Tiller & prop

[DHutch]

Make sure it stays within the profile of the boat.

 

Yeah, i guess thats pritty essential, could end up with a rather miss-shapen tiller!

[a1mac]

Is there a problem in having the prop to close to the rudder? My old boat steered wonderfully with a 9'' gap. My new boat with a 3'' gap is pathetic. Should there be enough room for the prop to create a flow before hitting the prop?

[ChrisPy]

  The boat handles beautifully, is just very very heavy on the tiller at high speeds and fast turns.

 

11211[/snapback]

............ bl**dy h*ll ............... !!

[John Orentas]

Is there a problem in having the prop to close to the rudder? My old boat steered wonderfully with a 9'' gap. My new boat with a 3'' gap is pathetic. Should there be enough room for the prop to create a flow before hitting the prop?

34023[/snapback]

 

 

Al.

 

I would say a 2-3 inch gap was about a minimum to avoid the possibility of anything jambing between, nine inches seems a very big gap and I really can't see any advantage in that, the closer it is to the propeller the more force you will get onto the rudder for manoeuvring.

 

I'm not sure you can make such comparisons between two boats, there will be a million variations between them all of which will have an effect on the handling.

[DHutch]

Ours is about 2inchs.

- One of the things you notice (mainly only when using the tiller) is the pulse of the prop blades passing by.

- They form a very clear beat, 5/6 times a second as the prop turns round!

 

 

Daniel

Edited October 8, 2005 by dhutch

[alan_fincher]

This is how ours currently is, and it's not good....

 

Link to gallery image of prop & rudder

 

Our surveyor was more unhappy with the length of unsupported shaft between stern tube and propellor, than with separation twixt prop and rudder.

 

But he also correctly said that you would get an unpleasant vibration, through the tiller. It's prpbly no worse than some others I've felt, but it doesn't need to be there.

 

Shortening the shaft, to move the prop back towards the boat, and away from the rudder is part of a package of improvements we expect to make when she is pulled out in a month or so.

[a1mac]

Can we have opinions of the % of balance blade to ruddersize

Alex

[DHutch]

Ours is pivoted at about 1/4 i roughly.

- ie theres about 4times more out the back than at the front.

 

 

Dnaiel

[a1mac]

I think that 25% is a good average. But will this be relative to the prop size?

If you have a 16 inch prop then either side of the prop will be 8 inches (at 90 degrees) and you make it long enough for coverage of 4 inches on the non directed drive side you will have 4 inches of prop pushing you forward?. If you increase the prop to 18 inchs then you will have 6 inches of non directed drive. Surely the more you have of non directed prop exposed the more it will push forward and not turn.

[John Orentas]

I would say it should be more like 3 : 1. But the best test as I have said before, when looking from the stern the propeller should be fully obscured. My friend added 2" to the balance blade on his 57' new style Liverpool boat recently. It made a big improvement to the general handling and improved manoeuvrability.

[bottle]

I would say it should be more like 3 : 1. But the best test as I have said before, when looking from the stern the propeller should be fully obscured. My friend added 2" to the balance blade on his 57' new style Liverpool boat recently. It made a big improvement to the general handling and improved manoeuvrability.

 

Hi John

 

When you say fully obscured at what angle should the tiller be (straight ahead being 0 ). I have in the back of mind that tillers are more 'efficient' at say 45 if this is so would that be the total obscure position as going beyond would be of no advantage.

 

In my little experience the tiller seems to lose effect when pass about 45 degrees.

[John Orentas]

Hi John

 

When you say fully obscured at what angle should the tiller be (straight ahead being 0 ). I have in the back of mind that tillers are more 'efficient' at say 45 if this is so would that be the total obscure position as going beyond would be of no advantage.

 

In my little experience the tiller seems to lose effect when pass about 45 degrees.

 

 

Hi Bottle.

