This bulletin board, hosted by the CDSOA, Inc., is the on-line meeting place for all Cape Dory owners and groups. We welcome everyone's questions, answers and comments about Cape Dory sailboat
Carl Thunberg wrote:All I know is, if I trim my sails ASSUMING the venturi slot effect is correct, my boat goes faster. If I do anything different, I slow down.
The slot still matters to the extent that it helps to have the genoa's trailing edge in the main's leeward airflow. Again in very simple terms that allows airflow to "exit" the genoa at higher speed which increases the rate of airflow over the genoa generally which increases the lift generated by the genoa.
In my opinion, if you properly trim your genoa (i.e. neither luffing nor stalling over the whole hight of the sail) and then you properly trim your main (same thing), the slot will take care of itself.
let out the sails untill theystart to luff then pull them in until it just stops. this is the starting point
you then want the sails to luff evenly as you head up . to see this just look at the luff of the sails.
if they luff at the top first, then it means that the sheet on the jib is too far back , move the car foward to adjust.
the vang should just be snug enough so that when you releas the main the boom will not rise....the vang keeps the proper twist that we are trying to achieve Ovwerall you ar etrying to achieve a "nice " shape that does not flutter (too soft) or too tight of a shape that will also not give the optimum power needed.
all of this trim stuff depends too onh the amount of power you need.. in chop you want to favor towards a fuller shape.
if you are moving at hull speed then anything you do to improve the sail shape is really a waste of time ..... Any questions?????
There is a story about a little girl and believing in Santa Clause and asking a newspaper if Santa really existed. The upshot of the story was the newspaper columnist writing back something to the effect that if you believe in Santa and it makes you happy and presents show up under your tree with Santa's name on them then he does, in fact, exist.
When in one of my many aerodynamics courses the existance of vorticity was questioned, the professor had to break down and say 'Yes, Virginia, there is vorticity'. It is a model to explain certain behaviors, and it breaks down if examined too closely. Since it is good enough to explain things most of the time we use it. Yes, Virginia, there is vorticity.
Same goes for 'circular flow' and other buzz words that are sometimes used to explain flow over sails. Aerodynamics has not changed, just the way we explain it.
Usually you will not go wrong with lots of telltales to help you understand twist and angle of attack. In light air you will want to increase camber to increase your maximum lift coefficient. Do this by easing luff tension, headstay tension and outhaul tension. Balance your sails so that there is not too much load in the helm, that rudder can really cause a great deal of drag. keep trying things.
I would let out the traveler - no need to use the vang until the mainsheet loses control when the traveler has been eased. Then adjust the main sheet/traveler until optimum shape and speed are achieved. Also - move the jib block forward 23 noches - or until the jib telltales break uniformly. Keep the leach of the main and genny almost parallel. Ease the halyard until you just start to see the crinkles in the luff of the main. Looks like the backstay needs to be eased to tighten the main leach. Then get in a race and see how she does! Boat looks great with a bone in her teeth! Install 3 leach tails on the main one at each batten and 3 on the genny - upper mid and lower third. Then adjust the sail so all are streaming aft.
