question about rudder axis of rotation

[Roger Pellett]

Snipes were, and still are, great boats. At one time they were the USA’s most numerous one design class.

As a summer sailing school, one exercise was to sail boats with the rudder removed. This is much easier than it sounds as the boat can be steered by trimming the sails.

Roger

 

[icecreamviking]

Why worry about the dynamics of the rudder other than getting it to look correct on the model? Unless you're building RC boats, the rudders may occasionally see some fluid pressures when you turn on the ceiling fan......

my model is digital and will in time be animated. I'm something of an imposter here, I've never made a physical model. But it does allow me to play as the master of my own little world where I can get as granular and specific as I care too. hinges and the like are fascinating to me

 

[Smithy]

my model is digital and will in time be animated. I'm something of an imposter here, I've never made a physical model. But it does allow me to play as the master of my own little world where I can get as granular and specific as I care too. hinges and the like are fascinating to me

Same principle, I guess. It’s arguable (and will probably be argued here) whether virtual modelling is more complicated, ‘better’, more ‘artistic’, difficult, ‘worthy’ etc than ‘traditional’ manual modelling.

Then again, someone will point out that the guys who use digitally controlled milling machines or 3D printers or even power tools to make their physical models are hardly ‘traditional’ at all.

Digitus, the Latin root of digital, means finger (or toe) so in an etymological sense we all make digital models.

Personally, I lean towards ‘traditional’ but use plastic rigging thread, acrylic paints, glues made from who knows what and an electric drill. I also build plastic kits made out of oil! It would be hypocritical of me to condemn your digital (= numbers) modelling in any way, even though it doesn’t float my boat.

(I don’t think I ever wrote a post with so many words in ‘scare quotes’.)

 

[Senorbob]

Tooter Turtle (I looked it up as it was a new one for me.) Sorry to go off track, but I am still not convinced the extra movement provided was there for no reason.
Allan

there is a reason. when a vessel is barely moving it can then utilize the full rudder travel to steer

 

[MCPWilk]

There are many variables at play, depending on the type and size of vessel. A square rigged ship is heavy and the rudder would I supect play only a part in manoevering, the trim of the yards and backing of the foresails also being an important part. Modern yachts and dinghies behave somewhat diferently, as do catamarans. Large sea going ships and smaller vessels with fixed propellors will rely on the efect of prop wash on the rudder. A forward raking rudder attached throughout its length to the sternpost will behave entirely differently to a forward raking rudder which projects below the keel.

 

[fred.hocker]

An interesting question. The rise of the rudder heel is even more pronounced in galleys with curved sternposts, where the effective rudderpost rake created by the gudgeons is approaching or even exceeding 60 degrees to the horizontal.

For most normal rudders with rakes near vertical, the rudder effectively stalls at about 14-15 degrees, ceasing to act as a foil and beginning to act as a spoiler. It still makes the ship turn, but the excessive drag acts as a brake as well. This can be used effectively in some situations, and can be useful at very low speeds where water flow over the rudder is minimal. There is a nice little book by Larry Mott about historical rudders and their behaviour, from Texas A&M Univeristy Press.

In looking at a number of different historical vessels from the 16th century and later, I noted maximum rudder angles anywhere between 17 and 45 degrees, but most were in the range of 20 to 35 degrees. The limiting factor is usually internal, when the forward end of the helm contacts the inside of the hull or some other obstruction. For example, the maximum rudder angle on Vasa is 23 degrees, limited by the helm meeting a rider futtock (not in any way by the whipstaff).

Fred

 

[Smithy]

ceasing to act as a foil and beginning to act as a spoiler.

Ah, of course. Good point.

 

[Roger Pellett]

My response to Andy’s post #20 above:

The small boats that many of have sailed have a point about which the boat pivots; usually the centerboard or fin keel and the center of effort of the rig is close to this point. This makes it easy for a well balanced boat’s momentum to carry it through the eye of the wind and on to the next tack.

Square rigged vessels are different. They have long straight keels and the rig is more widely distributed. They tack differently. The following is confirmed by Harland’s Seamanship in the Age of Sail and personal experience as a teenage crew member trying to tack an 80ft square rigged ship at a summer sailing school.

Best case, their backed foresail aids the turning force from the helm to push them on to the other tack. That’s if all goes well. More often than not the ship looses way and begins to back down. When this happens the helm is centered and again the foresail is backed to push the ship on to the new tack.

Roger