 

I don't know where this 45 deg. thing comes from, most narrowboats will get close to 90 deg. How do you measure efficiency in this case, you will probably get maximum sideways thrust at around 45 deg. but that is not really the point, in that case you will also be getting a lot of forward drive but you may not want to go forward.

 

With a bit of experimenting you can with many boats, including mine, find a tiller position where you are getting a significant sideways thrust with close to zero forward drive, but you need to 'feel around' for it, it is usually only one or two degrees from the position of zero side-thrust.

 

For anyone familiar with Whaley Bridge basin, Anthony managed a 180 deg turn there in one go with my 50ft boat.

 

The prop. fully obscured I mean at full tiller and sighting directly from behind five yards away.

[a1mac]

can any one comfirm my theory that a smaller pitch prop will increase rudder responce. ie a 18x10 is better than a 18x12

 

Hi Bottle.

 

I don't know where this 45 deg. thing comes from, most narrowboats will get close to 90 deg. How do you measure efficiency in this case, you will probably get maximum sideways thrust at around 45 deg. but that is not really the point, in that case you will also be getting a lot of forward drive but you may not want to go forward.

 

With a bit of experimenting you can with many boats, including mine, find a tiller position where you are getting a significant sideways thrust with close to zero forward drive, but you need to 'feel around' for it, it is usually only one or two degrees from the position of zero side-thrust.

 

For anyone familiar with Whaley Bridge basin, Anthony managed a 180 deg turn there in one go with my 50ft boat.

 

The prop. fully obscured I mean at full tiller and sighting directly from behind five yards away.

 

Can you give me details of the prop and engine fitted to your boat. The only boats I have seen do this are the Canal Time boats. They seem to turn faster the more power you apply! - perfect

 

Alex

[John Orentas]

can any one comfirm my theory that a smaller pitch prop will increase rudder responce. ie a 18x10 is better than a 18x12

Can you give me details of the prop and engine fitted to your boat. The only boats I have seen do this are the Canal Time boats. They seem to turn faster the more power you apply! - perfect

 

Alex

 

 

Hi Alex.

 

It is all a bit of a 'black art' propeller selection, some chandelers and other people in the trade have charts and formula's they can use (or they pretend they have and take a quess). Crowthers at one time would give out this information to anyone who phoned but they have stopped doing it probably because they were being taken advantage of.

 

An 18 x 10 prop. is not equivalent to an 18 x 12, the former would need to rotate at a higher speed, have a larger diameter or a greater circle ratio to give the same thrust. For what it is worth my boat is fitted with an 18 x 13 prop. but my engine is bigger than most, being a fairly low revving 2.2 litre 48 horsepower.

 

All this is a bit like tuning a racing car, there are a thousand factors to be taken into account.

[a1mac]

Hi Alex.

 

It is all a bit of a 'black art' propeller selection, some chandelers and other people in the trade have charts and formula's they can use (or they pretend they have and take a quess). Crowthers at one time would give out this information to anyone who phoned but they have stopped doing it probably because they were being taken advantage of.

 

An 18 x 10 prop. is not equivalent to an 18 x 12, the former would need to rotate at a higher speed, have a larger diameter or a greater circle ratio to give the same thrust. For what it is worth my boat is fitted with an 18 x 13 prop. but my engine is bigger than most, being a fairly low revving 2.2 litre 48 horsepower.

 

All this is a bit like tuning a racing car, there are a thousand factors to be taken into account.

 

 

Hi John

 

Thanks for the information.

I do know they are different from a performance piont of view but I am trying to confirm a theory of mine that a prop has to complete one revolution that is 10inchs or 12 inches before the 'stream' of directed water hits the rudder. I suspect that engines have become more powerful and therefor need bigger props without consideration to the traditional designs and relative distance between rudder pivot and prop, and that propellors are just spashing around behind the rudder and are given no room to become a directional flow.

[John Orentas]

Hi John

 

Thanks for the information.

