Subject: Re: [harryproa] Re: Bruce No.
Date: 4/23/2006, 6:48 PM
To:, Mike Crawford

Bruce No. is a very blunt instrument for comparison of just about anything, even
light weather performance. It takes no account of righting moment, heeling
force, rig efficiency, hull shape efficiency, wetted surface, parasitic windage
etc. 2 boats of the same LOA and same Bruce No. may have radically different
light weather performance and heavy weather safety characteristics based on
differences in all of the above.
Paul Nudd

Quoting Mike Crawford <>:

<<Personally, I wouldn't recommend much more than that unless you either sail a boat small enough that a capsize is a mere nuisance,
or you are an adrenalin addict with exceptional skill and attention span and you can afford the cost of the occasional salvage.>>

 Or, if you frequently deal with light winds and need to be able to cover some distance during those days.

 We rarely cruise, but I can't begin to recite the number of summer days we used to plod around while daysailing on our old monohull when the wind gods weren't helping out.  Granted, it wasn't a racing boat, but on a lot of those days no one under 45 feet was doing much of anything.

 Our current catamaran has a Bruce number of 1.9 empty, 1.8 with my wife and me on board, and I have to say that it's absolutely wonderful to be able to sail on days when our old boat wouldn't even move, and sail quickly when we would have been crawling.  Not only is it more fun, and cooler with more apparent wind in our faces, it allows us to cover a lot more ground and see something besides the same old bay.  It's also nice if we need to cover some real distance in order to get to another harbor before supper time, or perhaps get home before a change in weather makes us miserable.

 The flip side  is that only a fool would carry full sail on a BN 1.7+ boat once the wind kicks up.  15 knots is about the top end of where we can sail without a reef and still remain calm.  We purchased our boat with new sails, and the main came without any reef points.  We had one added before we used it, and are now adding two more based upon our sailing experience.

 So the price of a high Bruce number is the requirement of reefing early and often.  Not everyone is willing to do this, but that's true for all sailboats.  Some folks just don't want to reef.  I can't say why.  I'm personally happy to pay the price of reefing if that means I'll be able to sail in winds that are too light for others to bother with.

 Alternately, as Roger points out, the price of a high Bruce number is white-knuckle sailing and the willingness to risk capsize when a gust comes by.  That's certainly a legitimate use of a boat, but it's a bit too exciting for my tastes.


 One thing to note with the Harryproas is that a higher Bruce number will be a bit easier to handle than other boats.  This is for two reasons:

 a)  The majority of the weight is in the windward hull, creating a greater righting moment than a standard cat or trimaran with the same Bruce number.

 b)  The masts flex readily in gusts, making it much less finicky when you're not dealing with perfect trade winds.  Some have criticized the masts for being too flexible, but the appear to be looking at the boat from more of a pure racing perspective.  While there's no substitute for an overpowered stiff rig and an expert crew to man it, there's a lot to be said about a rig that will help absorb some of the gusts.  This is especially true if you want to use a boat with such a high Bruce number for something other than racing.

 More Bruce number information on the larger Visionarry Harryproa at:

      - Mike

robertbiegler wrote:

--- In, "oceanplodder2003"
<dana-tenacity@u...> wrote:
> Just been reading an article on these, seems 1.9 and above is a fast
> boat, do we know what the no. is for Harry?

The Bruce number is simply the square root of sail area divided by the
cube root of displacement.  The numbers caclulated from metric or
imterial units differ by about a factor 4.  For Harry, calculated from
sail area and displacement in metric units and racing trim (empty
weight plus two crew) it's 7.02, calculated from imperial units it's
1.76.  At designed cruising displacement, the numbers are 6.12 from
metric units and 1.53 from imperial units.

As for a Bruce number of 1.9 or above, that's for pretty extreme
racers requiring a lot of attention and skill.  Ellen Macarthur's B&Q
comes in at 7.95 in mteric or 1.99 in imperial.  Fujifilm, one of the
ORMA 60 trimarans is at 9.52 or 2.39.  That's the kind of boat which
is fully powered up in 8 knots of wind.  Most monohull cruisers come
in at 4 to 5 or 1 to 1.25, reasonably fast multihull cruisers at about
6 or 1.5.  Personally, I wouldn't recommend much more than that unless
you either sail a boat small enough that a capsize is a mere nuisance,
or you are an adrenalin addict with exceptional skill and attention
span and you can afford the cost of the occasional salvage.  Though if
you do crave that sort of excitement, I am sure Rob can provide it.


Robert Biegler

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