What Do Sponsons Do on a Jet Ski?

The word sponson has a variety of meanings depending mostly on the object to which the word is being attached, and if that sounds like Greek to you, you’re in good company.

The word doesn’t even appear in the Oxford Dictionary & Thesaurus (2011 edition). 

So, what does a sponson do on a jet ski? Sponsons aid in the balance and flotation of a PWC. They extend out from the sides of a jet ski (PWC) to provide stability for the vessel. Sponsons help the driver of the PWC with handling and cornering. At high speeds, sponsons become vital safety features that help keep riders safe.

You may be puzzled to learn that the word derives from a variation of “expansion” and that a sponson is sometimes described as a “buoyant appendage or flotation collar that boaters often attach to the upper edge of the gunwale of a personal watercraft to aid in flotation, prevent capsizing, and provide collision protection” (source).

But this doesn’t nearly describe a jet ski sponson.

So, what are sponsons and what do they do on a jet ski?

Despite the impeccable source of the Google Patents definition above, this is just one of the many applications of the word sponson.

Consequently, this particular definition does not describe what a jet ski sponson is designed to do.

These “projections” on a jet ski are externally mounted blades or wings made of molded plastic or metal, bolted to the underside hull of the craft, and varying in length and shape from less than a foot to three feet and more.

Sea-Doo RXT sponsons are not adjustable
Sea-Doo RXT factory sponsons are not adjustable

Most are factory-fitted these days, but they can also be bought in the aftermarket and self-fitted quite easily with the hardware, silicon, and instructions provided.

Customizing your self-fitted sponsons has the advantage that you can adjust their placement to your individual riding style and water conditions. Why would you want to do this?

Sponsons play a vital part in the handling and cornering of your Jet Ski, especially at speed, so much so that the sponson on a jet ski has become a highly desirable—nay, necessary—appendage in competitive jet ski racing, contributing to both speed and handling performance. 

No self-respecting jet ski pilot rider be seen in the car park without his or her customized sponsons attached to their craft.

You’ll probably only notice them when they are being hauled because sponsons on a Jet Ski in the water aren’t usually visible below the waterline.

Without them, the competitive jet ski rider would almost certainly lack a competitive edge.

What do sponsons do on a PWC?

Like most sports, jet ski riding is full of jargon. PWC, standing for personal watercraft, is the proper name for this type of craft which has become known by a number of different names: Jet Ski, Waverunner, Sea Doos, water scooters, and so on.

The first three titles are all brand names popularized by Kawasaki, Yamaha and Bombardier respectively.

Then there is a variety of jargon boating terms such as porpoising and trim, chine walking and oscillation, transom and gunwale, strake and venturi.

But before we get side-tracked, let’s get back to our primary point of interest: jet ski sponsons.

Jet ski designers started experimenting with sponsons as an aftermarket bolt-on in the late 80s and early 1990s.

This was as a result of attempts to improve handling by generating more lift at the rear of the craft, thus holding the bow down and reducing the tendency to porpoise. 

The Sea-Doo RXP-X has standard 3 position adjustable sponsons

A common problem with both jet skis and more conventional speed boats, porpoising is the tendency of watercraft to bounce up and down in the water if the craft is not properly in trim with the speed at which the boat is traveling.  

The introduction of sponsons not only added lift to the back of the craft, but it also had the added advantage of providing much-improved traction when cornering at speed.

Similar to a skeg on a surfboard, the sponson allows the boat to grip the water when cornering, reducing the tendency to slide around corners.

Does Sponson Design Matter?

Variations of a type of sponson on PWCs have been around for over twenty years.

Kawasaki first started experimenting with a kind of outboard mounted tab many years ago, and aftermarket bolt-ons have proliferated into a big business. 

They come in two (perhaps three) basic designs: symmetrical and parabolic.

The old “block” sponsons are now largely a thing of the past. Both designs serve the basic purpose of providing more lift, thus reducing the surface area in touch with water, controlling porpoising and usually improving speed and lateral stability.

A pair of sponsons can set you back anything from about $18 to over $400. WORX sells this mid-priced universal kit on Amazon that fits many skis.

The parabolic design is a turned down or hooked wing which channels displaced water backward, producing yet more lift and creating a skeg effect which adds to grip (source). 

There has been an awful lot of debate about exactly where the sponsons are best located on the hull, both in terms of distance from the transom (the flat surface forming the stern or rear of the craft), and placement in relation to the keel—not to mention the size and shape of the sponson itself, all of which impact upon the effect it has on the performance and handling of the craft.

