a1mac

Hi John You do seem to be comfirming my suspicions and the prop will act like an aircraft propellor. My comments about the tips of the prop was that this outside drive was insignificant -ie, the only bit of drive that reached the 'to close' rudder and as such could not provide enough stream for steerage. Do you have any knowledge of older boats and the distance between prop and rudder. My old 12 year old 50 ft Liverpool boat ran a 33hp nanni so only required a 16x10 prop but the distance from prop to rudder pivot was almost 18 inches and it steered perfectly. It would certanly turn 180 in Whaley Bridge basin - at any speed. Once again John, thanks for all your time Alex

Hi John Thanks for all your time on this one but your are the first person I have found with a knowledge and an opinion. Would you suppose that the first part of the flow begins at the outside tip of the propeller, if so the water hitting the rudder in the first, say six inches it is only from these outside edges. Going straight ahead the tube of thrust of the 'stream' must form around and beyond the rudder. I must admit I am still confused about the thrust of propellers. I also feel that if the rudders balance section is not big enough, the stream because it is only from these outer edges is having no effect on the balance beam and not even hitting it. I suspect older boats with smaller engines and smaller propellers probably didn't suffer any steering problems because the prop had more room to work. Alex

Hi John Thanks for the information. I do know they are different from a performance piont of view but I am trying to confirm a theory of mine that a prop has to complete one revolution that is 10inchs or 12 inches before the 'stream' of directed water hits the rudder. I suspect that engines have become more powerful and therefor need bigger props without consideration to the traditional designs and relative distance between rudder pivot and prop, and that propellors are just spashing around behind the rudder and are given no room to become a directional flow.

can any one comfirm my theory that a smaller pitch prop will increase rudder responce. ie a 18x10 is better than a 18x12 Can you give me details of the prop and engine fitted to your boat. The only boats I have seen do this are the Canal Time boats. They seem to turn faster the more power you apply! - perfect Alex

I think that 25% is a good average. But will this be relative to the prop size? If you have a 16 inch prop then either side of the prop will be 8 inches (at 90 degrees) and you make it long enough for coverage of 4 inches on the non directed drive side you will have 4 inches of prop pushing you forward?. If you increase the prop to 18 inchs then you will have 6 inches of non directed drive. Surely the more you have of non directed prop exposed the more it will push forward and not turn.

Can we have opinions of the % of balance blade to ruddersize Alex

Is there a problem in having the prop to close to the rudder? My old boat steered wonderfully with a 9'' gap. My new boat with a 3'' gap is pathetic. Should there be enough room for the prop to create a flow before hitting the prop?