Understanding Porpoising In Aluminum Boats

If your aluminum boat is porpoising, it can be a scary and uncomfortable experience. Porpoising is when a boat bounces up and down like a porpoise, and it can happen when there is disproportionate weight distribution in the boat, or issues with the trim angle and mounting height of the engine.

To prevent porpoising, you can try slowing down the boat, as this type of bouncing often begins when the motor picks up speed. You can also try trimming the motor down, moving passengers or cargo towards the bow, mounting trim tabs on the stern, or adding a wing or hydrofoil to the anti-cavitation plate. Additionally, ensuring that the weight in your boat is evenly distributed and that the mounting height of the motor is correct can help alleviate this issue.

Weight distribution

To test if weight distribution is the issue, add some temporary weight to the bow and do a test run. If the boat still porpoises, the problem is likely to be the engine height.

The weight distribution of people and gear on the boat is vital. If everyone is standing at the back of the boat, the bow will be extremely light and the stern will be heavy, causing the back of the boat to sink into the water and the bow to stick up in the air. Even the smallest change can cause a boat to porpoise.

To prevent porpoising, the weight of the boat should be evenly distributed so that the front does not fall lower. Heavy items like fuel and cargo should be moved to the back of the boat, and lighter objects should be placed towards the front.

Engine height

The anti-cavitation plate (also known as the anti-ventilation plate) is the key indicator of whether your engine height is correct. This plate should be flush with the bottom of the hull. If it is buried or below the waterline, it will pull the back of the boat down, causing the bow to ride high and resulting in porpoising.

To check if your engine height is correct, get your boat up to a good cruise speed and observe the anti-cavitation plate. If it is not visible, or is below the waterline, the engine is set too deep and needs to be raised.

It is recommended to raise the engine one set of holes at a time and test the boat after each adjustment to see if porpoising is reduced. This will help you find the optimum engine height, which will not only stop porpoising but also improve RPM, speed, and fuel economy.

In addition to engine height, weight distribution and trim angle also play a crucial role in preventing porpoising. Even small changes in weight distribution can trigger porpoising, so it is important to distribute weight evenly and adjust the trim angle as needed to maintain a stable ride.

Motor trim

If the trim angle is too far down, the stern of the boat will be forced up, and the bow will drop. This is known as plowing and leads to bow steering, where the vee of the bow digs into the water, making the boat difficult to control.

On the other hand, if the thrust angle is too far up, the stern will squat, and the bow will rise, leading to poor forward visibility and hull pounding. Both extremes can also lead to swamping.

To prevent porpoising, the boat should be trimmed properly. This can be done by following these steps:

• Utilise the trim switch to tilt the drive down completely while the engine is in neutral. Monitor the trim gauge or listen for the tilt motor's sound to signal that the engine is trimmed all the way down.
• Place the motor into forward gear and apply throttle until you reach cruising speed.
• Tilt the engine up with the trim switch. The boat's altitude on the water should change, causing a slight rise in the bow. You may also notice an increase in speed or the boat lifting completely.
• Continue lifting the engine until there is a slight drop in speed, the propeller starts to ventilate, or there is a quick rise in engine RPMs. Be careful not to trim the boat to the point of porpoising.
• Slowly tilt the engine down until a consistent RPM and maximum speed are reached again, with no ventilation noted. At this point, the boat is properly trimmed and is operating at maximum efficiency in terms of fuel economy.

Minor adjustments can be made after the boat is properly trimmed to achieve a more comfortable ride. For example, in choppy waters, trimming the bow down can reduce wave pounding, but it may also result in more spray.

It is recommended to experiment with different trim functions to find the optimal setting for your boat.

Hydrofoil

A hydrofoil is a skeg-mounted device that can be added to the outboard of a boat. It can help to solve the problem of porpoising, which is when a boat bounces up and down due to weight distribution issues, improper trim angles, and incorrect motor height.

While hydrofoils can be an effective solution, they are not always necessary to stop porpoising. If a hydrofoil does solve the problem, it is likely counterbalancing another issue, such as a motor that is mounted too low.

Some boat owners have reported success in reducing porpoising by clamping hydrofoils onto the cavitation plate without drilling, which allows for easy removal if they are ineffective.

Jack plate

A jack plate is a mounting bracket for an outboard engine that allows for setback and vertical movement of the engine along the transom. Jack plates are used for a variety of reasons and offer several advantages over boats without one. Jack plates can:

• Provide enhanced fuel economy
• Allow for safe navigation in shallow water
• Decrease drag, thereby increasing speed
• Provide adjustments necessary for transoms that were rebuilt (i.e. too low for the current engine)
• Offer better handling
• Allow planing with ease

While jack plates can help with porpoising, there are far better solutions to this problem. Porpoising is caused by weight distribution, trim angle, and mounting height. To prevent porpoising, you can:

• Adjust the weight distribution in your boat
• Adjust the trim angle of the engine
• Adjust the mounting height of the engine

Frequently asked questions