Subject: Re: Flat bottom hulls?
From: Mike Crawford
Date: 11/3/2018, 9:57 AM
To: harryproa@yahoogroups.com.au

<<This enables pinching to better than 40 degrees with control and best VMG is around 100 degree between tracks>>

  You can pinch to 40 degrees?  Nice!  That's trimaran territory.

  I don't think I've ever seen 100 degrees VMG on the catamaran.

  That's most comforting.

  My plan is to skip the daggerboard and go with longer rudders, raising them a bit most of the time.  Waters near us are either deep or not, so there's no penalty in drawing 4' instead of 3', particularly if the rudders can be raised.  Maybe we go for the greater depth 1/4 of the time -- when headed to windward in light summer winds.

  But we'll see.  A leeboard an always be added on later if theory doesn't pan out in reality.

        - Mike



Rick Willoughby rickwill@bigpond.net.au [harryproa] wrote on 10/30/2018 11:41 PM:
 

The 18m proa has evolved slightly.  It started out with two under hull rudders that were 900 span and 450 chord. With those, the successive tracks for best VMG were about 110 degrees apart.  If the hulls had any slimy fouling that would get out to 120 degrees.   


When we were working out how best to sail it I would not bother trying to get any data unless I knew the hulls were clean; usually by spending about 80 minutes in the water with a scrubbing brush.

After the rudders got bent for the second time they were replaced with two rudders of 600 span and 500 chord but included an end plate against the round hull to prevent vortex shedding over the top of the blade when the rudder was at an angle.  (The original rudders did not have the end plate and the loss of power in the steering once the vortex shedding over the top edge began was noticeable). The revised rudders also have a larger solid shaft that had sufficient strength to support the boat weight.  They have been grounded a few times without damage other than scuffing on their lower nose.  VMG with these alone gave tracks around 120 degrees apart.  VMG was as good as the deeper rudders with higher speed but pointing ability reduced.

A bi-directional cambered 2m deep and 480 chord dagger board was added on the windward side of the lee hull.  This enables pinching to better than 40 degrees with control and best VMG is around 100 degree between tracks.  Slimy fouling will knock that out to 110 degrees or more.  Severe fouling with some barnacles - it is not worth the effort trying to go to windward under sail.  It will still move OK but lots of distance with little to windward.

Rick



On 31 Oct 2018, at 12:53 pm, '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

Does the 18m HP follow this same pattern?

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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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