Guide to Jet Outboards

If you chase steelhead or salmon in any one of our river systems, chances are your boat has an outboard jet drive motor on it or you wish it did.

 Jet drive outboards are the motor of choice for almost every river fisherman for the simple fact that they allow anglers to motor through water that is too shallow for a propped outboard. 

Jet outboards, use a jet drive pump unit to propel boats forward by creating an opposite force in the water.

This allows boats to operate in shallow water and other locations where propeller-driven boats cannot, such as ankle deep water. Jet outboards can also provide more usable space inside the boat than inboard motors.

There are fishing spots out there that require you to go through inches of water to get there.

Jets also eliminate the chance of damaging a prop and lower unit on unseen hazards such as logs or unexposed rocks.

 

There are two options for getting to these fishing hot spots others can't reach:

Buy a jet outboard or convert an existing motor. Most people don’t know this, but many outboards can be converted to jet propulsion for a few thousand dollars, depending on their size. 

This conversion, along with the proper boat, can allow a boat to run in just a couple of inches of water — and potentially scoot over short sections of land (not recommended), if such an action is required. 

 

Combining the correct outboard power and boat design is the “secret” of successfully using an outboard jet to run in shallow water. 

These two pieces of information will assist you in selecting the best combination for your needs, thereby giving you the advantages of outboard power for shallow water use.

Most important is the boat. 

Keep it light as possible since, unlike a propeller drive, you can’t change to a lower pitch jet drive impeller to increase load capability.

Aluminum is usually the material of choice. It is light, tough and easy to form in the proper configuration. For outboard jets, the minimum recommended bottom thickness is .100 Gauge but thicker gauge or UHMW reinforcement can be useful in treacherous waters. Just be mindful about weight.

Aluminum is the hands-down winner for a jetboat’s material because of its light weight and durability.

Aluminum boats, on average, weigh 25 percent less than fiberglass models, and owners who go skinny tend to occasionally slide over rocks — bad for fiberglass.

The size and shape of the bottom is very important. The object is to ride on top of the water, which requires a good planing surface, and to float as shallow as possible when shut down.

Bottom should be at least 48 inches wide and boat length at least 14 feet long. Air entering the jet drive causes slippage, so the boat bottom needs to supply solid water, free of air, to the jet drive intake. 

A flat bottom boat runs shallower than a vee bottom, but slides on the turns more.

A vee bottom splits off air bubbles before they reach the jet intake. A flat bottom carries bubbles straight back.

A slight V of 6-10 degrees dead rise will enhance jet boat handling.

Deep vee is not desirable for the Outboard Jet, not just because of increased draft and drag, But because the Outboard Jet needs a flat apron of water about 10” wide leaving the hull on which to set the leading edge of the jet intake, to minimize air intake and frontal drag.

The dead rise should be fairly constant and not increase to a deep forefoot at the bow. 

 A deep forefoot can cause spinout on a sharp turn. Keels can be a problem, introducing air into the jet intake. Center keels vary in size and may introduce air.

If this is suspected, the keel should be removed 2-3 feet forward of the transom. Other keel arrangements which tend to funnel air to the jet intake should be avoided. The chines of the boat, where the sides meet the bottom, should be sharp.

Round chimes tend to suck the boat down in the water and cause drag. A properly designed tunnel, combined with a slight V bottom hull can greatly enhance jet boat performance. 

It should raise the motor 2-3 inches and place the heel of the jet intake flush or slightly above the bottom of the boat. A jet tunnel doesn’t work well with a flat bottom boat due to air ingestion. It’s imperative that the hull is designed correctly with the tunnel for the boat to operate properly.

 The tunnel needs to be just large enough to feed the jet drive its water requirements. A tunnel that is longer, wider or deeper than necessary wastes power in lifting excess water, tends to suck the stern down when planing and sits deeper in the water at rest due to lost buoyancy. The top of the tunnel width should be about 1 ¾ times the width of the jet drive water intake. 

The tunnel length doesn’t have to be longer than about 2 ½ times the water intake width.  

Pontoon boats don’t provide a defined height apron of water ahead of the jet intake. The water level between the pontoons varies with speed and load carried.

