Boatmad.com


Reply
 
Thread Tools Display Modes
 
Old 24-05-2007, 12:58 PM   #1
jw.
Registered User
 
jw.'s Avatar
 
Location: Scotland
Interests: Hole maker
Boat make: Humber Ocean Offshore
Engines: KAD 300/DPX

Join Date: Mar 2004
Location: Scotland
Posts: 954
Props - how do they do that?

OK, accepting that props can have different characteristics, what is it about the blade shape that produces a particular characteristic. For example, if a blade is raked and kinda spoon shaped it is likely to give bow lift, why? What's going on in the water?

And is it bow lift or does it actually drag the stern down and therefore lever the bow up cos I can't see how a prop on the stern can lift the bow?

And why, cos a blade is travelling down in the water on one side and travelling upward in the water on the other side so why don't the forces cancel?
__________________

__________________
JW.
jw. is offline   Reply With Quote
Old 24-05-2007, 01:32 PM   #2
Large member
 
Country: England
Location: On the farm
Occupation: General Trouble Causer Salterns Boatyard
Interests: Official smartass
Boat name: Seahorse.org
Boat make: a V24 and a SLOW unstable ICE Bladerunner
Engines: 2x300 promax, 320 Volvo
Cruising area: England/France & Med

Join Date: May 2005
Location: On the farm
Posts: 2,681
Quote:
Originally Posted by jw. View Post
OK, accepting that props can have different characteristics, what is it about the blade shape that produces a particular characteristic. For example, if a blade is raked and kinda spoon shaped it is likely to give bow lift, why? What's going on in the water?

And is it bow lift or does it actually drag the stern down and therefore lever the bow up cos I can't see how a prop on the stern can lift the bow?

And why, cos a blade is travelling down in the water on one side and travelling upward in the water on the other side so why don't the forces cancel?
Part of the issue is water is thicker lower down and thinner higher up. It has littel to do with the sides - more top and bottom.

Most ppl also run their props so that part of the blade is out the water on the top - ie in the air - and water is thicker than air by a little....
__________________

verytricky is offline   Reply With Quote
Old 24-05-2007, 06:20 PM   #3
Registered User
 

Join Date: Mar 2004
Posts: 1,016
It's Voodoo.....................
Jono is offline   Reply With Quote
Old 24-05-2007, 06:59 PM   #4
Registered User
 
Burnett's Avatar
 
Country: England
Location: Hertfordshire
Occupation: Airline Operations.
Interests: Rum. Pirates. And West Cornwall pasties.
Boat name: Any suggestions?
Boat make: Ring 18
Engines: Mercury 200 Black Max
Cruising area: The Bay of 'E'

Join Date: Sep 2005
Location: Hertfordshire
Posts: 369
Let me just consult my reference library...

__________________
Boat: (Noun) - A hole in the water, lined with fibreglass, into which you pour money.
Burnett is offline   Reply With Quote
Old 24-05-2007, 07:00 PM   #5
jw.
Registered User
 
jw.'s Avatar
 
Location: Scotland
Interests: Hole maker
Boat make: Humber Ocean Offshore
Engines: KAD 300/DPX

Join Date: Mar 2004
Location: Scotland
Posts: 954
Smarty pants...the pair of ya. I take it that's a, "How the feck should I know.".

__________________
JW.
jw. is offline   Reply With Quote
Old 24-05-2007, 07:16 PM   #6
Registered User
 
Burnett's Avatar
 
Country: England
Location: Hertfordshire
Occupation: Airline Operations.
Interests: Rum. Pirates. And West Cornwall pasties.
Boat name: Any suggestions?
Boat make: Ring 18
Engines: Mercury 200 Black Max
Cruising area: The Bay of 'E'

Join Date: Sep 2005
Location: Hertfordshire
Posts: 369
Yep. It's one of those things that I don't know how it works, but it just does, and I'm not smart enough to figure it out.
__________________
Boat: (Noun) - A hole in the water, lined with fibreglass, into which you pour money.
Burnett is offline   Reply With Quote
Old 25-05-2007, 07:48 AM   #7
Registered User
 

Join Date: Mar 2004
Posts: 1,016
Quote:
Originally Posted by jw. View Post
.........I take it that's a, "How the feck should I know.".


