You only need a volt meter. Adding the amp reading function will do you no good for your application. I've been involved with boating and boat building for a long time, including marine electronics. There is no practical purpose for knowing your real-time amp draw for a trolling motor on a kayak. Your battery voltage will dictate the ability of your trolling motor to work correctly and will be a factor long before any amp-hour considerations are had. The only time you need to consider amp-hours are when purchasing the battery and trolling motor, just so you can make sure that you're setting it up correctly.