I do know they are different from a performance piont of view but I am trying to confirm a theory of mine that a prop has to complete one revolution that is 10inchs or 12 inches before the 'stream' of directed water hits the rudder. I suspect that engines have become more powerful and therefor need bigger props without consideration to the traditional designs and relative distance between rudder pivot and prop, and that propellors are just spashing around behind the rudder and are given no room to become a directional flow.

 

 

Hi Alex.

 

Not sure that I follow your theory, I don't think that a stream has to develop or that the distance from the rudder is all that critical, the important thing is that there is a flow of water on both sides of the rudder in order that it will act upon it like an aircraft wing with unequal pressures on each side. That is why there is a point where the rudder suddenly ceases to be effective in the same way that a wing will stall.

 

Beyond that point the rudder serves only to direct the flow of water to one side or the other as in the case of 'stationary manoeuvring'.

[a1mac]

Hi Alex.

 

Not sure that I follow your theory, I don't think that a stream has to develop or that the distance from the rudder is all that critical, the important thing is that there is a flow of water on both sides of the rudder in order that it will act upon it like an aircraft wing with unequal pressures on each side. That is why there is a point where the rudder suddenly ceases to be effective in the same way that a wing will stall.

 

Beyond that point the rudder serves only to direct the flow of water to one side or the other as in the case of 'stationary manoeuvring'.

 

 

Hi John

 

Thanks for all your time on this one but your are the first person I have found with a knowledge and an opinion.

Would you suppose that the first part of the flow begins at the outside tip of the propeller, if so the water hitting the rudder in the first, say six inches it is only from these outside edges. Going straight ahead the tube of thrust of the 'stream' must form around and beyond the rudder. I must admit I am still confused about the thrust of propellers. I also feel that if the rudders balance section is not big enough, the stream because it is only from these outer edges is having no effect on the balance beam and not even hitting it.

I suspect older boats with smaller engines and smaller propellers probably didn't suffer any steering problems because the prop had more room to work.

 

Alex

[John Orentas]

Hi John

 

Thanks for all your time on this one but your are the first person I have found with a knowledge and an opinion.

Would you suppose that the first part of the flow begins at the outside tip of the propeller, if so the water hitting the rudder in the first, say six inches it is only from these outside edges. Going straight ahead the tube of thrust of the 'stream' must form around and beyond the rudder. I must admit I am still confused about the thrust of propellers. I also feel that if the rudders balance section is not big enough, the stream because it is only from these outer edges is having no effect on the balance beam and not even hitting it.

I suspect older boats with smaller engines and smaller propellers probably didn't suffer any steering problems because the prop had more room to work.

 

Alex

 

Alex.

 

I don't think the idea of the tips of the propeller being most significant is quite right, better I believe to think of the flow as a solid cylinder of water* moving rear-wards from the boat and it doesn't spread much either the mass of the surrounding water keeps it concentrated. A good analogy is a large rocket, as it goes through the atmosphere the jet-stream is quite concentrated because of the influence of the surrounding air, as it begins to leave the atmosphere and into the vacuum of space the stream fans-out very markedly.

 

The trust of a propeller is best thought of as a combination of the equal and opposite reaction of the mass of water being accelerated rear-wards and also the pressure difference on the front and rear as it rotates as with an aircraft prop.

 

Older boats, think about working boats they had very large slow moving props. they say that they are more efficient and as they cause very little incidental turbulence they no doubt are, but large props. have other problems. And no I don't have any special knowledge on all this, just thinking on my feet.

 

*Not sure you can have a solid cylinder of water but you will know what I mean.

Edited November 2, 2005 by John Orentas

[a1mac]

Alex.

 

I don't think the idea of the tips of the propeller being most significant is quite right, better I believe to think of the flow as a solid cylinder of water* moving rear-wards from the boat and it doesn't spread much either the mass of the surrounding water keeps it concentrated. A good analogy is a large rocket, as it goes through the atmosphere the jet-stream is quite concentrated because of the influence of the surrounding air, as it begins to leave the atmosphere and into vacuum of space the stream fans-out very markedly.

