Porpoising is the rhythmic bouncing of a boat’s bow up and down while on plane. It usually means the bow is riding too high: the motor is trimmed out too far, too much weight is at the back, the motor is mounted too high or the propeller is not holding the stern up. Trimming the motor down slightly is the quickest fix; weight distribution, motor height, prop choice and trim tabs or hydrofoils solve persistent cases.
Common causes of porpoising
| Cause | Fix |
|---|---|
| Motor trimmed out too far | Trim down (in) until the bounce stops |
| Weight concentrated at the stern | Move passengers, gear and fuel forward |
| Motor mounted too high | Lower the motor a hole; check for ventilation |
| Prop with too little lift or bite | A four-blade or higher-rake prop often helps; see 3 blade vs 4 blade prop |
| Hull design and speed | Trim tabs or a hydrofoil on the motor’s anti-ventilation plate |
Porpoising at high speed or when trimming up
Many boats run fastest with some trim out, but past a point the bow lifts and starts bouncing. Find the trim position just below where porpoising starts; that is usually close to the most efficient setting. If the boat porpoises even trimmed fully down, look at weight, motor height and the prop.
Motor height
A motor mounted too high can ventilate (lose grip on the water) and let the bow rise; too low adds drag and spray. Adjust in small steps and test; the anti-ventilation plate is usually set near the bottom of the hull, but the right height depends on the boat and propeller.
Trim tabs and hydrofoils
Trim tabs at the transom push the stern up and the bow down, which controls porpoising and helps the boat get on plane. Bolt-on hydrofoils on the motor’s anti-ventilation plate do a similar job on small boats with outboards.
Prop choice
A prop with more blade area or more rake changes how the stern rides. Four-blade props often hold the stern up better at lower speeds and reduce porpoising, at some cost in top speed; see the propeller sizing guide and stainless vs aluminum props.
Is porpoising dangerous?
Mild porpoising is uncomfortable and wastes fuel; severe porpoising can throw passengers around and make steering unpredictable. Slow down and trim down if it starts. If the boat also struggles to get on plane, see boat won’t get on plane; for propellers, see our boat propellers.
More guides: outboard trim not working · bow mount vs transom mount trolling motors.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Questions? Ask a parts specialist.