Adding an inboard to kayak?

How about a design that uses gears, shafts and chains to route power from low-hull mounted ICE up over rear deck, then down to water at stern. Put sprocket on Weedeater output shaft, then run chain up to another sprocket mounted on a long shaft that runs over rear deck, then down to prop. But that would be a few mountings and bearings, and how to make the final drive with prop that is also streamlined?

Maybe just use a big gas-powered weed eater with bent shaft run along the rear deck. Prop would be angled down a bit but that might be good or at least OK. https://www.lowes.com/pd/Weed-Eater...in-Curved-Shaft-Gas-String-Trimmer/1000284589 Plus you'd have a nice powerful weed eater. Such an app might require a few special parts to connect the prop, but those are cheap and fast from any machine-shop once you know exact dimensions with good drawing from CAD. The guys at the machine shop will be enthused. Would also need a few homemade parts such as wood or foam cradle for weed-eater, and a few more straps on your deck. I'm not seeing a high stern (or bow) on your boat like some ocean kayaks, which is good so it won't be interfering with draping a curved weed-eater shaft over the end.

Here is an interesting Weed-eater with what seems to be long flex-shaft from motor to long straight shaft that might be good for kayak. However, its a no-brand from China, so quality is unknown. Multi powerful 36cc gasoline brush cutter 4Stroker grass trimmer strimmer cutter | eBay https://www.ebay.com/i/303200256964?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=303200256964&targetid=797689825613&device=c&mktype=pla&googleloc=9032029&poi=&campaignid=8086082588&mkgroupid=82257584063&rlsatarget=pla-797689825613&abcId=1139466&merchantid=101689408&gclid=Cj0KCQiAxrbwBRCoARIsABEc9sjuac6odubNuBCg3KKxlYDf5IyrVZEHScjVrOuujSYXXeaevjTVLlMaAicpEALw_wcB

 

I think that the simplest and cheapest, while maintaining the least noise option, is to add more batteries. They would be about the same weight as a gas engine plus fuel. If you triple the capacity the range would be over 40 miles at low speed and about 21 miles at top speed. How far do you need to go?

 

From OP: "Buying more batteries is an obvious option. But at $570 per battery and 75 minutes at top (5.5mph) speed... I do not want to go this route."

Bixby appears to run on 26 volts, too bad, because 26v probably rules out using 36 volt lithium batteries available at box stores like HD for lawn tools and mowers at much lower cost. That 36v option is what I use simply by constructing a plugs and interlocks power cable to match the particular motor Appliance and Battery that I am using.

Seems like the lowest-cost and lowest fuss option would be to remove the cage around the existing Bixby setup and use an $8 model airplane RC prop. That change might increase the speed and possibly double the existing range. For my particular situation, I had a 15% increase in top speed, and a more than doubling of range at slightly lower than top speed when those kinds of things were implemented. Example: Efficient electric boat https://www.boatdesign.net/threads/efficient-electric-boat.27996/page-17 post # 252. Bixby is a generic drive system for many modular uses, and is not optimized or efficient for a streamlined kayak, is my opinion.

All the other options described so far would be much more costly and have serious drawbacks plus they would involve extensive use of a costly boat mechanic or Builder to construct the mounts- probably still leaving the beauty of the original kayak significantly Disturbed. Try to picture an appealing side mount or even a long tail added to such a beautiful kayak.

 

I referred to lead/acid, particularly AGM instead of a generator. If he can accept the weight of a generator, then they would be fine.

 

Agree AGMs (set of two 12 volt wired in series) save money, but it would be considerably heavier and probably not be a simple plug-and-play compared with the equivalent supplied lithium battery.

An electrician might have to be hired to work things out, if it's a proprietary plug or especially if it's a wire connection of more than two sockets, that feeds into the speed control from be supplied lithium battery. The existing lithium battery plug might have to be changed out so that the existing lithium battery and the AGM could alternately be quickly connected and disconnected when changing batteries. If there are more than two wires between the supplied lithium battery and the speed control in the original set up, it might mean using a plug with extra sockets to allow feedback from the lithium battery management system while disengaging same with the AGM. At least those are some of the things that I have encountered in my variety of set ups.

 

A switch that connects one or the other is all he needs. This is not a critical application where having the power off for less than a second would cause any harm.

 

OP indicated he needs to remove the batteries completely ("modular") from his kayak in order to transport to and from water. Separate battery plugs for each battery would make that process a lot easier, in my opinion. If all he needs is a wired switch to allow quick and complete removal of all batteries, as well as connecting one or the other, that is even better!

 

Realistically I could see myself running upwards of 30 miles a day if I could hit that 8-10mph range. I fish for striped bass on Jordan, Kerr, and Norman. Norman has 500 miles of shoreline. Striped bass move constantly and it’s useful to cover water and look for bait and marks.

Even while fishing I run 2-2.5mph in the winter. So like I’m going 10 miles even if I fish right by the launch for just 4 hours.

To me, the battery solution is not worth it at the price. It doesn’t give any more speed. And the cost for range at top speed is hilariously expensive. Say I’d like to run 5 miles to my spot, fish 4 hours, and run 5 miles back. Not even a long trip, I picked a 20 miler rather than the 30 mile one I wish I had range for. For me to run 5 miles at top (5.5mph) speed, fish 4 hours at troll speed, and run 5 miles back... I would need 3 batteries.

If there isn’t a solution that gives adequate speed and distance then I’ll just enjoy the boat for what it is and save up for the cost of a gheenoe and storage for it.

