What causes a boat to list?

 

What causes a boat to list? There was a time years ago when hardly anyone paid attention to their boat listing. Today it’s a common question, and often perceived as a problem with the boat. Keep in mind a boat can experience a list at rest, while running or both, but the list of possible causes is really short:

 

Weight is distributed off center.

Prop torque.

A hull anomaly.

A trim tab issue.

 

Taking these in order, weight distribution is the most common cause and easiest to cure. Move something; whether it be a battery or your big buddy, if it can be moved to level the boat out, why not?

 

Prop torque is the next most common cause and possibly the least understood, but it is only possible in a running list and most prominent in a single engine application. Simply put it’s one of Newton’s laws of motion. For every action there is an opposite and equal reaction. Most props spin to the right, or when viewed from the rear they spin clockwise. The “reaction” is the twisting motion the engine places on the boat’s hull to the left, or in nautical terms it creates a port list. Many years ago most small boats were a side console set up with the operator’s position on the starboard side. This was to allow the captain’s weight to offset the prop torque. They also would typically locate the battery on the starboard side, since the captain will usually have some passengers that offset his weight.

 

There is a trend in the last few years towards more centered console designs, and that orientation eliminates the ability to offset prop torque created list with the operator’s weight. The battery can be located to the starboard side, but with the console centered, and a great location for the battery, that weight is often centered as well. This leaves us with nothing much to counter the prop torque other than stored gear. If you can locate the heavy gear off centered to the starboard side, do so. Or, if the boat is equipped with trim tabs the tabs can be used to level the running attitude if necessary.

 

On the subject of a running list it’s worth noting that engine trim angle and speed can have a surprising effect on the tendency for the boat to list while underway. Hydrodynamics can be confusing at best, but simply put the engine’s thrust angle can force the bow lower or higher in the water. Since the bow is usually the sharper angle of entry to cut through the water, that part of the hull is less stable, and less able to resist the twisting force of prop torque to the port side. Trimming the engine out results in the bow running higher which shifts the lifting force of the water to the stern which is the wider and usually flatter part of the hull which offers more stability, and the running list can be reduced.

 

Speed affects the running list as well. Again it’s hydrodynamics at play, but at higher speeds the water becomes more like a solid in its ability to support the weight of the boat. Therefore as speed increases the effect of the prop torque will often diminish as the lift or support of the water increases. Once all of the forces at work on a hull are better understood it becomes easier to accept that a perfectly balanced boat running level at all times is very rare in real life, and often it comes from an operator who understands what is happening and how to balance the boat with the controls at his disposal. For those acutely bothered by a running list trim tabs and proper use of the tabs are a great asset.

 

A hull anomaly? This could be something as simple as a transducer, or as unusual as a misshaped runnin surface. I’d like to cover the transducer possibility first since it is so common. Transducers for recreational use have historically been relatively small and flat on the bottom, but that has changed dramatically in recent years and some of them are huge, while practically none are flat. A flat transducer can be mounted flush to the bottom of the hull where they have very little effect on the running attitude of the boat, but those with rounded bottoms are often mounted below the running surface to some degree. If they are angled down this creates lift, and the bigger the transducer the more lift it can create. Uneven lift from one side or the other of the boat creates a running list. Unfortunately for these transducers to deliver the side scan images, they must be underwater, and incorrect transducer installation is one of the problems I most often see.  

 

Moving on to a misshaped running surface. This can be distortion caused by the pressure of sitting on a poorly adjusted trailer or lift bunk, or is less common cases, flotation foam that over expands or shrinks. Whatever the cause, if there is distortion in the hull it changes the pressure the water running over it creates. More pressure on one side or the other can create a running list. Some hulls will have a hook or wedge designed into them, so not all irregularities are necessarily a problem.

 

And finally a trim tab issue. Tabs are installed by real people and sometimes human error will result in a tab installation issue causing a running list. A slight difference in angle on the tabs when fully retracted will result in more lift on one side than the other. With everything else balanced this results in a running list. I check them with the tabs fully retracted and look for a difference from one side to the other. That being stated, it is not uncommon to install the port side slightly lower than the starboard to offset the prop torque. It is a legitimate and useful tool to combat the list.

 

I’ll wrap this up with a short commentary. Recreational boats have developed over the years into some very impressive pieces of machinery, and are much more complex than they were years ago. Of course they are much more expensive as a result, and with the increased cost the consumers are more demanding than ever and deservedly so. Water sports are a great way for an active family to enjoy being outdoors, but while water is great and absolutely necessary in our lives, it is still an imperfect means of support. As a result, almost every boat will list to some degree. Sometimes we just need to embrace it.