March 10, 2021

Performance and jet safety

This is the first in a 4-part series looking into technical detail showing why the Seakart is described as a high end, high performance craft. 

Why does the Seakart provide such a different experience on the water compared to a Jet Ski or a rigid inflatable boat?

This week, we are getting inside the hull of The Seakart 335 and explain how the Yahama engine and jet drive system works so well.

Part 1 – Performance and jet safety

ACTIVITIES WITH THE SEAKART®

The high performance Seakart has a top speed of 45mph or 65mph depending on choice of either 110bhp or 18bhp 4-stroke Yamaha jet engine.

Thanks to the advances in hull designs, propulsion systems wake-enhancing technology, there are really no limitations to what you can do with a Seakart. Slow speed manoeuvrability, such as when docking, has been greatly improved over the years as has fuel consumption.

Did you know that the Seakarts Yamaha steering nozzle is also flyboard compatible for extreme watersports experiences?! Cool huh!

The rear platform is a great space to fit water skis, wakeboards and water toys to ski hook.  There is also a trim and tilt control on the steering wheel to add waves at the back of the Seakart for the wakeboard enthusiasts.

Jet drives offer a lot of practical advantages over propeller driven stern drives or outboard motors, mostly related to the fact that they are safer in conditions where impacts with foreign objects are likely. As mentioned above, they’re great in shallow waters, but inboard jets can also be backed up on to beaches given the absence of any protruding parts including rudders.  The jet drives give greater acceleration and with the centre of gravity being in the middle of the boat (instead of being at the back like an outboard engine) the handling is more precise and better balanced.

Jet propulsion not only gives the Seakart its unique performance and agility, it provides a great safety factor for a craft that is so versatile. 

Jet drives are certainly safer than props from an embarkation/disembarkation point of view for water skiers and wakeboarders, in as much as the engine can safely be left running at all times without the risk of anyone being seriously injured by the incased impeller.

Image above: Rear view of The Seakart.  Here you can see the steering nozzle and reverse bucket as well as the splash proof transom and headrest.

UNDERSTANDING JET PROPULSION

Jet pumps tuck all the whirling parts within the cylindrical pump enclosure, safely out of reach. This configuration also means a Seakart can operate in shallower water, and should it run aground, likely won’t suffer serious damage.

But just exactly how does jet propulsion work? Here’s a brief explanation of this critical component of our favourite watercraft.

UNDER PRESSURE

Engines get much more of the spotlight, but it’s the pump that actually translates that engine’s potential into usable power to push your craft forward through the water. As the engine turns over, it rotates, or spins, a driveshaft extending backward from the engine and passing through the hull. At the end of that driveshaft, within the safe confines of the jet pump, is the impeller which, when connected to the driveshaft, spins in unison. Give your craft a blast of throttle and that impeller spins faster; release the throttle to idle and that impeller spins slower. Unlike a gear-driven propeller, however, that impeller is always spinning when the engine is running. It doesn’t stop or reverse direction, it just spins.

The result of all that whirling, at least on the inlet side of the pump, is the creation of negative pressure. It’s this pressure that “pulls” or sucks water into the pump cavity. As the water passes through the swiftly rotating impeller, that pressure becomes positive. The impeller is now “pushing” the water back toward the pump outlet.

Before it gets there, however, it must be straightened. If left to leave the pump in a spiraling motion, it would cause the craft to literally tilt to one side from the force. The task of straightening this flow falls to the stator veins, short, fin-like protrusions within the pump. The stator veins not only straighten out the flow, they also enhance its velocity. What really provides the force, however, is the portion of the pump that follows – the venturi. The funnel-shaped cone that most people equate with the jet pump’s outlet or “nozzle,” the venturi effectively acts in the same way your thumb would when placed over the end of a fast-flowing garden hose. It reduces the line size, and causes the water’s velocity to increase. This accelerated blast of water then pushes your craft forward.

Before it gets there, however, it must be straightened. If left to leave the pump in a spiraling motion, it would cause the craft to literally tilt to one side from the force. The task of straightening this flow falls to the stator veins, short, fin-like protrusions within the pump. The stator veins not only straighten out the flow, they also enhance its velocity. What really provides the force, however, is the portion of the pump that follows – the venturi. The funnel-shaped cone that most people equate with the jet pump’s outlet or “nozzle,” the venturi effectively acts in the same way your thumb would when placed over the end of a fast-flowing garden hose. It reduces the line size, and causes the water’s velocity to increase. This accelerated blast of water then pushes your craft forward.

REDIRECTING THE FLOW

If left alone, that force would just push the craft forward. Unlike traditional boats, Seakarts don’t have rudders to control their direction. The only way to steer the craft is to redirect the pump’s thrust in the appropriate direction. Enter the steering nozzle. Linked via cable to the sports steering wheel, this nozzle is attached to the venturi and pivots from side-to-side to redirect water flow according to the driver’s input at the helm. Turn the wheel to the left or right and the steering nozzle responds in kind, redirecting the thrust to push the craft in the desired direction.

So how does a Seakart move in reverse? The same concept applies. Remember, water is always flowing out of the pump. To push the craft backwards, that water must be redirected forward. The steering nozzle couldn’t possibly handle this task. Instead, a reverse “bucket” drops into the flow and, with its curved shape, redirects the thrust forward. Steering still applies. Water is simply sent forward in relation to the angle it reaches and is deflected by the reverse bucket.

Neutral? Like reverse, there’s really no such thing, at least in the gearing sense. To achieve a neutral-like position, the reverse bucket just partially deploys. Water is redirected down, rather than forward or back, effectively keeping the craft in a stationary position.

Yamaha’s system on the Seakart is called RiDE®. It has the throttle paddle on right and the left paddle engages the reverse bucket. When you want to stop, ease off the throttle, squeeze left paddle to engage the reverse bucket as a brake. The RiDE® system is genuinely effective in slow-speed manoeuvring, such as getting away from the slipway and docking.

Look out for part-2 next week when we will be looking at Handling, ergonomics and comfort.

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