Sounds like a great idea.

Kayaking is mostly upper body strength, so his condition is not going to be a major problem -- once he is in the boat.

As I see it, his problems will be in launching and landing. These can probably be solved with hardware, though. If your father must have a wheelchair in order to move then you have to consider the terrain around launch and landing sites. Can his wheelchair move through sand on a beach, or rubble and gravel on a rocky launch site? an the wheelchair go into the water without damage? There are some models which can go into the water. The local college permits these in their pool, and that pool has a ramp for the wheel chairs. The swimmer rolls their chair in and can almost float out of it. They float back to the chair to remount it.

If so, then your father may be able to temporarily connect the kayak to the wheelchair so he can move himself from one to the other without the boat getting away. You would have to figure out some arrangement of ropes or straps that would permit this.

There are commercial kayak trailers or dollies which your father could hitch to the back of his wheelchair so he could tow his boat to the water. You could also make something like this. People have modified two-wheel golf-bag carts and baby carriages. A second wheelchair might even work.

You might want to create a large, self draining, cargo area where the wheelchair (and the kayak dolly) could be stored. Then it would be with him should your father land at some point other than where he launched.

Normally you would want your kayak deck to be smooth, but for your father's boat you might consider building in numerous handholds to make it easier for him to reboard from the water should he need to. You might consider using handhold such as rock climbers use on iondoor climbing walls. these would work, but be ugly. Or, you could cut holes in the deck, glue a block of wood about 5 cm thick under the hole, carve out a 4 cm deep hole in that block, and create handholds that way. With the second method the deck would be smooth, but look like a miniature golf course.

Also, you would want to be a bit more careful in locating the ropes for the deck lines. I'd chose ropes that were designed for rock climbing and which were VERY STRONG for their size as they would more likely need to take your father's full weight should he use them to pull himself around. Likewise, the fittings on the boat where these ropes are attached should be larger and stronger than what is common.

Many rudders are controlled by foot pedals, but there are other brandss and designs which allow you to control the rudder cables by hand. If you use a rudder you would probably want to find these, or adapt them yourself.

I agree with the comments from another post about fitting the cockpit to you father's body. You don't want merely a good fit, you want a great fit. Even without control of his legs he should be able to make the kayak behave as if it was part of his body.

Classes, classes classes. If he can learn all the safety skills of other kayakers -- and he should be able to -- then he'll be just fine. He'll also be able to see where he has to grab the boat to reboard it -- and that information could help you in deciding where to put the handholds.

By the way. Can he swim? I have seen a modified version of the breast stroke which is used by people with paralyzed legs. If he doesn't know it then he should learn it. Otherwise, he should attach a leash to his boat so it won't float away from him should he exit it.

Good luck with this.

PGJ