The Jet Unit

The jet unit is basically a pump which draws the water from under the boat and forces it out the rear through a nozzle which is turned to from left to right so that the boat can maneuver. Reverse is achieved by placing a "bucket" in front of the nozzle to redirect the water under the boat. With the reverse half on or in the "natural" position the boat can be turned around without going forward or back.

There are many jet unit manufacturers producing different units for different applications. If you need speed then a small nozzle is an advantage. When quick acceleration is needed a large nozzle works best.

A motor matched to the size of jet unit directly drives the impeller although some "sport jet" and other special setups may have a gear box. Impeller sizes can vary depending on what HP the motor is. Like with cars the more HP you have coupled with the least amount of weight the quicker the boat is, but in saying this the jet unit and hull design also has a lot to do with this. For example, a motor with the same HP in a sprint boat has fast acceleration and will get to its top speed of say 140KPH very quickly, where as a marathon boat will be much slower at getting to 140KPH but will reach speeds of 170KPH plus.

Whether used for racing or pleasure the principles are all the same. If you are not putting water out the back you have no steering and if you have no steering you will come unstuck. That is why it is essential to check all control equipment and the motor to make sure is is it top condition. If in doubt replace or fix it properly.

Inside the Jet unit

The information contained on this page is designed to give you a better understanding of how a jet unit works. It is not here to compare performance by rating different units against each other. If you have a different unit than one listed here why not take a few photo's and fill in a few details so that other jet boaters can learn and identify the many units out there. Contact Us

Hamilton 212
Hamilton Colorado

Power / RPM curves for Hamilton 770

The Hamilton 212 Jet unit mixed flow pump

Power / RPM curves for 212

The latest in the Hamilton range of jet units designed for the smaller craft ( 8M / 3 tonnes). The 212 is a single stage, 215mm (8.5") impeller and a 108mm nozzle and weighs 70kg.

A variation of this type of unit has also been used for jet sprinting in the past.
It is a simple jet unit to pull apart and can be done even on the river bank if needed. There are just seven nuts to undo to get to the last stage show here and usually only takes a 5-10 min. It is a well designed unit and performs well.

Click on the image to enlarge.

One obvious feature is the large split deflector or "reverse bucket" which is very effective, so well infact that the bow of the boat can be put under water !! It gives excellent maneuverability in reverse

 

Looking inside the tailpipe you can see the stainless steel wear ring , cutless bearing (black) and stator vanes which correct the water flow after the impeller. The diameter of the pipe is 215mm.

 

With the tail pipe removed you can see the impeller on the shaft along with the cutless bearing sleeve and impeller nut. Note the water supply feed coming from the tail pipe section through the transom to be used for cooling the motor (just above the spring).

 

This is a cast 2.8kW turbo impeller made from stainless steel. It differs slightly from the standard type of impeller by basically having a second impeller bolted to the back of the first and also having a more curved front edge. The turbo impeller is designed to give better "grip" in white water. It is also less prone to slippage.

 

Looking into the intake housing you can see gravel caught in the grill or intake screen. If too much gravel is present it can lower the performance of the unit but the odd stone is ok. The screen can get a hard time if a lot of shallow rock hopping river boating is done and may need to be rebuilt at some stage.

 

Minor problems:

Small stones can cause the the spring holding the water seal to wind over the retainer causing it to leak. To fix this don't pump too much gravel or cut the rope guard off.

A stone can get caught between the 1st and 2nd stage of the turbo impeller resulting in a vibration and loss of performance.

A flat stone can make it all the way through the grill and impeller and get trapped in the stator vanes. Most of the time it is hardly noticeable so it pays to check the tailpipe when you have it apart for inspection.

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The Hamilton Colorado Jet unit

This is an older jet unit found in such boats as the Hamilton Jet 30. This one has the T3 nozzel. You will note that much of the jet unit is within the boat.

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