A ship or boat have limited maneuverability ( the ability to change direction ) with rudder and propeller. In congested waters, near port area’s and emergency condition a ship may be required to quickly move backwards. While the popular way to do is to take Williamson turn but the quickest and easiest way is to reverse the engine.
Reversing the marine engine turn the propeller with the same force but in opposite direction then before. For an ship with fixed pitch propeller this is done by; reversing the engine direction or using an reversing gear all together. On another hands controllable pitch propeller give freedom to achieve same results by rotating the propeller pitch itself.
Large ships with two stroke marine diesel engine are reversed by changing timing of their fuel injection. Let us understand in details why we need to reverse ship in first place; and how it affects the working of the engine.
Why Are Ships And Boats Reversed ?
We all know that the ship moves forward in the water pushing it backwards by the help of its propeller. In the process it produce huge energy in engines which it then convert into rotational torque. The rotational energy transmitted to the shaft then transmit it to the propeller giving it motion.
The propeller with its churning action cuts the water backwards creating equal and opposite force in opposite direction. This happens due to the newtons third law of motion. Now imagine if the ship only had propeller it can only move forward in one direction. So rudder are installed to help steer the ships.
But even they can’t do much when it comes to quick maneuverability specially in shallow waters. So bow thruster are installed to assist in the job. But still they can’t stop a ship and thus reverse thrust is required to slow down and stop the ship. It did the functions similar to what brakes did in cars in emergency condition.
Thus seeing its importance it is required by law to test reversing of engine in commercial ships before leaving the port or harbor. A ships informs all about the astern propulsion of ship by blowing three short burst of air horn followed by a sierra signaling flag.
Reversing Two Stroke Marine Diesel Engine
Two stroke marine diesel engines are simply reversed by changing the fuel injection timing. This is possible as unlike four stroke engine a two stroke engine have almost identical timing diagram with an exception of fuel injection. Thus the fuel pump timing is reversed by either changing the cam follower relative position; or moving cam of fuel pump.
We have already discussed the scavenging and exhaust timing for two stroke engine in astern and forward direction are identical. Let’s see in details how this happens and helps in reversing process.
Role Of Timing Diagram In Reversing Procedure
It takes complete 360 degree of rotation to complete one cycle. If you closely observe the timing diagram for an two stroke marine diesel engine; with an constant pressure turbocharger you will see the following things:
- Compression starts at 110 BTDC and lasts till 10 BTDC when fuel is injected within the cylinder.
- Now since the fuel is ignited 10 degree before TDC it ignites under pressure and power stroke starts at 12 ATDC.
- The power stroke last between 12 ATDC and 110 ATDC till the exhaust valves is not open.
- The exhaust valves opens at 110 ATDC and hot fumes get out of the engine in a process known as exhaust blow down.
- The exhaust blow down last for 110 To 140 ATDC.
- Scavenge port opens at 140 ATDC and the whole scavenging process complete from 140 ATDC to 140 BTDC.
- There is a brief period from 140 BTDC till 110 BTD called post scavenging which then succeeded by compression stroke.
For normal operation of an engine the correct order of important events at exact BTDC and ATDC position is as discussed above. Now If the engine is reversed the whole process reversed on timing diagram. For example the exhaust valve will open at 110 BTDC while Scavenge ports Closed at 110 ATDC. This ensures the same timing for exhaust and scavenging process in both direction saving the need to have two different sets of cam.
The only problem of fixing the fuel injection timing is then solved by changing the relative position of the fuel pump cam follower; rather than the whole cam assembly.
Reversing Main Engine In Simple Steps
Any marine diesel engine weather be two stroke or four can be reversed by shifting the operational cam axially. In this arrangement the engine have two separate cam’s one for ahead and one for astern direction. Both these cams sits next to each other on the same shaft and are equipped with hydraulic cylinders.
The hydraulic cylinder put appropriate force required to shift the operating cam to the left or right based on current direction. Many engine also use an air cylinder with high air pressure to do the same job. Once replaced the camshaft is locked to its position with the help of locking device.
But for modern two stroke engine this can be easy done by either of the two technique; by rotating the fuel pump cam by its lost motion as in sulzer or simply moving the relative cam follower position on fuel cam as in B&W engines.
