Going backwards

[Nb 68160]

Why is going backwards on a 30ft boat so much harder than a bigger boat to keep in a straight line , my 30ft boat wants to go in a big circle 🤔

[Arthur Marshall]

Presumably because the prop shoves the blunt end sideways, and a small boat has less weight so is easier to shift?

[Jen-in-Wellies]

Have you been going backwards so far you've gone in a big circle round the world, back to where you started?

[Mac of Cygnet]

My 27ft boat, Cygnet, was always a devil to steer backwards, and required constant correction (quick burst forward on the opposite tack).  The upside was that correcting was much quicker and easier than on a longer, heavier boat.  I got so that I could usually back into my finger mooring quite successfully, despite gongoozlers.

[David Mack]

The other side of the coin is that a boat that short can turn in many places, so you can turn around to go back the way you have come and not need to reverse, unlike those of us with longer boats.

[cuthound]

17 minutes ago, Mac of Cygnet said:

My 27ft boat, Cygnet, was always a devil to steer backwards, and required constant correction (quick burst forward on the opposite tack).  The upside was that correcting was much quicker and easier than on a longer, heavier boat.  I got so that I could usually back into my finger mooring quite successfully, despite gongoozlers.

 

That is how I steered our first share boat (a 578 foot Pat Buckle hull) in reverse.

 

The second share boat (a 58 foot Graham Reeves hull) and my current boat ( a 60 foot Alexander hull) all steer reasonably well on reverse provided there is no wind.

[MPR]

7 minutes ago, cuthound said:

 

That is how I steered our first share boat (a 578 foot Pat Buckle hull) in reverse.

 

 

Wow!

[cuthound]

1 hour ago, MPR said:

Wow!

 

I always was prone to exaggerating and typos! 🤣

[BEngo]

The ease of going backwards is inversely proportional to the number of spectators.⁸

Edited May 2 by BEngo

[IanM]

2 minutes ago, BEngo said:

The ease of going backwards is inversely proportional to the number of spectators.⁸

Ours seems to go backwards perfectly until I think about what it is I’m doing to make it do that… then the inevitable happens and it all goes sideways. 

[Mac of Cygnet]

4 hours ago, David Mack said:

The other side of the coin is that a boat that short can turn in many places, so you can turn around to go back the way you have come and not need to reverse, unlike those of us with longer boats.

Absolutely! I just laughed at winding holes,

[Peugeot 106]

I’m pretty perfect with my 25’ Narrowboat as long as there is NOBODY watching. Otherwise I often have to put in little bursts of forward to straighten out. 

i find when reversing the revs are quite important for any steering in reverse. Make sure you are completely stopped before reversing and experiment with revs. Also have a look at any wind direction

 

In forwards I can spin on my own length

[Tacet]

10 hours ago, Peugeot 106 said:

I’m pretty perfect with my 25’ Narrowboat as long as there is NOBODY watching. Otherwise I often have to put in little bursts of forward to straighten out. 

i find when reversing the revs are quite important for any steering in reverse. Make sure you are completely stopped before reversing and experiment with revs. Also have a look at any wind direction

 

In forwards I can spin on my own length

How can you not be stopped before reversing?

[WulfNut]

Just under 50ft.  I've come to the conclusion that she will make her own mind up where she is going in reverse and my job is to just let her do it slowly before slowly correcting her.  Main thing I've found is to make sure I haven't dumped stuff on the semi trad seats as there's a pole up front for a reason, and that reason is she has a mind of her own going backwards. 

[Peugeot 106]

39 minutes ago, Tacet said:

How can you not be stopped before reversing?

Ha Ha I suppose I could have said if you are tanking along forward and slam into reverse you might not stay straight for reversing when you have stopped. You might. 
It definitely helps to have the boat lined up ( taking into account wind) before reversing
On my 25’ boat it’s easier to reverse when the water tank in the bow is full

i have an ancient well maintained charity owned plywood cruiser next to me on the finger pontoon. It’s always a “ phew” moment when I’m in but I haven’t bashed it…..yet

[Chris John]

When I had a GRP on the Thames we were moored in an old mill stream. There was a narrowboat opposite and when they wanted to go out they had to reverse down the mill stream. They dropped a chain from the bow to drag on the river bed and it appeared to aid the reversing. 
Good idea until you trawl or catch on something I guess 

