Why? Prop Question

[LubeDude]

Why when I turn left, does my engine gain about 500 RPM? Not trimmed out too much, and I know the engine isnt too high, Im even thinking about raising it another hole.<br /><br />14.25, SS four blade Turbo 1

 

[mattttt25]

Re: Why? Prop Question

it has to be ventilation of some sort. too much "smarttabs" and "x dimmension" advice followed is raising your entire boat out of the water.

 

[Jilly - 5]

Re: Why? Prop Question

For a very technical reason. A single RH prop, running straight ahead, will try to push the stern of the boat out to starboard. That's why there is the small trim tab on the rear of the cavitation plate, aft of the prop. This turning tendancy comes from the greater opposite force on the lower prop blade cutting into "harder water" than the upper blade cutting through "softer water" (more airated). When you turn to port, you alleviate the "hard water" pressure on the blade making the bottom part of the revolution, giving rise to increased rpm. You should also notice a fall-off in rpm when you turn to starboard, as this helm move increases the lower blade pressure, thus increasing prop loading. Note that a prop does not have a "lower blade" vs an upper blade. I'm talking about the blade's position during a given revolution. <br /><br />You will also find that a four-blade prop has decreased tendency, and a five-blade has virtually noe at all. That's one of the major reasons submarines have 5 and even 6 blade props in a single screw situation. Don;t want the prop throwing off the stern on a tube-shaped vessel (they're also acoustically quieter, again for several technical reasons)).

 

[Richard Petersen]

Re: Why? Prop Question

Jilly is right. Unless you race the boat. Forget it.

 

[umblecumbuz]

Re: Why? Prop Question

Don't fully agree with Jilly this time round, although previous posts have been great.<br /><br />The principal reason a sub has large-diameter multi-bladed props is because they can turn slower for a given speed, adding to their silent-running characteristics.<br /><br />The old rule holds just as true of props as it does of bullets from a gun - 'Every action has an equal and opposite reaction.' <br /><br />If a prop is turning clockwise screwing into the water, it is trying to turn the water around it anti-clockwise. This produces a twisting moment in the direction of travel - ie. the boat tries to turn to starboard. The trim tab under the Anti-Ventilation (not 'cavitation' - that's a different thing entirely) plate is shaped or angled to minimise this tendency.<br /><br />This will happen whether the water is more 'aerated' at the top of the prop than at the bottom, or not.<br /><br />That is why it is often harder to turn to port than to starboard - because turning to port is going against the turning moment of the prop. As a result, it can lose 'bite' more easily when turning to port.