How a Propeller actually works

[Guest]

How a Propeller actually works & How to gain best low or high power performance, including the importance of pitch, and pros & cons against stainless steel.

Any takers from the technical Mafia, I would like to know in simple non gobbledegook,

Cheers

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

 

[hlb]

Grumpy. Just accept the one that comes with the boat. Use the little stick. It might have a Tortoise and a hare on it. Or say throttle. Just waggle it about a bit till best effect.

<font color=blue> Haydn

 

[Guest]

Haydn, I was just having a discussion with a firm about fitting on a new SS prop for me. We were discussing the max revs and whether I needed more lower end torque or top end speed. I know thia has been covered on another thread, but it was interesting when he was explaining about the screw action and what each extra inch did. Seemed facinating, I have not studied the physics of this yet. Anyway his suggetsion was to run it for 20hrs and see what I needed and check the revs. It would cost £360 incl p&p and VAT. BUT is it worth it? don't worry I'm not rushing out to get it, just researching, as for Bronze, Oh1 I'll leave that for another day /forums/images/icons/laugh.gif


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

 

[Col]

It's just a screw.
The bigger the pitch, the further forward it will go through the water per revolution.
Stainless props can be made with thinner blades, so performance gains can be had.
SS don't suffer with pings and nicks like an ally one will, but will eat your anodes quicker.
Prop should have been matched to your boat by the builders, so should be right. You only have 1 forward gear, so prop is sized to allow max revs/speed to be acheived.
You can put a bigger pitch prop on, and make the boat slower!
You can experiment a little, for example-
Your boat (2tons) safety gear (9tons)may need a shorter pitch prop than a 5 stone bloke out on his own /forums/images/icons/laugh.gif

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[Guest]

Thanks Col, Should a SS be the same size as a matched Ally one, or does the difference in thickness and texture make a difference that has to be allowed for.

So if I am 14.5" x 19" ally, will I be 14.5" x 19" SS ?



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

 

[qsiv]

By heck!

There are huge multi - volume tomes on this subject - and some still consider it a black art rather than science.

A propeller is a screw that winds it's way through the water - the boat is attached to the screw, so it advances. The rate of advance for a given RPM is governed by the pitch. The limiting factor for the pitch is cavitation when the pressure on the low pressure side of the blade drops to such an extent that it is below the vapour pressure of the water at it's ambient temperature, at which point bubbles form causing the flow to detach (i.e the water boils). High figures for pitch might give higher top speed, but at the expense of static thrust (poor acceleration).

In amongst all this there is skew and blade area ratio. The former has a large impact on noise, the latter on drag.

Suface piercing props are even more complex, and in simple terms are beneficial because they significantly reduce hydrodynamic drag (minimal immersed sterngear, and half the prop disc out of the water at any one time, and usually small fixed rudders with vectored thrust).

Add to that efficiency considerations (3, 4 or 5 blades), the fact that the prop tip speed needs to be kept within cavitation limits, and needs to be an adequate distance from the underside of the hull to avoid noise transmission and hydrodynamic interference.

 

[hlb]

No. You would want a 13.5 (Or is it the other way round)? Grumpy. Why dont you just get the boat. And see how it performs. Before adding safety gear and after. Use it for a year or two. You will probably have hit a few rocks by then and need new props anyway!! Just get on with it, like the rest of us

<font color=blue> Haydn

 

[Col]

Could try 19 1/2". You can get props "cupped" up to, I think 1" either way. So if it was a bit to tall (or not tall enough) Someone like Steel developments could *adjust* it.
May be worth seeking their advice, some thing like 20 years in the prop business.
Sorry, don't have phone no: to hand.

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[Guest]

SO, you could compensate for the slightly slower top end speed of a diesel, but using it's low end torque, by fitting a bigger pitch prop? Am I getting the hang of it a little?

As if you wanted to pull a water skier you would lower the pitch of the prop, for greater torque on a petrol. Is that correct?

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

 

[Guest]

You do not need to know, trust me. Knowing would only give you nightmares.

Are you planning to carry any spare sea, BTW, in case you run out?

 

[DepSol]

Get to grips with the boat first, then when you know enough about how she handles etc etc you will be in a better position to judge if you need to change the prop and then when you change it to judge if it is better worse or indifferent.

You need to walk before you can run. Dont worry about these things until you are certain you need to make an adjustment

Dom

2003 is going to be a good year for me

 

[wakeup]

Variable Pitch Props

Why don't sports boats have variable pitch props, they've been around for yonks on light aircraft. could get a whole lot of different peformance profiles out of a boat with variable prop pitch, throttle, tilt and trim.

 

[Col]

Re: Variable Pitch Props

You can.(but dont tell Grumpy/forums/images/icons/wink.gif)
Don't seem very popular, quite expensive probably.
They are basically a mechanism, that upon reaching a certain preset rpm (adjustable) flick the blades round to a higher pitch, again, adjustable.
Seem to introduce more problems than they solve.


<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[Guest]

Re: Variable Pitch Props

Where could I buy one of those? On the other hand don't worry, the forum mafia have spoken, equipment fun buying now over. I will now wait for the boat and see if I need any more safety gear first, before looking at props. Where can I book a sea safety check? I was just about to click 'go to checkout' on the purchase of NASA sx35 DSC VHF and aerial, you just saved me £240 thanks. I had already bought the motorola hand held walkie talkies though /forums/images/icons/laugh.gif Next stop, Experience !!

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

 

[Guest]

Re: Variable Pitch Props

SAW THAT, does it have an effect on reverse as it would / have the opposite effect?

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

 

[Guest]

Re: Variable Pitch Props

>> Where can I book a sea safety check?

Easy. Just go out to sea. You can arrange a safety check by calling the Coastguard on Channel 16, and using the keywords: "Mayday, Mayday, Mayday". They'll send a lifeboat out, and they'll be happy to advise on safety issues, your parentage etc etc

 

[Guest]

Re: Variable Pitch Props

You are so fuuny today Graham, my sides are splitting with laughter (not)

Lucky I'm all VHF'd up, could have fallen for that. I will just use the DSC transmitter & 406 EPIRB and wait for help as I was taught to /forums/images/icons/laugh.gif

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

 

[Guest]

Re: Variable Pitch Props

Glad to be of service. Seriously, you book a safety check with your local lifeboat station, generally. Get to know 'em, they're great blokes, and you may meet them for real one day, so it's nice to know them...

 

[hlb]

Re: Variable Pitch Props

Hard to know what there going to check on a brand new boat, and would need the whole UN inspectorate team to search through Grumpys list of articles of mass distruction!!

<font color=blue> Haydn

 

[Guest]

Re: Variable Pitch Props

I think he'll get first rate service. They'll follow him everywhere, probably out of morbid curiosity.