Is a jet right for you?

The advantages of jets	The disadvantages of jets
Shallow water operation (the main advantage)	Jets are typically noisy due to the exhaust outlet being out of the water on plane
Less complex than most propeller drives	Poor power efficiency (outboard jets lose around 1/3 of your engine HP vs. prop)
Require little maintenance	Jets do poor in areas with lots of sticks, weeds, or algae
Allow you to do things you could not do with propeller when handling (very tight circles, etc)	Poor gas mileage due to overcoming engine power loss with bigger engine
Safer around swimmers and people in water.	Propeller people will think they handle weird at slow speeds
No propeller torque to fight you at the steering wheel (or tiller)	Poor reverse characteristics

Jets handle "differently" at low speeds. This is something you get used to. Since they have no rudder below the water surface, steering is based completely on thrust. This means that if your engine is not running, you can not steer. Also, jet pumps do not have a true neutral; the impeller is always spinning. For reverse operation, jet pumps have a gate that extends over the stream of water and redirects it underneath the boat. Balancing the gate to direct water equally backwards and forwards is how a neutral setting is achieved, thought it is not a perfect neutral. Since the water does a U-turn directly out of the nozzle, reverse on jet pumps is somewhat poor. Newer inboard jets, such as the Hamilton 212, have overcome this somewhat with huge reverse buckets that redirect more water, thus creating a better reverse.

Your outboard jet is perhaps one of the least efficiency jet drives on the market. Due to limited space, these outboard jets must be small and therefore sacrifice efficiency. Water is sucked up the bottom of the shoe, then immediately does a 90 degree turn into the bowl of the pump. It is then forced to do a complete loop, then exiting out a small nozzle at accelerated speeds. All these twists and bends lose efficiency, and that is where the 30% of your horsepower went. I have an 85 HP Force outboard on my little 16' Crestliner, and it is probably the equivalent of a 50 HP outboard powerwise, but you are packing more weight as well with the bigger engine.

A frequent misconception I see about jet pump power is how the craft is propelled. A jet powered craft is not propelled by the water coming out of the jet and pushing against the water in the lake/ocean/river. It is propelled completely on the rapid displacement of water: (i.e. the water being accelerated and pushed out at a high velocity). To understand this, say you are in a chair with wheels on a smooth floor, and you have a bowling ball in your lap. If you throw the bowling ball in front of you, two things will happen: the bowling ball will move in the direction you threw it, and you (and your chair) will move in the other (or opposite direction). This is how jets work, but the boat is the chair and the water is a whole bunch of bowling balls. Because of this, don't be concerned if your jet stream is coming out above the waterline; it should be.

Newton's Third Law of Motion: "For every action, there is an equal and opposite reaction. "

In order to keep efficiency as high as possible, there are a few things you can do to improve and keep the performance of your outboard jet. Below are some instructions and images on how to take your jet unit apart. It is not hard, and can be done in just a few hours. All you will need is a screw driver, and a wrench and socket set. Click on any of the images to see them enlarged.


Note: My jet is on a 1987 85 HP Force outboard. There may be slight variations between different models. Please take this into account. I suspect the main difference is the adapter plate.

The first thing one could do is write down the serial number of your outboard jet. This can help identify it if you need to get any parts. The serial number is located where the reverse gate attaches to the jet pump. It will probably be a four to 7 digit number. Write this number down.
Your Outboard Jet should also have a model number. This is on the other size of the jet, there the reverse gate attaches as well. For instance, my jet is a model "O", so I just have a letter "O" stamped in this location.

The first thing you may want to do to inspect your jet is to remove the gate control cable from the jet. This is just a matter of taking off two screws on the cable mount, and the cable is off. Very easy.

Now you will want to take the shoe off of the jet. To do this, just look around on the bottom ring of the shoe and you will see 5 or 6 bolts all the way around. Just losen these bolts and the shoe will come off. It requres some force to jiggle the shoe off, but keep working at it and it should eventually pop off. One thing you can do to increase performance is to make the fins of the intake grate a "teardrop" shape. This means pulling out your grinder or hand file, and grinding off some metal. If you don't want to do this, at least make sure the fins are straight and that water can flow through them easily. As far as I can tell, you can not easily remove the fins to straighten or sharpen them.

