Subject: Re: [harryproa] Re: rudder-prop combination |
From: Rick Willoughby |
Date: 6/17/2013, 7:54 PM |
To: harryproa@yahoogroups.com.au |
Reply-to: harryproa@yahoogroups.com.au |
Luc
Thank you, Rick. The Seabbatical design, however, featured a prop traveling up and down the rudder, and thus could be kept in or out of the water as required. This would eliminate the need for a folding prop.
The other points are well taken.
What about pods with the motor in line with the prop, thus, obviating the 90 degree gearing and vertical drive shaft?
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> 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
> rickwill@...
>