 

Bixpy responded to my questions regarding the generator idea.

They do not recommend charging the battery while discharging it. They say this shortens life span rapidly, and can cause a fire. This has become an issue to the point where they have made the newer (as of 6 months ago) batteries incompatible with charging and discharging. They said a lithium battery system doesn’t work in parallel with load and generator unless you have a battery management system (BMS) designed for this. Presumably this is why a Bixpy costs $1000 and the price of a torquedo deep blue system requires reaching out for a quote lol.


I have one of the OG batteries. But I don’t feel like destroying it by explicitly doing the opposite of what the manufacture recommends. Plus how would I even get a new Bixpy battery compatible with charging/discharging in the future - assuming the cost to battery performance incurred by disobeying the recommendations could be justified.

So the generator idea, at least in conjunction with the factory (Bixpy) battery is out of the picture.

 

Regarding the weed eater idea, I fail to see the advantages over a longtail. Seems like longtail motors exist in the same price range (maybe only slightly more) than a quality weed eater, and don’t require as much extensive modification.

I don’t mean to be dismissive of the idea. I just don’t get it and figure I just be missing your point.

 

This gives me anxiety. I don’t know much about this. My dad holds a graduate degree in EE and I read this out loud to him and asked if he could help me. He said absolutely not and told me to just buy a boat already.

 

I like the idea of swapping out the prop for one better designed to match the hullspeed of the watercraft. This is a far faster kayak than what Bixpy was designing for.

 

Remaining options:

Generator and DIY energy storage system independent of the Bixpy battery.

Longtail

Side-mounted outboard

I think we have been quite thorough going through these possibilities. I appreciate everyone’s help with that. I have a buddy with a small outboard and I’ll find access to a longtail somehow. I’ll likely try these options out before breaking down and trying to make a DIY hybrid system (which I doubt I even have the technical knowledge nor skill to do) or more likely get a Gheenoe or small center console down the line.

 

I'm very dubitative. A kayak is a displacement boat optimized for the 250 watts that can produce a normal guy paddling over a few hours. A very trained athlete can produce 400 watts. So by nature and physics the boat is limited in speed and whatever the amount of power it will go only marginally faster. No need of a herd of ponies to move it at its optimal speed.
I have not made detailed calculations and I won't do, but I smell that a 250 watts electric engine with reduction and a carefully optimized propeller will do the job of keeping the kayak at a steady speed close to the hull limit speed. You'll be close to the 200 watts most of the time.
These are very crude first calculations. 250 watts with 12 volts is 21 amps. For a 8 hours autonomy you'll need 170 total ah in the batteries, so 2 batteries 12V 100 ah will do the job, and respecting the rule of thumb of never discharging a battery totally.
A short circuit on a 100 ah bat can produce 500 amps of instant discharge so breakers or fuses are mandatory. The connections are simply XT90 gold plated.
These batteries do exist LiPo, reliable, strong, capable of enough rates of discharge and high rates of charge if needed, with BMS and 120 V charger... The weight of one battery is around 12 kg. You can find it on DHgate and Aliexpress.

The simplest solution is a small electric pusher as it's adjustable in height, inclination and rotation on a small rig on the side close to you. Kayaks are double enders so a propeller at the stern is not the best place while the kayak is pitching on the waves Maybe, and it's maybe, it will need an optimized propeller that can be made if you have some knowledge about calculations of boat propellers. Plus the 2 LiPo batteries.

If you want DIY, think about 2 light wheels paddles with a brushed and a reducted 250 watts 24V motor for bicycle you can find everywhere in the States plus its controller for a few bucks. That's simple, dirt cheap and primitive but works pretty well on a "slow" displacement boat. As you need 2 12V 100 ah bats ( 2400 watts/hour total capacity), you can have 12V or 24V you'll have the same total capacity so the same autonomy.

I laughed imagining a Predator engine running, vibrating and smoking in the hull after the tiring starting it in the crampy hull. You'll have to install it on silent blocks or it will destroy the hull. To install also a (good) generator. The electricity generated will be very "dirty" so very good and expensive electronics are needed for charging the bats without destroying them. In fact trying to charge the bats while sailing is no sense as the batts are charged at only 1/10 of their capacity during 12 to 14h. That means simply that you will always consume more electricity than the charger can give to the bats.
Or you make an hybrid gas generator for an electric engine. In this case for a 250/350 watts motor you need to produce continuously around 450 watts, 2/3 of HP so a 35cc Honda 1.3 HP will be largely sufficient for the task. And a hybrid system is more complicated to make than you may think.

Whatever the gas engine, you'll need the entries and exits of air with ventilation, plus the insulation of the exhaust ready to burn everything including the kayaker, and worst the stocked gas. I imagine also the smelly 5 gallons gas jerrican, and the need to stop for refilling the engine's tank as the these carburetors are only fed by gravity unless you have a recirculating pumping system for the gas...
The Predator weights already 18 kg, is absolutely not made to be close to water, it's not spark proof. The carb can leak...A no-marine gas engine is just the best way to have fire in the kayak. Same punition with the Honda.

I would hate kayaking with the noise of a cheap industrial engine in my back while smelling the spilled gas and the exhaust fumes...

If you don't want to spend the needed money forget it, but transforming an old kayak out of its normal range of use is always expensive whatever the solution.
If you want to go faster go for a small boat with a 5 to 10 HP 4S outboard.