Outboard Jet Drive

This simple conversion kit can transform your boat and get you in to locations where normal propeller driven hulls are unable to operate, such as shallow rivers, white water rapids and even over sandbars or where there is a high risk of propeller injuries to personnel or marine mammals. Jet drive conversion kits are available for most Yamaha models from 25hp up to 300hp. The Jet drive conversion is particularly well suited to Yamaha 4-strokes, they come as standard with the same dependability, fuel efficiency and quiet that you've come to expect from the leader in outboard engine technology. 

Newton's 3rd law
“For every action there is an equal and opposite reaction”

Unlike a conventional outboard which uses a propeller to screw it way through the water a Jet outboard works by sucking it into a chamber where it’s is compressed and ejected through a nozzle at high velocity. 

For more information on outboard jet drive conversions contact Alan on 01463 236539

Frequently asked questions

1. What is the best type of boat to use with an Outboard jet?
The choice of boat is very important. The best is a flat bottomed or dory style boat with or slight vee of less than 10° per side maximum. Polyethylene or Aluminium is the best as it is light, tough and easy to repair with minimum maintenance. Fibreglass is heavier and shatters on impact. Inflatables with flexible bottoms introduce air in to the intake and RIB's tend to have too much vee.

2. Does the weight of my boat matter?
With a jet drive weight is extremely important. Contact Alan (01463 236539) for an indication of power that you will require for your boat. Remember to allow for the weight of passengers and equipment.

3. Will I lose horsepower if I convert my propeller to a jet?
Yes, approximately a 30% HP loss - multiply your motor HP by 0.7 - example: 50 HP with jet is approximately equal to 35 HP prop drive ( 50 x .7 = 35)

4. What speed will I get?
It depends on the boat and the weight and trim of the boat. A good setup will get about 80% of propeller speeds: i.e a boat fitted with a 70hp propeller outboard might achieve 30knots but the same boat fitted with a Yamaha F100DETL with a jet conversion producing the equivalent thrust as a 70hp propeller outboard will still only achieve 25knots.  

5.  Can we change jet impeller pitch for load or speed?
No, unlike a propeller drive, a jet can develop full power at a standstill. The impeller that comes with the outboard jet conversion kit gives the rated factory RPM at full throttle. RPM hardly varies between a heavy load and running fast with a light load.

6.  How about a heavy load?
A heavily loaded prop boat using a low pitch prop can carry much more weight than a boat with an outboard jet.

7. Will I have to raise my motor up on the transom for a jet?
You must raise the engine approximately 6 inches higher on the transom than with a propeller until the leading edge of the intake is flush with boat bottom. 

8. Can I put a short shaft on a long shaft motor so I won't have to raise the transom?
No. The jet drive shaft length must match the motor shaft length, or you would have to change the entire mid section of the motor.

9. Why isn't there any kits for engines less than 25 HP?
We lose about 30% of the HP when converting from propeller to jet drive. With less than 25 HP, it is difficult to plane a boat with more than two people. 

10.  How long does it take to get a kit?
All outboard jet kits are built to order therefore it can take between 4-8 weeks to fulfil an order.

11. How far does the jet go down into the water?
The water intake grill slopes down to the rear to give a scooping effect. The leading edge is flush with the bottom of the boat. The trailing edge is about 50mm lower.

12. Does a stainless steel impeller improve performance of a jet drive?
Stainless steel impellers work well on motors above 100 HP. They give less slippage on start up and therefore can pickup a heavier load. Top speed of, maybe, one to two mph or better. Below 100 HP, the main advantage is longer wear life.

13. How do you flush a jet that doesn't have a flushing hole?
On jet drives in which the cooling pump is attached to the top of the bearing housing, it is not possible to provide a flushing inlet. Immersing the jet drive in a barrel of water doesn't work either, because the water is thrown out. Backing the boat into some fresh water is the only solution. 

14. How long does it take to change from prop to jet?
About 2 hrs; more the first time, less after you have done it. Raising the boat transom approximately six inches takes longer, depending on how you do it. If you plan to switch between prop and jet, depending on water conditions, winter, summer, etc., purchase a second cooling water pump so that each, the prop and jet drive, have one installed and you don't have to transfer the pump back and forth.