How a Propeller actually works

[Col]

Re: Variable Pitch Props

No. Only work going fwd. They are very complicated, can't find any info on them now. Found this however Cols Picture Album

 

[spannerman]

Interesting to read the replies regarding how a prop works, and yes likening it to a screw going into wood is a good illustration. But what no one mentioned that for a prop to work it actually has to slip, so no prop is 100% efficient, you see this if you times your prop speed by the pitch by 60 ( min) to find out how many inches you will in theory travel in 1 hr, convert it to mph and you will see that you should have a race boat, but it isn't so. The reason is that the water in front of the blade experiences a lower pressure ( hydrodynamic lift, same effect as an aeroplane wing) so the blades continually advance forward in an effort to equalise the pressure on both sides, hence we go forward. If the were no pressure differential the boat would hardly move. And thats one of the reasons our shiny high performance SS props aren't so good in reverse, they are totally the wrong shape to generate decent reverse thrust.
Don't worry about it Grumpy just put the gas in an trust in Newton.

 

[Guest]

Spanner, I don't think I will be converting to gas (LPG) as it will not comply with the BSS rules for inland waterways. It is OK to have LPG on its own, or petrol on it's own, but not dual fuel. But I will put trust in Newton /forums/images/icons/wink.gif

"You only see what you recognise, and you only recognise what you know"

 

[jfm]

slip is becoming rare

Agree about slipping but there isn't that much slip these days, considering how sqwishy water is. When calculating speed from rpm x pitch use 17% slip and you will be close, at planing speeds. Smart props will be down to 13%. That's almost like turning through wood.......

 

[Guest]

Re: slip is becoming rare

STOP talking about this kind of stuff on my boring useless threads /forums/images/icons/laugh.gif Actually I think it is brilliant, we will be the most knowledgable forum going /forums/images/icons/wink.gif. The raggies will have no chance in catching up with our knowledge. Well done the experts (with the experience to go with it) for giving me an insight into what goes on in making a boat go.

"You only see what you recognise, and you only recognise what you know"

 

[BarryH]

Grumps, I've read all the posts with interest. All quiet in depth and teccynikal, So in laymans terms, this is how they work. Engine goes brrmm...brmmm.... bbeeerruuummmm....bbeeerrrruuummmm, gears go clunka clunka gnashh gnashh, prop goes whizz whizzz whiiiizzzzz, boat goes woooooooooosssshhhhh!

So what else can I help you wiv!!

 

[Guest]

Barry, you dissapoint me somewhat, I had you down as one of my technical advisors in my 'favourites' category'. God, I may have to 'refresh the favourites' now. /forums/images/icons/laugh.gif

"You only see what you recognise, and you only recognise what you know"

 

[hlb]

Grump would you mind putting that caption line of yours in<font color=red> red.<font color=blue> It will save me reading it thirty times a day. Taa.

<font color=blue> Haydn

 

[Guest]

You cheeky monkey/forums/images/icons/wink.gif you only did that to keep me busy in the 'user options' for an hour. OK own up HOW do I do that?

"You only see what you recognise, and you only recognise what you know"

 

[hlb]

Get the little brackets like this [ and] now write red in the middle or blue or black. But not white cos kim Kimmerised it cos we were using secret writing and talking about him!!
Like this <font color=red> See or even <font color=yellow>!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

<font color=blue> Haydn

 

[Guest]

Like this ?

<font color=red>"You only see what you recognise, and you only recognise what you know"<font color=red>

 

[Stingo]

Not that I should tell you how to do this because you think raggies don't know anything and I'd hate to spoil the illusion.

But, do this {red]whatever it is you have to say{/red]

Now don't use the {, but rather a [ and everything between the ] and the [ will be <font color=red>in red</font color=red>

i.e. replace the { with [ or click http://www.stingo.co.uk - now showing at a computer near you

 

[Guest]

What is the efficiency of a boat prop? I ask because aircraft ones are very slender to minimise tip losses where the high pressure nips round the tip to neutralise the low pressure lift (like glider wings).

Obviously you have to make boat props stubby so they are strong enough to work in water but would putting a ring round the whole thing be beneficial? or bending the tips up like jumbo jet wings?