I’m in the process of replacing the refrigerator and freezer on my 2005 Grand Banks Europa. With respect to battery drain, is it usually more energy efficient to go with AC/DC units or AC-only units and run them through an inverter? —Frank Brown, Grand Banks Europa 46

Broadly speaking, AC/DC refrigerator/freezers intended for the marine market are maximized for efficiency, since there may be periods of time when they run solely on DC and possibly at lower voltages. All AC home fridges have an unlimited supply of AC power, so while they have become much more efficient, they will often have automatic defrost circuits that use heat strips and can really run up the amperage use. Newer units can also have computer monitors and Wi-Fi connectivity, at additional amperage cost. For any electrical device you install on your boat, it is always a good idea to check the spec sheet for how much wattage (watts = amps x volts) the unit will use and do some comparison shopping. —Max Parker, Zimmerman Marine

My dad has a production cruising boat with a bridge clearance that’s clearly stated in all the specs available to the public. Despite all that, he recently ended up with a bent Windex [wind indicator] on his masthead after brushing the underside of a bridge, which he thought he had enough room to clear based on the stated bridge clearance at the waterline and his knowledge of his own clearance requirements. How is that possible? Better still, how can he be assured it doesn’t happen again? Any best practices? —Sean MacPherson, Savannah, GA

Production boats are not made on an assembly line like cars, with mass-produced parts of exact dimensions. Countless variations can occur from boat to boat within the same model. Also, if your dad is not the original owner, he may not know what modifications a previous owner may have made, So, your dad should measure and record the height of his specific boat, and not accept a manufacturer’s specifications for that model.

Next, encourage him to be conservative when estimating the vertical clearance of bridges. It’s indicated on tide boards at the base of the bridge; on drawbridges, the clearance at “low steel” is given (if the bridge has an arched shape, the height is for the lowest point near the edge, with additional clearance in the center). Markings on a bridge should be in half-foot increments, but they are frequently obscured by water stains or marine growth, so it wouldn’t be difficult to misjudge the exact clearance by a few inches, resulting in a bent Windex or worse.

It is also important to know how much the weight of the fuel, water and waste affects the boat’s displacement. One of the hydrostatic data points a naval architect uses when designing a boat is pounds per inch of immersion. PPI is the weight required to sink the boat an inch. As an example, the PPI on our 54-foot trawler is 3,160 pounds. On some trawlers with tankage potentially in the thousands of gallons, the difference between full and empty can be several inches. If the height measurement is taken when the boat is fully loaded and your dad attempts to clear a bridge lightly loaded, this could also make a difference. (PPI can be less of an issue on sailboats with less tankage, and because PPI increases the lower the boat settles in the water due to the shape of the hull.)

If there is any doubt about a bridge clearance, wait for an opening on a drawbridge or for lower tide on a fixed bridge. There are occasions when it’s OK to take a little off the top, but I hope in the future, your dad is able to limit that to the barber chair. —Bob Arrington