[MHml] spray rails

[Bill Gibbs]

How about a high chine at the bow that only sees water when the speeds are
20+ and the bows are depressing downwind?

I agree with you about the spray.

 

Bill

AB

 

From: multihulls-bounces at steamradio.com
[mailto:multihulls-bounces at steamradio.com] On Behalf Of Silas Spence
Sent: Monday, March 16, 2009 2:59 AM
To: multihulls at steamradio.com
Subject: Re: [MHml] spray rails

 

The spray rails provide a benefit by generating some lift at the bow.
Whether you use this to provide a bow up trim moment or combine it with
increased interceptor immersion at the stern, as most high speed vessels do,
to lift the entire boat and reduce wetted surface is simply a question of
design objectives. The fact is, according to our data, you won't get either
result until you hit 18-19 knots on a 12m vessel or about 24 knots on an 18m
vessel and almost 30 knots on a 33m/110ft maxi. Even then, this is just
barely when the rails start to make lift - to get really definite,
noticeable, positive results (ie, 2-3% reduction in resistance, which is due
to the rails finally generating substantial lift) you have to make about
5-8% more speed than the thresholds I listed. 

I do believe better fairing as in the case of making the rail almost into a
chine would help with the nose diving tendencies quite a bit but I would
still expect the turbulence cause by the abrupt chine (at least one large
enough to provide useful amounts of lift) to cause a slower recovery from a
nose dive than a vessel without such chines.

As far as rails or chines for a drier ride, of course (given proper
implentation) - but this is down to personal preference/experience. I have
been on several powerboats where a well designed and placed chine or spray
rail made for noticeably drier ride, but to date no sailboats (mono or
multi) where I have noticed a clear benefit under a broad range of
conditions. Based on my experience I would not want spray rails on my own
boat. If a designer could convince me that a chine would actually provide a
tangible benefit without compromising the buoyancy distribution or
hydrodynamics of the bow I would consider it, but I think most yacht
designers lack access to the kind of empirical data I would want to see.
But, thats just me and I'm not too bothered by spray, yet.

Silas Spence

 


--Forwarded Message Attachment--

From: billg at gibbsCAM.com
To: multihulls at steamradio.com
Date: Sun, 15 Mar 2009 13:51:43 -0700
Subject: Re: [MHml] Prop Pitch for Multihulls, and spray rails





How about spray rails also for a bow up vector at speed, or just a 
drier ride?

 

Some of the big monos have faired the top of the spray rail into 
the hull, more of a chine than a rail I guess.  This eliminates the "water 
on top" problem with waves etc..

 

 

 

Bill

 

Afterburner





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