Luc
Starting from scratch and a fat budget it would be possible to engineer a reasonable system. Some of the pluses and challenges are listed.
1. The prop would need to fold otherwise the added resistance when sailing would be intolerable. In 7 to 10 knots of wind our speed doubles once the props clear the water. In fact one of the units is impossible for me to lift if the boat speed exceeds 6kts due to the prop torque holding it down and this is just due to the motor freewheeling.
2. The rudder would need to have about 300 degrees of rotation so the prop could be folded whenever sailing. This means that the no-go angle needs to be marked up and the rudder is always returned the same side during a shunt. In this way the cables would not need any fancy slip-ring set up to allow continuous rotation. By the way, I would not recommend bi-directional rudders on any boat where the helm is unlikely to be constantly tended. Having experienced the sudden yaw due to rudder instability I recommend that the rudder are quite strongly self-centring.
3. It would be great to have vectored thrust from the props. It would make tight maneuvers so much easier. Side windage on the 18m proa is significant. It also means that the motor controllers would not need reversing. This reduces their cost.
4. With both thrusters on one side of the boat the motoring efficiency would be lower due to misalingned drive and drag. For a set speed the loss would be of the order of 10%. On the other hand there would be no need for separate drive legs that offer extra resistance when motoring.
5. Sailing performance would not be quite as good with in-rudder thrusters. This would depend on how carefully the detail was engineered around the fairing of the gearbox and transition to the prop hub. The folded blades would always be added drag but likely small compared to the other drag components.
6. The motoring efficiency and thrust for power are highly dependantg on the diameter of the prop and the reduction ratio. Finding or making a suitable folding prop is likely the most challenging aspect for the in-rudder system. The gear size to spin the prop will have a bearing on the rudder thickness. Note that the 18m proa how has 75mm thick rudders (15% of 500mm). The gearbox we are using is 90mm thick. So finding suitable gears to fit the rudder could be another challenge.
Rick
On 16/06/2013, at 9:39 PM, LucD wrote:
Rick, you were not instantly convinced by the rudder up/down traveling prop combination for the Seabbatical design.
The Promas design http://www.rolls-royce.com/marine/products/stabilisation_manoeuvring/promas/index.jsp treats this issue, although in another context. An alternative setup also shows a rudder with a flap.
For the Seabbatical what about a Speer like foil with the side with the prop fixed acting like a board and allowing the prop&pod to travel up/down, and the other side act as a flap/canard depending on direction?
It does seem simpler to build than the original.
Needless to say, I don't have the numbers ;)
Luc
PS: thank you for the numbers on hull fouling. It makes sense in hindsight.
Rick Willoughby
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