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 Post subject: What is "Hull Speed"?
PostPosted: Mon Dec 14, 2009 2:07 am 
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Joined: Sat Nov 12, 2005 10:46 pm
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Location: Escondido
We see or hear this term used a lot and unfortunately it is widely misunderstood as it applies to kayaks. The traditional formula for hull speed in knots is 1.34 times the square roof of the waterline length (in feet) of a displacement hull. The concept was demonstrated by William Froude working with models of naval ships for the British admiralty about 150 years ago.

This does not directly apply to kayaks since they are not displacement hulls, but rather semi-displacement hulls. There are other limitations -- catamarans and surf skis for instance where the length to beam ratio is very large; surfing hulls and/or planing hulls and their kin are other examples that don't fit this formula.

An excellent example of a displacement hull is shown here (no, Jason, this is not a Pro Angler) -- a 73,000 ton aircraft carrier, which happens to be temporarily blocking the path of this somewhat lighter kayak.
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At the traditional hull speed, boats are sailing in a trough formed by the bow and stern wave and depending on the hull shape, displacement and power available, may be trapped or even (rarely) sailed under or capsized at faster speeds.

For light boats the trough isn't deep enough to trap it if there is enough power to climb out. For a Hobie Adventure for example, "hull speed" is about 5.36 knots or 6.2 MPH, yet sprint speed is closer to 7.5+ mph and sailing speed (Adventure Island) can exceed 10 MPH with no ill effects. So obviously 6.2 MPH is not really the hull speed at all!

Most naval designers have long ago abandoned the traditional formula for hull speed and adopted a more appropriate and realistic Froude number to describe the great variation of hull performances that are achievable in modern boats. As kayak users, it would be desirable for us not to allow our thinking to be limited by outmoded and misapplied theories. 8)


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PostPosted: Mon Dec 14, 2009 8:40 am 
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Location: Canyon Lake, Tx
How did you get that close to an aircraft carrier?...Nice explanation on hull speed...

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PostPosted: Mon Dec 14, 2009 10:35 am 
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Joined: Sun Mar 16, 2008 3:51 am
Posts: 153
Location: Grantham, NH
RR, is that the Midway Museum? I'm going to be out to S.D. in April and was thinking of trying to make that a thing to do. I have been on the Kitty Hawk before but was thinking I might get to see more on an inactive ship.

I had never heard that explanation of Hull Speed before but found it quite interesting and explained a few things I had kind of wondered about but never knew for sure.

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PostPosted: Tue Dec 15, 2009 10:54 pm 
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Joined: Tue Dec 15, 2009 10:48 pm
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If that is the USS MIDWAY, I served 2 tours on her when she was homeported in Yokosuka Japan. She was a rough ride. Hoping my new pro angler will be smoother


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PostPosted: Wed Dec 16, 2009 1:08 am 
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Just can't pull anything over on you guys -- that is the USS Midway, now a museum in San Diego. Built as a straight deck WW II carrier on a Montana class battleship hull, you can see history in her lines as successive modifications almost doubled her displacement over the years.
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lgbyron46 wrote:
She was a rough ride.
As I understand it, a couple of the major refittings were a bit overly ambitious, resulting in some undesirable handling characteristics at times in her history.

NH Explorer wrote:
I'm going to be out to S.D. in April and was thinking of trying to make that a thing to do.
If you're coming to San Diego, you won't want to miss it. Of course, right next door is the San Diego maritime museum with all kinds of cool ships from subs to schooners, America's Cup racers to ferries. Here's the Star of India -- over 1 1/2 centuries old and still sailing!
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Nearby is the fishing fleet -- always colorful.
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Of course the wife will want to stop at Seaport Village for a little shopping and fine dining.
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Be sure to fly in for a nice close-up tour of Downtown on your way to the airport.
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Come to think of it, the best way to see the many sights of the bay is by kayak! 8)
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