Model Boat Mayhem - Forum

Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: dreadnought72 on April 10, 2017, 11:52:15 PM

Title: Propellor drag
Post by: dreadnought72 on April 10, 2017, 11:52:15 PM
I learned today that a stationary prop on a sailing boat generates less drag than a windmilling one.


It seems counter-intuitive, but a windmilling prop is a stalled prop: the drag is not far off the area of the disk of the propellor, while the static prop has drag equal to the blade area.


Who'd have thought it? ...well, not me, at any rate.


Andy
Title: Re: Propellor drag
Post by: derekwarner on April 11, 2017, 12:53:26 AM
Well I hope your wine glass is full  %)...as Mr Google found 55,100 responses to your question...I certainly will not read them all  {-) ............... Derek

Title: Re: Propellor drag
Post by: Martin [Admin] on April 11, 2017, 03:53:56 AM
 
Interesting!   I was on a Tug (many years ago) with two Azimuth drives, the skipper demonstrated that the drives aimed out sideways to the tug, was a more effective 'brake' than both of the drives aimed forward against the force of the pull. He said he rarely used it as it was it was 'too counter intuitive for his liking!'.......

http://www.modelboatmayhem.co.uk/Gallery/Felixstowe/Story.htm (http://www.modelboatmayhem.co.uk/Gallery/Felixstowe/Story.htm)

(http://www.modelboatmayhem.co.uk/Gallery/Felixstowe/images/Felixstowe%20014.JPG)
Title: Re: Propellor drag
Post by: Tug Fanatic on April 11, 2017, 08:12:40 AM
It does seem odd. The question that I would like answered regards the comparative load on the engines of braking by sideways thrust or reverse thrust. If you get better braking by applying sideways thrust does that mean a higher engine loading? Those with intuitive answers need not reply!




Interesting!   I was on a Tug (many years ago) with two Azimuth drives, the skipper demonstrated that the drives aimed out sideways to the tug, was a more effective 'brake' than both of the drives aimed forward against the force of the pull. He said he rarely used it as it was it was 'too counter intuitive for his liking!'.......

http://www.modelboatmayhem.co.uk/Gallery/Felixstowe/Story.htm (http://www.modelboatmayhem.co.uk/Gallery/Felixstowe/Story.htm)






Title: Re: Propellor drag
Post by: rnli12 on April 11, 2017, 10:36:21 AM
It all depends on whether the ship is on constant course and speed as to the effectiveness of a trailing shaft as opposed to a braked or stalled shaft. The hydro dynamic imbalance can cause excessive wear on the shaft bearings.


On numerous occasions we would trail one shaft and maintain 50 rpm creating limited thrust.


R
Title: Re: Propellor drag
Post by: derekwarner on April 11, 2017, 10:50:20 AM
mmmmmmm....I could understand a propeller swung athwart and 'traleing' could introduce "brinneling" of the shaft white metal bearing element >>:-(

So whilst this would not necessarily be evident upon the next first set of revolutions :P...it would eventually

My money is letting the propeller 'trale' ...less maintenance costs would outweigh any non sensual savings in fuel costs  %%

Derek
Title: Re: Propellor drag
Post by: Colin Bishop on April 11, 2017, 11:19:39 AM
This is not a new problem.

The old liner Empress of Britain in the 1930s was used for transatlantic runs in the Summer and cruising in the winter. She had a four shaft installation and they took two of the props off when she went cruising!

Early steam warships, including HMS Warrior, had the facility to raise the propeller up into the stern to reduce drag when proceeding under sail.

Many leisure yachts have folding propellers. The blades normally lie folded and pointing astern but starting the engine forces them outwards by centrifugal force where they then provide thrust.

Colin