Paddleducks
Old Yahoo Group => Yahoo Messages => Topic started by: Derek Warner on June 17, 2005, 07:45:25 PM
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We have seen many postings re the increase the rudder scale sizing to turn a fixed shaft paddler in a
respectable manner etc, but have we considered increasing the rudder sweep included angle?
The photograph I took (of the hulk of PS Decoy) 's rudder looks grossly over length & prone to mechanical
damage (in a scale format)
PS Decoy current oversize rudder will present a block 80 mm high x 60 mm long resistance to the water,
but at swept angle? - regards Derek
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Well Derek!
This IS a knotty question! :))
The answer is very simple - but also quite complicated. As I know a little about fluid mechanics and hydrodynamics ( NO! I am NOT a clever clogs - just remembering education burnt into the brain many years ago!!)
In simple terms I would not recommend a rudder deflection of more than 40degrees - if this does not give the turn rate desired then increase the AREA of the rudder.
That's the simple answer - now for the more complicated bit!
Given the same deflection on any rudder there are five forces/characteristics that affect the rate and behaviour of turn of any ship or boat. In no particular order they are these:
1. The speed of the vessel. It may seem obvious but the faster a hull moves through the water the greater any turning moment produced by the rudder. This is where we get the term 'steerage way' being the minimum speed at which any ship is controllable. The rudder on a ship does not just work by DEFLECTING water, it acts in a similar way to an aircraft wing by producing a high pressure area on the deflected side of the rudder and a low pressure area on the other. Nature abhors a vacuum (whether in air or fluids - though in fluids it isn't quite the same - but enough!!) so the stern of the vessel will deflect to try to equalise the different pressures - hence the turn. The higher the speed the greater the pressure difference hence the 'tighter' the turn.
2. The speed of the water moving PAST the rudder. This is different to the above as in a single screw driven vessel quite a high flow of water can be produced over the rudder by the screw even when the ship is stationary - this factor does not apply to paddle steamers as any 'water movement' has generally gone over the distance to the stern. But it is, I always think. quite magical to watch a largish ship manoeuver its' stern away from a quayside just by turning the wheel and applying power.
3. The hull length to beam ratio. Generally speaking one can consider the underwater portion of the hull very like an aircraft wing - with the rudder as the flap if you like. Long thin wing sections (High L-B ratio) are far more efficient than short thick ones (Low L-B ratio) especially in fluids. So, all other factors taken into account the rudder will act much more effectively on a long thin hull (cross channel paddle steamer?) than on a short fat one (Tug?)
4. Turning moment of the hull. Every hull will have a point about which the lateral forces of the rudder will act and this will NOT necessarily coincide with the centre of gravity of the hull. A hull with much more displaced volume towards the stern will tend to pivot around a point farther back than one with a higher volume of water displaced by the forward part of the hull. Tugs for instance tend to turn very smartly and 'swing' their bows because generally the maximum displacemrnt of the hull is rearward of midships.
5. Mass (weight distribution) balance of the hull. A ship will float level in the water whether it has most of its mass concentrated amidships or at either end. The weight of the ship remains the same. Imagine it as the difference between holding a 10 pound ball of lead or two 5 pound balls at each end of a pole. It's very easy to turn the 10 lb ball in your hand. To turn the two 5 lb balls will require much more initial force because of the moment arm of the mass - it will also be much more difficult to arrest the swing on the bar once you decide to STOP spinning it. So a ship with its mass in the centre responds much more readily to rudder commands than one with its mass at either end. This distribution of mass also affects the turning moment of the hull.
Hope this helps answer your question - My brain hurts!!!!!!!!!!!!!!!!!!!
Stuart Badger
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Thanks Stuart - proposal here for PJ - could I suggest that the comments & points here from Stuarts should form the basis of a
folder in the Files Section under say "Paddle Rudder - Size & Scale & Rudder (Sweep Arc)" - I think any member considering such issues will certainly benefit by reading such a clear, concise and understandable text message
So whilst this has rounded off most of the edges of my questions I am still a little unsure of the limitation of say 40 degrees, however
I am assuming this is 40 + 40 = 80 included - regards Derek