H4B
Well-Known Member
A Jeanneau Sunfast 3200.
I am getting the idea that you have to be bold and 'gun ' the boat around and I shouldn't expect to turn the boat in its own length.
Last edited:
Twin rudders manoeuvrability
- Thread starter H4B
- Start date
A Jeanneau Sunfast 3200.
I am getting the idea that you have to be bold and 'gun ' the boat around and I shouldn't expect to turn the boat in its own length.
Laurie
Well-Known Member
I think you are right....that is going to have much less lateral grip on the water(lighter, less hull immersed, shorter chord keel) when manoeuvring slowly compared to your Fulmar, in any case. Never mind just astern.
Birdseye
Well-Known Member
Should make no difference going backwards. The only issue is that with a single rudder going forwards you have water from the prop flowing over the rudder and dont with twins. In reverse, both are equal
Southerly used to recommend bow thrusters for their twin rudder boats. I used to have a cat with twin rudders and going forward you needed decent speed to turn. At one knot your steering capability was pretty limited and in cross winds non existant.
Fourbees
Well-Known Member
I don't know why, but it is so. After years of twin rudder fun I chartered a single rudder fin keeler in the Canaries, and was astonished at how easy it was to reverse it into a berth. Went in sweet as a nut, every time, which would not have been true of my own boat...
Ian_Edwards
Well-Known Member
going astern
When you go astern, the water is drawn into the prop' (the low pressure side) over a larger volume. At bollard pull (advance coefficient=0) and in free water, no hull or water surface, it's theoretically a hemisphere immediately on the low pressure side of the prop'.
As boat makes way astern, the volume from which the water "sucked" gets smaller and look more like a cone or a funnel, the faster the boat goes the smaller the angle of the cone. Adding the free water surface and the presence of the hull makes the picture much more complicated, but the basic idea is much the same.
But the critical point is that water is moving towards the prop' and over a single rudder on the centreline. Not anything like as fast as it does on the output side of the prop', where it forms a "jet", but it is moving.
So you get more response from a single rudder on the centreline and the closer it is to the prop' the more response you get.
Twin rudders, tend to be outside the main movement of the water towards the prop, they are off the principal axis of the prop' and the water movement is much slower than on the axis of the prop' where a single rudder is placed.
If you have no water movement over the rudders, you don't get any lift from them, so the boat is less maneuverable.
When you go astern, the water is drawn into the prop' (the low pressure side) over a larger volume. At bollard pull (advance coefficient=0) and in free water, no hull or water surface, it's theoretically a hemisphere immediately on the low pressure side of the prop'.
As boat makes way astern, the volume from which the water "sucked" gets smaller and look more like a cone or a funnel, the faster the boat goes the smaller the angle of the cone. Adding the free water surface and the presence of the hull makes the picture much more complicated, but the basic idea is much the same.
But the critical point is that water is moving towards the prop' and over a single rudder on the centreline. Not anything like as fast as it does on the output side of the prop', where it forms a "jet", but it is moving.
So you get more response from a single rudder on the centreline and the closer it is to the prop' the more response you get.
Twin rudders, tend to be outside the main movement of the water towards the prop, they are off the principal axis of the prop' and the water movement is much slower than on the axis of the prop' where a single rudder is placed.
If you have no water movement over the rudders, you don't get any lift from them, so the boat is less maneuverable.
D
Deleted member 36384
Guest
This would explain a comment made by 'jimi' I think. he was on a course and the instructor was teaching how to the yacht going backwards more effectively, I think on a windy day. The instructor held the yacht on warps and applied a lot of reverse thrust as the yacht strained at the warps. The next bit I am not so sure but the warps were slipped and the throttle cut back, the speed of water over the rudder caused by the engine at a big throttle initially was what allowed the rudder to work better at slow speeds until suitable way was achieved.
PITCAIRN
Active Member
Irrespective of the difficulties of reversing with twin rudders, the real downside is you loose the protection afforded by the keel should you run into something floating in the water, a single rudder may be saved by the keel, but with twin rudders one or other may be snapped off.
afterpegassus
Well-Known Member
What I've been able to deduce from this thread is that relatively few twin ruddered vessels go astern. The majority seem to reverse.
maxi77
Well-Known Member
At the end of the day steering performance is down to how much side ways force you can develop. There are a number of factors, area of the rudders, shape as an aerofoil can improve lift, flow of water over the rudder and angle of the rudders. Within a given hull profile a single rudder may not give adequate area to develop enough sideways thrust. Thus more than one rudder may be required, even if this loses the benefit of aditional water velocity from prop wash. On Dutch barges I worked on systems with up to 8 rudders, mainly to get the required rudder area. Thus a single rudder on oneboat would out perform a double on another which in turn may out perform a single on a third boat. When making comparisons one must ensure you are always comparing apples with apples
Melleman
Well-Known Member
May be you can reverse in to your spot and then the twin rudders will be no ploblem anymore.