Hull speed

[R. Cooper]

I have a First 285. coming home from the Beneteau Reunion, with terrific winds 20 knots (gusts to 27). Have just install brand new Garmin 180 GPS. It was reading 7.1 to 7.2 knots of speed.( we were close hauled) I have calculated my hull speed to be 6.6 knots. Is it possible to exceed hull speed close hauled? What would explain the vibration in the boat at 6.5 to 6.7 knots. Is this like breaking the sound bearier in an airplane?

 

[Jon Bastien]

Hull speed is a theoretical number...

If you search the archives on the Hunter Owner's Web, there was a thread on this topic about 6 months ago. Basically, Hull speed is the product of a complex equation involving several measurements of your boat; Some boats will never see hull speed, and other boats laugh at the theoretical number (Catamarans or other multihulls come to mind). Another consideration is that the GPS will only tell you your speed over the surface of the Earth. If you are making 6 knots through the water (indicated on a knot meter), and the tide/current is running in the same direction at 3 knots, you will be move over the Earth's surface at 9 knots (as seen on a GPS).Go searching in the HOW archives- There's a lot of info on this topic.Good luck!--Jon BastienH23 '2 Sheets to the Wind'H25 'Adagio'

 

[Guest]

Don't forget to add tidal current...

...to your speed to determine speed over ground(SOG) which shows on your GPS.

 

[ROGER POTVIN]

DID YOU FIGURE IN THE CURRENT SPEED ? VIBRATION

VIBRATION FROM THE PROP ?? WAS IN NEUTRAL? THEN IT WAS SPINNING GIVING A VIBRATION, SHIFT TO REVERSE

 

[Bob Robeson]

Yes you can go faster!

Hull speed a result of the hydrodynamic forces at work. Like previous posts, this theoretical speed is determined by the (length of the water line)cubed, times 1.34.As a displacement hull goes through the water it creates a pressure wave. You can see this wave on any boat moving though the water. The faster the boat goes the longer the wave gets. Hull speed is reached when the wave (the high sides) becomes the same length as the boat. When you are going slower than hull speed the boat is going down the backside of the wave (down hill), requiring little power.To go faster than hull speed takes power, lots of power. As the wave gets longer, the boat must try and climb the front part of the wave. It’s like going up hill. And faster you go the steeper the hill, but if you add enough power you can go faster.Run down wind, put up a huge spinnaker, adding tons of power and you can go considerably faster than hull speed.Hope this helpsBobC-36