......... Yeah... but ask me about something I DO know about....like...errr.... oh Feck'.... just don't ask me nuffink.........Ok?
Jono is offline   Reply With Quote
Old 25-05-2007, 04:45 PM   #8
Registered User
 
TheOrs's Avatar
 
Country: England
Location: Plymouth
Boat make: Fletcher Arrowflyte
Engines: Mercury 75

Join Date: Nov 2004
Location: Plymouth
Posts: 402
I would imagine that Matt is researching & writing the perfect reply.

I'm going to wait & read that one.

(no pressure...)
TheOrs is offline   Reply With Quote
Old 25-05-2007, 05:07 PM   #9
Registered User
 

Join Date: Mar 2004
Posts: 6,862
Ere, I'll not have my name used in vain. I'm proud and fiercely defensive of being ignorant.
Matt is offline   Reply With Quote
Old 25-05-2007, 06:58 PM   #10
Registered User
 
WightStuff's Avatar
 
Country: England
Location: Cowes
Occupation: Marine engineer
Interests: Anthing that goes fast
Boat name: Offshore Monkey Business
Boat make: Buzzi 32
Engines: Grimes 1050
Cruising area: Solent

Join Date: Feb 2006
Location: Cowes
Posts: 116
The cutting edge of the blade causes a indifference in the water pressure at the front of the blade and at the rear, the lower pressure at the rear of the blade causes the propulsion of the forward motion. The different shape of the blade causes different characteristics, i.e. bow lift is from the blade trying to pull the stern downwards thus producing a higher bow lift. I hope this helps, studying this at uni
WightStuff is offline   Reply With Quote
Old 25-05-2007, 10:15 PM   #11
jw.
Registered User
 
jw.'s Avatar
 
Location: Scotland
Interests: Hole maker
Boat make: Humber Ocean Offshore
Engines: KAD 300/DPX

Join Date: Mar 2004
Location: Scotland
Posts: 954
Cool. Ta. But I think you mean, 'the lower pressure at the FRONT of the blade causes the propulsion of the forward motion.'

So, the blade forces are in balance while the blades are to the sides; the water is always more disturbed near the surface (thinner, as Marc suggests); therefore the blade characteristic is dominant while the blade is low in the water. I can see that the lift forces will act at a right angle to the blade so a raked blade will produce a considerable downward pressure while at the bottom whereas the upward pressure while the blade is at the top will be less because of the water disturbance.

We seem to agree that there is no bow lift, it's the stern being hauled downward. Now, why is this effect more pronounced on a blade which has some curvature, ie. spoon shaped.?

What about blades that are said to give stern lift? Are they just hauling the stern down less rather than lifting it?

How about other shapes of blade and their characteristics?
__________________
JW.
jw. is offline   Reply With Quote
Old 25-05-2007, 10:34 PM   #12
Large member
 
Country: England
Location: On the farm
Occupation: General Trouble Causer Salterns Boatyard
Interests: Official smartass
Boat name: Seahorse.org
Boat make: a V24 and a SLOW unstable ICE Bladerunner
Engines: 2x300 promax, 320 Volvo
Cruising area: England/France & Med

Join Date: May 2005
Location: On the farm
Posts: 2,681
A spoon will cup the water and drag the blade into the water....
verytricky is offline   Reply With Quote
Old 26-05-2007, 11:55 AM   #13
jw.
Registered User
 
jw.'s Avatar
 
Location: Scotland
Interests: Hole maker
Boat make: Humber Ocean Offshore
Engines: KAD 300/DPX

Join Date: Mar 2004
Location: Scotland
Posts: 954
Quote:
Originally Posted by verytricky View Post
A spoon will cup the water and drag the blade into the water....
Mr Verytricky, would you like to elaborate on that please?
__________________
JW.
jw. is offline   Reply With Quote
Old 27-05-2007, 03:46 AM   #14
Large member
 
Country: England
Location: On the farm
Occupation: General Trouble Causer Salterns Boatyard
Interests: Official smartass
Boat name: Seahorse.org
Boat make: a V24 and a SLOW unstable ICE Bladerunner
Engines: 2x300 promax, 320 Volvo
Cruising area: England/France & Med

Join Date: May 2005
Location: On the farm
Posts: 2,681
Quote:
Originally Posted by jw. View Post
Mr Verytricky, would you like to elaborate on that please?
Aish!

