Need a Boat to Pass Under a Bridge? There are several things to consider when going under a bridge with your boat, some of which are draft and bridge clearance.

Draft is a term found in the specification sheet of most boats and is a measurement that can be very important when buying your boat. A boat’s draft is the distance measured between the top of the boat’s highest point and the waterline.

Vertical clearance is the distance above the draft that allows the boat to pass safely under a bridge. Failure to consider the draft can result in a failure to calculate the vertical clearance of a vessel during a crossing and can be disastrous.

The span of a bridge can be shown on maps as measured from the water surface to the underside of the bridge at Mean Highest High Water, the most restrictive span. Mean Highest High Water (MHHW) is the average height of the highest tide recorded at a tide station each day during the designated recording period. It is used to measure the navigational clearance or draft under bridges so that ships can pass safely.

For our MAKAI M37 Power Catamaran, the measured draft from the waterline to the top of the roof is 3130 mm, or 10 ft 3 in. The loaded waterline is the point on the hull where the water reaches the boat loaded with fuel, water and other small items such as safety equipment.

When passing under a bridge, there are various things that you will need to take into consideration, one being the air draft.