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Old 03-02-2009, 04:05 PM   #21
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RE: Propellers/Handling

I've got the drawings to do this on my Krogen, but haven't yet. One question occured to me... to any of you who have this setup, does it push the rudder around in reverse? I know that if I get too much way on in reverse, the rudder will be pinned hard over and I have to stop or slow down to move it again. Does the fishtail rudder make this worse, or have any other negative effect in reverse?

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Old 03-02-2009, 05:45 PM   #22
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RE: Propellers/Handling

I haven't seen any but then I've not backed a long ways.* Just into a slip and the usual hop/skip/jump approach seems to still work well.* Wheel hard to strb/back slow/quick blast fwd & continue.

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Old 03-02-2009, 06:51 PM   #23
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RE: Propellers/Handling

Hi Eric,

Gotta make a couple of observations on your comment* below:

"... but the prop wash of both air and hydralic screws cones inward just aft of the blades. The prop wash is largely fwd or aft but lots of water is slung outward, radially off the ends of the blades."

The wash doesn't cone inward. Due to what is called "tip rolloff" a vortice is created by chordwise flow rolling off the tip and that flow moves the wake back toward the hub but only by a few percent of the blade span and then it quickly stabilizes in less than a blade span to form a cylinder. That is what you see in all those neat pictures of the spiralling*trail of bubbles behind a propeller in a test tank.**That cylindrical*flow*spreads out as it leaves the influence of the hull wake.

There is very little*(virtually no)*spanwise flow on a marine propeller. What little there is is very localized, mostly*in the root, and is reduced to zero at the blade tip. That is what creates the tip vortice you see in all the pictures.**It moves inward as I described above, not outward at all so there is no water slung radially outward from the blades to cause any effect at all. There are pressure pulses as a blade approaches the hull, that is what causes noise and vibration when the*blade tips are too close to the hull but there is no outward flow.
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Old 03-03-2009, 11:33 AM   #24
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RE: Propellers/Handling

Morning Rick,

*** I agree with a lot of what you said but if there is pitch there must be spanwise flow. The " tip rolloff " you mention must be a result of the spanwise flow that causes the tip vortice. There are many very wide blade tips on many commercial propellers that leads me to agree with you about the spanwise flow being small. I'm buying into that to the extent that I'm leaning closer to a propeller with large circular blades. I belive they are smoother and have better backing manners. The inward cone prop wash recolection may have come*from my aviation past. So you may have caught me posting a falsie* .. shame on me.

Eric Henning
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Old 03-03-2009, 12:52 PM   #25
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nomadwilly wrote:


all except for the spiral part.

The bit about the spiraling propwash and its effect on a hull I got from an MIT study, some of which was quoted on the Grand Banks Owners forum a number of years ago. It was the same study that also laid to rest the business about whether a freewheeling prop has more or less drag then a locked prop.

All I remember about the spiraling propwash explanation is that the movement of the "column" of water being thrust back from a prop has an overall twist to it that is imparted by the rotating action of the prop.* I do not recall any discussion about what happens to the water right at the prop blades, only that the*"column" of*water being moved back by the prop has a slight spiraling motion.

When*a boat is moving through the water*at whatever speed the prop is going to move it, there is probably*not much, or any, "column" of water coming off of the prop.* But when a boat is*more or less stationary, as it is at a dock, the props will move water*quite a distance behind or in front of it depending on which way they're turning.** For example, when we run our boat in the slip with the transmissions in gear, even at only 1,000 rpm, the prop in forward moves a lot of water all*the way across the fairway to the slips across from us.

If winter winds conspire with my work schedule to prevent us from taking the boat out for four weeks, we will run the boat in the slip for about half an hour.* We put the port transmission in reverse and the starboard tansmission in forward and run the engines up to about 1,000 rpm to put a load on them so they'll get up to operating temperature.* With one in forward and one in reverse there is mot much load on the docklines.*

If I stand on deck and look down at the port side of the*hull at the waterline, I can see the water that's being moved*toward the*front of the boat*"piling up"*against the hull and then*"ricochetting" (sp?) off forward*at a very*shallow angle.* So there is obviously some sideways force being generated there.* And since our boat has a relatively prominent keel for this type of boat, I presume the same thing is happening with water being spiraled into the side of the keel ahead of the prop.

I'm no physicist or hydrodynamics expert, so all I can do is repeat what I have read about this spriraling water business.* But I've read about it in enough places that so far I have given the idea credibility.

Of all forces acting to move the boat sideways with one prop in forward and one in reverse, the spiralling water effect is undoubtecly the weakest, even with the boat relatively stationary as during a docking or undocking.* But it seems to be there, at least from what I've observed by watching the water against our hull.

As I said, I believe this is a force that's*present only when the hull is relatively stationary. The props are moving water but the hull hasn't been accellerated yet*to match the thrust being generated by the props.* Once the hull starts to move, I suspect this spiraling force rapidly diminishes since the props are now moving the boat, not moving the water.


-- Edited by Marin at 15:11, 2009-03-03

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