Jump to content

Tiller/Rudder pressure


Richardcn

Featured Posts

I was going into explaining paddle wheel theory.

Think that the propeller is a centrifugal fan, in a housing where one side is expanding the air toward the outlet, and then sharply close the gap, a propeller working in a such matter that it is not cavitating as mush as stalling the blades, it happens on high rev and low or negative movement (opposite the propeller) specially easy going forward and in reverse gear, when close to the hull going up the water thrown out by the centrifugal force is diverted in a different direction, this is not happening on the other side.

 

what is thrust?, it is a fluid that get displaced in a hurry, we move a mass of something more in one direction then in other directions, and we get a force that can move things.

 

so I would be suspicious if prop walk has only one source.

 

we have some pilots onboard here, everyone that have flown a single engine propeller plane know why we need a lot of rudder during take off and climb, we have the swirl that hit the fin on one side, we have the P-factor, and we have the gyroscopic force when we rotate (or lift the tail wheel from the ground)

Link to comment
Share on other sites

This didn't start as a 'theory' of my own, this started as an attempt to explain an effect that I had experienced, but you had dismissed as a myth. It is described as a 'paddle wheel effect' to try to simplify the explanation as to why the stern of a boat can be (slightly) pushed sideways by the rotation of the propeller. The simplified description of 'paddle wheel effect' is irrelevant to the discussion, you had declared that Prop walk was a myth, not paddle wheel effect, so is seems that you have now changed your 'argument' in an effort to claim some sort of 'victory'.

What victory? Well I suppose enhanced knowledge and understanding could be considered a victory but it's not a term I'd use. Oh and if you check back to my first mention of it, you will see that I said prop walk was "mostly myth", which is not really the same as "declaring that Prop walk was a myth".

 

My ironic rubbishing of the theory of lunar tides was intended to display that most people will repeat '..something that they have heard someone else say without any understanding of what they were saying..' when it gives a reasonable explanation of their experiences. I haven't seen any practical visual demonstration of lumps of rock attracting water and have no real understanding of how it happens, but accept that it is a reality (unless someone can come up with a better explanation!). So your ridicule of this acceptance is a bit misplaced, I don't understand atomic theory but I'm sure that those who were at Hiroshima wouldn't doubt it's reality.

 

 

However I think we are all familiar with the effects of gravity and so once you pause for a moment to consider it, tides easily fall out of that thought process.

 

And finally your approach to discussion of,"... my discussion style is to make a bold statement and await being proven wrong....." is the total opposite of both legal and scientific method. In science you propose a theorem that fits with observations and then try to find out where and why it doesn't fit, in legal circles you have to prove something happened since it is impossible to prove something didn't. If I were to make the 'bold statement' that your ancestor was Jack the Ripper it would be impossible for you to disprove it. It is from this line of reasoning that we get the crop of various idiotic conspiracy theories that the Internet is plagued with.

I don't recall the forum rules requiring scientific method to be used in discussion. Anyway, as we all know scientific method doesn't work anyway (viz Zen and the art of Motorcycle maintenance). As to legal method, never really heard of that but in law one party usually makes outrageous claims of innocence etc, and the other party has to prove they are lying. Kind of like... Edited by nicknorman
Link to comment
Share on other sites

What victory? Well I suppose enhanced knowledge and understanding could be considered a victory but it's not a term I'd use. Oh and if you check back to my first mention of it, you will see that I said prop walk was "mostly myth", which is not really the same as "declaring that Prop walk was a myth".

 

Still backpedalling then, so to clarify, are you saying that prop walk is a myth or not? You can't really have half a myth, or sort of a myth, or type of a myth, mostly a myth. It either exists or it doesn't. Your 'argument' seems to be that it only happens to defective boats which is arrant nonsense unless the majority of all boats built are 'defective'.

 

However I think we are all familiar with the effects of gravity and so once you pause for a moment to consider it, tides easily fall out of that thought process.

 

Perhaps you could show me a physical example that I can see of water being amazingly attracted to a lump of rock (sea crashing against cliffs doesn't count!). The effect of gravity of me dropping something to the floor has no relevance to water being attracted by something tens of thousands of miles away.

