Actually, a hub motor may not be very good at handling axial loading on a continuing basis. They are mostly meant for use on e-bikes. On a bike, there is actually very little axial loading on the hub in a turn. This is because the bike is leaning into the turn. It may work but I think I would go for something a bit more robust. If you do this, please keep us updated on how well the motor holds up over months or years of use.
You may find that a trailer stub axle and bearing would provide axial stabilization and serve well enough as a thrust bearing. Otherwise, Glen-L offers a couple of bearings and housings useful as thrust bearings for small craft. Or you could fabricate something that would work... a flange fixed to the shaft with a ball bearing race cut into each face, and a stationary round plate mounted forward, another aft, of the rotating flange, with corresponding bearing race cut into the face. Add a housing and some lubrication and you could have the king of small boat thrust bearings. Overkill, yeah, maybe LOL. But while the RPM and power rating might work nicely for that hull, depending on the prop used, I think a thrust bearing of some sort is called for.
The prefab belt drive reduction units offered by some ev vendors not only provide a thrust bearing, but also completely isolate the motor shaft from the prop shaft, eliminating effects of poor alignment or stabilization on the motor. Radial loading however is something that hub motors are designed for. So any wobble in the pulleys would be pretty small potatoes to the motor. You could even use soft mounts for the motor. No reason why you couldn't have any reduction ratio you want, by changing pulleys, even 1:1. Just sayin.
In another thread I present a means of direct drive coupling a motor to the prop shaft that allows for small alignment errors and needs a thrust bearing only to hold the shaft sort of in the ballpark. I will be doing this, and the coupling system is a splined coupling collar slid onto splined stub shafts fixed to prop and motor shafts. This I am counting on to give me good isolation of the motor from real world issues that would otherwise threaten motor lifespan.
72v is a perfectly acceptable voltage from a purely electromechanical standpoint. However, electrical standards on boats differ a bit between lethal and nonlethal voltage electrical systems. Below 50v is somewhat arbitrarily regarded as nonlethal, and above 50v considered lethal. This makes 48v sort of a sweet spot for small boats up to maybe 35 foot. If you are barely going over 50v, then you may as well go to 96v, 120, 144, or even higher. What this does for you is reduces the current needed to get the same amount of work done. 72v offers some advantage over the more popular 48v, but enough to justify the extra expense of engineering the system up to proper standards? Sure, maybe. Up to you. But especially for a 17 foot double ender, I would seriously consider sticking to 48v. There are a lot of chargers, inverters, controllers, breakers, etc that will work fine with a 48v system. Choices at 72v will maybe be slightly more limited and limiting.
Is your hub motor brushless, or brushed? One consideration is that propane or gasoline accessories must not be used on a boat with a brushed motor. The reasons are obvious. Brushed motors do offer one small advantage though. If your controller fails, you can temporarily connect the motor directly to battery. The only way to adjust speed is to tap up or down the series. The reversing relay might be made to work, or you would reverse polarity on the connection. This is of course extremely not a good idea but like a lot of not good ideas, it might get you home some day. All in all, I prefer brushless. You can, after all, keep a spare controller aboard. Even a spare motor.
The typical 5kw brushless motor would work well for your hull. You might be surprised to find that despite the numbers, you can go direct drive due to the fairly low loading presented by the small hull you are pushing, depending on prop size and pitch. I can push my 7600lb 27' boat with a 5kw motor in direct drive operation just fine. In fact, preliminary tests early this year showed a significant improvement in efficiency over the theoretically correct reduction range, using mechanical down gearing. Yes, I realize that the hub motor is probably cheaper, but the difference probably not at all worth the extra engineering hassles. Take a hard look at a more conventional motor, before pulling the trigger on the hub motor, is my suggestion.
If you want a motor with an integral thrust bearing and your pockets are deep enough for LiFeP04 batteries, you might consider a Lynch motor, They are designed for direct drive applications, particularly marine traction use. YMMV.
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