If this doesn’t create enough variables, then consider also the weight of the rider, and whether or not they are standing or seated on the Jet Ski.

Little wonder that designers have created sponsons which, in many cases, can be easily adjusted in five minutes on the beach or riverbank, to cater for different surf and water conditions.

What is the main purpose of the sponson?

By now it should be clear that its most important function is that of a stabilizer, and that where the sponson is fixed to the hull will have a marked effect on the quality of the ride, and on the PWC’s stability and handling, especially in turning and cornering.

The extent of this effect will still depend on a range of other factors such as the size and weight of the craft itself, the design of the hull, the lift generated at different speeds and the weight of the rider and passengers.

Like their larger motorboat counterparts, modern jet skis may have a variable trim system (VTS) that facilitates onboard adjustment of the craft’s trim to the water conditions and speed of the boat.

Adjusting the craft’s trim also affects the flow of water over and under the sponsons, and hence their impact on performance.

Optimum conditions are really only achieved with the experience of the rider and the skill which comes with this experience.

Are all sponsons equal?

Clearly they are not. Their design, shape, and placement on the craft will determine to a large extent how they impact the performance of the craft.

It’s a fine balancing act that requires experience, some skill, and a lot of knowledge about how your individual craft is designed to perform—and, of course, what you want to use your jet ski for.

If you’re highly competitive and take part in regular competitions requiring skill and speed, you will want to spend time choosing and experimenting with the range of sponson options that are available in the aftermarket.

Even entry level models from Sea-Doo, Kawasaki and Yamaha can be fitted with aftermarket sponsons (all 3 of those links are to Amazon products for each model line) if they aren’t standard on the model you are interested in.

If you’re simply a recreational jet ski rider who enjoys “messing about on boats” (albeit with a powerful engine under your seat), then fitting sponsons may not even be a consideration.

Whether or not you decide to fit sponsons may also depend on the type of water you’re on. Flatwater riding—as in lakes and some rivers—may not call for sponsons for the recreational rider who is just looking for some fun in the sun.

As you gain experience and skill in handling your craft, however, you will probably feel the need for the added maneuverability and control which sponsons will provide.

This will certainly be the case for the more competitive rider looking to improve on the speed and tight cornering required on a competitive slalom course, for example.

Add-ons like this Riva Racing kit on Amazon can give you a performance edge.

Are there safety issues with fitting sponsons?

It seems that just as motorcyclists tend to upgrade to bigger and faster motorcycles, so do jet ski enthusiasts advance to bigger and more powerful machines.

As mentioned above, fitting sponsons to your jet ski, if anything, should provide added maneuverability and control—provided you have already acquired the skill to handle your craft under most conditions. 

The most threatening safety issue in handling a PWC has nothing to do with whether or not sponsons have been fitted. The real danger is that, without power, a PWC loses steerability.

Since the first reaction of a novice rider is to de-throttle in the face of danger, the craft will continue in the direction it was going—but without steerage.

Anything—or anyone—in its way is going to get run over. See (source).

What of the future?

The effect of water passing rapidly over or under a surface is part of the study of hydrodynamics or fluid dynamics.

Clever folks in laboratories and white coats are studying the way in which sponsons may be used to provide the lift necessary to reduce bow rise and porpoising, to keep water flowing steadily underneath during normal tracking, and to ensure the hull “sucks” down in cornering in order to maintain grip.

The intention is that all of this may be adjusted by the PWC operator “in-flight”, as it were, either mechanically, hydraulically or thermodynamically.

A so-called “modular adjustable sponson system” is already off the drawing board, but it’ll cost you.

Final Thoughts

“On a purely functional level, probably no other single part has played as big a role as sponsons in the quantum leaps in performance we’ve all enjoyed over the past few years,” says Brian Bevins of Beach House Express, one of the leading manufacturers of aftermarket sponsons (source). 

So, if you’re into motorized water sports in any way—and it’s not just for the competitive enthusiasts—take a look at how sponsons can improve your craft’s handling and performance.

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Tim Conner, M.D.

Tim Conner, M.D. started boating in 1974. He has been involved in recreational boating continuously since then. Dr. Conner has been active in boating and watersports safety education for decades. He rode his first jet ski in 1997, and rejoined the personal watercraft arena in 2012 with a Sea-Doo GTX 155, followed by 2 supercharged SeaDoos. Scuba certification came in 1988, and he and the family have traveled the world snorkeling and scuba diving for decades. The family has recently taken up paddle boarding, with wakeboards and kitesurfing the next challenges for my adult kids. Click the photo for a lot more.

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