[PCW] windage.

[Alan Bliss]

Wind and current effects make maneuvering interesting, and with  
practice, fun. Because water is more dense than air, the dynamic  
pressure that a moderate current exerts on a hull is much greater than  
that of a comparable breeze. On the other hand, we can sometimes find  
ourselves maneuvering in decidedly fresh winds. That's when it's  
useful to reflect on Bernoulli's Theorem, which Crenshaw's Naval  
Shiphandling cites to explain that the dynamic pressure of wind (or  
any other fluid) increases as the square of velocity. Put differently,  
when the wind picks up from ten to twenty knots, the force exerted on  
your hull does not merely double - it increases fourfold, if I  
understand Capt. Crenshaw and Monsieur Bernoulli correctly.

It sure feels like that's so. In my former career, I ran a three- 
decked dinner and sightseeing boat with a water draft of six feet, and  
an air draft of over thirty feet, distributed along a hull length of  
135 feet. Obviously, wind speed and direction were factors in every  
movement. While I won't deny that there were occasional thrills, it  
helps to remember that you can make the wind and current work for you  
as easily as against you. Just form a mental picture of where the  
outside forces are coming from, and use your propellers, boat speed,  
momentum, and rudder effect accordingly. E.g., try to approach  
anything from downwind, and let its force work as your air brake. Same  
applies with current.

Jonah Jones mentions bow thrusters, and I can't refrain from throwing  
in my two cents worth on that subject, as well. Bow thrusters are like  
insurance companies, which is to say they are great, until the day  
comes (and it will) when you really need them. When the wind is  
cranking, or the current is running enough to make you wonder whether  
you even ought to be trying to dock or get underway, you'd better be  
prepared to handle your boat with its main engines and rudders. Use  
spring lines to kick one end of the vessel around and get it aimed in  
your intended direction of travel, and then go. The power of most  
thrusters is adequate for moderate conditions, but if you install a  
large enough thruster for those truly hairy moments, you'll have two  
big holes in your boat beneath the waterline, a suite of heavy  
machinery to power it, and still no guarantee that it'll preserve your  
dignity. To depend on a bow thruster is to invite trouble on board.  
Trouble sneaks on easily and often enough without any engraved  
invitations.

Alan Bliss

On Apr 22, 2009, at 3:37 PM, Jonah Jones wrote:

> With regards to windage.
> We have an endeavour 48.
> She's high at the sides, so yes, there is windage.
> The engines being far apart ,though, give you a lot of  
> manouverability.
> Personally I found that learning to dock in current was more of an  
> issue than wind.
>
> Two other chaps who have 48's have put bow thrusters on and swear by  
> them.
>
> With regards to ocean crossing,
> A big serious tennant design cat would be the way to go, or  a  
> sailboat.
>
> regards Jonah.
>
>
> www.jonah-art.com
> www.jonahs-whale.com
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