Disclaimers:
* If you follow any of my advice then it is assumed you are willing to blow a few motors or controllers up
The primary failure mode of over-volting brushed equipment will be in heat generation. The increase in voltage over the basically fixed load will increase current draw. Increased current draw will generate heat to the square of current.
Heat = Power
Power = Current Squared x Resistance
Heat on the motor windings is not the issue here (as it is with Ebikes). The motors are water cooled. The real damage will build up at the Brushes. They are now commutating much more current... and... they are seeing a bigger spark at every jump
So
* Run the best brushes you can find
* Try to find a way to heat sink the brushes
If you use the factory PWM controller, it may need additional heat sinking. Just crack the case and fix a big aluminum heat sink on there and for the most part it will probably be fine. Very few 12V rated parts will blow at 18V or 20V. A lead acid battery can be 14.4V hot off the charger, so no supplier of "reliable equipment" is going to populate with 16V components. Not enough margin*
You can almost always sneak "18V" (so 21V hot off the charger - 5S) on old 12V equipment*
So...
* Get the heat out of the brushes
* Inspect your brushes regularly
Mind your wires!
You want to use wires rated for 90C or 105C wherever you can. Strip out and cut away the junk wires that these things come with. They were Value Engineered to run off of a saggy 12V battery. Go heavy! 10AWG minimum. Wiring acts as a heat sink for Burst.
* Be Smart!
* Run as close to nominal where you can
* Keep burst power down to a minimum
I commonly FLY off of the dock to show off for the other boaters, but as soon as I am in the channel I back my throttle off to something manageable. If you keep your heat generation (Average) close to factory, then you can enjoy BURST performance with Reliability. Exactly the same as ebikes.
Ebikes
I commonly run 88V+
I commonly run 100A+ current limit
This is on a standard 36V 1hp ebike hub. OF COURSE... if you point it straight up hill and bog it... it will destroy your controller or motor pretty quick. Bursting Power is what this is all about once you get things to not instantly explode due to voltage in excess of rated parts*
So...
* Mind you Temperatures!
Take it apart!
These machines are serviceable. You want to get very comfortable with disassembling your brushed trolling motor. If you do not, then... you will end up with a surprise failure. When you are assessing reliability on an over-volted device you want to be able to get in there and inspect.
* remove the prop
* remove the fasteners
* inspect the seals and ingress
Use a Marine Rated grease on all o-rings. This GREATLY increases the sealing performance, especially on older equipment. As the motor heats and cools it will expand and contract. This creates odd vacuums and pressures, allowing more liquid in than normal. Until you have a baseline you are going to want to tear down your motor, clear out any ingress, re-pack with grease.
Use a Dielectric grease anywhere near exposed electrical terminals. Use a Marine Grease anywhere near the water interface. Different greases wash away in different ways.
eh...
Always have a Plan B
If you are over-volting a trolling motor then you better have a plan in place
* Oars at a minimum
* Backup motor
Unless you like rowing, I suggest bringing along some secondary propulsion source... at least until you have determined a reliability regime that can get you out and back. I go out into Open Water (ocean) and the weather sometimes changes. So...
* Wear a Wetsuit
* Bring Fins
I wear a 4:3 wetsuit that doubles as a flotation device. I bring swim fins... as... I am the secondary propulsion device.
Prepare to get wet!
If you are attaching 48V to a 32lb thrust trolling motor on a Sea Kayak... you better be experienced and ready to get wet
* dont bring anything that does not float
* Keep the batteries in something that can fall in the water
The little boats get squirrely! If you goose it 48V with the prop to the side it will move you quick... so... I go out with the ASSUMPTION that we are doing exercises and that WE ARE going to go into the drink.
Disclaimer Done
For waterproofing you can use the knock-off "Zero Case". These are the locking cases with an O-ring seal and check valve. You then drill those for two brass studs - they are easy to find. Brass studs have an o-ring seal or mounting surface. In this way, your lithium stays dry and you bring out only two wires. BEWARE... of the following
* Batteries generate heat
* Contactor Coils generate a lot of heat
So - think about getting heat out of the battery box if you are going sealed. A lot of cooling happens Radiant - A LOT. I typically use DT bulkhead connectors for IP67 needs. They leak a little, but - we are not looking to go under water. We are looking to survive a rollover and get back to shore.
eh...
Split your Battery in Two
Where boating is concerned, it is far better to have TWO small batteries than 1 large. You will learn this the hard way when you have to paddle 3 miles. Just like Ebiking... it is easy to lose track of time. Remember that the following affect your range:
* Currents
* Wind
So... when you head out, head out up current and into the wind. Use only half of your pack and when that runs dry, turn your ass around and head back in. There is no shame in burning only 60% of your capacity. There is great shame if the Coast Guard has to come out and get you.
-methods