Surface propeller and low speed displacement boat

[AndrewGilchrist]

Rick W the prop in the pic you put up is not a surface piercing prop. It appears to have absolutley nothing in common with one.

 

[AndrewGilchrist]

Alberto are you competing in the US universities solar challenge or is it an italian program? Andrew

 

[Sketchy]

Hi there, been lurking on these forums for a while now and finally have something to contribute.

We've adapted a thai long tail work with an electric motor out of an electrical scooter, motor is rated at 6kW but doesn't really ever go above 5kW during operation.

Using the drive system on our 16ft sea skiff we managed 10.2kts with two up/260kg all up weight and using 4.4kW.

hull is a planing hull and not the most efficient if looking for minimum power requirement/maximum speed but its what the customer wanted.

Adam.

 

[johneck]

Frosty, what do you mean by your comment
:Brunello your pictures of the twin surface prop boat appears to have the rotation opposite to a normal rotation, Is there a reason for that?"

The props appear to be outboard rotation, is that what you are referring to?

They also have forward skew, is that what is unusual? I suspect that this has to do with reducing the drag associated with the blades entering the water.

I believe that there are numerous patents about moving the propellers aft of the hull in order to allow increased diameter and avoid hull pressure pulse problems.

 

[blisspacket]

Curious to know, Brunello, if you swapped wheels LH to RH and determined any difference in thrust between the two setups? Yes, this is five years back....

 

[blisspacket]

Calling attention to the page 1 post of Brunello, in Dec 08: his photos show counterrotating props, and props that are the reverse of present configurations. Brunello has apparently gone high commercial. Have these "scooper props" I'll call them, shown up elsewhere in the surface-piercing mode?

 

[mogman]

Counter rotating... why?

For what it is worth, in my reading on the tests, videos and commentary by the designers... the two propellers had rotated inward toward each other. The result was a marked reduction in projected thrust and efficiency, probably a disturbance with the water flow toward the stern at the hull's centerline.

The propellers were then switched so they turned outward, thus rotating away from each other. The improvement was dramatic and exceeded projections. Why this had such a positive result, I do not know and will leave it to others to explain.

 

[Rurudyne]

For what it is worth, in my reading on the tests, videos and commentary by the designers... the two propellers had rotated inward toward each other. The result was a marked reduction in projected thrust and efficiency, probably a disturbance with the water flow toward the stern at the hull's centerline.

The propellers were then switched so they turned outward, thus rotating away from each other. The improvement was dramatic and exceeded projections. Why this had such a positive result, I do not know and will leave it to others to explain.

My guess: consider which way the water is being shed by the props.

When the semi-submerged blade hits the water it will have a minor perpendicular paddle effect. As it exits some the water that would have otherwise been carried around with the blade (were it submerged) is shed because of centrifugal force as well (having been accelerated by the prop). With the blades rotating outwards the shed water would be into a higher pressure area between the blades and should add to thrust better rather than just being shed.