The Steamboating Forum

DetroiTug wrote:Reverse puts as much thrust on the engine, and you are far more likely to catch things with the prop' in astern.

That depends on how someone chooses to operate. Typically a small launch is backed up under low power. And most props are cupped or at least shaped to be more effective (more load) in one direction of rotation i.e. forward.

-Ron

Ron,

Newton came up with a very useful law- 'Every action has an equal and opposite reaction'- to push a boat through the water forwards at 1mph will require the same effort to go 1mph astern, albeit slightly different variations as boats are shaped to go fast forward, not astern, although at 1mph the resistance curve of any hull is only just biting, if not at all, in either direction.

I agree in the sense that most launches go astern slowly, but in actual fact the engine is usually doing more work because, as you say, the prop' is nowhere near as effective in astern. This means you have more power going through the shaft, however more of it is lost to slip- but this is academic as Newtons third law applies overall, whatever force is applied on the propeller will push on the shaft in the opposite direction with equal force.

Also, when it comes to an emergency stop to avert disaster, the last thing you want is an underrated bearing giving up- I have seen this happen before with the result the prop pulled itself and it's shaft through the sterntube and into the rudder gear.

Greg

Most propellers are geometrically designed for one direction of rotation. In other words to be most effective or push more fluid per rotation in one direction.

It is impossible to have a propeller optimized for one direction of rotation and have it equally effective (torque loading) in both directions of rotation for the same RPM.

-Ron

Ron,

I think we are both seeing the same point from different angles- to go a certain speed in either direction a boat will require a certain amount of power, that's a given.

Eg. In forward, if an engine is putting out 1hp @ 100rpm, and .75hp is effective through the prop with a resultant speed of 1mph- or 1mph @ 1hp with 25% prop slip.

In astern the boat will require .75hp to go astern at 1mph, however, as you say, the prop' is not as effective in astern, lets say slip is 50% in astern. This would mean that 1.5hp would be required from the engine to go at the same speed in astern with increased torque and power loadings on the shaft being the result.

Of course, as you say, for the same RPM the boat will be going slower in astern than in forward, but that is still the same RPM as forward, for which you are quite clear a thrust bearing is required.

At the end of the day the prop is more likely to hit the bottom in astern, and in an emergency stop situation the transmission should not be a point of worry if stopping suddenly is required, and bi-directional thrust bearing is almost certainly a necessity for true protection of the engine bearings.

Greg

Yes, it needs a proper thrust bearing. What I proposed secondly was not an ideal solution rather a may work if no other solution is possible approach other than relying on a snap ring alone - no external thrust bearing.

-Ron