Re: [Amel Yacht Owners] Re: Flexofold Prop


greatketch@...
 


James,

There are so many variables... there are always exceptions to the simple rules...

If I wanted to design a boat with as little prop walk as possible I would put the prop deep on a horizontal shaft (like an SM) so the slipstream stayed as far away from the hull as possible.  I would put the prop as far back from a very narrow keel as I could to give the slip stream time to lose energy before it interacted with the hull.  Kind of like some of the ocean racing monohulls that use saildrives.

Or...  make the keel so big, deep and broad that the momentum transfer was the same on both sides.  I drove a Tayana 37 that was like this. Full keel, prop in an aperture between the keel hung rudder and keel.  Prop walk was not really noticeable. 

Some boats that have a reputation for very limited propwalk actually just have very large and responsive spade rudders that get a good bite at low speeds and overwhelm the walk as soon as the boat starts moving.

This is one of the reasons that twin screw boats almost always have counter rotating props: All the off axis yaw forces cancel out.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL

---In amelyachtowners@..., <lokiyawl2@...> wrote :

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


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