Jet Boat Performance

Know Your Jet Boat

Courtesy of American Turbine

Most boat owners like to perform at least a certain amount of their boat maintenance themselves to ensure continuing good performance and reliability.

To avoid unnecessary trips to the service center, it might be useful to have a checklist which can be run through systematically, in the hope of pinpointing the problem quickly. The boat can be divided into three categories - jet unit, engine, and hull.

If something is "wrong" with the boat it is usually poor acceleration and load carrying, coupled with excessive fuel consumption or engine RPM's. It could be unusual noise coming from the mechanicals, or possibly just poor top speed. All of these things may be present to some degree, but the usual complaint is that the boat is just plain "gutless".


The most important single instrument on the boat when considering performance drop-off is the engine tachometer. The great thing about jet boats is that the engine RPM's should remain the same throughout the life of the boat, regardless of age, loading, water conditions, towing, whatever.

There is no situation where the RPM's should be different from when the boat was new, and as an owner, you will know what these are. At any time, you should be able to open the throttle fully and get exactly the same maximum reading you have been used to. Or perhaps you are finding it needs more RPM's to cruise your normal load? RPM's are a most important indicator of proper operation of the boat.

It is important at this stage to feel confident that your tachometer is reading accurately.

Now we come to our check list, and determination of which major area is the problem. The simplest way is a check on the RPM's first. They could be normal, high, or low, and this will pin point the place to look:

A. Normal maximum RPM = hull problem

If the boat is performing poorly and the maximum RPM's are normal and what you are used to, you can look to the hull and some of the external parts. These include:

1. Overload: Too much weight aboard.

2. Balance: Either too much weight aft, which will cause the nose to be too high and make planing difficult, or if the weight is too far forward, it will cause the nose to plow, difficult steering, wetness, and poor top speed.

3. Reverse bucket: Is the bucket dragging in the reverse stream? Make sure the control is fully up.

4. Excessive hull drag: Is there some external hull obstruction such as rough surface, broken keel strips, or other reason impeding the smooth flow of water over the hull bottom? A visual check on the trailer should reveal if there is. Metal hulls can have a "hook" bashed into them forward of the transom which can cause the bow to plow. The planing surface forward of the transom six to nine feet should be true and flat.

B. High rpm = jet unit problem

Higher than normal RPM's, lack of thrust, slipping clutch feel, engine racing and no go?

1. Blockage: The most common problem is weeds and stones blocking the intake grate. Also be aware of ski rope, fishing line and plastic bags winding around the pump shaft. Small sticks and stones can become lodged in the impeller affecting the performance dramatically, objects trapped in the impeller can cause the rotating assembly to be out of balance, causing severe vibration. Make sure the water passage through the jet is clear.

2. Impeller wear: The heart of the jet is the impeller, and its condition. If you run in shallow gravel beds or across sand bars the leading edges will become dull and inefficient. Pumping sand will increase the wear-ring to impeller clearance. Causing cavitation and loss of performance.

3. Bowl / stator vanes: Its not too much of a problem, but the leading edge of the fixed stator vanes can become blunt and damaged.

4. Air leaks: If excessive air leaks into the intake ahead of the impeller, the jet unit will "slip". Possible sources of air leaks are though a faulty gland seal, which is usually accompanied by a static water leak into the boat when standing idle.

So if the gland is worn out and leaking into the boat with the engine off, it can also suck air when accelerating on to plane, and if this happens, then the thrust is reduced dramatically. Air can also be introduced into the system via the inspection cover, so you will want to make sure the cover is tight.

C. Low RPM = engine problem

There is generally no way the jet unit can overload the engine and bring the RPM's down. If the RPM's are down from usual, it is almost certain to be a engine problem. A compression check will usually reveal leaking piston rings or valves, but the most common reasons for reduced engine power are:

1. Throttle: Check that the throttle is opening fully.

2. Fuel: The fuel supply must be adequate for the engine size. Racing boats frequently have a fuel pressure gauge which is, with the tachometer, probably the most important engine instrument. Sufficient fuel must be reaching the engine.

3. Air to the engine: The carburetor must be getting its full quota of cool air. If the engine has to work to get adequate air, and if it is hot air, this will reduce power.

4. Ignition spark: Be satisfied the ignition system is operating properly. A problem here is usually indicated by a rough running or missing engine.

5. Exhaust: Check for a free flowing exhaust system. Some silencers can become blocked, rubber hoses disintegrate internally, or there is excessive water injection. Such things can cause excessive back pressure and reduce power.

Provided your engine is getting its full quota of air and fuel, and is getting enough spark and at the right time, the engine will usually be OK, and maximum RPM's will result. However if the RPM's are down and you believe the tachometer, look for an engine problem.

The hull, engine, and jet unit are the three main areas to look at when your performance is down.

D. Excessive noise

This can often be a concern even if there is not a reduction in performance. The most common causes of noise are:

1. Cavitation: The jet unit is starved for water, and usually sounds like a rattle or a can of loose bolts in the back of the boat. Most likely a blocked intake grate.

2. Moan or whine: The jet unit can exhibit some "turbine whine" not unlike a turbocharger noise but you will know what is usual with your boat. However, if you have a new more obvious moan/whine, especially if it is a very low frequency grumble at idle, that increases with engine RPM's then it is likely to be a rough/worn/water damaged thrust bearing. If water has gotten into the bearing, it is usually as a result of a flooded bilge at some time on a warm bearing, then water can be sucked in as it cools.

3. Periodic vibration: Often at specific RPM's and disappearing at other throttle openings is probably a torsional vibration emanating most likely from the universal joints on the drive shaft.

Check them for worn/slack joint needle rollers, or if they have been installed incorrectly after an overhaul.