# Propeller diameter influence on turning

Discussion in 'Props' started by TallyHo, Sep 2, 2014.

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### TallyHoNew Member

Hello,

New here, nice forums.
I was wondering if anyone of you would like to start some discussion about the propeller diameter influence, during turning.

I am a mechanical engineer, so i apologize in advance ... bear with me, this is mostly theoretical:

Assumptions:
- same hull
- same operating point of propeller (same open water efficiency in the working point)
- same rudder & rudder turn rate
- same environment (water depth, current, wind)
- same entry conditions (velocity, resistance of vessel, thrust)

Variable:
- only propeller diameter and rpm, say for example from 0.5 [m] / 200 [rpm] to 1 [m] / 100 [rpm], same thrust.

Setting:
Vessel enters turning area with..say 5 [m/s].
Rudder starts turning with 10 [deg/s].
End is whichever boat first completes 90 [deg].

Question:
Which boat does the turn in the least distance or time? AKA which one is faster?

Reasoning:
Con Larger propeller: less swift turning:
The larger propeller will have more drag, and will lose speed faster during the turn, hence rendering the rudder less efficient. Therefore the turn will be completed later.

Pro Larger propeller: swifter turning:
The larger propeller displaces more water hence pushing more water against the rudder, hence turning faster.

Just out of curiosity, what are your ideas? I think the larger propeller will be less effective, but I am not sure. A literature search developed little.

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### gonzoSenior Member

Why would a larger propeller have more drag? Your specifications say they both have the same thrust, hence, no drag.

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### TallyHoNew Member

Hm i guess you are right there, but you would mean no difference in drag right?... good point.
But if the same entrance thrust is assumed, and when the turn is started they are ''allowed'' to differ?

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### jonrSenior Member

Like a car without a differential, I think a wide propeller would slightly resist turning. But more critical would be if one propeller creates more velocity over the rudder (perhaps you excluded that).

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### rwatsonSenior Member

What about drag of the prop cross section - hence 'prop walk' that is most evident at slower speeds, usually when trying to dock

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### gonzoSenior Member

On the other hand, if they are installed in the same shaft, a larger diameter propeller will have less loss due to ventilation in really hard turns.

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