 

I don't recall the forum rules requiring scientific method to be used in discussion. Anyway, as we all know scientific method doesn't work anyway (viz Zen and the art of Motorcycle maintenance). As to legal method, never really heard of that but in law one party usually makes outrageous claims of innocence etc, and the other party has to prove they are lying. Kind of like...

 

'....As we all know scientific method doesn't work anyway...", frusty.gif and you then quote a book of fiction es 'evidence'????? I suppose religion answers all questions (not). And your knowledge of the legal process is astounding, I had of course totally forgotten that people ask to appear in Court to prove that they haven't committed any offence and we have to Summon the Prosecution to appear to disprove their denial, obviously my mistake I thought it worked the other way around.

Link to comment
Share on other sites

Still backpedalling then, so to clarify, are you saying that prop walk is a myth or not? You can't really have half a myth, or sort of a myth, or type of a myth, mostly a myth. It either exists or it doesn't. Your 'argument' seems to be that it only happens to defective boats which is arrant nonsense unless the majority of all boats built are 'defective'.

"Still back-pedalling then" is a bit like "when did you stop beating your wife?" I can't be still back-pedalling since I never started. What I did do was to remind you of my initial statement, which you have taken and altered so as to make your point. Really, if you have to pretend I said something I didn't say just to make your point, you are getting a little desperate!

 

I said it was mostly myth. That means a lot of it is myth, some of it is real. For example "paddle wheel effect" so often glibly quoted as the explanation, is myth. Spiral flow is real and probably accounts for some prop walk effect in some boats. But not all boats.

Link to comment
Share on other sites

RYA are people who sail boats, they are not fluid dynamicists. And anyway, don't yachts have sails not props?

 

 

 

Many offshore yachts have both.

Engines are needed to maneuver in harbours / ports as well as being used to motor sail in no or very little wind.

As in the case of no wind a yachtie does not really relish being taken towards rocks on a tide.

 

Incidentally sailing craft once moving create "apparent wind." Nick, you should know this from your gliding activities.

 

JHH5-14760.jpg

 

Having sailed extensively offshore in a variety of yachts I can state in my experience, no I didn't take videos to back my statement up, as many now demand evidence rather than believing a man's word, offshore yachts are prone to paddlewheel effect, more so than a narrow boat.

 

I have sailed a variety of narrow boats as well, I have noticed that nb's are less prone to this phenomenon. captain.gif

 

Sorry I don't have evidence.

 

I will quote my dear departed Dad

 

"I do not need to justify what I say, my friends don't need it,

my enemies wouldn't believe me anyway."

Edited by Ray T
Link to comment
Share on other sites

Perhaps you could show me a physical example that I can see of water being amazingly attracted to a lump of rock (sea crashing against cliffs doesn't count!). The effect of gravity of me dropping something to the floor has no relevance to water being attracted by something tens of thousands of miles away.

 

No relevance to those who can't see the big picture, but it is a small step from realising that massive things attract each other to working out why tides occur. Well Mr Newton managed it a very very long time ago and he didn't even have a computer!

Link to comment
Share on other sites

 

'....As we all know scientific method doesn't work anyway...", :banghead: and you then quote a book of fiction es 'evidence'????? I suppose religion answers all questions (not). And your knowledge of the legal process is astounding, I had of course totally forgotten that people ask to appear in Court to prove that they haven't committed any offence and we have to Summon the Prosecution to appear to disprove their denial, obviously my mistake I thought it worked the other way around.

 

I was certainly not quoting Zen... as evidence since as you say it is not a "scientific" book. It does however contain a treatise on the flaw in the scientific method and it is easier for me to make a reference to this than to spend many pages repeating it on here (and no doubt getting it wrong and/or missing bits). Sorry if you didn't understand the difference.

 

On court proceedings perhaps you have never heard of a case when the defendant stands up and says "not guilty" whereupon the prosecution set about proving him to be a liar?

Edited by nicknorman
Link to comment
Share on other sites

Many offshore yachts have both.

