Off the top of my head there are 3 ways of propelling a boat, either through the use of a conventional propeller, an impeller (or ‘jet drive’), and aircraft style propellers typically found on flat bottomed air boats used in marshlands where below waterline propulsion is impractical.
For most of us the choice will come down to propellers or jet drives, with props generally ruling the waves in most harbours.
But why is this the case? Why don’t jet drives enjoy the same level of popularity?
Well in simple terms jet drives are great, but only in very specific circumstances, much like airboats. The impeller of a jet is generally a smaller item than a prop designed for a boat of the same size, and being fully encased, it is well protected from its environment. As such jet drives are perfect for flat bottomed vessels (Jon Boats) used in shallow, rocky rivers, or in applications where personal safety around the prop would be an issue, such as when water skiers climb in and out of the stern of the boat.
Advantages of Jet Drives
Jet drives offer a lot of practical advantages over propeller driven stern drives or outboard motors, mostly related to the fact that they are safer in conditions where impacts with foreign objects are likely. As mentioned above, they’re great in shallow waters, but inboard jets can also be backed up on to beaches given the absence of any protruding parts including rudders.
At very high speeds jet drives have been documented as being more efficient than props, and they’re certainly quicker to accelerate.
In addition if you ever need to stop quickly, then in theory a jet drive is the best means to do so. Why so? Well by slamming the deflector plate over the jet stream and immediately reversing the flow of water you get a much more immediate effect than having to wait for a prop to get up to speed in reverse.
Just be sure to hold on tight if you ever do this!
Disadvantages of Jet Drives
Whilst jet drives operate well at speed, when it comes to travelling slowly they aren’t great at all. The reason for this is that the effectiveness of the directional control of the ‘jet’ (the flow of water expelled by the jet that provides thrust) is more or less proportional to the speed.
Thus, lower speed = poorer directional control.
Jet boats aren’t too hot when travelling in reverse either; given the jet can only be directed aft, you’re reliant on the redirection of thrust as a result of lowering the deflector plate over the jet exhaust. This isn’t as effective or efficient as simply reversing the direction of rotation on a normal propeller, and the results can be sluggish and difficult to control.
Outboard motors and stern drives have the distinct advantage of being possible to trim; that is to say you can tilt the engine or prop with the effect of raising or lowering the bow of the boat. This enables you to tailor the trim of the boat to suit the conditions or speed, for example by trimming the bow up to cope with a choppier sea or travel up on plane at a higher speed.
Fixed jet drives do not have this luxury, and most outboard jet drives are usually mounted so high on the transom that trimming is ineffective. This make jets a far less comfortable ride in more challenging sea states.
Jet drives are certainly safer than props from an embarkation/disembarkation point of view for water skiers and wakeboarders, in as much as the engine can safely be left running at all times without the risk of anyone being seriously injured by the incased impeller.
However the actual performance for anyone being towed by a jet boat is far from ideal. Jets produce a a very aerated ‘bubbly’ wash that can easily throw a skier/boarder off course.
To make matters worse inboard jet drive boats don’t feature rudders or skegs, making them far less stable in the water, and consequently far more likely to be pushed off course by the action of the person being towed pulling hard on their towing line. If the boat then loses control you can imagine what happens to the person behind the boat.
How Does Jet Drive Compare to Prop Drive?
The major difference between propellers and jets is in the output performance. Inboard jets are slightly more favourable in this respect, but comparing like for like with an outboard with a prop and the same outboard with a jet conversion, you’ll generally find about a 30% loss in horsepower in the jet version, largely because the path the water takes from the intake to the jet exhaust is somewhat convoluted, making for an inefficient transfer of energy.
Inboard or Outboard Jet Drive?
As touched on above, in much the same way as inboard and sterndrive/outboard propeller drives, jet drives broadly come in two distinct flavours: inboard and outboard.
Outboard jet drives, whether in the form of a converted outboard or an off the shelf model are the most popular choice, and indeed if you wish to experiment with a jet, opting for a conversion to the lower unit of your outboard is going to be the easiest and most cost effective way to do it.
As far as I know Outboard Jets in California are the only company offering outboard jet conversions. For a couple of thousand bucks you can convert your standard outboard to a jet outboard. This could be particularly useful if you’re planning on taking your boat to shallower, rockier territory, but with a view to converting back at a later date.
Outboard jets protrude below the bottom of the transom, and as such are prone to being knocked in shallow waters.
Inboard jets are more of a commitment, and if retro-fitted require extensive cut and shut work on your boat’s hull.
However the major advantage is that the water intake is flush with the hull underside, whilst the jet outlet is on the back of the transom, all of which means that the boat has no protrusions below the hull, making inboard jets the ideal solution for flat bottomed Jon boats in shallow river waters.
Maintenance Compared to Inboard and Outboard Propeller Driven Engines
Jet drives typically require a service every year or every 100 hours, depending on the manufacturer’s instructions. Typical service maintenance includes an oil change, oil and air filter changes, greasing of moving parts.
If you plan your service along with ‘winterizing’ your engine you’ll also need to top up the antifreeze (if your engine has a closed coolant system) and any other procedures in the manufacturer’s instructions.
These procedures apply regardless of whether your engine has a prop or jet attached to it, however there are a couple of clear advantages of jets that make them slightly less of a headache than outboard propeller driven engines in particular:
- On a standard outboard motor the lower unit houses the gear case and propeller; as such it requires its own oil change, which must be factored into a maintenance schedule, both in terms of time and cost.
- Propellers are prone to being damaged, often beyond repair, if the propeller unintentionally strikes a solid object when in use. This can make for a very expensive repair indeed!