Paul
Once speed gets above say 20kts a prop has to be designed with cavitation in mind. The high lift sections I use for low speed applications loose efficiency compared with their slow speed performance.
Cavitation is a key driver of marine props and why you see very thin sections on the outer portion of most marine propellers.
Above 30kts some part of any prop blade will be cavitating. Around 40kts the whole lifting face is cavitating so props for this speed and above are designed to work with cavitation. Around this speed there is an advantage going to surface piecing to remove the drag associated with underwater parts of submerged prop. Also the front face of the blade is encouraged to be fully ventilated so it only has air drag and the back face is the only part producing thrust.
Hence the propeller efficiency is only part of the story. Drag associated with submerged parts to drive the prop also has to be considered in the overall calculation. However I would have thought that for Planet Solar with its low speed there would be great advantage in using high lift submerged blade sections. But it is not unusual to find marine propulsion experts who cannot work from first principles. They use empirical charts to match the prop to the boat. You need to find specialist ship propeller designers to get back to first principles that underpin their design software like this:
There is much higher fatigue loading on surface piercing blades because they load up and completely unload each revolution, which is another design issue for them.
The shape of the Planet Solar blades look like typical surface piercing blades but they may actually work as submerged blades because the hub is so large. The hub could limit blade ventilation. The hub houses blade pitch control, which is probably useful to get the best from the low power drive system through the wide range of loading from downwind to upwind on a craft with a lot of windage.
The best example of a well designed high lift, high efficiency blade I have seen for a low speed marine application is used on the tidal turbines in Scotland like the one shown here:
There is no fundamental difference between a marine propeller and an air propeller for low speed applications but you will not find data sheets for high lift sections on high aspect blades for marine applications. The Wageningen series has lowest 2-blade area ratio of 20%:
My 2-blade pedal boat props are only 5% BAR and blades aspect ratio of 6.
Rick
On 08/10/2012, at 9:06 AM, Paul Wilson wrote:
I agree with you Rick, I can't see how a blade chopping the surface of the water would be efficient compared to a prop getting some real bite in to deeper and more solid water. Yet they make great claims....
http://www.arneson-industries.com/page.php?type=products&id=drives
Surface piercing props have never really caught on so what is the real story behind their claims of efficiency? It doesn't make sense to me.
Paul
On 8/10/2012 10:04 a.m., Rick Willoughby wrote:
Luc
The hard data available suggests the propulsion system is not very efficient.
They took 585 days to circle the globe. The best sailing boat has done it in 45 days.
The best efficiency I have seen for a surface piecing prop is 70%. The best for a submerged prop is 87%.
If you can find any actual prop data on planet solar I would be interested.
Rick
On 07/10/2012, at 11:12 PM, LucD wrote:
I just happened to watch Planetsolar - what did they do right with surface piercing props if I assume efficiency is prime for that boat? Luc
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Luc
> Low speed surface piecing props are hopelessly inefficient - maybe
> 20% if it is constrained by depth. It is better to use a smaller
> prop and enclose it in a nozzle to lift efficiency - basically a jet.
>
> In fact it is difficult to better a paddlewheel for shallow water
> operation but they would be an ugly addition to a proa.
>
> Rick
> On 09/09/2012, at 9:00 PM, LucD wrote:
>
> > While Nol was discussing how Zeilen met Visie progressed optimizing
> > Blind Date, he mentioned readying her for sailing the shallow
> > waters the Dutch coasts are famous for. I guess originally kick-up
> > rudders were meant to solve that.
> >
> > If one were to build a harryproa dedicated for waters like the
> > Wadden Sea http://en.wikipedia.org/wiki/Wadden_Sea with beaching
> > capability would it make sense to consider:
> > 1. an asymmetric lee hull?
> > 2. and ww hull (also flat to lee)?
> > 3. Low aspect schooner rig (for occasional rudderless steering) ?
> > 4. As apparently these boats motor more often: Rick, you were
> > involved in a discussion on slow turning surface piercing
> > propellers in 2008. Would this application make sense to offset the
> > loss of efficiency, in your opinion?
> > 5. A double hull would not require lots of flat bottoms as a monohull
> > 6. While transitioning from beached to floating the boat can bang
> > the sea bottom without being best positioned with regard to tide
> > and wind directions - Would there be a way to hold down the boat
> > some longer on the bottom and let loose to have it float higher and
> > have more room for maneuvering?
> >
> > Luc
> >
> >
>
> Rick Willoughby
> rickwill@...
>
Rick Willoughby
Rick Willoughby
__._,_.___
.