Dear Boat Doctor,

I’m the kind of person who likes to know how things work and a few systems on my boat have me stymied. A few years ago I installed a Frigoboat keel-cooled refrigerator. It works great, but I am trying to figure how many times per day it runs and for how long. It’s so quiet that I can hardly tell when it is running. 

I am also trying to figure out if my alternator is running correctly. I have a Balmar 100-amp alternator with an external regulator. There is an ammeter cabled into the output but it just gives the immediate amperage, I don’t know what it looks like over time.

Jim Allaway

Milwaukee, Wisconsin

Dear Jim,

I applaud your interest in the performance of your systems. I don’t have any permanent instrumentation ideas for you, and your interests are bordering on engineering level, but I have some ideas on how you can sample data in the short term.

The fridge information is simple. I think the easiest way to approach it is to monitor the temperature of the fridge over time.

I like to use the USB500 series loggers from Measurement Computing (www.mccdaq.com) for doing this. You can get loggers for temperature logging as well as a few other parameters. These are low-cost standalone data loggers that cost about $80 and will periodically sample data (from once a second to once every 12 hours) and allow you to upload it to MS Excel for data analysis. You just set the device up with your computer and it will collect data until it fills up, plug it back into your computer and analyze your data.

Starting from a warm box, you’ll see a moderate down-sloped cooling line until the box hits the set point. After that you’ll get a sawtooth shape—a long up sloping line as the box warms, and sharp down sloped cooling line as the temp pulls back down.

You can measure the time between the points to see how long and often the compressor runs. The time from the low set point to where the compressor kicks back in will let you know how well your box is sealed and insulated, the longer it takes the better the box is.

You can do something very similar with your alternator. Just connect a USB500 series DC voltage logger across the ammeter shunt. You will be measuring a small voltage but you can correct this to an ampere reading when you do the analysis. Shunts are specified for a given voltage drop at a given current—if yours reads 50mV at 100A, just correct the values with a formula like this (100A * observed value / 50mV = actual current). By plotting the values you’ll be able to see how the current output changes over time, and correlate that to engine speed and battery charge level.