It’s necessary to build an inclined plane ahead of the jet intake attached to the motor mount, about 16” wide inclined at about 15 degrees with the leading edge above water level, fully loaded at rest, trailing edge lined up with the leading edge of the jet intake.

COSTS?

Jet Pump engines are considerably less expensive, since they don’t have expensive stainless steel shafts, gears and props, but there’s a price to pay: usable power.

Take, for example, the Yamaha F150. It uses a 2.7L in-line four-cylinder four-stroke to make 150 hp — with a prop. 

Equipped with a jet drive, however, Yamaha puts something else on it: a cowling with the number 105, because that’s how much power it generates due to a jet’s relative inefficiency. It goes down the line: F115 (80 hp), F90 (65 hp), F60 (40 hp) and F40 (30 hp).

 The other major engine brands experience a similar reduction in thrust, and the selection is limited. Mercury has five but in the narrowest range of horsepower, with models producing 80, 65, 40, 35 and 25 hp. 

There is about a 30% power loss when converting to jet drive. A suitable outboard jet boat carrying an average load will provide about 80% of propeller speeds.

It’s important to decide what load you want to carry how many people, gear, fuel, etc. and add the weight of the boat, motor and battery. Look up this gross weight on the chart below to determine the minimum HP you need. The chart HP is power head horsepower.It’s important that the boat-motor combination plane the boat quickly. Planning would normally be in the mid 20 MPH range and up. This is necessary for optimum jet performance and shallow water running. At low RPM, when the boat is not planing, you would need about 12 inches of clearance between the bottom of the jet and river bed to maneuver.

A few things things to consider...

Does the boat plane quickly and not cavitate easily?

(listen to the motor for the change in RPM’s) a destructive and negative performance trait. 

Will the boat maneuver through obstacles typically found in your environment without breaking loose or skating? Is handling predictable?

Poor performance will show up in a jet boat taking a long time to get up on a plane, breaking loose early in cornering and cavitation. 

The advantages of Outboard Jet boat include maximum space inside the boat while still having the ability to operate in less than 6” of water, relative ease of cleaning out a clogged pump and adjustment for wear on the impeller and intake liner. So select a suitable boat with sufficient HP to handle your total weight, then enjoy fishing and hunting in areas you couldn’t reach before.

Yamaha has a company division that makes jets for personal watercraft, but it’s interesting that they, along with Mercury, Honda and Tohatsu, source their jet drives from Outboard Jets, which has been making them since 1960 and is the king of jet outboard components. 

Almost every jet drive pump on the water today, no matter what brand motor, is made by a single company: Outboard Jets out of San Leandro, California. The company was started in 1960 by Dick Stallman, who worked closely with the Wooldridge family to form the product that you know today as the jet drive. Today, Outboard Jets build over 80 different models to fit 6 of the top name outboard manufacturers in the marine industry.

 Jet drives aren’t compatible with every type of waterway. Mud, for instance, is a jet killer, so a better solution for going into muddy areas is an air-cooled motor such as a propped Mud Buddy. Jets work best on hard bottoms such as rocky riverbeds. 

They work OK over sand, but it, like small rocks, tends to abrade parts such as the impeller and the wear ring that surrounds the impeller. If the impeller doesn’t fit tightly within the wear ring, power is lost, like with a worn ring on a piston in a combustion engine.

The impeller can also get dinged up, which costs power, but carefully filing rough spots can bring it back to life. The wear ring is a sacrificial part that costs about $50 to replace when it gets grooved. 

The use of outboard jets is definitely on the rise and we will be seeing a lot more of them on the rivers in the years to come. If you are an angler that is thinking about purchasing an outboard jet, these are some great things to keep in mind when looking.

Looking for a JET OUTBOARD ENGINE to replace your PROP OUTBOARD ENGINE?

Marine Parts Warehouse can order brand new Mercury Outboard engines. Please click here for a quote!

If you are wanting to convert from a jet to a propeller lower unit please click HERE. 

For more information on outboard motors, contact us today!

Posted by Dr. Johnny Fever