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

The gearing is not just 90 degrees.  It provides a reduction ratio of 3.3.  This enables us to swing the 540X520 prop to get high prop efficiency.  The 350X300 60HP outboard prop was giving less than 50% efficiency.  

With regard to the submersible pod.  These are commercially available.  I think retail is around $9000 a unit for 10kW but not certain:
http://www.mastervolt.com/marine/products/podmaster/podmaster-10/#specifications
They have direct drive props that are smaller than what we have so motoring efficiency would be lower if used in our application.  Bollard pull is likely lower as well although our gearing is still too tall to allow us to put full 10kW motor rating through the big prop at zero speed.  We would need controllers with a higher current limit and push the motors into overload using the existing gear ratio - likely tolerable for as long as the battery can supply the current because the motors have good cooling.

The 18m proa has high frontal windage.  With existing thrusters we could make way in winds up to 40 knots.  Hence bollard pull is a critical requirement for this boat and something to consider with any multi-hull where most of the boat is exposed to the wind and may not be particularly streamline.

With regard to building your own submersible unit using the ME1003 motor - It still needs a gearbox.  Suitable planetary box is around $700 versus the $130 box we used although we added a stainless output shaft and bearings with adequate thrust capacity. which added $80 to each box.  The motor would require a substantial shroud that would increase drag somewhat over what we have now although the existing fairing is quite large.

We also considered bore pump motors as an option for submersible drives.  They also required the planetary box.  The electrics were more complex because standard units are high voltage.

Lithium batteries and low cost solar panels are enabling technologies for electric yacht drives and it is early days in their development.  The possible commercial options will increase and prices should become more competitive. The current cost of the battery forces more consideration to be given to prop efficiency as the combination of battery capacity and efficiency will set range per charge at acceptable speed (also more consideration on hull drag).  The components we chose were intended for other applications; already competitively priced making the overall cost reasonable.  Time will tell if the system is reliable.  One benefit is that we have intimate knowledge of all the components used.

Rick



 
On 18/06/2013, at 2:40 AM, LucD wrote:

 

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@...
>


Rick Willoughby




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