 

The trust of a propeller is best thought of as a combination of the equal and opposite reaction of the mass of water being accelerated rear-wards and also the pressure difference on the front and rear as it rotates as with an aircraft prop.

 

Older boats, think about working boats they had very large slow moving props. they say that they are more efficient and as they cause very little incidental turbulence they no doubt are, but large props. have other problems. And no I don't have any special knowledge on all this, just thinking on my feet.

 

*Not sure you can have a solid cylinder of water but you will know what I mean.

 

 

Hi John

 

You do seem to be comfirming my suspicions and the prop will act like an aircraft propellor.

My comments about the tips of the prop was that this outside drive was insignificant -ie, the only bit of drive that reached the 'to close' rudder and as such could not provide enough stream for steerage.

Do you have any knowledge of older boats and the distance between prop and rudder.

My old 12 year old 50 ft Liverpool boat ran a 33hp nanni so only required a 16x10 prop but the distance from prop to rudder pivot was almost 18 inches and it steered perfectly. It would certanly turn 180 in Whaley Bridge basin - at any speed.

 

Once again John, thanks for all your time

 

 

Alex

[enigma]

Alex

 

The "Propeller Handbook" suggests an average gap between the rudder and propeller of 15 percent of the propeller diameter.

 

The propellers on an aeroplane and a narrowboat are very different. An aeroplane propeller both pushes air rearwards (positive pressure) and sucks forward (negative pressure).

 

A boat propeller only pushes water rearwards it does not produce any forward negative pressure. To do so would induce propeller cavitation and that is to be avoided.

 

A narrowboat rudder is a flat plate it has no aerofoil shape. Therefore it cannot operate like a wing. An aircraft wing produces less pressure on the top of the wing than on the bottom. The equivalent of the top on a narrowboat is the rear face. As it is a flat plate the aerofoil affect is not there. A narrowboat rudder merely deflects the water sideways off its forward face.

 

When the rudder is deflected about 60 degrees and the propeller is obscured by the rudder maximum water deflection will be achieved. Rudder deflection towards 90 degrees does not turn the boat.

 

The main part of the rudder lies behind the shaft, the balance part is in front of the shaft. If there is no balance part the rudder is hard to move. If the balance part is too large the rudder is too easy to move and may snatch the tiller. The solution here is trial and error.

Edited October 23, 2005 by enigma

[dor]

I agree with what you say enigma. Negative pressure gives rise to cavitation which is inefficent at best, damaging at worse. It does little to increase the thrust, as opposed to a deflecting blade in a commpressible medium, i.e. a aero prop in air.

 

My rudder had about 25% area forward, and used to wobble uncomfortably. I had about 3 cm cut off the front and it improved enourmously, without being too heavy to turn. Maximum turning efficiency is at about 70%, and if I get it right I can almost spin the boat in its own length. For reasons I have yet to establish, I find waggling the tiller when I start to turn increases the turning action considerably.

 

But the best balance area of the rudder relative to the main area is dependent on many factors, based on the geometry of the swim etc.

 

What is right for one boat is wrong for another.

 

Like so many things in boating, it is not an exact science and a certain amount of intuition and trial& error is worth much more than a lot of theory.

Edited October 23, 2005 by dor

[enigma]

Rapid pumping or waggling the tiller.

 

When you push the tiller away from you, you speed up the rate of water flow across the rudder. More water, faster, means that the sideways thrust is greater and the turn accelerates. When you pull the tiller towards you the water slows down and the rate of turn reduces but at a slower rate. The net result is a faster turn and sore muscles.

[dor]

Thanks Enigma, I kenw there had to be a logical reason somewhere!

[Hairy-Neil]

I believe that the first welded steel boats were the Admiral Class built between 1957 and 1960

 

 

 

Charles Hill & Sons of Bristol built eight all welded steel boats in 1934 for the Severn & Canal Carrying Company all named after species of tree. One of them, Oak is an exibit at the National Waterways Museum at Gloucester.