[blackrose]

20 hours ago, Nb 68160 said:

Why is going backwards on a 30ft boat so much harder than a bigger boat to keep in a straight line , my 30ft boat wants to go in a big circle 🤔

 

Part of it is simply due to the length: width ratio of the boat. If you think about a boat with a 1: 1 ratio such as a coracle it spins on its own axis, whereas a 70ft narrowboat which has roughly a 10:1 length: width ratio, is relatively easy to keep in a straight line whether in forward or astern, and easier than a shorter narrowboat. Conversely, all other things being equal, the 70ft narrowboat is more difficult to turn than a coracle or a 30ft narrowboat.

[cuthound]

14 hours ago, Peugeot 106 said:

In forwards I can spin on my own length

 

Yes, but what about your boat... 🤣😂

[Peugeot 106]

9 hours ago, cuthound said:

 

Yes, but what about your boat... 🤣😂

Of course my boat can do it. But it’s not so good in reverse.
 

I’m perfecting the Eskimo roll

 

[Bee]

Some boats just will not go backwards in a straight line. Bee (10m x 3.4m) with a double chine and alightly V bottom  will not steer backwards, although she looks like a 100 year old tug below the waterline she is more akin to a cruiser. The rudder, which is a big thing, makes very little difference to what the front end does. A bow thruster would help but retro fitting and paying for it is not an attractive proposition. Narrow boats are usually flat sided, draw quite a lot of water and have more resistance to skating sideways. Bee will turn in a tight circle going forwards which is great but will also turn in very nearly its own length in reverse which is impressive but not so good when trying to hold a position in a current with other boats. Somewhere in Holland there is a lock keeper who wanted Bee to reverse 25 metres to rearrange the flotilla of small boats in his huge lock. We were seriously concerned about his health as he grew ever more angry at Bee's antics making Spirograph patterns all over the place.

[Mike Todd]

A boat turns about its centre as the result of a force applied to some other point along its length - usually at the stern but at the bow in the case of thrusters. Steering in a forwards direction whilst motoring is largely the result of the pressure of the prop wash onto the rudder plate when it is held across the flow - at its best when the flow is minimally disturbed. Hence turning the rudder too far at an angle eg a right angle, gives much less turning effect. The speed of rotation about the centre is dependent on the force which is related to the speed of the prop, so long as the flow is smooth. Hence, at low speeds the rate is reduced and traditionally steerers used fish tailing to improve the rate of turn. Note that in this case very different physics is involved as it largely depends on the non-linearity when the rudder is turned through the almost still water. Pulling the tiller harder produces more force than when returning it slowly.

 

The effect of the prop flow when in reverse are not the negative of when moving forwards. The prop sucks water from behind the boat and pushes it to one side or the other. The turning force comes from a combination of the flow over the rudder and the being 'jetted' to the side. Props are not designed to work with the same efficiency in reverse - the hydrodynamic flow through a prop is very complicated which is why prop design has long been a black art and still is to some extent.

 

A boat in a canal is also a complex behaviour as it is partially dependent on water depth which is rarely a constant from side to side, Hence there will be turning forces independent of the prop forces. One common example is how a boat is drawn to one side when in a confined width such as going through a bridge hole. 

 

A further factor is that most steerers spend much more time in a forward direction rather than reverse. Hence the instinctive skills are different. (Not dissimilar to learning to ride a bike) Even in quite benign stretches of the canal, the steerer will be making unconscious changes to the tiller position which are determined through looking at how the boat is moving. Only in special manoeuvres is this process at all conscious. This is helped by the fact that we steer from the back and can see how the boat is moving - not so in reverse. (All this is different with centre cockpit steering, of course as also with outboard propulsion without a rudder when only the prop flow is the significant turning force. Likewise with an unpowered butty and a larger rudder)

 

Narrowboats can be controlled in reverse but it is a much less practised skill and most of us do not get to a 'master' level! The common practice of combining reverse with occasional forward thrusts is based on the differences in the forces in the two directions.

 

If you think that predicting a boat movement either forward or backward is simple, try laying a pencil on a table top and 'steer' it by pushing on one end!