Engines are needed to maneuver in harbours / ports as well as being used to motor sail in no or very little wind.

Incidentally sailing craft once moving create "apparent wind." Nick, you should know this from your gliding activities

 

Yes I knew that really. I think I first helmed a motor-sailer when I was about 5 (St Mawes, Cornwall), then every year until I was about 18, then a flotilla sailing holiday in the Aegean and a bare boat sailing holiday in the BVI (18 metre Italian rocket ship - very nice!)

 

JHH5-14760.jpg

 

Having sailed extensively offshore in a variety of yachts I can state in my experience, no I didn't take videos to back my statement up, as many now demand evidence rather than believing a man's word, offshore yachts are prone to paddlewheel effect, more so than a narrow boat.

 

I have sailed a variety of narrow boats as well, I have noticed that nb's are less prone to this phenomenon. :captain:

 

Sorry I don't have evidence.

 

I will quote my dear departed Dad

 

"I do not need to justify what I say, my friends don't need it,

my enemies wouldn't believe me anyway."

You just have to look at that hull to see that it is a completely different kettle of fish to a narrowboat, with its curved bottom, keel, fin etc etc. and relevant to the preceding conversation, a significant angle on the propshaft. TBH I can't remember whether any of the motor sailers exhibited significant prop wash but in general we tried to "do it properly" by picking up the mooring or dropping anchor without using the engine, and therefore rarely needed reverse. Edited by nicknorman
Link to comment
Share on other sites

Have just taken 2 inches off the front edge of the rudder but not welded any to the trailing edge.and what an improvement. Rudder now self centering and normal steering requires very little pressure on the tiller. Full lock requires a bit more pressure than before because I have lost mechanical advantage for the balance blade but what a difference.

Link to comment
Share on other sites

Have just taken 2 inches off the front edge of the rudder but not welded any to the trailing edge.and what an improvement. Rudder now self centering and normal steering requires very little pressure on the tiller. Full lock requires a bit more pressure than before because I have lost mechanical advantage for the balance blade but what a difference.

I'm very interested to understand why people are doing this. I mean, what is prompting this action and how are people determining what adjustment to make also what are the risk factors. I think I'd be happy to do the same (once we HAVE a boat and IF we had a problem) to achieve the results that others have but why are there (apparently) so many poorly designed rudders out there? There surely must be a reason why they have an 'oversized' rudder and balance blade in the first place?

Edited by Richardcn
Link to comment
Share on other sites

I'm very interested to understand why people are doing this. I mean, what is prompting this action and how are people determining what adjustment to make also what are the risk factors. I think I'd be happy to do the same (once we HAVE a boat and IF we had a problem) to achieve the results that others have but why are there (apparently) so many poorly designed rudders out there? There surely must be a reason why they have an 'oversized' rudder and balance blade in the first place?

Personal preference, I think. The 45' boat next door can be steered with literally one finger- long tiller and a very balanced rudder. My old boat had an overbalanced rudder, which whilst it made going along harder work, made e boat very, very manoeuvrable- to the extent of doing a U turn, without reverse, with the 48' boat on the 60' wide river. I delivered a 62' boat earlier this year that also had a very balanced rudder, to the extent that there was no feedback at all and you could steer it easily with one hand, and probably one finger.

 

My current boat has no balance whatsoever. It is very, very heavy to steer as a result, but it goes in a straight line without any control input and won't wander off course. I find myself, during manoeuvres, working very hard to turn the rudder against the force of the prop wash, and often brace against the cabin to push, or use my whole body, rather than just one hand.

 

On the cut, because it's deep drafted at 3', it is very hard work to steer if you don't agree with the boat about where it wants to go- but if you do it right and get in the channel, it steers itself.

Link to comment
Share on other sites

We have several saying they took off 2" on the balance and did or did not welt it at the back, but I would like to ask how long was the balance from start from centre of stock. and how long is the trailing plate?

