Boats & Porpoise? 6 Answers (For Beginners!)

If you’ve been sailing for quite some time now, it is without a doubt that you’ve experienced your boat bobbing up and down with the waves while you have no control over it.

What you’ve experienced is called porpoising, and it’s a pretty concerning event.

Mainly because it doesn’t just lead to seasickness and discomfort, but can have significant consequences.

Porpoising is a common phenomenon, and an awful one because it often ends up with injuries to the people on board and damage to cargo.

In this article, we answer some burning questions about this situation and give you insights as to how you can go about solving this problem in your own boat.

What Exactly is Porpoising?

Porpoising is essentially a rhythmic bobbing of your boat, almost as if it is bouncing up and down against the waves.

To an observer on shore, it appears as though you are diving in and out of the water to crest repeatedly, as playful sea creatures are known to do.

To the unfortunate passengers, it’s turbulence meets whiplash on the high seas.

At times, this motion could be a temporary action caused by errant winds.

However, more often than not, you can notice this bobbing motion progressively getting worse.

It can eventually reach a point where your boat truly is leaping aggressively, almost flying over the surface of the water and barely making contact.

The Dangers that Come With Porpoising:

Riding a boat while it is porpoising is an incredibly dangerous hazard.

Not only does it make your ride tense and uncomfortable, but it can also lead to you losing control of your boat.

It can also damage the boat’s payload, as well as putting everyone on board at risk of getting injured due to the hostile manner in which the boat is tossed about in the waters.

Porpoising can also damage the hull.

This is because the downward joggling motion of the ship adds more stress to the bow, making it more susceptible to cracking or even breaking apart.

Even more terrifying, it can also cause your boat to flip over when the nose of the boat hits the water, especially if the intensity of the porpoising motion is exceptionally high.

What Causes Boat Porpoising?

Most of the time, the boat is porpoising because its center of mass is too far out in the front.

That makes the forward tip fall lower than it should, causing it to porpoise.

There are many aspects that can trigger this happening:

This can range from the motor’s position to the weight distribution of everything and everyone on board, to boat speed and wind conditions — multiple interplaying factors may contribute to it.

This unstable bobbing is usually caused by the outdrive, which should be just below the main motor.

If it is not at the right place, the outdrive could push down the boat’s back and force the front upwards.

This push lasts for a short while before sending the boat back to normal position, then restarting the cycle of pushing and resting, making a regular bobbing motion that sends the front flying up and down.

Unless you slow it down, the thrusting and bobbing will get worse with each cycle.

How Do you Fix a Boat Porpoising?

Before you go off buying new stuff to fix the porpoising, you need to check first if there are things around your boat that you could move around.

This will save you time and money and greatly helps you out if the porpoising happens in the middle of the sea.

1. Monitor Boat Organization & Weight:

  1. You could start by checking on the locations of dead weights.
    • It could help to move heavy stuff like fuel and cargo to the back of the boat and leave the front out for lighter objects.
  2. You can also try experimenting with other setups.
    • The point here is to evenly distribute the weight over the boat so the front won’t fall lower.
    • Rearrange cargo, seating, or even luggage on board to achieve this.

Sometimes, the design of the boat itself causes it to porpoise.

2. Choose a Good Hull Design & Deadrise:

A concept called “deadrise” shows you the angle of the boat’s bottom compared to the horizontal plane.

This affects your boat’s overall riding performance.

A good deadrise angle gives you a smooth ride, but a bad one will make you porpoise, among other things.

The next time you choose a hull design, you should also keep a few things in mind.

  1. Different designs are suitable for different water conditions.
    • Going fast in rough water, for example, needs deep deadrise angles to get a smooth ride.
  2. Trim: If deadrise is the angle of the bottom, trim is the angle of the sides.
    • Manufacturers design the trim so the boat can move above the water smoothly.
    • But a damaged trim might cause it to bob up and down because the water pushes on it at awkward angles.
    • You should slow down when this happens and have it fixed when you can. Moving faster will make the bobbing action worse.

3. Check for A Dirty Hull:

Finally, you might have a dirty hull.

Because your boat is always submerged in water, it can collect small microorganisms under it, making tiny bumps that can be too small to see.

This presents a problem because the boat needs a relatively smooth hull to move around the water comfortably.

The small bumps cause drag and turbulent flow under the hull, pulling the front down with the hooking action they make.

This is why you should always keep your boat clean, mostly it’s bottom.

Will a Jack Plate Help with Porpoising?

Typically, you will want to use a jack plate to get you through shallow waters.

These jack plates are handy things: they are hydraulic jacks that allow you to move your motor up and down so you could control how close the propellers are to the bottom.

Likewise, you could also use this in deep water when your boat porpoises.

By adjusting the propeller’s distance from the transom, you are also affecting the boat’s center of mass.

Lowering it may put that center a little bit toward the back, lifting the hull and helping to avoid porpoising.

Lifting it too high, however, can push the center of mass forward and lower the boat’s hull.

What Are the Best Trim Tabs for Boats?

The best trim tabs are those that help adjust your boat’s balance fast enough to keep the boat from going overboard.

Their main function is to roll your boat to port and starboard, but a good pair can also help with porpoising by affecting how the water flows at the back of the boat.

Trim tabs do this by pushing on the flow of water at the back at such an angle that it makes the front hull turn up, and the back goes down.

If you want to add trim tabs now, you must take a look at your boat’s make first:

Automatic Trim Tab:

For smaller boats like dinghies and motorized kayaks, you will want to use an automatic trim tab like the Bennett AC3000.

Automatic trim tabs are best in boats where there is no need to manually adjust the boat’s angle, or when there is no space for a hydraulic pump.

Electric Trim Tab:

However, if your boat is big enough to have its passengers move around and change seats during the ride, you might want an electric trim tab.

Lenco Marine is a good brand for electric trim tabs, which allow for quick adjustment because of their electric hoses.

Hydraulic Trim Tabs:

Meanwhile, bigger boats might need hydraulic-hosed trim tabs instead.

These are stronger and let you move your trim tabs even if one side of the boat becomes very heavy.

Bennett’s Classic Hydraulic Trim Tabs are best in this case.

No matter what kind of system you use, the trim tabs themselves should always be as big as your transom allows.

Any less will reduce the effectiveness of your trim tabs.

Will a Hydrofoil Stop Porpoising?

A hydrofoil literally lifts most of the boat out of the water.

This reduces drag on the front, which partly causes porpoising.

Any porpoising that occurs in a hydrofoil boat is usually caused by the propellers being too low.

A jack plate installed with hydrofoil is an almost perfect – albeit expensive – way of taking away porpoising altogether.

Final Thoughts:

Porpoising doesn’t really have to be part and parcel of your sailing experience.

It can easily be avoided for the most part, by simply following a few precautionary measures while sailing.

Most importantly, make sure to use the correct equipment for your boat.

Hydrofoils and jack plates will ensure that your boat never porpoises, but this approach towards the problem can be quite expensive.

A cost-effective method of avoiding this problem is merely maintaining your boat well.

Cleaning out your hull and distributing weight evenly is the best way to go in order to ensure a smooth sail, every single time.

Resources:

How To Prevent Your Boat From Porpoising – PerfProTech.com

The Hull Truth Boating Forum – HYPERLINK

Understanding Porpoising – Boat Design Net

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