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Doctor Doo is here to help cure what ails you!
A problem we hear from time to time with Sea-Doo watercraft and boat owners is the congestion of debris in the direct drive water jet propulsion system but this is something that can be minimized with some understanding of what is going on underneath your vessel.
Lets first look at what a Direct Drive Water Jet Propulsion system consists of.
A Direct Drive Water Jet Propulsion system starts at the intake, this is a tunnel or track that enters the bottom of the hull to feed the encased impeller. Now hold this vision for just a few sentences… In front of the intake inside the vessels engine compartment rest the engine, mounted directly onto the bottom of the hull with the in-line drive shaft angled straight back (towards the transom) traveling through the hull and connected to the front, center of the impeller. No transmission here, it is a “direct-drive” system.
Okay back to the water flow. The impeller is a bladed or veined component that when turned transfers water from front to back, this is what creates the water propulsion. But there is more to it than that, behind the impeller blade are directional veins angled the opposite as the impeller blades to straighten the twirling water flow to force it straight back to use that energy as efficiently as possible to create maximum forward thrust. Behind the jet pump is the Venturi. The venturi is shaped similar to a cone, with the inlet being much larger in opening diameter than the smaller, tighter exiting outlet. This compresses the water and provides greater thrust when forced out. Imagine holding a garden hose with water running freely out of the end. now squeeze your thumb over the end and wow, it shoots a stream of water a lot harder, faster and further, this is what the venturi does.
Now to provide steering (and trim on some models) there is a pivoting nozzle on the rear of the venturi, this redirects the water flow from side to side to push the transom to side to side to change direction – or up and down to change/trim planning angles.
Okay so that is direct-drive water jet propulsion 101, this video offers a visual look at everything we just described.
This system draws water up into the water jet pump and pushes it out the back to propel the vessel. The pump is creating suction to draw that water into the system and anything else that might be in its negative pressure proximity. This could include sand, trash bags, sticks, Sunglasses, rope, but designed to efficiently process water and it does this very well. To help ensure only water enters the system an intake grate is mounted over the inlet. This is usually comprised of three hydrodynamically designed “bars” that run from the front of the opening to the back and are nearly flush with the bottom of the hull. The intake grate ensures large objects do not enter the propulsion system.
The intake grate is designed for hydrodynamic performance and the greater the covering, fencing or screening of the intake substantially impedes water flow and ultimately performance suffers. There is a balance but with this small objects can be drawn into the intake system. Here is where education can save you headaches.
First you should always follow your owners manual and operate your vessel in 3-feet (1m) of water depth or more but if you do find yourself in shallower water and your Sea-Doo vessel’s propulsion seems “funny” get to a safe location (one with out current and clear of potentially dangerous objects) and turn off the engine(s). The worse thing you can do is try to power through it. Why? Okay, imagine you’re vacuuming the interior of your car. You accidently run the hose nozzle over a rouge gas pump receipt and it is sucked to the end of the hose impeded airflow. Bumping up the vacuum power to max only holds the paper to the hose tighter. Shut the vacuum off and watch the paper fall off the vacuum hose un aided.
A common mistake of Sea-Doo owners that get into vegetation and it begins entering the jet pump system is powering up and hoping it will be processed. What this can do is pull the debris into the system deeper increasing the odds it will get stuck. Another common mistake is thinking by putting the vessel in reverse and powering up it will blow out the debris that way.
Remember a few paragraphs up when we discussed “direct drive”? This system works off of redirection of waterflow. In reverse the water coming out of the venture is redirected forward, similar to technology of jet airplanes when they slow down after landing. The intake is still drawing in water in the same direction even when in reverse. Both of these actions can also cause the exhaust to overheat as the cooling water flow is hindered. The best course of action is to turn off the engines and more times than not the debris will simply fall away.
In 2012 BRP takes this thinking a step further with the addition of the weedless system. This new system works with the iTC (intelligent Throttle Control) system and offers a button located on the dashboard of select Sea-Doo boat models and when activated (engine must be turned off) hinges the intake grate downward to utilize gravity in the aid of debris simply falling away.
That was a long way of simply saying, to keep your propulsion system clean, simply suck less!
This installment of Doctor Doo was supplied by Willy Carmine a former member of the BRP/Sea-Doo R&D team and current President of Route 1 Motorsports an authorized BRP Sea-Doo and Can-Am dealer in Grant, Florida where his customers boat in the Indian River, the 120 mile long lagoon that averages 4.5 feet (1.4m) in depth, so he knows a thing or two about shallow water boating.