 

I have some 24% balance on my Shilling rudder, but it is a manual hydraulic wheel steering, but at large angles and lot of power on it takes more force to get it from full rudder left or right, back to centre, because at big angles (alpha) the left centre moves forward, before I re made the rudder from a simple plate to Shilling it was about 21,5% balance and the steering forces was to high, it was 200mm balance out of 925 mm total, the schilling is more effective, so can be smaller, now 204mm balance and some 845mm total, with a slight fishtail, first the Schilling did not have the fishtail so was 800 total and was very neutral at small angles but way to heavy to get centred.

 

I can think that a smaller NB rudder with tiller steering can use a smaller percent balance, saw some number of 17% for large sailboats, but I am sure it is different from case to case.

Link to comment
Share on other sites

We have several saying they took off 2" on the balance and did or did not welt it at the back, but I would like to ask how long was the balance from start from centre of stock. and how long is the trailing plate?

 

I have some 24% balance on my Shilling rudder, but it is a manual hydraulic wheel steering, but at large angles and lot of power on it takes more force to get it from full rudder left or right, back to centre, because at big angles (alpha) the left centre moves forward, before I re made the rudder from a simple plate to Shilling it was about 21,5% balance and the steering forces was to high, it was 200mm balance out of 925 mm total, the schilling is more effective, so can be smaller, now 204mm balance and some 845mm total, with a slight fishtail, first the Schilling did not have the fishtail so was 800 total and was very neutral at small angles but way to heavy to get centred.

 

I can think that a smaller NB rudder with tiller steering can use a smaller percent balance, saw some number of 17% for larg

e sailboats, but I am sure it is different from case to case.

I'm sure I've seen 20% of rudder area in front of the stock mooted as the perfect amount.

Link to comment
Share on other sites

I was going to make a new rudder for a SAGA 26, it had a poor rudder, but was very light on the wheel and had a 20% balance as you say, newer models of the same boat had a rectangular rudder instead of the taper, the owner bought a different boat, so didn't come to any material work, just a study.

Link to comment
Share on other sites

I'm sure I've seen 20% of rudder area in front of the stock mooted as the perfect amount.

Of course, the old working boatmen hated balance. Any balance uses energy that would otherwise be providing forward motion.

 

The old guys would prefer to "put their backs into it" and "get 'em ahead".

 

George ex nb Alton retired

Link to comment
Share on other sites

How so? A blade in the water aligned with the flow is a blade - it doesn't make any difference where the pivot point is surely?

I'd have thought the only time energy might be 'lost' would be when moving the rudder to turn as some of the thrust would be assisting the rudder position by hitting the balance blade. However, surely most, if not all, of the energy is being deflected in the required direction anyway to push the stern around?

Link to comment
Share on other sites

How so? A blade in the water aligned with the flow is a blade - it doesn't make any difference where the pivot point is surely?

Apparently it does matter. I don't know the exact mechanics of it, perhaps someone else does?

 

A simple question comes to mind. On a perfectly balanced rudder, a slight pressure will turn the boat. On an unbalanced rudder a lot of effort (energy) is required to push the rudder over and keep it there. The balanced rudder is taking energy from somewhere to achieve what my back is doing to the unbalanced one. Where does that energy come from? I contend, as did the old lads, it is at the expense of forward movement.

 

Rotation is of course unaffected by the percentage of balance but that is a different question.

 

George ex nb Alton retired

Link to comment
Share on other sites

Apparently it does matter. I don't know the exact mechanics of it, perhaps someone else does?

 

A simple question comes to mind. On a perfectly balanced rudder, a slight pressure will turn the boat. On an unbalanced rudder a lot of effort (energy) is required to push the rudder over and keep it there. The balanced rudder is taking energy from somewhere to achieve what my back is doing to the unbalanced one. Where does that energy come from? I contend, as did the old lads, it is at the expense of forward movement.

 

Rotation is of course unaffected by the percentage of balance but that is a different question.

 

George ex nb Alton retired

Apparently it did matter - in the minds of the old lads who probably were used to what they were used to and never tried any new fangled ideas!

 

Anyway, two points, first that when the rudder is straight you presumably agree that there is no difference?

