Outboard jet propulsion systems

Outboard jets work best on flat-bottomed boats with a bottom width of 48 inches or more. The shape of the hull cannot be easily defined.

When it comes to bottom width, the wider the bottom, the more water it displaces. Because a shallow draft is important, the wider the boat, the better the draft. A wider hull sits higher in the water, so it exhibits less drag. You would conclude wider is better.

However, is not entirely true, in the sense that the shape of the hull helps direct the flow of water. Flat hulls tend to cause more cavitation. A deadrise hull manoeuvres better and cavitates less than a flat-bottomed boat.

Many aluminum hull manufacturers have introduced a whole new line of extra-wide modified john boats that offer a consistent line of 4 to 6 degrees of deadrise from bow to transom. It is important that the deadrise falls all along the hull because the "V" provides a smoother ride and splits off air-filled water before it reaches the jet intake.

Eliminating cavitation

All this information is great if you have not selected your outboard jet hull yet. But how do those who already have a jetboat handle cavitation? As outboard jet standards go, all the hull types cavitate under extreme conditions. However, cavitation under normal running conditions should not be tolerated, and should be eliminated to maintain the life of your outboard.

If your outboard jet is cavitating, the jet pump is pulling more air than water. It will not allow the pump to produce adequate water pressure to bring the boat on plane. The engine works hard and erratically and this could cause damage if let go for too long. This type of cavitation is likely to be a result of improper mounting height of the outboard or the hull bottom characteristics causing a poor flow of clean water to the jet intake. There are several solutions to enhance performance.

Install intake fins

Some companies offer an intake fin kit as an accessory designed to reduce cavitation. This kit is easy to install and offers the advantage of directing a larger volume of water into the jet intake. The increased volume of water reduces the amount of ingested air, allowing the pump to operate efficiently. The added set of fins even enhances the grip in manoeuvring and assists in reduced cavitation in tight turns and rough water.

Transom jacks

Transom jacks can help. They come in a variety of sizes and styles from manual adjustable to powered hydraulic models. They are easy to install, but you may have trouble moving the outboard unit off the transom. Your local marine dealer can install a transom jack for you with very little effort and take any concern of managing the 200-pound weight of the outboard.

Beyond the ability to adjust the height on the fly, if needed, the transom jack extends the intake foot about six to eight inches farther back from the transom. The water flow there is more likely to be clean, providing better intake efficiency. Many of today's fiberglass bass boat hulls are designed with a step transom that places the engine farther out to take advantage of the power increase that results in faster lift.

On the negative side, the transom now acts more as a fulcrum because the outboard weight is shifted farther back. This might require minor adjustments to your payload placement. This mounting placement also adds more torque to the transom.

Even though modern hull transoms offer multiple knee braces for additional support, be sure to check with the hull manufacturer first, before adding a transom jack mount. A transom saver, which is not often associated with an outboard jet, may also be a smart choice if you travel over uneven ground on land.

The jet intake requires a clean flow of water free of air bubbles to allow the pump to operate at peak efficiency. Flat-bottom john boats tend to carry these air bubbles straight back into the intake. Keels can also aid in increasing this effect because they introduce even more air into the intake. The center keel can be removed two to three feet from the transom forward. The goal is to produce an apron of clean, air-free water just wider than the intake.

Doing this requires some special equipment and no doubt renders most, if not all, hull warranties null and void.

Reducing excessive spray

Spray is a problem that can plague outboard jet-powered hulls. Water deflects off the transom and the intake foot below the bell housing, and sprays into the back of the boat. While annoying, it can also cause some cavitation, and should be resolved.

Even though for some the idea of adding a spray deflection plate to the transom is not appealing, I have seen some splash plates mounted higher and directly to the outboard. This keeps the water from spraying into the boat, but it doesn't do much to direct water or prevent any cavitation. However, it is an option. It is also worth mentioning that some hulls made exclusively for jet outboards even have a spray deflection plate welded to the bottom of the transom.

The stock impeller on all outboard jets is a lightweight three-blade design that is easy to machine because it is made of aluminum. Because it's aluminum, it is soft and prone to dulling and nicks. After time, this wear can cause significant loss in performance. Proper maintenance requires that the impeller be removed, sharpened, and have a shim(s) removed to ensure that the proper clearance between the impeller blades and intake housing is met.

A newer design is a stainless steel impeller that offers four thinner but stronger blades. This design feature enhances payload capabilities in two ways. First, the area is increased because of the extra blade.

This added area increases the traction and improves the water flow to the pump. Second, the blades are thinner, so they cut through the water with less resistance, even though there is more blade area. A stainless steel impeller also features better tolerances.