The why I 'll leave unanswered but at a moment I wanted an electric pump in my kayak.
Not an easy project because there are quite a few requirements. Such as:
After long consideration, I choose to place it behind the seat, since there is quite a lot of empty space and placement of the battery in the day hatch is difficult because there is no glue that will provide adequate adhesion to PE. In addition, the bulkhead, like the rest of the boat is somewhat flexible and to thin to a heavy battery to be screwed on. Fortunately, the seat is sturdy enough and several tensioning straps for the seat can be used for tying up.
I could choose between Rule 500 and L450 Johnson, chosen for the latter because of the integrated electronic sensor. The sensor however apeared to be a little float with a magnet which controls a reed relay. Perfectly usable but not as great as expected. Unfortunately, many electrical wires that need to be connected (seawater resistant).
The material of choice was Lexan or something similar. However it appeared that quite to the price and nobody could confirm the adhesives. Someone gave the tip to epoxy the plywood trial and it sounded feasible. The first trial version was rejected and largely rebuilt. Of construction are no pictures but from the results there are.
The loose piece of hose that you see is holding the box in place when rolling, actually a point for improvement, but I never did.
A nice detail is the watertight mini hatch for the fuse and plugs, made of a PET milk bottle, excellent waterproof and a very cheap solution.
The most exciting was the hole drilling or sawing, after long consideration, first drilled with three mm and then with the big hole saw made to measure. Unfortunately the 3mm drill had run away so the hole is not quite ideal.
Things that have been considered:
The box has been extensively tested on the channel, the cockpit is empty within a few minutes. The left over water is not unpleasant but slightly more than the hand pump. After two hours of wet practicing everything was still in place. Edit: Meanwhile tested on salt water in all kinds of conditions.
Oh, and the why. A boat full of water is always a bad thing, with a hand pump in waves your cockpit gets already full during pumping, moreover, you do want a paddle in your hands for balance in that situation. Due to sail immediately after boarding you waste less time and energy to pump and youre blow away less from the right direction. It is not necessary that a rescuer should first empty the boat by pulling it across the deck. Despite those two kilos extra weight so I can see the benefits though.
The first summer is the thing worked well, it was indeed unfortunate that there are no decent on / off button, removing and replacing the fuse is a bit a hassle and tricky and had the risk that you would lose the fuse or hatch. While the boat is upside down the pump is running dry, as wears of the pump and the battery runs low.
In November something made a cracking sound, the signal to rebuild the box. An electric water sensor (M158 KEMO) was ordered and then left the matter to March :-)
The box is mainly the same, a bit more space because the float has gone, on the other hand the inside is slightly fuller. Unfortunately the hose runs along the operation, stupid! The switch is mounted on the outer wall of the box, the neck and cap of a milk bottle covering it, a fuse can no longer fall in. A pet soda bottle top is covering the mainswitch, may be difficult to operate with very cold fingers but to leave it on costs only 10 mA, so leaving it on for a day or two is no big deal. Forgetint to turn it on is more difficult. The box is again put into the epoxy, reinforced with some glass lining. A peephole to be able to see the sensor LEDs.
One leg of the sensor is just through the bottom of the box and the other is placed high at the rear, So I can adjust the distance between the legs afterwards. The 10 mA continuous current, I take for granted, 1 minute pumping costs as much as two hour standby. The relay also uses some extra power, the power consumption in operation is 10% higher. Two or three times a season charging the battery should still be quite adequate.
Unfortunately there was trouble right away after installation, for lack of power. At home it worked again. Bad sign, a version 3 still necessary??
All worked fine, at least on fresh water. In salt water the sensor is so sensitive that the pump never went off again, only if the boat was completely dry. The plan was to build again with the floating switch and of course an on-off switch. The box is never completely finished, I learned to roll and had another kayak (Anas acuta - with foot pump). The buyer of the Nordkapp got the components of this system but never rebuilt it.