Stop Boat Porpoising: Tips And Tricks For Smooth Sailing

Porpoising is a common issue for boat enthusiasts to experience on calm waters, where the boat bounces up and down in a cyclical motion, similar to the way a porpoise swims. This can range from mildly uncomfortable to dangerous, with the potential for loss of control, structural hull damage, payload damage, and passenger injury.

There are several reasons why a boat may porpoise, including disproportionate weight distribution between the bow and stern, the trim angle of the engine, and the mounting height of the engine. To fix this issue, adjustments can be made to the weight distribution, engine trim settings, and motor height.

One solution to reduce porpoising is to lower the trim angle using the boat's controls. While this does not address the underlying issue, it can provide a quick fix for a more comfortable ride. Another solution is to adjust the distribution of static weight in the boat by moving heavy components such as the battery and fuel. Additionally, changes to the motor height, engine setback, and propeller selection can also help mitigate porpoising.

In some cases, adding an outboard motor hydrofoil or a hydrofoil with a jack plate can be an effective solution to reduce porpoising.

Adjust the weight distribution in your boat

Adjusting the weight distribution in your boat is crucial to prevent porpoising and ensure a smooth ride. Porpoising occurs when the weight in the boat is disproportionately distributed between the bow and the stern. Even a slight change in weight distribution can trigger porpoising, so it's important to carefully distribute the weight of passengers, gear, payload, battery, and fuel.

If the weight is concentrated at the back of the boat, the bow will be light, causing the stern to sink and the bow to lift out of the water. In this case, you can adjust the weight distribution by having more people move to the front of the boat. Conversely, if the bow is heavy, you can ask some passengers to move towards the middle or back of the boat.

The weight distribution of static weight components, such as the battery and fuel, can also impact porpoising. Moving these heavy components towards the back of the boat can help reduce the tendency to porpoise. Additionally, adjusting the dynamic forces, such as engine setback, propeller selection, and motor height, can further minimize porpoising.

It's important to note that weight distribution is not the only factor contributing to porpoising. The trim angle, motor height, and hull design also play a significant role. Therefore, a comprehensive approach, including minor adjustments to multiple factors, is often necessary to effectively prevent porpoising and ensure a safe and comfortable ride.

Lower the trim angle

The trim angle of a boat is the running angle of the boat as it moves through the water. It can be adjusted using a toggle button on the control lever or the dash. Lowering the trim angle will bring the propeller closer to the boat and tilt the bow downwards.

To lower the trim angle, follow these steps:

• While the engine is in neutral, use the trim switch to tilt the drive down as far as it will go. Monitor the engine trim gauge or listen for the change in sound from the tilt motor, which signals that the engine is trimmed fully down.
• Place the motor into forward gear and apply throttle until you reach cruising speed.
• Tilt the engine up with the trim switch. The altitude of the boat on the water should change, causing a slight rise in the bow. You may also notice a complete lift of the boat or an increase in speed.
• Continue tilting the engine up until you notice a slight drop in speed, an increase in engine RPMs, or the propeller starts to ventilate.
• If the boat begins to porpoise as a result of these changes, slowly tilt the engine back down until the consistent RPM and maximum speed are reached again, and there is no ventilation.

By lowering the trim angle, you can reduce or eliminate porpoising. However, it is important to note that lowering the trim too much will result in a loss of speed. Therefore, you should only lower it slightly until you find a balance where the boat no longer porpoises but still maintains its speed.

Optimise the propeller selection

Propeller selection is a critical factor in optimising the performance of your boat and minimising the porpoising effect. Here are some tips to help you make the right propeller choices:

