.
The kayak ran for some weeks with a 30 Ah lithium-ion battery. It is very light but some cells bulged under charging conditions. Lacking confidence (I’d heard about these type of cells exploding!) I opted for a lithium phosphate type, which are considered very safe and stable.
Can you use a car battery for a trolling motor?
A car battery is designed to deliver large currents for a very short time and then it is immediately placed under charge by the car’s electrical system. It is never discharged more than a few percent of it’s capacity, while a marine deep-cycle battery can be discharge up to 80% of its capacity. A car battery should not be used for trolling motor applications.
However, there’s always the exception to the rule! For some weeks I used a very cheap 44Ah small car battery on my solar panelled kayak to travel between towns at either side of a bay. The journey took about 40 minutes and I had 300 watts of solar panels fitted using the Speed 4 (20 amp) setting of a 30lbs trolling motor. The numbers look like this:
- Current taken by motor = 20 amps
- Current supplied by solar panels = 16 amps (2 x 150 watt panels in ideal conditions)
- Ah left in battery after 40 mins = 44-((20-16) x 40/60mins) = 41.33 Ah (94%)
Even with just one 150 watt panel the battery would only be 10% depleted. The kayak could easily travel twice the distance before endangering the battery, and would probably last several years – it’s a cheap option.
For serious cruising over several hours a deep-cycle is required. Lithium phosphate batteries are naturally deep-cycle as they can be discharged up to 100% without damage.
What gauge wire for 12v trolling motor battery?
If you have an existing trolling motor you will need extra wire to connect it to the new solar control elements. This useful cable size calculator will tell you the correct size to use without overheating.
I use 60 amp Anderson fast-connectors for connecting between the motor, speed controller and battery.
They are big enough to manipulate without damaging them and make a secure connection.
Don’t forget to wire in a 50 amp fuse between the battery and trolling motor.
Trolling motor amp draw chart
In case you already have a trolling motor or intend to purchase a bigger one, the chart below shows the maximum currents pulled by common sizes of motor. Thrust sizes between 18lbs and 44 lbs tend to be 12 volts, 55lbs to 90lbs usually 24 volts and over 90lbs thrust 36 volts.
As the voltage increases, the current reduces. It’s the current we are interested in here, because it determines cable size.
Trolling Motor Thrust Size in lbs/f
Maximum Current in Amps (motors may be different voltages.)
Step 6 – How To Choose The Solar Panels For Your Trolling Motor
How to choose a solar panel to charge a kayak battery
In the case of inflatable kayaks and canoes solar calculations are hardly necessary. The space available on these boats is very limited, so I decided to use as much of it as is available to maximize the amount of power I could feed into the battery while running, to extend running time as much as possible.
In my case it’s possible to mount 2 x 150 watt solar panels overhead. This also gives the benefit of providing shade from the sun at the hottest part of the day.
Weight is at a premium, with respect to the carrying capacity of the canoe and also the thrust of the trolling motor used. Rigid solar panels are much heavier at 11kgs than flexible panels at 2kgs.
The panels used in the photo above are 1120 mm x 550 mm. Ironically, a light frame is needed to support these flexible solar panels, which was fabricated out of aluminum angle pieces.
Flexible panels are not as efficient as rigid but have the advantage of being able to bend through 30 degrees.
Warning: If you bend them beyond this limit, particularly in cold weather, you’ll probably hear a crack – this sound is the bus-bars that link the individual solar cells breaking and the output of the panel will be severely reduced.
For simplicity’s sake, assume maximum insolation of 1000 watts/square metre so the panels will give maximum rated output. Realistically it will be much less than this most of the time. This is a best case scenario and will be degraded due to the season, the location where you live and the angle of the panels (usually horizontal).
This table shows the run time of a 30 lbs trolling motor running at Speed 4 connected to a 40, 60, 90 and 110 Ah deep-cycle lead acid batteries without solar panels and with. Two cases are shown, 150 watt and 300 watt panels. A 150 watt solar panel should deliver about 8 amps in ideal conditions.
Traditional deep-cycle lead acid cells can be depleted up to 80% of the time, but this can be significantly reduced if current draw is aggressive – one of the reasons I use a speed controller. Assume no losses.
The option you choose depends on a variety of factors:
- Will the kayak be used for fishing or as a cruiser, with the trolling motor as the main source of propulsion?
- Will the battery need to be lifted in and out of the kayak after each trip?
In my case, the trolling motor is used as the main source of propulsion and I’m getting too old to lift 30kg in and out of a canoe, so my priorities are long run-time and light-weight battery.
