Pancake Perkins diesels

So it's a boat already fitted with the Perkins. Interesting first post was they are counter rotating engines. I never gave that a thought at first.

Normally counter rotation is from the gearboxes ........counter rotating motors often bring big problems like unobtainable starter components

Been around a lot of boats for a lot of years and from my experience most twin-screw boats have counter rotating engines.

Both these have been rebuilt and run with around 10 low hours while the hydraulic pump injection system is new.
The other flybridge cruisers with the twin 504 Cummins weren't opposing rotation, nor the Volvo TAMD 60;s, in fact looked over a Detroit diesel twin installation which wasn't reverse rotation either.

All shaft drives. Thanks everyone.

Here is the story on twin props. This is on aircraft but boats the same.

 

Because the shaft on a boat slopes down to the back (in an aircraft the nose sits up) the pitch on the down going prop is more than the pitch on the upgoing prop = more thrust.

In an aircraft with same direction props one side is called the "critical engine" because the thrust line is out further as in the diagram. On these aircraft - most light twins with same direction props, you run out of rudder to keep straight at a higher speed. That is why the aircraft is held on the ground longer than strictly necessary until a safety speed is reached. The aircraft will fly below this speed but if you lose an engine you will not have enough rudder to stop the aircraft turning and certainly will crash.

On a boat with same direction props, the combined thrust line is outside the keel line to one side as in the aircraft picture. The rudder will be offset to counter the turn effect. You can see the rudder on aircraft "A" trying to counter the swing from an off-set thrustline. You do not notice this at the helm of a boat but it certainly creates inefficiency and drag. Such boats turn better in one direction (particularly in reverse) than the other. For most pleasure boat use this does not cause great problems other than having to be aware in tight situations.

With contra-rotating props you have a choice of the downgoing blade (extra thrust) on the inside or outside. He has drawn the right prop on aircraft "B"  incorrectly but if the blades turn toward the centre the boat will track nicely in a straight line more like a single prop. If you want maneuverability say in a game fishing boat or a tug the props turning outward may be the go placing the thrust lines further apart

Stirling bombers from early WW2 were the first 4 engine bombers to be used in WW2, maybe just the allies, anyway 
As I understand it these had all 4 engines spin the same direction and could be quite handful when pooring on the power as the aircraft would shift sideways and could easily be crashed 

I'm fairly sure that's correct, someone can correct me if Im wrong
later in the war these were taken off bomber duty and used to tow gliders, when  the gliders were released the plane would also shift sideways, with the change in load I guess 
The Stirling was huge, bigger than Lancaster and Halifax and flying Fortress, maybe the Super Fortress got as big as the Stirling 

But the Stirling had a very low ceiling and suffered huge losses because of this 

I'm no expert on any of this stuff but I have done some reading over the years, so please forgive me if I have this all hashed up

 

Paul

A lot of WW2 bombers had one direction props including B-17 Flying Fortress. They had a tail fin not only huge with lots of authority but offset slightly a few degrees to compensate. Many aircraft have this compensating fin offset. As I said with boats the rudder is permanently slightly offset to compensate on same direction props. Some rudders have a little trim tab at the back to offset the load so it cannot be felt through the steering. All aircraft have this trim tab because of the high sensitivity. Advanced aircraft can adjust the tab as speed changes for aerodynamic compensation as the speed and thrust change so you do not have to keep your feet on the pedals to hold the aircraft straight. The little blokes have that rough fixed tab and you just wind the hand wheel to pull on springs or bungies in the rudder control wires to take the pedal pressure off your feet.

This simpler permanent trim system (a bit of tin that is bent to take out the greater turning effect of offset thrust and aerodynamic pressures) is common to almost all light aircraft. The same thing is seen on boat rudders to take our asymmetric turning loads - saves you having to hold wheel pressure in one direction.

 

Of course we all know the trims on outboard motors that compensate for asymmetric prop thrust - downgoing blade has more thrust than upcoming blade. Anyone who has done water skiing with a high powered outboard (single or worse twin) will know the huge heave required to keep straight on takeoff until the water flow starts acting on the hull.


 

If the boat engines are hooked to a gear box there may be an option to reverse rotation.

Easy

  www.youtube.com/shorts/XSoOg9VX_sQ?feature=share

Obviously you need a whole new prop as you can not run a prop backwards with any efficiency.

mammoth wrote:

If the boat engines are hooked to a gear box there may be an option to reverse rotation.

A lot of marine gearboxes do have the ability to provide a reverse rotation.