Marine transmissions

1. Explain the different type of gear selection systems (in outboard gearcases and stern drivers and Inboard marine gearboxes)

For outboard… a dog clutch is used which consists of a gear which is on the output shaft that slides in either direction to lock the forward or reverse gear to the shaft. the gears are in what’s called a gear case which is in the bottom of the outboard system, different to the set up of a sternleg or an outboard where the gearcase is in the top of the leg. The stern leg uses a similar dog clutch system, except it uses coned syncro hubs similar to that as the syncro cones on a manual gearbox. this is so there is smooth sync as the gears are selected instead of the rough clacking made by the teeth from dog clutch as on an outboard. The gear is engaged once the cone/selector is pushed more into the gear and becomes tight enough for the two components to engage.

A dog clutch system on an outboard

2. What type of gears are used in outboards and stern legs? (Give an explanation of why manufacturers choose this type of gear)

Helical type gears are used for gears in the outboards and sternlegs. The gears are angled at 45 degrees so that they can mesh properly as there is one shaft, meshing with another shaft at an angle of 90 degrees.

3. Explain what a duo propeller system is and how it works/operates

 

(Image from: http://www.volvopenta.com/SiteCollectionImages/VPC/Drives/758x228/ML_DPS_Duoprop_758x228.jpg)

Duo prop design:

• One of the main benefits of the IPS and DuoProp is highly increased propulsion efficiency. This means that less engine power and fuel consumption. In water, propellers function better when they are designed to pull, instead of push. Because of the placement of engines at the rear of most boats, however, this propeller orientation is rare. The DuoProp overcomes this limitation by using two propellers instead of one.

  Like traditional shaft-driven boat systems, the system features propellers that are fully submerged. This, however, is where most similarity ends. Instead of standard propellers, IPS employs two propellers per unit. These duo props are counter-rotating. One propeller spins clockwise, while the other rotates counterclockwise. This reduces wasted energy and results in better efficiency through the water.

(Info from: http://www.ehow.com/how-does_5336911_volvo-outdrive-duoprop-works.html)

Volvo IPS system:

(Image from: http://www.tomjenkinsyachtsales.com/img/news/13_Volvo%20Penta%20IPS.jpg)

4. Why would a V drive be used instead of a standard shaft drive system 

V-drive is a propulsion system for boats that consists of two drive shafts, a gearbox, and a propeller. In a "V-drive" boat the engine is mounted in the rear of the boat and the front of the engine faces aft. Connected to the rear of the engine is the transmission. The first drive shaft connects the rear of the transmission to a gearbox mounted in the centre of the boat. The second drive shaft extends from the gearbox to the rear and out the bottom of the boat to where a propeller is mounted.

(Info from: http://en.wikipedia.org/wiki/V-drive)

V drive diagram

 

(Image from: http://www.gknservice.com/typo3temp/fl_realurl_image/for-remote-v-drives-3c-17-91.jpg)

With the standard shaft drive setup, the engine is further forward in the hull with a shaft driven straight to the propeller.

(Image from: http://www.michiganmotorz.com/images/article/Inboard.jpg)

5. Compare a jet unit with a surface drive unit and give reasons why you would choose one system over the other

Jet unit:

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 below 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 (F) to turn the impeller.

Reverse is achieved by lowering an astern deflector (E) into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.

(Info and image from: http://www.hamjet.co.nz/hamiltonjet_waterjet/how_a_waterjet_works)

Surface drive:

(Image from: http://www.francehelices.fr/surface-drive-system-uk.htm)

What are surface drives, and how do surface drives work? Put quite simply, surface piercing propellers operate half in and half out of the water in the planing wake region of a boat just behind the boats transom. Having the propeller at the surface level reduces drag and reduces the vessels draft, making the surface drive suitable for many applications where the owner is looking to reduce fuel costs, increase speed and create a more efficient vessel.
There are advantages and disadvantages for all propulsion systems for water craft. But for planing and semi-planing vessels, surface propulsion has several distinct advantages over its competitors in the propulsion marketplace.

(info from: http://www.seafury.com/a/About-Surface-Drives)

Over all, I would choose to have a Jet unit rather than a surface driver as the propeller is inside the mechanism and wont be disturbed by any external obstructions, and because the water is being sucked from under the hull,  so it doesn’t matter how shallow the water can be, it will still be able to drive without a propeller hitting the sea floor.