Luc
I read this when sent and forgot to reply.
The curved shaft is aimed at operating a high aspect prop without vibration by aligning the prop to flow. The efficiency of such props is mid 80s or higher. That means the velocity ratio of water entering the prop field is not much slower than the water exiting. Without aligning flow there is vibration due to uneven flow around the prop disc due to the misalignment. It is referred to p-factor with aircraft props and you can find explanations on it in that situation.
Most marine props are not very efficient in their normal operation. Meaning they suffer high velocity ratios. So if there is some misalignment with flow the resulting vibration is hardly noticeable because the change in flow due to misalignment is low relative to the induced change in flow from incoming to outgoing.
When looking for props for the proa we were after something capable of handling 3kN thrust. The 540x520 nylon prop we are using have relatively high aspect blades but nothing like I use on the pedal boats. We tried to get larger diameter but none had suitable pitch. They also had the same blade area ratio so designed for heavier loading than needed for larger diameter on the prop.
At normal cruise the proa props operate at a velocity ratio of 1.12 - not particularly high but considerably higher than the 1.03 on the pedal boats. Hence they are not as sensitive to misalignment.
Also with the thrusters there was no problem getting the flow aligned anyhow. The reason for looking at curved shafts was to get the drive components out of the water and make a simple system for lowering and raising the prop.
I have not found a source of suitable stainless steel in Australia. I use spring steel for the pedal boat drives and sheath it with glass. Pedal boat builders in USA and Europe have been able to source 8mm round bar in 17-7 stainless and that has high endurance limit suitable for the pedal boats.
For the pedal boats torque is not a consideration from a shaft loading perspective. The torsional stiffness is important for this application. With higher power the torque would be the deciding factor in diameter - probably a lot smaller than you think would be needed. Once the shaft diameter is selected the geometry has to considered to ensure the shaft is operated within the fatigue endurance limit. If you were using a standard marine prop then it is likely a simple long-tail arrangement would work OK. Accept higher losses for the simplicity.
There are also more electric pod drives becoming available and these are worth considering.
With a sailing boat the thruster design needs to consider the motoring ability in strong head winds so bollard pull is key consideration. With the 18m proa it cannot make way in wind much over 40kts. The motors are capable of 9kW each on 48V but under bollard pull conditions the motors become torque limited if operating within rated current when swinging the big props, limiting input power to 6.6kW. So motoring efficiency under good conditions is traded off for thrust under extreme conditions.
Windage on the boat hampers pointing ability when sailing and motoring/manoeuvring ability when motoring. Likewise for hull surface finish. Getting a good result for these can reduce sail area and motor size while getting better overall performance.
Rick
On 07/07/2014, at 4:33 AM, lucjdekeyser@telenet.be [harryproa] <harryproa@yahoogroups.com.au> wrote:
So, scaling up the original may quickly hit its limit.
I would like to see it work first on a boat the size of the 18m Melbourne although the target is rather the lighter schooner Vis.Are you saying that if you could have sourced the steel spring closer to home it could have been cost effective enough to use?I realize that the prop dimension would be far from ideal, but would this setup also be stable enough from a hydrodynamics point of view to be used as a generator?Or would that be as problematic as going in reverse?Do you see any inherent problems using two of those?What about docking in close quarters using frequent short bursts?
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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