Its late ( or early, depending on your point of view.... )

This is a very much complicated topic, and only partially understood. There is also no such thing as a 'standard prop' to discuss, as surface piercing props, duo props, submerged props etc etc all work on different principles.

Basically a prop consists of a dynamic between diameter, pitch, number of blades, blade area, cupping, skew, rake, thickness and sectional shape. Now you have to choose a specific prop to discuss, and then chat around the dynamics of that prop design for that prop application.

I could write several pages to give the answer, as it is not just a simple one word answer.



Some things with a common or standard outboard engine:
A right hand rotation blade tends to 'walk' to the right when viewed from behind. This is sometimes called the paddlewheel effect. Some skippers can being the boat alongside a dock, and at low revs 'walk' the boat closer to the dock. At low revs the paddlewheel or walk effect is greater than the forward propulsive effect of the blade. This is provable in practice, so lets accept this as fact.

Now explain this fact! The prop is fully submerged, and you would expect that the prop turns in a nice circle, and there are three or four blades ( or two in a smaller boat ) so the prop should be 'balanced' in the water, and providing motion should generate propulsion, not 'walking', 'crabbing' or 'paddlewheel' like we see it, so there has to be some force acting on the props out of balance. This is mostly due to the different 'thickness' or resistance to the blade provided by the water.

To simplyfy things tremendously, and make life simple for indication only, assume that the blade actually travels in a square motion, and not in an every changing circular mothin. This simplyfies the theory of the blade into an up, right, down and left motion to complete the cycle.

The up stroke counters the down stroke ( mostly and not at 90 degrees, but assume it to simplyfy the model ) what you have is the differance between the left stroke and the right stroke causing your effect. These are the strokes deep in the water and shallow in the water. There is less resistance in the water to the blade travelling from left to right as it is on the surface, and more resistance to the blade travelling from right to left as it has more resistance to the blade. A difference between the 'power' requred to move the blade from left to right vs the 'power' required to move the blade from right to left creates an imbalance which tries to act on the central point in the 'square' and when it cant resolve the balance differential in the center ( as this is a fixed metal object connected to a power source ) the only thing it can do to resolve the imbalance is to move more water out of the way of the blade on the right to left stroke. This increased movement of the water from right to left causes the entire unit to move from left to right, ie the stern of the boat.

We can see that this is the case in reality, so the theoretical model holds up.

Simplyfying it even further, you can arue that you can offset or cancell out 100% of the action of the left to right motion and be left with a residual right to left motion only. Doing so you can then examine only what effect your prop design is having at that moment of rotation where it is at the lowest point in its rotation. So if at that point it is pulling 'down' then the sern goes down and the bow lifts. If it pushes 'up' then the stern lifts and the bow goes down.

This does not work for a duoprop, which really screws the cat if you attempt cupping, so this theoretical model is out the window for a duoprop.


I hope that helps.
verytricky is offline   Reply With Quote
Old 27-05-2007, 10:01 AM   #15
Registered User
 
kitten's Avatar
 
Location: devon

Join Date: Mar 2004
Location: devon
Posts: 556
As I understand it,
All propellers produce a cone of force behind them, this will in all cases of props that run near the surface try to lift the stern as the top half of the cone has nothing to react against (compared to the bottom) this cone can be reduced by cupping the prop and trying to send the force in a straight line, therfore not lifting the stern so much and giving the effect of bow lift as compared to an uncupped prop.
All IMVHO
kitten is offline   Reply With Quote
Old 27-05-2007, 12:45 PM   #16
jw.
Registered User
 
jw.'s Avatar
 
Location: Scotland
Interests: Hole maker
Boat make: Humber Ocean Offshore
Engines: KAD 300/DPX

Join Date: Mar 2004
Location: Scotland
Posts: 954
Quote:
Originally Posted by verytricky View Post
Aish!

Its late ( or early, depending on your point of view.... )
Firstly Marc, thanks for taking the time to write all that.

