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Re: How fast ?
Posted: Sat Feb 27, 2010 11:51 am
by DaveB
I know a Sailfish has the record of 51 knots, now is this a semi displacement fish? The bottle nose Dolphin can do 35knots plus and I would consider that mammel to be a displacement body.
Point is, the power needed to move the object beyound it's displacement speed .
I know that the S will do circles around my Mac.X except for downwind were I can surf.
The S is designed more ike a Dolphin compared to my Blue Whale boat.
Dave
kadet wrote:Yeah I noticed he was talking about an S shortly after I posted
However don't kid yourself that an S is a pure displacement hull. Given its weight and water line length I think they should be termed a semi-displacement hull like most modern fibre glass trailer sailers without full keels and loads of lead. Hull speed is generally predicted by the formula Anthony Deane worked out in 1670 for British Men O'War, Hullspeed(in knots)=1.34*(LWL^1/2), (LWL is hull length at the water line)
If your speeds ever exceed the theoretical speed for a displacement hull then you have a semi-displacement (S) or planing hull
.
I have seen a 36ft keel boat (6.9 knot hull speed) under tow by an overeager coast guard crew traveling at 12 knots after a break down. It is a big hole they climb out of but if you give anything that floats enough power it will plane.
One of my favorite destroyers is only 437 feet long and according to Deane's law can only achieve 24.5 knots but in reality can exceed 33+ knots.
A more accurate way to determine speed maybe Froude's Law concerning the characteristics of ship design and the speeds which can be achieved.
see
http://www.sjsu.edu/faculty/watkins/froude.htm
Re: How fast ?
Posted: Sat Feb 27, 2010 4:12 pm
by ferd johns
437 feet? Adams class? I served as a young division officer on the Jolly Charlie from 1964-1967. You are correct- we did over 33 in full power trials w/ all 4 boilers on the line. She hummed like a bowstring. And she was beautiful.
Ferd
Re: How fast ?
Posted: Sat Feb 27, 2010 5:51 pm
by kadet
437 feet? Adams class?
Yes an Adams or more correctly a modified version of it called the Perth Class built for the RAN.
HMAS Brisbane (D41) is credited with doing 41 knots using 2 × General Electric steam turbines providing 70,000 hp
Point is, the power needed to move the object beyond it's displacement speed .
Exactly what Froude's law refers to.
Re: How fast ?
Posted: Mon Jul 19, 2010 11:20 pm
by alaskasailor
The concept of hull speed tends to cause a great deal of confusion. First let us note that hull speed is not an absolute limit on the speed of a boat. It is merely a point at which the resistance to motion increases significantly. It is possible for a boat to exceed its hull speed. Hull speed is related to the waterline length of the hull as well as the shape of the hull. This hydrodynamic resistance is dependent on the shape of the waves around the hull. A hull displaces the water as it moves. This displaced water parts and is pushed around the bow, causing waves to form. The bow wave is quite noticeable if you look at the side of the boat. The back of the wave forms a trough and then a second wave and so on. The stern also creates a wave. The distance between these waves (the wavelength) is a function of the speed of the wave. Once the wavelength increases to match the waterline length of the hull, resistance increases. This is when the bow wave and stern wave collide. The boat is not trapped between the waves for evermore, but it does take an increase in power to overcome this resistance.
Re: How fast ?
Posted: Tue Jul 20, 2010 4:09 am
by Catigale
I think you will find anyone who has sailed the S and D boats will say they are smoking fast compared to the X and M boats..all theory aside.
Re: How fast ?
Posted: Tue Jul 20, 2010 8:05 am
by jschrade
I was amazed to average 4.5kts and peak in gusts at 6+kts using just the Genoa on a beam reach with the motor up out of the water.
This was on the way back after a long day of island hopping with the kids and their friends. The winds were forecast to be perfect to set a new speed record for our boat but, alas the winds decided to die as we were heading north in the Gulf.
When the winds are primarily East or West, we can reach up and down the ICW or the Gulf depending on wind direction. Using the land, you can get all of the wind on flat water!
I am assuming that I ran into hull speed as you could feel the boat digging in some of the gusts. The Genoa likely doesn't have the power to overcome this. The winds were gusting around 15kts or better.
Jim