• Consider the boat's engine and horsepower: The propeller you choose must match your boat's engine and horsepower. Selecting a propeller with the right amount of "lift" is crucial. For example, choosing a propeller with more aft-lift will change the dynamic balance of the hull, reducing the trim angle and the speed range of the hump zone, which should eliminate the porpoising effect.
• Examine the propeller's diameter and pitch: The diameter and pitch of the propeller influence its performance. A propeller with a larger diameter and higher pitch will provide more lift, which can help reduce porpoising.
• Match the propeller to the boat's intended use: Consider the type of activities you plan to use your boat for. For example, if you participate in watersports that require high speeds, you'll need a propeller that performs well at higher RPMs. On the other hand, if you prioritise fuel efficiency for cruising, a propeller with a lower pitch may be more suitable.
• Take the boat's hull design into account: The shape and design of your boat's hull will impact the propeller's performance. For instance, a boat with a flatter bottom may require a different propeller than one with a steep rise in its hull to minimise porpoising.
• Experiment with different propellers: Don't be afraid to try out different propellers to see which one works best for your boat. Minor adjustments in propeller design can have a significant impact on the boat's handling and performance, so it's worth testing a few options to find the optimal match.
• Seek expert advice: Consult with marine engineers or experienced boaters who can provide guidance on selecting the right propeller for your specific boat. They can offer insights based on their knowledge and experience to help you make an informed decision.

Raise the motor

Raising the motor is one of the most common solutions to fixing a boat's porpoising problem. This issue is often caused by a low-mounted motor, which creates unnecessary drag and an incorrect angle of thrust.

To check if your motor is mounted at the appropriate height, observe the anti-cavitation plate (also known as the anti-ventilation plate) when your boat is on plane and at operating speed. This plate is flat and found on the lower unit of most outboards. If it is below the waterline, it will pull the back of the boat down, causing the bow to ride high and the boat to porpoise. The plate should be skimming the surface of the water when the boat is at operating speed.

If you notice that the motor is set too deep, you will need to raise it. This can be done by raising the motor one set of holes at a time and testing the boat after each adjustment to see if it reduces porpoising.

When you get the engine height to the optimum level, you will not only stop the boat from porpoising, but you may also see improvements in RPM, speed, and fuel economy.

In addition to raising the motor, you can also try adjusting the trim. Trimming down will usually minimise porpoising in choppy waters, while trimming up will maximise speed in flat water.

It is important to note that there is no one-size-fits-all solution to porpoising. The best fix will depend on the specific design, size, and weight of your boat.

Add trim tabs to the boat

If you're experiencing porpoising, it's important to first identify the root cause. Porpoising can be caused by a variety of factors, including weight distribution, engine trim settings, motor height, and propeller selection. Once you've identified the cause, you can then explore solutions such as adding trim tabs.

Trim tabs are adjustable plates that extend from the transom of a boat and can be used to adjust the running angle of the vessel. They are an effective way to correct listing (roll) and improve visibility and efficiency. When it comes to porpoising, trim tabs can help to tame a bobbing bow without the need to slow down or make significant adjustments to the engine or drive trim.

• Trim tabs cannot bring the bow of the boat up, so ensure you adjust your outboard or sterndrive trim accordingly.
• Fully deploy the trim tabs downward to help the boat get on plane and reduce bow rise.
• Adjust the trim tabs to keep the hull level across the beam once the boat is up on the step.
• Trim tabs are most effective when the boat is already on plane and at its optimum cruising speed.
• Use trim tabs in conjunction with power trim for maximum performance. Adjust the trim tabs to achieve a planing attitude, then use the power trim to position the prop path parallel to the water flow.
• When operating in a head sea, trim the bow down to improve comfort and handling.
• In a following sea or when running an inlet, keep the trim tabs fully retracted for better maneuverability and steering control.
• When operating with side winds, use the trim tabs to raise the windward side of the boat and block incoming spray.
• When operating in reverse, keep the trim tabs fully raised to avoid drag and strain on the tabs.

When installing trim tabs, it's important to select the appropriate size and type for your boat. Here are some guidelines:

• Choose trim tabs with the widest span that will fit on your transom, as this will provide the most effective lift.
• For boats with maximum speeds of 40 MPH, select trim tabs with a span of at least one inch per side for every foot of boat length.
• If your boat has twin outboards or I/Os, or limited transom space, consider using 12" chord tabs to achieve maximum lift.
• Slower boats, semi-displacement hulls, boats over 50 feet, and boats with outboard brackets may benefit from the increased surface area of 12" chord tabs.
• Bennett Marine offers a range of trim tab systems, including spring-loaded, electric, and hydraulic options, as well as custom solutions for unique applications.

By adding trim tabs and adjusting them appropriately, you can significantly improve the performance and handling of your boat, reducing porpoising and providing a more comfortable ride for you and your passengers.

Frequently asked questions