Quote:
This is a very much complicated topic, and only partially understood.
Well, that's kinda why I started the thread. There are companies who design props so someome must have a good grasp of the theory and I was hoping to tap into that for a bit of enlightenment.

I did understand the principle of propwalk but you've made me realise just how much out of balance the forces must be top to bottom and how this must also load the propshaft and it's bearings.

Quote:
Simplyfying it even further, you can arue that you can offset or cancell out 100% of the action of the left to right motion and be left with a residual right to left motion only. Doing so you can then examine only what effect your prop design is having at that moment of rotation where it is at the lowest point in its rotation.
I don't understand this bit.

Quote:
This does not work for a duoprop, which really screws the cat if you attempt cupping, so this theoretical model is out the window for a duoprop.
Are you conversant with the theory of duoprops because I've been unable to find much information about them.

In an earlier post you said,
Quote:
A spoon will cup the water and drag the blade into the water....
Are you able to explain the forces acting upon the blade to give this effect?
__________________
JW.
jw. is offline   Reply With Quote
Old 05-06-2007, 08:20 AM   #17
Traveling
 
Country: UK
Location: Alderholt
Occupation: Aerospace
Boat name: T/T D2S
Boat make: Midas 27' Cat, Argo 16 Cat. Avon Rib Thingy
Engines: Merc 280-ROS -JSRE,65Xs, 75 Stinger, Yam 60
Cruising area: Any Seedy Bar

Join Date: Sep 2004
Location: Alderholt
Posts: 4,223
Jimbo of Aeromarine Research (Member Here) Has written a small package about this very subject . It\'s a free download on subscription to the newsletter e-mail service offered

can subscribe at
http://www.aeromarineresearch.com/popup.htm

main site
http://www.aeromarineresearch.com/
BluFin is offline   Reply With Quote
Old 05-06-2007, 09:48 AM   #18
Registered User
 

Join Date: Mar 2004
Posts: 6,862
jw. I've been thinking about how to demonstrate this in yer bathtub. I'll probably need some help to refine it, but like you I can't find a definitive article that explains it (all articles simply refer to cup and rake as statements of fact)

I reckon the following helps simplify it.
a) The prop is linear only (see below)
b) The prop shaft is parallel with the water surface (neutral trim)
c) The prop semi submerged with 50% of the prop below water surface.
d) The prop is in clean, undisturbed water.
e) Slip=0

a is the most difficult to imagine. Consider that the prop blades moving in a vertical plane on both entry and exit from the water are counteracting each other in all vertical forces. Therefore we'll ignore all the vertical components here since they balance each other. (Though thrust and torque are obviously not balanced to create thrust forward).

So we can simplify the prop to being a single fixed blade moving linearly across the surface with 2 components - a sideways component which is the applied force (normally propshaft torque), and a forward component which is generated by the angle, pitch etc of the prop.

On a graph of force this would be x & y axis covered.
We want to look at the z-axis as well.

Making sense so far?
Matt is offline   Reply With Quote
Old 05-06-2007, 10:12 AM   #19
jw.
Registered User
 
jw.'s Avatar
 
Location: Scotland
Interests: Hole maker
Boat make: Humber Ocean Offshore
Engines: KAD 300/DPX

Join Date: Mar 2004
Location: Scotland
Posts: 954
Thanks for that BluFin. I've seen the article before but it was worth a re-read. However, it doesn't attempt to explain how and why the more, err, exotic features of a prop work.

An interesting bit was the expected gain from blue-printing a prop. Since the gain is got by trimming the prop shape as close as possible to the original design, the original designers must have a good handle on what features to design in to give the required characteristics. The search continues...
__________________
JW.
jw. is offline   Reply With Quote
Old 05-06-2007, 10:17 AM   #20
Registered User
 

Join Date: Mar 2004
Posts: 6,862
Not necessarily. Props are always very empirical.

IMVHO, Blueprinting actually means is that all the blades follow exactly the same shape. Otherwise the slightly different shape of each blade means they'll all be counteracting each other (like a car with too much toe in), reducing efficiency.
__________________

Matt is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Powered by vBadvanced CMPS v3.2.3

All times are GMT +1. The time now is 06:20 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
×