On 8/13/06, Terry <[EMAIL PROTECTED]> wrote: > I have get cavitation at about 1100 revs (tickover 850 revs, shallow canal > running 1050 revs to 1300 revs,, deep canal 1050 to 1400 revs river running > 1500 to 1800 revs) and also get vibration frm the rudder on shallow canals > at 1050 to 1300 revs and on deeper canals at similar revs. Ususally on > rivers and very deep canals at higher revs the vibration disapears. Having > just been on a slipway for a wek for bottom blacking the rudder has been > checked out by the boatyard and assessed to be OK, there is a little bit of > play in the cup on the skeg but they say that play is necessary and is not > too much or too little. > > The distance from rear of prop to start of rudder is 240mm - this to me > appears to be a large gap - I had considered having a plate put onto the > front of the rudder to reduce this distance. > > Has anyone got experience in rudder design or can anyone give me details of > any books I can read, or internet links that will help me.
Sucking air in to the prop has caused great problems on Kismet. At rest there is plenty of water for it, and generally doesn't cause us a problem when manoeuvring for example. It's only when we get some speed up does the water line drop below the front edge of the counter, and cavitation (for want of a better word) happens. This happens at above 2,000 rpm, unless waves (even small ones) cause air to be sucked in. To make matters worse the prop of Kismet is very high, and very close to the counter (less than 1/2 inch), so we also get lots of deep hull vibrations at even higher speeds. What you seem to be suffering from is what I've noticed on Shoestring, and the Narrow Boat Trust's Nuneaton, where when maneuvering at low speed, especially turning I find, causes air to get in. It's usually worst on Shoestring with a full water tank, and an empty diesel tank. Once moving I've not noticed a problem on either boats. So with that information, is it the same as any of the above, or does it continue while underway as well? Re, your rudder problem, you may have a bent blade that is producing uneven wash (maybe even a plastic bag! :-) I'm not sure having the rudder a good distance from the prop is a bad thing, but all rudders need a balance of suitable size for the rest of it - i.e. the bit in front of the tube needs to be in proportion to the bit behind it. If the balance is to big, then the prop wash could pull the tiller around to the side. There is a rule of thumb for this, but I can't remember what it is. Mike -- Michael Askin http://shoestring.zapto.org/ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/canals-list/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
