Cavitation Explained

Cavitation

The image above represents the same principle in pump cavitation “As water slips around the propellers of a boat, there are places where it experiences sudden, extreme loss of pressure “cavitation“. The propeller is making a ‘hole‘” in the water, and the more powerful that the propeller is the more water it clears away. Water appears — for the most part — as a liquid on Earth because the atmosphere on Earth keeps it under pressure. Drop the pressure and the individual molecules of water come apart. The water boils, even at very low temperatures. When propellers lower the pressure on certain areas of the water, the water boils, and we see these bubbles.” In its worst form, and when pushed to a saturation point, cavitation can cause severe harmonics, As the bubbles collapse, they can create powerful reverberating shock waves, which can not only disrupt the flow of water around the propeller but actually stress, weaken and break the prop itself. This effect has been commonly attributed to a loss of acceleration, traction, and thrust in PWC.

impeller cavitation

The First aspect to look at = Sea-doo cavitation is the Wear-Ring Condition.

The wear-ring is Sea-doo’s design that incorporates a removable “ring” that is sacrificial. Sea-doo used a plastic liner with a fiberglass core in earlier designs, this designed was successful for many years and we still receive 2- stroke watercraft with the original wear ring installed. The majority of wear ring failures was caused by debris ingestion through the pump or the plastic liner dry rotted on earlier model’s wear rings. The newer designed wear rings from Sea-doo are solid plastic and the majority of aftermarket wear rings are plastic. Aftermarket companies have designed a stainless wear ring / insert for all sea-doo watercraft. The wear ring can fail in multiple ways: the majority of cases there will be a cut groove around the impeller blades where a rock or debris has lodged or processed through the pump housing and this will be visible by inspection. When the wear ring has an increased space between the impeller and housing “wear ring” this will cause cavitation.

Pump cavitation

A Second cause of Cavitation

The other leading cause of cavitation in Sea-doo Watercraft is directly related to the drive shaft thru hull fitting. Earlier 2 stroke jet-ski’s used a bearing carrier style thru hull fitting. This fitting worked wonderfully for many years, Sea-doo redesigned the thru hull fittings because no one ever greased the bearings and the system failed. The newer designed thru hull fittings are a carbon seal design. The carbon seal uses a machined carbon flange that aligns against a machined aluminum flange, the unit incorporated an accordion style boot and moon key design to keep pressure on the carbon seal, this prevented water from entering into the hull and kept air from escaping through the hull around the driveshaft into the pump cavity and causing cavitation ! Once the carbon seal is damaged from overheating, wear, or a loose accordion boot the watercraft would malfunction.

Seadoo carbon seal

Cavitation can cause:

  • Slipping feeling when accelerating “slipping clutch”
  • A loss in speed on top-end or high RPM’s
  • Water appearing the hull – carbon ring failure
  • Engine overheating

Cavitation Can Cause Engine Failure!

Excessive Cavitation will cause engine failure from overheating. We receive several skis’s every year with holes in exhaust resonators and engine damage from cavitation. The cavitation causes an air pocket of sorts inside the pump housing. If water is not pushing through the pump and through the engine it will overheat! Sea-doo watercraft with closed loops systems will overheat and sink ! the resonator is level with the waterline and will steadily flood the ski while sitting if the resonator has been damaged.