1 ) MAN B&W Type Engine
In MAN B&W engines fuel pump operates by raising plunger on each each peak of the cam follower. This ensures fuel injection till the cam follower remain up top till the next stroke starts with the follower returning to its base circle. Based on design the engine would have either reversible pneumatic cylinder or an hydraulic one.
Now when air is passed to these pneumatic cylinder from the starting air line; the piston within the pneumatic cylinder moves to the other side moving the follower position relative to cam position. Additional safeties and locking arrangement then ensures that the follower remains to new position; if not the fuel injection will be halted.
The reversing of engine should be done only while the engine is operational ( i.e turned by starting air ) and not while its stopped. This ensures that the follower does not moves beyond a desired degree producing undesirable results damaging valves and systems.
2 ) Sulzer RTA Type Engines
A sulzer engines use the technique known as lost motion control by rotating the cam itself in astern direction with the help of hydraulic pressure. The hydraulic motors used for this purpose is called lost motion servo pumps. To understand the concept of lost motion let us consider a simple example of engine with piston just before TDC.
Now if the engine is rotating in forward direction the cam follower will operate the fuel pump as it start to reach the peak of the cam profile. Similarly if the engine is in reverse the engine will just be going to stop injection as the follower just moves out of the cams peak. Now this angular difference between two cam positions from peak to peak is called lost motion.
High pressure oil is applied to the vanes in between the cam profile to rotate by the hydraulic lost motion servo pumps. It rotate the cam by an angle of lost motion making the engine to switch injection timing to astern direction.
Reversing Small Engines In Boats And Small Ships
Small ships and boats does nor really reverse the engine to reverse the propeller motion. Instead they use gear box assembly with a reversing gear fixed with an idler. The shifting between the gears are many a times achieved by using reversible hydraulic coupling or by using friction clutch.
In simple the complete system consists of at least five gears; forward pinion, reverse pinion, idler, counter shaft gear and driving gear. Together they are connected with the drive or power shaft with forward and reverse clutch to make or break contact. For simple design many small ships also use electrically driven shaft.
They can be easily be reversed by changing the direction of current thus to produce revere torque; rotating shaft and then propeller in opposite direction. Similarly ships with controllable pitch propeller change the pitch of propeller to perform reversing operation. This is achieved by rotating propeller blades normal to the hub of propeller.
Best Reversible Motor Engine for Your Recreational Boat / Yachts
I know what you expect from your boat or yachts; to be fast, runs great, is economical, came with an easy to follow instruction, can be easily reversed and does not require frequent repairs or maintenance. Although there are plenty of engine out there in the market which satisfy one or another above criteria; i personally go for two stroke inflammable boat engine specially that of 6 H.P from “hangkai” or “suzuki”.
The reason i like and recommend it is the dual method to reverse the engine. Either you switch the knob to reverse and let the gear box do the job or move the propeller all together on a 360 degree steering rotation. These are the outdoor motor drive type and are self contained requiring less care.
These are mounted at the rear of your recreational boat, small yachts or fishing boats; ready to be used in both fresh and salty water condition. Usually they came under the range of 800 to 1500 dollars depending upon season and time. So its wise to keep an eye on their current price on amazon before making decision; to just buy at the right time. You can check on its current price from link over here on amazon.
Practical Implication Of Reversing Ship In Emergency Condition
Many a times during emergency to avoid collision or possible grounding a ship is required to quickly operate under reverse condition to stop as quick as possible. Such a process of moving from ahead or forward direction back to reversed direction propulsion is also known as crash maneuvering.
This first reduce the speed of ship drastically then stop and move in reverse direction. Since the engine is normally runs at near its maximum continues ration. We simply cannot reverse the engine at same speed. Even if we change the cam or its follower position the fuel will not inject; thanks to the running direction interlock.
The engine speed is first reduced by passing short bursts of starting air in reverse direction. Once it reach zero fuel is injected according to reverse injection timing. Since its done in emergency many secondary alarms and trips are often bypassed during operation of such a maneuvering.
Note: The article is produced on request from Saurav kumar.
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