If you have raced sailing dinghies you know that bow down trim is fast in light air (going slow) and stern down (within reason) is the mode for breezy conditions. The same rules apply to larger sailboats and displacement power boats.

The Norwegian trawler in the photo above has quite a broad stern, so it will benefit more from this approach than would be the case with a fine rear end.

On our last sailboat, a 78 foot (24m) ketch we had three tons of water ballast in hull side tanks. Close reaching and beating we’d fill the forward tanks first, then as the wind built and we wanted more stability, the aft tanks would be brought into play.

Broad reaching the opposite was true. We’d start filling aft first, then move to forward tanks.

Now that we have switched to power the same rules are being used for trim. On Wind Horse we bias to forward trim in all modes except when running in a breeze. Right now, for example, we are almost full of fuel (having topped up in Reine, Norway). As we are not covering many miles, we are carrying more fresh water than usual, as much as 1000 gallons (3750 liters), so we can take really deep baths and long showers. But when we are covering long distances we do not like to drag the extra weight of all that water. So at present we are really heavy.

This fresh water is in the forward tank. As such, we have a slight bow down, stern up trim. This gains us enough efficiency that we can carry the extra weight without paying a significant drag penalty.

When we are crossing oceans, if we are not running in a good breeze (and surfing) then we tend to burn the fuel in our aft tanks first.

In heavy weather some designs will benefit from stern down trim. Stern down gives better prop cover so there is less tendency to cavitate. The rudder is more deeply immersed and will ventilate less at high angles of attack. And most sailboat hulls will generate more form stability when trimmed by the stern.