A. "What is a Jet-Boat??? A Jet boat is nothing less than a boat, which is propelled by a jet of water. These vessels do not have a ship propeller underneath or at the back. They do not even have a rudder. There is a hole underneath the hull where water is sucked in. A reduced tube sticking out of the stern leads the water out at a high velocity. Steering is achieved by bending the water jet in the counter direction where you want to go. Jet boats are commonly used where boats with ordinary propellers cannot be used. For instance safety is an important reason to use a jet, like in jetskis. Or vessels that have to operate in shallow water where airboats are not an alternative, like for example rescue boats of the coast guard. Another application of jet drives can be found at large high-speed ships. At a certain level of water movement a ship propeller cannot push the boat any faster, regardless of the amount of power that will be used. The propeller blades cause a vacuum, which is called "cavitation". With a jet this problem does not occur. A jet drive is generally even more efficient than a conventional ship propeller. But the cost of a single unit does not cope with the advantages

B. Advantages of a model jet-boat: No parts are sticking out or under the hull. Compact mounting of the drive is possible due to the missing of the propeller axis. It is possible to run these models in shallow water (*). Many conventional models can easily be converted to a jet drive, see also the site of Allan Teo. The "propeller" is located inside the model. No damage is possible (*). You can run these models in white water. Just be careful not to lose your model when the battery runs out. A boat display stand is not really necessary. (* not completely true, see disadvantages)

C. Disadvantages of a model jet-boat: Model Jet-drives are not available in many model hobbyshops. The jet inlet can suck waste in which can cause the model to stall. Objects in shallow water up to 20cm deep can be sucked in the jet drive. This can damage the impeller A model jet-drive is not easy to install. If you built in a jet-drive inside a model it cannot be taken out without damaging both your model and the jet-drive. Model jet-drives need good maintenance to prevent trouble. Almost every electric jet model can only be operated with a speed 700 class motor. This results also in the use of at least 8 NC cells. Expensive motors and speed controls are inevitable.

D. Is a jet-boat good for beginners??? Yes. Personally, I think a electric motor driven jet boat is an ideal first timer fast model ship, even if this person has no RC experience whatsoever. Due to the compact dimensions of the jet drive there is enough space left inside the hull. These models can achieve high speeds. This will appeal to first timers for sure. In addition to this these models are also highly manoeuvrable due to the vector steering, as this is used also in the most modern jet fighters like the X-33. All moving objects are located inside the model. When a crash is inevitable (and being a first timer you will) the damage will be very limited. This also concerns transportation of the models. A model jet drive is friendlier for your fingers also. It has happened to all ship modellers once that their models started to operate still on top of the boat stand (caused by yourself). Unfortunately, there are no beginner models for sale today. For a showcase see "kits for sale". Model jet boats are still not very common. There are not very many modellers who can help first timers with problems. Even at the hobby shop they cannot provide much information. A tip: buy as a first timer a model where the jet is already build in.

2. CONVERTING TO JET-DRIVE SYSTEM A. What kind of hull does the model have??? Before you can think of a conversion you should first check if it is possible to build in a jet-unit in your model. It will be possible in most cases. If you want the Mini-JET for instance you need a model with a thin ABS hull. The jet inlet has a forge, which has to be glued inside the hole in the hull. Almost every other jet-unit must be bonded inside the hull with Epoxy resin and glass fibre reinforcement. After that you cut out and sand the inlet from outside the hull. The wall thickness of the hull is no barrier anymore. If you want to use a single jet-unit you must take a good look at the hull. The unit must be built in the middle of the hull. Most boats have a V-shaped hull. This makes it often difficult to bond a jet-unit on top of it. Its need a flat surface so that it can be glued on top. A solution can be to flatten the surface inside the hull where you want to build in the jet-unit. After that you can glue in the jet and cut out the inlet. The only problem can be turbulence before the jet inlet during high speeds. It is also possible to build in 2 or more jet-units. An even number eliminates the problem with the V-shaped hull. To start your boat correctly the model must be far enough submerged so that the whole jet-drive is under water. Otherwise you could get problems with getting on step. Do not forget to close and to smooth the hole for the propeller axis. This is situated before the location of the jet-unit in most cases.

B. Propellor vs. Jet-You cannot copy everything from the configuration of the propeller driven version of your model. The centre of gravity must be more to the front since the jet tends to suck the stern downwards. You can say that the centre of gravity will be at the half of the length of the vessel in most cases. With a propeller driven version this would be at 1/3 of the total length from the stern. Another big issue is which motor to use. Cheap speed 600 type motors often propel prop driven boats. Because of the small size a jet-drive creates more space in you hull for Batteries. So it will be possible to use a speed 700 motor in a model which was designed for a 600. This causes the model to run much faster than it actually was meant to go. I do not think anybody will complain about that :) A good example is the SeaJet from Robbe. Prop version: Speed 600 8.4V with 7 NC cells, total weight 1450 grams. Propulsion system: 2 contra rotating propellers. Jet version: Speed 700 9.6V type with 12 to 16 NC cells, weight approx. 3200 grams. Propulsion system: Kehrer 28mm jet. All the running hardware will be more expensive which is a disadvantage. An alternative can be the use of two Mini-JET drives. You can still use the cheap engines, but this does not eliminate the need of double batteries. You can easily connect two batteries in series. If you have two 7-cell batteries you can turn them into 14 cells by just connecting them to each other. Make sure the batteries are of the same type and capacity. Otherwise you could get serious trouble.

Cooling- Electric motors as well as gas engines must be properly cooled. The jet units suitable for gas engines do have a standard connection for water-cooling inside the jet outlet. They should also provide this with electric jet-units, manufacturers!!! A water-cooling for an electric motor can easily be made. Just take 4mm aluminium tube for speed 600 and 5 up to 6mm for speed 700 type of engines. Some engines do have their own cooling coil built in. Just wrap the tube kindly perpendicular around the casing of the electric motor. To fill the tube with sand is not needed. These tubes can be bought at your local hobby shop. You can buy a lot of different "water-scoops" on the market these days. These scoop the water out of the pond just by the speed of your model. You can install these with extra wide fuel tubing. Be sure this system is well installed. If there is a leak or a tube slides off, your model will be filled with water in just 10 seconds!!!" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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