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On Wed, 23 Apr 2003 11:15:52 +0100, George Huxtable wrote:

>
>However, many boaters change their craft almost as frequently as they
>change their socks. Can anyone venture an opinion, based on the craft they
>have owned or known well, which were most and which least affected by
>prop-walk, and if this related to the detailed shape of the back-end, or
>indeed any other factor?
>

I think the worst walker I have ever steered in or out of a marina slip
was a Bristol 35.5, fixed-keel version. It was equipped with a Maxprop,
quite possibly maladjusted.

Our J36 has the least. After 13 seasons I still can't tell from the
wheel which way it rotates. Others with more moderate reactions were
Sabre 34 and Cal 2-30.

I think the submarine argument establishes firmly that the hull and
appendiges near the prop produce geometric asymmetries. In fin-keel
spade-rudder boats, the top of the prop is close to the hull and the
bottom is not. A skeg has significant impact.

The difference between the Cal 2-30 and the J36, as far as I can see,
results from the fore-and aft position of the respective props. In the
Cal, and most spade-rudder boats, the engine is aft of the companionway
and the prop is near the stern (and the rudder). In both, friction
against the hull bottom inhibits flow across the top of the prop, while
the bottom kicks the other way. This corresponds to the "wheel rolling"
offered by one poster. and is indeed in the observed direction of all
prop walk, AFIK.



>==========
>
>If my suggestion is correct, much of the prop-walk derives from the
>interaction between the hull/rudder and the spiralling backwash from the
>propellor, and from differences in the geometry above the prop axis and
>below it. In that case,  prop-walk wouldn't be a problem in vessels
>propelled by an outboard. The water flow as it leaves the prop of an
>outboard is (in forward gear) very clean; it has no further interaction
>with the vessel. So if the pivoting of an outboard can be locked up,
>exactly in the fore-and-aft direction, then I suggest that with the vessel
>stationary, there would by little or no sideways kick when forward gear is
>engaged. Has anyone observed this (or the converse, even)?
>
>==========


Rodney Myrvaagnes     NYC                                           J36 Gjo/a


The sound of a Great Blue Heron's wingbeats going by your head

   

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