[YachtForums]

I have no problem with the preceding hypothesis or statements. It’s quite accurate, however I take exception with…

A water jet protects the engine from changes in boat weight. It presents a predictable load to the engine and that load does NOT change with changes in the boat.

This is not correct. It’s actually quite the contrary. Jet pumps will encounter a greater degree of loading and unloading because they are recessed inside the hull, as opposed to a prop placed underneath the hull. The higher the x-factor, the sooner ventilation is incurred. This is compounded by intake gullet vacuum, which is essentially artificial weight… and is QUITE significant with jet pumps.

Let me give an example… in IHBA Drag racing (using jet boats), when the pump unloads at high speed, you’re essentially driving a kite. You’ve lost a huge amount of downforce and vacuum (or weight). The forward velocity remains relatively unchanged for the moment and the hull still has the same level of wind speed passing under it. You can do the math from here…

With their exposed, prop driven counterparts, when the prop unloads… the downforce on the hull remains constant. However, this can result in some wild stern walking! Both of these scenarios, at the speeds these boats are traveling, can result in the boat swapping ends. No need to go into the horrific details that follow.

Water jets also vary considerably in handling. Two cycle water jets are not as responsive as any of the other systems because the small diameter water jet operating at high rpm and pressure does not move enough water to provide crisp response to steering changes.

This is NOT entirely correct. There are multiple variables. This has more to do with the boats length, displacement and hull design. Conventional rudders generally offer greater deflection, because they have greater travel than the typical jet pump steering nozzle allows. The reason pump nozzles have limited travel is because vectored thrust can hydraulic at the venturi’s orifice under extreme angles and high pressure. This is a matter of controlling hydraulics and optimizing the connection between the venturi and the nozzle.

As for smaller diameter orifices under higher pressure not being as effective… False. A jet driven boat CAN turn faster because it does not have an appendage protruding beneath the surface that will hold a designated track. Pressurized thrust with sufficient deflection can provide equal or better better turning. Again, the hull plays an equally critical role in the performance of either propulsion system, or it’s form of thrust vectoring. Certainly the mass of water being moved is important, but pressure and deflection play important roles as well.

Inboard water jets, while having larger diameter jets and moving more water, have a disadvantage because the steering nozzle is usually at or just behind the boat transom. This does not give it the same steering leverage as an I/O or outboard where the propeller is usually 24 inches behind the transom providing more steering leverage

This is not entirely correct either. A shorter distance between two leverage points provides a tighter turning radius. A greater distance can provide increased leverage, but this does not equate to faster turning. Vectored thrust under high pressure, will more than make up for the leverage lost from a further forward exit point. Again, this can have as much to do with the hull than the factors cited.

Also, when referencing the leverage an outboard can create... say on an extended bracket, that same outboard uses a skeg on the lower using that will reduce side slip. Jet boats generally don't have an appendage of this type, allowing more slip at the stern and thus a quicker change in heading.