Posts : 20358 Join date : 2013-01-16 Age : 70 Location : Seattle
Subject: Hull Speed Mon Aug 19, 2013 2:19 pm
A boat displaces its own weight in water. When the boat is moving, it must push that much water out of the way as it goes forward. Since a heavy boat has to push more water out of the way, it makes bigger waves. (As a boat moves faster it has to push aside more water in less time, so that makes the waves bigger too.) Each boat creates a bow wave and a stern wave. When a boat reaches "hull speed" the bow and stern waves coincide to make one huge wave system. A heavy boat gets trapped in its own wave system. (For a 20 foot boat, hull speed is about 6 knots. For a 30 foot boat, hull speed is about 7.3 knots.) The best example of this is a tugboat. Tugboats are very heavy, since they have huge engines for shoving ships around; and when they are not shoving a ship, they are racing as fast as they can to the next job. That's why you see them with a huge bow wave, a huge stern wave, and a deep wave trough in between. In spite of their enormous horsepower, they can't break loose from the trap of their own wave system. They dig a big hole in the water, and can't climb out of it. A light displacement boat such as a dinghy, a ULDB, or a multihull doesn't have so much water to move out of the way - so they make smaller waves. When they reach the speed that would be hull speed for a heavy boat the wave system is not big enough to trap them. They are able to exceed the "speed limit" where bow and stern waves coincide. A planing hull actually climbs up its own bow wave and is lifted partially out of the water. Obviously ocean waves affect a light boat more strongly, since the weight of the wave is bigger compared to the weight of the boat. Consequently light boats surf more readily; but are often slowed down more when going against the waves. The upwind loss is diminished though, because light boats tend to be narrower and more maneuverable. Therefore, they can sometimes slither through and around waves a bit better at the hand of a skilled helmsman.
_Howard Admin
Posts : 8734 Join date : 2013-01-16 Age : 79 Location : California
Subject: Re: Hull Speed Fri Aug 23, 2013 10:07 am
NoCoPilot wrote:
A boat displaces its own weight in water.
Yes. Thank you, Archimedes.
NoCoPilot wrote:
When the boat is moving, it must push that much water out of the way as it goes forward.
Are you sure about this? Or is the displaced water equal to the immersed portion of the boat? I have seen the bow waves of ships six and seven hundred feet long, weighing tens of thousands of tons, and the bow waves seemed quite small for that mass.
NoCoPilot
Posts : 20358 Join date : 2013-01-16 Age : 70 Location : Seattle
Subject: Re: Hull Speed Fri Aug 23, 2013 10:18 am
_Howard wrote:
NoCoPilot wrote:
When the boat is moving, it must push that much water out of the way as it goes forward.
Are you sure about this? Or is the displaced water equal to the immersed portion of the boat?
Well of course "the immersed portion of the boat" is equal to the weight of the boat, in cubic feet of water.
I posted this because I saw an unusual boat being trailered through Seattle, funny-shaped hull. It reminded me of discussions I'd had with my late father just a few months ago where we were discussing hull speed, and how the Americas Cup racing schooners are computer-designed to have minimum drag/maximum hull-speed, in fact catamarans have taken over the race in the same way picklefork hydroplanes have taken over hydroplane racing, because two narrow hulls beat one broad one every time due to the greater efficiency in not having to push aside a big volume of water as you move through the water. Large ocean-going oil tankers are long (which increases the hull-speed) but they have to be wide (which decreases the hull speed). A standard oil tanker, though you don't realize it, moves fast enough to ski behind.
Incidentally, this came up with my dad because I was reading an article about hypersonic airplanes, and wondered if "hull speed" was an issue with aircraft design too.
My dad said, "Not really." There are issues of extreme heating, and laminar airflow, and sonic booms but not with standing waves in the air.
The text BTW is not mine, I copied if off the internet -- source of all wisdom.
NoCoPilot
Posts : 20358 Join date : 2013-01-16 Age : 70 Location : Seattle
Subject: Re: Hull Speed Thu Sep 19, 2013 8:37 am
Watched a bit of the Americas Cup this WE and the catamarans have airfoils (waterfoils?) under the hulls so that once they get up to speed the hulls don't even touch the water. Everything is flying on computer-controlled underwater wings.
Which leads to the occasional spectacular crash.
NoCoPilot
Posts : 20358 Join date : 2013-01-16 Age : 70 Location : Seattle
Subject: Re: Hull Speed Tue Oct 02, 2018 5:04 am
David Kaplan wrote:
Froede’s Law states that maximum hull speed is the square root of a boat’s length at the waterline multiplied by 1.34. A heavy hull, or one manufactured with an inefficient shape, obviously couldn’t approach that full hull speed. And the benefits of length were diminishing, since the relationship between speed and length is not linear, but based on a square root. But even so, Froede’s Law was clear: the longer a boat the faster it can go. Thus, a 100-foot sailboat in theory could go no faster than 13.4 knots; a 200-foot sailboat, no faster than 19 knots; and a 300-foot sailboat, 23.2 knots.
A speedboat of course could go virtually as fast as it wanted as it planed over the water, as long as its engine provided enough juice.
The main reason that many sailboats had a sloped transom—the surface at the stern of the boat—was to make the boat’s waterline longer and therefore better able to handle the bow wave.
NoCoPilot
Posts : 20358 Join date : 2013-01-16 Age : 70 Location : Seattle
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