Drop The Prop

Many areas of the country have waterways that are too shallow for conventional outboards. In other places, rocks, stumps and sandbars just beneath the surface conspire to “help” boat owners send their local mechanic’s kid to college. There are two options for remedying such situations: 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 between $1,700 and $2,700, 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. 

Boaters who ply skinny or debris-strewn bodies of water on a regular basis are probably better served buying a jet outboard from Yamaha, Honda, Mercury or Evinrude. Suzuki doesn’t have one (though there is a way to convert one). Most are considerably less expensive, for the engines themselves, 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. Evinrude offers just three models based on its E-TEC direct-injected two-strokes: 105 hp, 60 hp and 40 hp, which are the same jet outboard options Honda offers. Mercury has five but in the narrowest range of horsepower, with models producing 80, 65, 40, 35 and 25 hp. 

Yamaha and BRP (Evinrude) both have 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. They have conversion kits for Suzuki too, but Suzuki doesn’t market them directly to the public like the others. To exactly match the factory motor’s color, Outboard Jets gets paint chip samples from the manufacturer. 

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. Aluminum thickness matters greatly.

because of its weight. For smaller boats powered by 40 hp and less, a quarter-inch thickness is good. For larger boats powered by up to 200 hp, 3/16-inch aluminum is more appropriate.

So what type of hull makes a good jetboat? The whole point of owning a jetboat is to ride on top of the water, not in it, so the best hull bottom shape tends to be flatter, to provide more lift. But totally flat bottoms tend to be less efficient, since air bubbles on these hulls go straight back, which reduces a jet pump’s efficiency. They also tend to skid in turns, which makes them unsuited for tight, twisty rivers.

V-hulled boats shunt bubbles to the side, which helps feed the pump mostly un-aerated water for better thrust, but too much deadrise makes them settle into the water deeper, which causes more drag. Boats that have six to 10 degrees of deadrise provide the best of both worlds. It’s better if the deadrise remains constant rather than being sharper at the bow, to avoid spinning out. Pronounced keels can be problematic, since they can introduce air into the jet intake, although removing the rear three feet can solve this issue.

Most “normal” boats are stern heavy, so weight needs to be pushed farther forward, which is why jetboat helms are often near the front, although tiller steering can still be used if other measures are taken. Placing heavy items such as fuel tanks and batteries toward the bow helps improve weight distribution.The main difference between prop and jet rigging is jet outboards need to be mounted about six inches higher, so owners who want to shift back and forth from prop to jet need a jackplate. The front of the jet’s intake grill needs to be slightly higher than the hull bottom, so its leading edge can’t snag anything, which could damage it. The back of the intake grill sits lower than the hull, so it can scoop water for the impeller, which looks like a big screw and is usually made of durable stainless steel.

Jet drives aren’t compatible with every type of waterway. Mud, for instance, is a jet killer, so a better solution for going skinny over it 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.

Owners who have an outboard that they want to convert to jet should go to outboardjets.com and see if it’s compatible. The site provides information about how to install them. Surprisingly, it only takes about an hour to change from prop to jet, for anyone who knows which end of a screwdriver to hold. Some outboard owners claim that after a few tries, they can do it in less than half an hour.

For more information on your outboard motors, contact us today!