Now that you have the shoe off, you can clearly see the impeller. The impeller is held on by a big nut on the drive shaft that has a cotter pin running through it. The clearance between the impeller and the wear ring in the shoe is critical to performance. Outboard Jet Corp. specifies this tolerance as .0030", but you need not get out your feeler gage. Basically, if you can slip a piece of paper between you impeller and your wear ring, that is about as much clearance as you want. Any thing less, and you will lose a lot of speed. Any thing more and you will wear out your wear ring more quickly. The impeller has a simple yet ingenious way of keeping tight clearnances. When you use your jet, the wear ring will gradually get bigger due to friction with water, sand, etc. To counteract this, your outboard jet has shims that can move the impeller small distances down the tapered wear ring. After the end of a season, it is advised to check your tolerances and re-shim as necessary. To move the impeller further down into the wear ring, all you need to do is move a shim form the lower side (nut side) of the impeller to the top side (motor side) of the impeller. Moving shims like this helps keep all your shims together, and makes it easy to adjust.

	Here you can see how the impeller fits into the wear ring of the shoe. Remember, tight clearances are essential! I re-shimmed my impeller after someone had ignored it for a few years and got another ~10 MPH out of the boat!
You will also notice that your impeller drive shaft is most likely a D shaped shaft. In this picture you can see the metal block and nylon sleeve that go inside the impeller when you are mounting it in your jet.
They are pulled out in this picture so you can see them better. You can also see an extreme closeup of the end of the shaft.

	Here you can see the 5 shims, the washer, the eared washer, and the nut used with the impeller.
It is important to inspect your impeller at the end of every season. You don't want to see any major pitting, and you want your edges to be nice and sharp. Stainless steel impellers are available on some models, but not my model. They provide better performance due to less flexing, and are more durable than aluminum.
	Here is a close up after sharpening the impeller. To do this, simple get the impeller and a metal hand file, and try to match the exising angle. The aluminum is soft, so I would not advise using a power grinder. It doesn't take much and you will have your leading edges knife-sharp and ready to really bite into the water. If your impeller is damaged beyond repair, there should be some numbers stamped into it. With these numbers and the model number of your jet, Outboard Jet Corp. Will be able to tell you which impeller you need to buy and where you can get one. Just give them a call.

On my outboard jet, two bolts need to be removed to take out the wear ring. As I understand it, most shoes have wear rings, but the really old shoes did not, and a complete shoe replacement was needed when the gap was too big between the impeller and shoe. You can see the wear ring comes out with some light tapping of a screwdriver handle. Replacing wear rings is much cheaper than replacing shoes, as shoes go for around $300. Wear rings usually cost around $40. Aluminum impellers are around $170.
Now, you want to take your jet unit off your motor. When the jet is mounted, the process is basically like this: The original lower unit and propeller drive is taken off, then an adapter plate is put on, then the jet drive mounts to this adapter plate. You need not take off your adapter plate, just your jet unit.
	To take off jet unit, take off the shoe and impeller, then look into the jet. You will see four large bolts and four small bolts. Remove the four large bolts, and one bolt near the nozzle on the outsize of the jet, and your jet should come off. Be carefull as it weighs around 20 or 30 pounds. You might have to work a little to get it off, but don't force it too much.
The four smaller bolts you see probably do not need to be removed. These bolts hold in the main bearing of the jet unit.While you are this far, push up and down on the shaft to make sure the bearing is in good condition. There should not be any slop either way; it should feel tight.

	This picture is looking up at the bottom of motor and adapter plate after the jet unit has been removed. You can see the six bolts that used to mount the original propeller lower unit to the motor. Now, these six bolts mount the adapter plate to the motor, and the jet unit bolts into this adapter plate. Seems to work pretty well.
	Once you get your jet unit off, it would be a good idea to replace your water pump assembly and impeller. These sit right on top of the jet unit as you can see here.

At the very least, inspect your impeller. But, as long as you got this far, you might as well replace it as preventive maintenance. Water pump impellers are much cheaper than your engine overheating, so replace them every season or two depending on use. And NEVER run your engine out of water, it will chew these rubber impellers up in seconds.

Close up shot of my water pump.|

Here you can see where the water pump goes in relation to the drive shaft and jet pump unit, as well as the adapter plate that is still on the motor.
When you get it all back together, remember to grease your jet periodically! Outboard Jet Corp. recommends that you grease your jet bearing every 10 hours of use. Just grease it until you can see grease come out the outlet and you are done. It takes just a few seconds to do, and keeps it well lubricated.

Bottom Line: Take care of your jet once in a while, and use it often. See you fishing!

Relevant Links: Enhancing Your Outboard Jet Performance Marine Engine Books and Manuals Bentz Boat's Jet Outboard Sled (PDF File) Steve Anderson's Page on Outboard Jets Epinions Outboard Motor Reviews Western Whitewater Association