Close quarters handling

[benjenbav]

I have noticed that, if I am making a tight turn at very slow speed with one engine astern and the other ahead, I can get the boat to turn even more quickly by nudging one into neutral for just a second or so before re-engaging.

It seems quite a nifty trick but I don't really understand why it works.

I have twin shafts with a keel between them (extending just below the lowest point of the blades). Maybe with the two screws rotating in opposite directions the water that each one is turning in is too churned up to get a decent grip and putting one side in neutral briefly allows the blades to bite more effectively before the force from the other side is re-introduced?

 

[Renegade_Master]

Its down to the two different pivot points. Pivot point fwd is fwd of midships, pivot point astern is aft of mid ships. So say stbs in fwd and ports astern as she starts to turn i.e once you have momentum, moving stb out briefly will increase the bow turn from the port astern engine briefly.

 

[benjenbav]

Thx, Sun Coast. It works a treat but it was puzzling me as to why it works, because it seems a bit counter-intuitive.

 

[nonitoo]

What is your prop rotation when going ahead ?

Both outward turning, both inward turning, both right handed, both left handed or are they CPP and, if so, what rotation are they ?

How large and how close to the props are the rudders ? Are they spade or semi-balanced ?

All these alternatives will alter the slow speed handling characteristics in differing ways.

As regards the pivot point, as has been said, the faster you go ahead the further forward the pivot moves from the middle of the boat and the same applies when going astern only the pivot moves aft of the middle.

Tom

 

[benjenbav]

There are so many things in your post that I don't know the answer to that I feel a bit of a dummy. I think the props are outwards rotating, they're certainly not CPP and they are probably about a metre from the rudders. I guess it's like the effect of rudders on manoeuvring: you need to get the feel of the particular boat before being dogmatic about whether there's any point in doing anything other than leaving them straight and steering on the sticks.

 

[thefatlady]

Sorry, but disagree. Nothing to do with pivot points. It is the reaction to the torque applied to the propeller on starting it rotating. The reaction tries to rotate the boat the other way and gives it a kick.

If you don't believe me, try it on two similar boats, one with single props and one with dual props. The dual prop version will give almost no kick. Applies with shafts or with outdrives with drives held straight ahead.

 

[carlton]

Sorry, but disagree. Nothing to do with pivot points. It is the reaction to the torque applied to the propeller on starting it rotating. The reaction tries to rotate the boat the other way and gives it a kick.

If you don't believe me, try it on two similar boats, one with single props and one with dual props. The dual prop version will give almost no kick. Applies with shafts or with outdrives with drives held straight ahead.

Are you talking about 'prop-walk'?

 

[oceanfroggie]

You can also use the rudders with asymetric thrust to move the pivot point forward and aft at low tick over speeds.

 

[thefatlady]

Carlton, no. Prop walk is a continuous effect due to asymetric prop thrust (prop geometry).
Benjenbav was asking about a transient effect due to engaging and disengaging drive. That is a torque reaction.

 

[carlton]

Thanks - wasn't aware of 'torque reaction'!

 

[hlb]

Dont see as it matters, long as it works, I just add a few revs on one engine, or take it off another. Boat goes round in it's own space, if thats what I want and at any speed, though usually slowish is best.

 

[benjenbav]

Dont see as it matters, long as it works,

I agree, but, having spent physics lessons at school pushing trolleys with tickertape attached to them out of the lab windows, I find myself, late in life with a curiosity as to how things work.

 

[hlb]

Best thing, is whatever keeps you in total control, usually that means, only two options, thats why I dont like bow thrutchers on twin engines. To much to control in limited time. Learn one course of action for nearly all conditions. Cock ups happen when folk loose control.

 

[nonitoo]

I find myself, late in life with a curiosity as to how things work.

Perhaps this may help,

Transverse thrust - think of the prop as a wheel, whichever way it is turning the stern will try and move in that direction.

Outward turning screws - Using the transverse thrust (prop walk) with this configuration assists turning in either direction.
Turning to port, stbd (ahead) and port(astern) props both turning in the same direction (right handed) so stern tries to move to starboard assisting a port turn.
Equally, turning to starboard, both screws are turning left handedly and so the stern tries to move to port assisting the turn again.

Inward turning screws - the opposite effect to the above and when turning in either direction the transverse thrust operates against the required turn reducing the effectiveness of the manoeuvre.
Traditionally inward turning screws were used mainly by RN vessels when they were required to patrol at slow speed for long periods. On station they used only one screw and so the rotation assisted the helmsman/autopilot load by reducing the rudder inputs required. It was felt that this facility outweighed the disadvantages found in close quarter manoeuvres when tugs would be available.

You can work out the advantages/disadvantages of the other configurations I mentioned in my last post yourself.

At very slow speeds the rudders can be used effectively with the ahead moving screw and due to the negligible ahead/astern motion during a tight turn the rudder serving the astern turning screw will have little effect.
I have never seen either a Becker or Schilling rudder on (smallish) leisure craft (although I am sure someone will prove me wrong) and a Kort, while giving vastly improved manoeuvring suffers from increased water resistance and therefore increased running costs when on long passages.

