We have all found ourselves in a situation where we just wanted to enjoy some speed on the water only to find that our boat merely bounced up and down like a jack-in-the-box. What causes a boat to porpoise like this when you increase your boat speed? Here we outline some key causes for a bouncing boat and show you how to stop a boat porpoising once and for all.
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What is porpoising on a boat?
Boat porpoising refers to the phenomenon of a boat bouncing on the water.
Boat porpoising usually occurs when under power while trying to build up speed.
Instead of the boat riding the waves, or sitting on top of calm water (known as planing), the boat bounces off the water into the air and comes down with force to bounce back up again.
Boat porpoising under power (video)
The cause of boat porpoising lies with the motor
What causes a boat to porpoise?
The cause of a porpoising boat lies with the outboard motor – both the motor type and size and also how it is used.
Porpoising occurs when the motor’s drive pushes the bow of the boat upwards and out of the water.
The thrust from the motor is actually providing a boost that pushes the boat up and out of the water.
However, unlike planning when the boat is pushed out of the water and lands on top of it for less water resistance and smoother movement forward, porpoising bounces the boat up and down on the water.
This porpoising effect (named after the way porpoise move in the water) occurs when the “push” from the outboard motor forces the boat up and out of the water for only a short period of time before it falls back down into the water – usually with an uncomfortable splash.
Once the boat is back down on the water the motor once again pushes it back up and out of the water.
If the motor speed and trim is maintained this lifting and falling sequence becomes a recurring cycle that is called porpoising.
So how do you stop this from happening?
Well the first thing to do is to identify why the motor is causing the porpoising effect in the first place and then you can apply the simple fix to stop it happening.
Let’s do that now.
How to stop a boat from porpoising. 4 Solutions
If you have a boat that continually porpoises then you most likely have an incorrect motor mounted on the boat.
To stop the boat from porpoising be sure you have the correct motor.
This is especially true for Jon boats and other flat bottom vessels (see our Jon boat motor guide).
A motor that is too big and heavy for a boat will way down the stern and cause porpoising under power.
Similarly, a motor that under-powers the boat will cause porpoising when trying to get the boat on plane.
An uneven load can also cause boat porpoising. So, ensure the load is distributed properly on the boat or try moving gear and passengers forward.
Other fixes include raising the motor, adjusting the trim correctly and adding trim tabs to the boat.
You may find that several of these fixes may be required to stop your Jon boat from porpoising so don’t be afraid to try a few different approaches.
Having said that, you will probably be able to identify the specific cause of the porpoising on your Jon boat as you move through these fixes though.
1. Correct bad weight distribution
A very common cause of porpoising on a Jon boat is bad weight distribution.
If your Jon boat keeps porpoising when you increase your speed then your boat may be too heavy at the aft.
This can have several causes.
A Jon boat that is too aft heavy may be due to having the wrong sized outboard motor fitted to the boat.
An outboard motor that is too big and heavy for your boat size can really way down the stern causing the bow to rise out of the water.
Other causes for an aft heavy Jon boat may be due to poor distribution of load.
The boat may simply be too heavy at the rear because the load distribution is uneven.
You may need to redistribute weight by ensuring you move gear and passengers forward.
If you experience porpoising when the boat is empty, and you are sure you have the correct sized outboard motor fitted, then you may want to consider moving the battery to a more forward position.
2. Adjust the trim correctly when increasing speed
Another common cause of porpoising is poorly adjusted motor trim.
You may be employing the wrong technique when you trim your motor.
To stop porpoising when it occurs you should gradually lower the trim until the boat is no longer bouncing on the water.
Trimming it too low though will result in other problems.
As you probably already know, lowering the trim too much will result in a loss of speed.
So, if you have to lower the trim so much that the speed of the boat reduces significantly then this approach alone is not enough to fix your porpoising problem.
This is because your trim technique is probably not to blame.
However, lowering your trim in conjunction with a few of the other fixes may be all you need to stop your Jon boat from continually lifting out of and then lowering in the water.
So be sure to try it along with other fixes.
3. Ensure you have the motor positioned correctly
You will also find that raising the motor will help.
When you raise the motor on your Jon boat you move the thrust line higher which will, in turn, force the bow of the boat down into the water.
Try both raising the motor and trimming in to see if this helps eliminate the porpoising.
Be careful though with your position of the motor.
You should not raise the motor too much as this may cause the prop to come out of the water when trimming up.
This results in propeller ventilation where the propeller starts sucking in air.
This will have a major negative impact on the performance of your boat.
4. Add trim tabs to the boat
Adding trim tabs to a Jon boat is the last option you want to explore. Only if everything else has failed should you consider adding trims.
A Jon boat should not need trim tabs to stop porpoising nor should it need them to get on plane.
If your are experiencing porpoising on a Jon boat then the odds are that one of the above solutions will sort your problem out quickly.
However, all boat types benefit from trim tabs and for the same reasons.
Trim tabs eliminate porpoising and help a boat to get on plane quickly.
Trim tabs will also correct for uneven weight distribution.
However, you should not rely on trim tabs to fix your poor positioning of gear and passengers on your boat nor should they be relied on to counteract an overly heavy outboard motor.
Strive to ensure your Jon boat has the correct setup before resorting to attaching trim tabs.
Porpoising when trying to get a flat bottom boat on plane
If you only experience porpoising when you are trying to get your Jon boat on plane there could be a few different causes.
Your boat could be too heavy and your load may exceed your Jon boat’s weight capacity, or the boat may actually be too heavy at the bow.
Another common cause of porpoising when trying to get a Jon boat on plane is lack of power; you may not have a powerful enough outboard motor to successfully execute the maneuver.
For any readers that are experiencing porpoising mostly while trying to plane a Jon boat read the article how to get a Jon boat on plane which explains how to fix this problem.