The table shows that the range of even a small battery can be extended considerably but these are still quite heavy. I opted for a 30Ah lithium phosphate as described in Step 6.
Have a different size battery? Enter the amp-hour rating into the calculator below and see the runtime gains you could make with 100 watt solar panel if running a 30lbs trolling motor at Speed 4 (assume max current of 20 amps).
Trolling Motor Run-Time Calculator With 100 Watt Solar Panel – 30lbs Motor At Speed 4
Run time for a 30 Ah lithium phosphate battery is shown below. This type of battery can be discharged 100% with no damage:
- Full capacity available is 30 Ah x 100%
- 30 lbs motor draw at Speed 4 = 20 amps
- Provided by 300 watt solar panels = 16 amps
- Hours run-time = 30 Ah divided by (20 amps + 16 amps) = 30/4 = approx 7.5 hours
Step 7 – Do I Need A Solar Charge Controller?
Solar boat battery charger
A solar charge controller is essential for safe and efficient operation of any solar charger circuit. As it name suggests, it samples the battery voltage and regulates the voltage from the solar panels to the right level for that particular battery type.
It’s generally not recommended to charge any kind of battery with a solar panel without charge controller, unless the solar panel is so tiny that its output current is at or below the battery’s float charge current.
How to select a solar charge controller
There are two kinds of SCC – Pulse Width Modulation (PWM) Maximum Power Point Tracking (MPPT).
You can read more about them here but basically the MMPT can be up to 20% to 30% more efficient, which is essential when the area you have for solar panel mounting is limited.
For small power applications it’s a matter of choice between PWM or MPPT, but as you’re limited for solar panel space I would strongly recommend MPPT controllers, even though they are more expensive.
MPPT charge controllers are also more expensive than PWM but a reliable model can be found for around $60. 300 watts of solar panels would deliver 16 amps at the best of times, so be sure to purchase one with a minimum 20 to 30 amp rating – just in case you expand in the future.
Solar charge regulators have an input voltage rating which is not normally an issue as the canoe has a limited number of panels. Normally the voltage input from the solar panels will be between 18 to 21 volts.
I recommend connecting the two panels in series, doubling the voltage and halving the current. As most controllers have an input PV voltage rating of 60 volts this is perfectly in order.
The last thing to note is to make sure the solar charger has a setting for your particular type of battery. The charging voltages and charging profile for lead-acid, lithium phosphate and lithium ion batteries are all different.
Step 8 – Mounting The Solar Panels And connecting the solar panels to the battery
Building The Solar Panel Frames
Flexible solar panels are lighter than rigid panels, although not as efficient. Another disadvantage is that they are difficult to mount unless you intend to lay them across the tubular sides of the kayak on a out-rigger.
I prefer to mount them overhead. In this way the canoe occupants get some relief from the sun and the panels are not likely to be shaded, thereby reducing their charging power. Shading is a particular problem if you have solar panels connected in series, which is my preference.
A light-weight structure is necessary as the solar panels are overhead. I made the frames out of 13mm aluminum screwed together at the corners and fitted clips for fitting the uprights. The uprights are made out of metal tubular broom handles – very light and rigid.
Solar Kayak Circuit Diagram
Now it’s time to connect everything up and take it on the water! The very simple circuit diagram below shows the connections. In addition, I added a wattmeter and a voltage/capacity meter for the battery.
These are not strictly necessary, but I like to know what’s happening around the circuit and what the state of the battery is. The solar charge controller has blue-tooth and also a remote monitor to show charging current, amp-hours and energy transmitted. The speed controller has a remote control feature – very useful for keeping in a water-proof bag around the neck.
All the components are mounted inside an aluminum equipment case, together with externally fitted Anderson plugs for battery and motor and MC4 connectors for the photovoltaic panels.
Can a solar panel run a trolling motor?
A solar panel can run a trolling motor if it’s output rating is significantly more than the d.c. motor rating. A buck voltage regular is required to ensure the motor is not subject to over-voltage and resulting high current.
Can a solar panel charge a trolling motor battery?
Solar panels can charge trolling motor batteries. Normal practice is to incorporate a suitably-sized solar charge controller (SCC) to ensure safe charging voltages so that the battery isn’t damaged.
What size trolling motor for kayaks?
A 30lbs/f trolling motor is sufficient for most kayaks. Allow 2lbs of thrust for every 100lbs (45kg) weight carried, including boat, all equipment and passengers.
What size battery for canoe trolling motor?
Trolling motor battery size depends on the trolling motor current draw and the run-time required. Lithium phosphate batteries give the best performance, with a high energy density, light weight and 100% discharge capability.