[Rhodes22-list] Sailing suggestion.

Hank hnw555 at gmail.com
Thu Apr 20 13:46:13 EDT 2006


Ed,

The following article explains how hull speed can be exceeded in a small
boat.

Hank

 Heavy Boats, Light Boats, and Hull Speed Here's an attempt to explain these
things in layman terms.....

by Jim Antrim

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.


http://www.antrimdesign.com/articles/hullspeed.html


On 4/20/06, Tootle <ekroposki at charter.net> wrote:
>
>
> Dave:
>
>        Dave said, "By heeling a typical keel boat aren't you moving mass
> above the water line?" Dave what mass are you referring to?  What comes up
> on one side is going down on the other.  Only Rummy has recorded putting
> keel mass at water level.
>
> Ron,
>       Ron said, "The amount of energy will depend on the boat
> displacement,
> the height of the wave generated, and the shape of the hull.  That's a
> hard
> thing to calculate.
>       Surely a Physicist could calculate that?
>
> This has been a great discussion, too bad old Roger P is not in it this
> time.  Has it been decided whether the Rhodes 22 can plane?  Can it really
> exceed hull speed?  What does it take to achieve going faster than hull
> speed?
>
> Ed K
> Greenville, SC, USA
>
>
> --
> View this message in context:
> http://www.nabble.com/Sailing-suggestion.-t1468265.html#a4011038
> Sent from the Rhodes22 forum at Nabble.com.
>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>


More information about the Rhodes22-list mailing list