 

Secondly, on your "conservation of energy" argument how much energy is required to hold a tiller against a force. Answer, none. Energy is admittedly required to move the rudder into that deflected position but once there, it could for example be secured with a rope and last time I looked, ropes don't make energy! Of course the vagaries of the human frame means that you are expending energy to keep your muscles taught, but that energy is not transmitted to the boat.

 

This definitely sounds like an old wives tale - well, old boatman's tale anyway!

Link to comment
Share on other sites

 

 

A simple question comes to mind. On a perfectly balanced rudder, a slight pressure will turn the boat. On an unbalanced rudder a lot of effort (energy) is required to push the rudder over and keep it there. The balanced rudder is taking energy from somewhere to achieve what my back is doing to the unbalanced one. Where does that energy come from? I contend, as did the old lads, it is at the expense of forward movement.

 

Simplistically with an unbalance rudder, when it is put over only half of the prop thrust is redirected to one side - the rest is still providing forward thrust. With some balance ahead of the pivot point a greater amount of the total thrust is redirected in the direction necessary to make the boat turn.

 

It is very noticeable that a motorised sailing barge such as a tjalk is far less handy than one built as a motor vessel in the first instance. They will simply have a large rudder blade hung on the back of the stern post. This can be countered to some extent by carving the stern about to be able to put some leading balance to the rudder. The stern shape will still be different and the water simply can't get to the prop (and rudder) as efficiently as a motor with a fine swim, but it is the leading edge that really makes the difference.

 

 

edit for added clarity

Edited by Tam & Di
Link to comment
Share on other sites

 

Simplistically with an unbalance rudder, when it is put over only half of the prop thrust is redirected to one side - the rest is still providing forward thrust. With some balance ahead of the pivot point a greater amount of the total thrust is redirected in the direction necessary to make the boat turn.

 

It is very noticeable that a motorised sailing barge such as a tjalk is far less handy than one built as a motor vessel in the first instance. They will simply have a large rudder blade hung on the back of the stern post. This can be countered to some extent by carving the stern about to be able to put some leading balance to the rudder. The stern shape will still be different and the water simply can't get to the prop (and rudder) as efficiently as a motor with a fine swim, but it is the leading edge that really makes the difference.

 

 

edit for added clarity

Thanks for that. Tell me if I am wrong but for any given area of rudder, including any balance, the more balance that is built in, the more water is diverted into turning rather than forward motion.

 

This is probably what the old lads were referring to. A big unbalanced Josher rudder took some heaving about, but more water was left to do its primary job of propelling the boat forward than on a later balanced rudder.

 

George ex nb Alton retired

Link to comment
Share on other sites

Thanks for that. Tell me if I am wrong but for any given area of rudder, including any balance, the more balance that is built in, the more water is diverted into turning rather than forward motion.

 

This is probably what the old lads were referring to. A big unbalanced Josher rudder took some heaving about, but more water was left to do its primary job of propelling the boat forward than on a later balanced rudder.

 

George ex nb Alton retired

But surely when turning you want the boat to turn and not go straight ahead! With an unbalanced rudder you would need more deflection to turn the same amount for the reasons described, that can't be efficient!

 

It still sounds like some macho BS to me - there was plenty of that around then and still is!

Edited by nicknorman
Link to comment
Share on other sites

But surely when turning you want the boat to turn and not go straight ahead! With an unbalanced rudder you would need more deflection to turn the same amount for the reasons described, that can't be efficient!

 

It still sounds like some macho BS to me - there was plenty of that around then and still is!

Fair enough Nick.

 

I bow to the superior knowledge imparted by the SMH school of motoring boating.

 

George ex nb Alton retired

Link to comment
Share on other sites

Nick is on to something here.

 

if using a dividing rudder! the water that goes straight back is pushing the boat straight forward, but the boat is turning, so the straight part of the prop wash get a vector that is counter productive.

 

a balanced rudder cover more of the prop disc and more force is going into turning the boat, or less rudder angle is needed. for the same turn rate.

 

a better shaped rudder will turn most of the prop wash as well as the free streaming water, without it Stall, the rudder.

see Schilling rudder...

Edited by Dalslandia
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.