Why does my boat porpoise at speed, and how do I stop it?

Porpoising is a rhythmic bouncing of the bow caused by the boat struggling to maintain a steady planing attitude, often due to incorrect engine trim or too much weight aft. You can stop it immediately by trimming your outboard or sterndrive down (in towards the transom) to force the bow down, or by moving heavy gear and passengers forward.

Mechanics of a bouncing bow

When a boat planes, it balances on a hydrodynamic fulcrum located near the aft third of the hull. If your engine is trimmed out too far, the propeller thrust pushes the bow up. Eventually, the hull lifts so high that it loses hydrodynamic lift. Gravity then slams the bow back down into the water. The moment the hull re-enters, lift is generated again and the bow shoots back up. This creates a continuous bouncing cycle that ruins fuel economy and makes steering unpredictable. Stay seated and keep a firm grip on the helm when porpoising occurs. Violent bouncing can throw crew overboard.

Adjusting engine trim and height

The fastest cure is to trim the engine down into negative trim. As you accelerate onto a plane, the engine should be tucked all the way in. Once planing, tap the trim switch up in short bursts until the steering feels light and the revs rise. If the bow starts bouncing, you have trimmed out too far. Tap the trim back down for a second or two until the hull settles.

If the boat still porpoises with the engine trimmed fully down, check your engine mounting height. Many boatbuilders and dealers mount outboards too low on the transom. A low drive creates excessive drag below the hull. This drag acts as a lever that forces the bow up. Look at the anti-ventilation plate (the large horizontal fin directly above the propeller) when running on a plane. It should be skimming the surface of the water. If it sits buried beneath the surface, raise the engine by one or two mounting holes. Each hole typically lifts the engine 19mm.

Propeller rake and design

Propeller geometry dictates hull attitude. Propellers with a high degree of rake (where the blades sweep backwards heavily) provide maximum bow lift. Models like the Mercury Tempest Plus or Yamaha Reliance suit heavy offshore boats. On a light 6-metre sportsboat, they provide too much lift and trigger porpoising. Switching to a propeller with less rake or a four-blade design provides more stern lift. A four-blade prop like a Mercury SpitFire or Solas Amita 4 will pin the transom up and the bow down. This often cures a porpoise entirely at the cost of two or three knots of top speed.