Re: [Amel Yacht Owners] Re: Flexofold Prop


James Alton
 

Bill,

  That is the most reasonable explanation that I have ever heard for why props walk.    You have knack for explaining things in an understandable form.  So if i have processed the information correctly, the only way to have a single prop that does not walk is to have infinite draft so that the two slip streams fully cancel?  If I am on the right track, does this also explain  the reason that the more shallow draft boats that I have handled seem to have more prop walk than the deeper keeled ones?  I always just thought that this was due to difference in the force required to move more lateral plane sideways but now you have me rethinking this…   

Best,

James
SV Sueno,  Maramu #220

On Nov 27, 2017, at 4:07 PM, greatketch@... [amelyachtowners] <amelyachtowners@...> wrote:

There have been many explanations for prop walk written over the years.  The best that can be said about most of them is that they are "creative."


The idea that it was caused by the angle of the prop shaft was a popular one that has appeared in a number of popular sailing books and instructional materials. If that was the reason for why we have prop walk then an Amel SM, with its horizontal prop shaft, would have none.  But of course it does.  Not a lot compared to some boats, but enough to prove that explanation is just plain wrong or, at the very least, incomplete.

Here is how I explained it to my students when I was teaching big boat maneuvering:
An ideal propeller in a perfect universe would move through the water like a screw moves into a wood plank. A propeller with a 14" pitch would move a boat forward 14&qu ot; for every turn.  But there are no perfect propellers, we do not live in a perfect universe, and water is not wood, so propellers "slip".  A propeller with a 14" pitch will move a boat (very roughly) 10" forward for every turn, and some water will be thrown in the opposite direction.  This is the "slipstream."

Because a propeller is spinning, its slipstream is also rotating as it move away from the prop.  Now comes the hard part without drawing on the napkin...  

Imagine you are behind a boat in reverse. the prop is spinning counterclockwise as you look forward. The slipstream is moving forward, and while it moves forward, it also is spinning counterclockwise.

The top of the slipstream is moving from your right to your left while moving forward.  It hits the hull and/or keel. Very close to 100% of its horizontal momentum is transferred to the boat, pushing the stern to your left (the boat's port).

The bottom of the slipstream hits your boat hull and/or keel moving from left to right.  Less than 100% of its momentum gets transferred to the boat because some of it escapes by going under the hull and/of keel.  It does push the boat somewhat to the right (starboard) but it is less efficient, so the stern of the boat experiences a net push to port.
There are probably other things going on here too, but this model seems to explain all the variations I see.  For example, when  you first shift into reverse, the boa t is not moving.  Slip is close to 100%, and prop walk can be dramatic. As soon as the boat begins to move, slip decreases, and prop walk decreases even if you don't correct at all with the rudder.  Maybe you could argue that this is the time you are most concerned with and the YM test is a reasonable model.  I would argue I prefer to know if the boat can be steered once it starts moving, because that is more important to me.  So...  maybe I was a bit harsh to say "almost useless" :-)  At least I didn't write "totally useless!"

In a typical sailboat propeller installation the slip is about 30%.  Yes, it will be higher in reverse, but close enough for argument.  The way YM ran their tests the boat was tied down and could not move.  So the slip must be 100%.  The slipstream is roughly 2 to 3 times stronger than it would normally be.  The difference between t he amount of propwalk in the real world (where boats actually move)  with different props is caused by the changing ratio of thrust to slip. The test they ran takes that completely out of the equation. In the test as they ran it the prop that moves more water will always show more propwalk.  

There are so many variables, that I would hesitate to ever predict the differences in prop walk magnitude between two different boats. But I'll say this, if there is a difference between a Maramu and a Super Maramu, it is not because of the angle of the prop shaft.

Bill Kinney
SM160 Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., wrote :

Bill,

   A great post!  Your points about the problem of load matching a particular prop to various conditions is on point I think.  I was thinking about what you said about the fixed bollard test and prop walk possibly being almost useless data and am wondering if that is always correct?  I thinking this because generally when I shift into reverse, the boat will usually have little or no forward motion so there will be a period of time where there is almost no flow over the hull in either direction.  So in effect would this period of reversing when the boat is reversing direction not be quite similar to being tied to a bollard?   

   Can you tell me if there is a significant difference in the amount of prop walk (sideways) between a prop shaft that is angled such as on my Maramu versus the shaft on the SM which appears to be in line with the WL?  

   Again, interesting discourse, thanks!

Best,

James
SV Sueno,  Maramu #220




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