Tuesday, December 9, 2008

forget planing, the foiling sportsboat

The next breakthru is not planing upwind-it is flying a sportboat upwind.....
30' LOA sportboat on foils:

My personal opinion is that chasing an upwind planing keelboat is a lot of effort to go a little faster-seems that a bi-foiler sportboat would be a better choice-a lot of effort to go a lot faster upwind and offwind.

BUT you should put some numbers on paper and see what it would take.
Bethwaites SCP/Total weight ratio predicts that a boat with a ratio over 30% will plane upwind. It goes like this:

RM(righting moment) divided by the distance between the CE(center of effort) and the CLP(CLR)(center of lateral pressure(resistance) equals SCP.

SCP divided by total weight is a percentage. It will work with metric or US measure-just use consistent units.

You can play with this to see what combination of SA,weight,RM will actually have a chance of working.

This also works(with other indicators) to predict "foilability" but was designed to predict upwind planing.

Another problem with planing monohull keelboats is the relatively wide hull that is generally thought to be requied. With a potential keelboat foiler you go with a 10/1 L/B ratio to start with(MAX width) so weight is already reduced a lot.

You can see below that its a bit tough to get good numbers.......


Numbers for a foiler-for a planing hull weight would have to increase but there is enough margin for that, I think:

---LOA 24'
---LWL 24'
---Beam 18.6' (overall, incl. racks)2.4' at waterline
---Draft 6.5' keel,rudder retractable
---Sail Area 586sq.ft. upwind and downwind (planing boat would need big spin)
---Boat weight,incl. rig and ballast-1100lb.
---Ballast 490lb. (110° canting keel)110° canting keel wouldn't work on a planing hull so keel might have to be deeper @ 55-60° cant.
---Displacement (incl crew)-1420lb.
SCP/Total weight=42%


And before you say that it doesn't fit the rules. So what? Rules are meant to be changed when they hold back developement with artificial restrictions.

No comments: