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Outboard, Sterndrive, Jetdrive?

Comments: 82 Comments
Upper page: Marine Engines

 

The three most popular types of propulsion systems for craft under 50 m in length overall are the Outboards, the Sterndrives, and the Jetdrives

Each boat, depending on its design and intended use, will require different types of propulsion. Most recreational boats in the United States today use outboard engines and are less than twenty feet in length.

 

 

Outboard

Outboard motors are popular and quite useful on smaller boats. They are light and powerful, and modern outboards are extremely quiet. The outboard provides a completely self-contained propulsion system from engine to transmission to shaft and propeller. They are most often mounted directly on the transom of the boat, however, you may find boat designs incorporating a motor well or bracket on which the motor mounts. The entire motor swivels to provide easy steering as the turning propeller pushes the stern.

Outboards come in a large range of sizes and horsepower and can use different fuel sources. From small electric trolling motors to gasoline-and-oil-mixture two-cycle engines to gasoline-only four-cycle engines to diesel-powered outboards, the selection is large.

 

 

Inboard/Outboard

These are also referred to as I/Os or stern drive engines. Stern drives are generally heavier than outboards. They consist of an engine mounted inboard and a lower unit attached low on the transom. This lower unit resembles the bottom part or lower unit of an outboard motor. The outdrive or lower unit part swivels from side to side to provide for the steering of the boat. It can also be tilted up and down to provide boat trim while underway.

I/Os come in both gasoline and diesel models, and larger ones generally have more power than outboards. Because the main power supply is similar to a small automobile engine, easily accessible and more powerful, stern drives are often favored over outboards, especially on larger motorboats.

 

 

Inboards

These engines are most popular on motorboats over twenty six feet in length. The engine, similar to the inboard/outboard, is mounted inside the watercraft toward the center to give good weight distribution.

The engine connects directly to a transmission, out of which comes a shaft which goes through the hull of the boat as it passes through the “stuffing box.” The shaft is then attached to a propeller that turns to propel the boat. (The stuffing box is a cylinder through which the shaft passes. The shaft is surrounded by a stuffing material that, when compressed between the cylinder wall and the shaft, prevents excessive water from entering the boat.) Because the shaft is fixed and does not swivel from side to side, a rudder is mounted behind the shaft and propeller to deflect the flow of water and provide steering direction.

 

 

Jet Drive

Diagram of a jet drive.

These propulsion systems do not have propellers, which are a potential danger to people in the water and to marine life. Jet drives are usually inboard engines that take in water that flows through a pump powered by an impeller. The water is then discharged at high pressure through a nozzle propelling the boat forward. The nozzle swivels to provide steering to the boat. Most personal watercraft use jet drives.

A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. It works in relation to Newton’s Third Law of Motion – “every action has an equal and opposite reaction”. A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful fire hose.

Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft’s hull, propelling it forward (see diagram below).

In a boat hull the jet unit is mounted inboard in the aft section. Water enters the jet unit intake on the bottom of the boat, at boat speed, and is accelerated through the jet unit and discharged through the transom at a high velocity.

The picture opposite shows where water enters the jet unit via the Intake (A). The pumping unit, which includes the Impeller (B) and Stator (C), increases the pressure, or “head”, of the flow. This high pressure flow is discharged at the nozzle (D) as a high velocity jet stream. The driveshaft attaches at the coupling (E) to turn the impeller.

Note: When power is not being applied, a jet driven watercraft loses its steering because it is the stream of water that steers the boat. Keep hands, feet and hair away from the pump intake and do not operate in shallow water.

Choosing the right type of propulsion system for your boat is a very important matter. Both its weight and horsepower will both have an impact on the performance of your boat. If your boat is underpowered, its engine will work hard continually and will provide poor performance. Additionally if your boat is overpowered, it may exceed the safe operating speed that was designed for the watercraft.

In recreational usage, a jet drive is usually coupled to a higher horsepower motor than would be used in a prop driven application to arrive at a similar speed.

 

 

Jet Drive Coupled to an Inboard Engine

Marine jet drives are typically powered by marine gasoline or diesel engines
ranging in size from 50 HP (38kW) to well over 500 HP (372kW), and are
used as an alternative to propeller systems.

 

 

 

 

 

 

 

 

 

 

Jet Drive Coupled to an Outboard Engine

Marine jet drives may be powered by two stroke outboard engines ranging from 25 Hp (19 kW) to 200 Hp (149 kW).

Jet drive outboards are suited for shallow-water applications where light weight and acceleration are desired.

 

 

 

 

 

 

 

Jet Drive Pros and Cons

Pros:

  1. the jetdrive adds almost ZERO draft to the boat
  2. jetdrives have less moving parts
  3. jetdrives accelerate quicker
  4. jetdrives are more maneuverable at high speeds
  5. jetdrives use smaller, lighter, less expensive engines
  6. submerged objects cannot create propeller damage (no propeller); minimal damage in case of grounding or beaching
  7. propellers can injure swimmers; jetdrives eliminate this potential problem

Cons:

  1. jetdrives have impellers that get damaged and clogged easily with weeds, sand and foreign objects like gravel or plastic bags, specially in shallow water
  2. many jetdrives can’t be stored in water, very susceptible to marine growth because of tight tolerances  –> impeller needs to stay dry or moving, boat needs to be trailered or on a lift; propellers can stay submerged when not in use
  3. jetdrives typically consume more fuel than a propeller driven outboard
  4. jetdrives have lower top speed capability
  5. jetdrives tend to use higher RPM two cycle engines tend to be less reliable
  6. jetdrives, without a propeller or sterndrive acting as an “anchor” are not very stable or useful for tow sports since the stern tends to wiggle
  7. jet drives are usually much noisier than propeller drives

 

 

 

 

read more at:

http://www.boatingbasicsonline.com/content/general/3_6.php

http://www.boatdesign.net/forums/jet-drives/jet-drive-426.html

http://outboardmotorsforless.com/articles/parts-of-an-outboard-motor/

http://www.seabuddyonboats.com/motors-and-power/outboard-or-stern-drive-io-for-the-boat-show/

http://www.arsmarine.com/Blueprinting.htm

http://logan.ieidev.com/BellHousingPTOMarineApps

http://www.boat-ed.com/nm/course/p1-4_whatkindofengine.htm

http://www.mercurymarine.com/engines/jets/optimax-jet-outboards/

http://www.yamahaoutboards.com/outboards/Jet-Drive/overview

http://www.mercurymarine.com/engines/mercruiser/video/

http://www.castoldijet.it/en/waterjet_en/intro1_en.html



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