 

[Tacet]

1 hour ago, Mike Todd said:

A boat turns about its centre as the result of a force applied to some other point along its length - usually at the stern but at the bow in the case of thrusters. Steering in a forwards direction whilst motoring is largely the result of the pressure of the prop wash onto the rudder plate when it is held across the flow - at its best when the flow is minimally disturbed. Hence turning the rudder too far at an angle eg a right angle, gives much less turning effect. 

 

Having previously noted the point about less rudder angle gives a greater turn, I experimented a bit.  On a tight turn, such as winding, I found myself unable to be persuaded to reduce the tiller angle.  Having boated for a good few years, I thought I must have some reason for adopting the full lock method.

 

I reckon too much angle indeed reduces the force perpendicular to the ahead axis of the boat - but it also reduces the ahead force by interrupting the prop thrust.  The net effect of which is that the resolved vector is more perpendicular to the ahead axis.  Hence a greater rate of turn.

 

On the other hand, if you're ploughing through the water at a good speed and need to make a rapid swerve ( a less usual thing than a low speed manoeuvre), the maximum sideways force is called for - so don't push the tiller too far over.

 

Now there must be limits to either case, and boats vary a bit - but it does align with how it works for me in practice.

[Tam & Di]

There are several factors including hull shape and length, prop size and direction of rotation, size of rudder, and steerer capability. A major issue too is the amount of counter balance on the rudder. If there is none at all it means that when the rudder is held over there is still at least 50% of the prop thrust acting directly forwards or backwards, according to what gear is engaged. The more counter balance the rudder has obviously affects how much of the thrust is being affected by the rudder.

[blackrose]

3 hours ago, Bee said:

Some boats just will not go backwards in a straight line. Bee (10m x 3.4m) with a double chine and alightly V bottom  will not steer backwards, 

 

10m x 3.4m is roughly a 2.9: 1 length width ratio so it's bound to want to spin in reverse. Maybe you have wheel steering but if it's a tiller then chances are it will go off in its own direction as soon as you take your hand off the tiller.

 

My boat is 17.3m x 3.6m so 4.8: 1 L to W ratio, but I have a BT so reversing is no problem. Again if I let go of the tiller it won't stay in a straight line.

1 hour ago, Tacet said:

Having previously noted the point about less rudder angle gives a greater turn, I experimented a bit.  On a tight turn, such as winding, I found myself unable to be persuaded to reduce the tiller angle.  Having boated for a good few years, I thought I must have some reason for adopting the full lock method.

 

I reckon too much angle indeed reduces the force perpendicular to the ahead axis of the boat - but it also reduces the ahead force by interrupting the prop thrust.  The net effect of which is that the resolved vector is more perpendicular to the ahead axis.  Hence a greater rate of turn.

 

On the other hand, if you're ploughing through the water at a good speed and need to make a rapid swerve ( a less usual thing than a low speed manoeuvre), the maximum sideways force is called for - so don't push the tiller too far over.

 

Now there must be limits to either case, and boats vary a bit - but it does align with how it works for me in practice.

 

I might be wrong but I thought the limits to the efficiency of rudder angle was simply dictated by the angle at which the rudder stalls, much like an aircraft wing? I suppose the rudder still acts as a deflector even after it's stalled and lost maximum efficiency.

 

On my boat maximum rudder angle is maximum turning at whatever speed the boat is moving. The stops are at about 80 degrees on each side and it's not possible to mechanically over-steer.

 

 

[wakey_wake]

4 hours ago, Mike Todd said:

The effect of the prop flow when in reverse are not the negative of when moving forwards. The prop sucks water from behind the boat and pushes it to one side or the other. The turning force comes from a combination of the flow over the rudder and the being 'jetted' to the side.

This points to the possibility of a rudderette, mounted forward of the prop.

In forward thrust one would want is it straight. In reverse thrust it should give significant rudder authority (quite independent of the main rudder) by directing prop wash port or starboard of the swim.

 

Do such things exist?

It's a tricky place to be installing another bearing but it might be less invasive than retrofitting a BT.

The risk of bashing, bending and weed entanglement seem significantly increased.

 

There would be a problem connecting the part of the rudderette below the propshaft, to the part above.

One solution would be to omit it and have ~half the authority.

Another solution maybe a pair of rudderettes, astride the prop width, hinged astern, connected at the top, and shaped to gate the propwash onto the swim.

Edited May 4 by wakey_wake
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