Much as Chipko says.
I've had a "ten pound" mini tach on the last 5 outboards, clipped to the tiller arm with cable ties initially for running in, prop decisions and getting used to the motor. Once that period is over I make up a bracket/holder under the hood with a section of kitchen chopping board and the tach lives there for recording hours plus service time idle checks etc. I've also not yet found one that's truly waterproof hence the relocation asap. They all have the same method of operation needing a few turns of their signal wire around one plug lead then you tab through the menu to set up for the firing cycle vs revs of your outboard.
I was very keen on the all black one in the image below as they have a sealed in 5yr battery so should be more waterproof and they have an earth as well as signal wire so I hoped would record pulses accurately. Over time having ordered 5 then needing to return 3 as their batteries had flattened in storage I changed brands. For no particular reason I chanced on the one with the blue front in the other image and I have these now on both outboards. This one has a replaceable battery so the (very) weak link is moisture ingress through the battery cover with its pathetic attempt at an o-ring seal. Silicone grease needed to make anything like a decent seal.
Lots of choices...
https://www.google.com/search?client...UTF-8&oe=UTF-8