But, as always, nothing beats getting to know your boat with all its foibles.

Tom

 

[jfm]

I dont agree any of the explanations above, especially Suncoasts pivot point stuff. And let's keep this to the transient effect that BJB is asking about, which is nothing to do with prop walk generally. And let's assume the boat is stationary in the fore-aft sense - it is spinning on the spot.

There are two possible effects I reckon. Not mutually exclusive - both are probably happening. The first is the one you offer BJB - the grip of the propellor (in this case it's the sideways, propwalk grip that we're interested in, but it applies equally to the axial thrust grip) is better in near-stationary water. Once the prop has been running a couple of seconds it is in turbulent water, especially the trailing prop that is in the lee of the keel.

The second effect might be an illusion. The acceleration generated by the prop is higher at the standing start than as you approach the terminal velocity. In this case the terminal velocity I'm referring to is the steady-state sideways swing of the boat's arse as you do the turn. Probably about 2 knots. As you approach that 2 knots, acceleration tends (asymptotically, for the mathematicians) to zero. Whereas from a standing start the acceleration (and hence the kick you feel) is at its greatest. So it's possible BJB you are being duped by an illusion that by letting the boat's sideways velocity fall as you slip into neutral, you are allowing yourself to re-feel some acceleration when you engage gear again. An effect that is turbocharged of course by the fact the prop is re-starting in less turbulent water, item #1 above

 

[Hurricane]

I've noticed another interesting thing that initially seems to catch you out.

If the boat is absolutely stationary the prop walk is less than if she is moving - I think this is more or less explained above and accepted by everyone here. In other words if there is some "way on" the effect is more.

But I've noticed that at higher speeds
- lets say 6 knots with both engines driving forwards at their lowest speed)
- an engine then put in reverse has quite a small effect - certainly to begin with anyway. I sometimes give a little extra power on the reversing prop to get it to respond. This is the ONLY time that I use more throttle when manouvering. Normally, everything I do is at ultra low speed.

It all seems to come naturally now but thinking about it, the prop walk is greatest at about 1 or 2 knots and you would think it would be most responsive at the higher speeds.

I assume that its similar to jfm's comments in that the reversing prop is slipping more and takes time to "grip".

 

[jfm]

If the boat is absolutely stationary the prop walk is less than if she is moving - I think this is more or less explained above and accepted by everyone here. In other words if there is some "way on" the effect is more.

I don't find that! I'd say the prop walk is practically the same (if not slightly stronger) when the boat is stationary (in the fore-aft sense) than if it has a few knots of way on (in the fore-aft sense). Likewise the bowthruster.

 

[Hurricane]

I don't find that! I'd say the prop walk is practically the same (if not slightly stronger) when the boat is stationary (in the fore-aft sense) than if it has a few knots of way on (in the fore-aft sense). Likewise the bowthruster.

Oh - for me, its definately more pronounced with a bit of "way on"

We are currently berthed at the end of a run of pontoons so we always dock sideways onto the pontoon and the amount of sideways movement (well actually stern swing) is definately more pronounced if the boat is moving forwards slightly than if she was static.

 

[jfm]

Oh - for me, its definately more pronounced with a bit of "way on"

We are currently berthed at the end of a run of pontoons so we always dock sideways onto the pontoon and the amount of sideways movement (well actually stern swing) is definately more pronounced if the boat is moving forwards slightly than if she was static.

Are you sure it's a like for like comparison? I mean if you pass your berth and want to reverse into it, then if you have some way on you can run the prop kick prop for longer, as it will be both kicking sideways AND stopping the boat. A handy technique, that can let you swing the boat's bum right into the berth. Whereas if you were stopped and prop kicked, you'd have to play with both props and maybe run them for less time to avoid putting reverse way on. So all I'm saying is, are you comparing like with like!

BTW, I think this thread is proving all boats handle differently! I'm not disagreeing with you at all on the effect you find on your boat

 

[Hurricane]

Are you sure it's a like for like comparison? I mean if you pass your berth and want to reverse into it, then if you have some way on you can run the prop kick prop for longer, as it will be both kicking sideways AND stopping the boat. A handy technique, that can let you swing the boat's bum right into the berth. Whereas if you were stopped and prop kicked, you'd have to play with both props and maybe run them for less time to avoid putting reverse way on. So all I'm saying is, are you comparing like with like!

BTW, I think this thread is proving all boats handle differently! I'm not disagreeing with you at all on the effect you find on your boat

Interesting way of thinking about it.
On our old boat in the Hamble we did just that.
Run down the fairway and engage the opposite engine in reverse until the boat had stopped and turned through 90 degrees.

Maybe we are seeing the same effect - putting an engine in reverse when the boat is under way (forwards) does, of course do two things - slow it down and turn in - bow moves the other way as well of course but not as much as the stern. It just feels to me that the stern moves more when the boat is under way than if it were stopped.