Just to throw a little more interest into the Footys of Florida.
I have been asked by Bob Boisvert of Titusville to design him a boat to (hopefully) excell in Florioda conditions. The result is a modified Moonshadow with a lower prismatic coefficient (i.e.more of a pot belly), shallower and slightly finer stern and about 50 g more displacement.
Here are some pictures and the basic drag curves. Note how Easter Saturday has appreciably less wave drag (the red lines) than Moonshadow in the medium speed range which we consider to be essential for East Coast of Florida conditions. This is despite thge fact the Easter Saturday has 15% more displacement.
We also hope hat the greater rocker will make the boat less prone to heave in a residual slop (described non-technically as ‘bobbing up and down’!).
Bob Boisvert is very anxious to point out that he is a beginner at boatbuilding, that his Razor sails like a US Army tank and that he told me all this at the beginning. Fair enough - he ds. We’ve tried to keep the thing reasonably easy to build and intitial pictures of the styrofoam hull are pretty encouraging.
With my notorious inability to get dates right, I managed to get the name of this thread wrng: the name of the boat is Easter Saturday, not Easter Sunday. As usual there’s a reason why. Anserrs on the back of a postage stamp please.
Thanks for posting the information. Bob is a great guy. I look forward to seeing your design on the water. It’s turning out to be an interesting summer here in Central Florida.
Your drag curves are extremely interesting. This is the first time I have seen any drag curves posted, other than my earlier fishing rod tests. Did these curves come from a simulation package, or are they actual test data?
The only test results that I have on a conventionally shaped hull (not a soda bottle) is a single test point, on a Razor hull, measuring 0.68 oz drag at 1.14 kt, with a total weight of 16.6 oz. This is about twice the drag shown on your curves. Possible reasons for the difference include:
A finer entry on your hulls (but it is necessarily deeper, so some of this advantage will cancel)
Less weight?
Form drag may not be included in your data?
Errors in my test data (the test was very unsophisticated)
It would be very informative if you could do a similar simulation on a Razor hull, to see if it agrees with my test data. I think Flavio is grappling with this same problem.
Wave drag is an area that has me completely flummoxed. I have been scouring the internet for information, and everything that I have found has either been irrelevant to our situation, or too complex to understand. I thought that I had hit the mother lode in some papers by Francis Noblesse, but his simple derivation for bow wave height uses a fudge factor which appears to give very wrong answers for a Footy.
The drag curves are purely theoretical and come with the Vacanti Prolines hull drafting package that I use. They are based on the well-known Delft test data (Technische Hoogschool Delft/Netherlands Ship Model Basin, now Marin).
In formal terms the size of a Footy is such as to be way outside the envelope for which the Delft model was designed. However, there is anecdotal information (Graham Bantock and Brett McCormack, although both tend to play their cards rather close to their chests) that the shape of the curves, although probably not the absolute values, does approximate quite well to Footys. In my limited experience at this size, I have found empirically that he correlation is quite good.
In the past I have used what is probably best described as an elaborate rule of thumb which I derived from a paper published by someone at he Wolfson Unit in Southampton and am now unable to locate.
For the sake of argument, suppose that we can model the wavetrain as the summation of five different wavetrains. The first is a simple hollow of fixed length extending over the waterline length of the boat. Viscous losses apart, it is symmetrical so the energy that goes in at one end is recovered at the other. In any case, we cannot do much about it except by reducing displacement and/or rocker. The second and third systems form a pair: they are the bow and stern waves and are generated at the forward and aft waterline endings. They have the wavelength of a free wave travelling at the speed of the boat and start with a peak. The fourth and fifth wave systems are generated at the first differentials of the curve of immersed cross-sectional areas (i.e. at the points where the change in slope of the curve is most rapid in respect of length). They start with a trough. The total wavetrain results from superimposing the five systems and drag is a function of the unsigned area under the curve (i.e. of the squares of the ordinates).
To make this work we have to build in a fudge factor for the viscous losses – that is how fast each sub-system decays – and make some sort of decision about where we think that the wave behind the boat ceases to have any effect on drag. Again, I make no claims whatsoever to the absolute accuracy of his technique, but boats designed using this principle seem to have waveforms that correspond reasonably closely in shape with the prediction.
I am the Bob Boisvert identified by Angus as the "builder’ of Easter Saturday and also known as FLABOB on this web site(Florida Bob).
On Thursday (2008-08-28) while preparing to apply the first coat of epoxy to the Styrofoam hull, I remembered that Angus had requested photos of my progress.
I returned to our apartment to obtain my camera, and on exiting to our patio (my work shop), heard a “CRUNCH!”
The crunch was the result of my stepping into the center of the hull. Apparently the winds had blown it off my work table.
The only loss was the time involved in shaping the hull and the demolished hull itself, now in the local Dust Bin (trash can in US parlance).
Fortunately, I have “back-up” copies of the basic hull. I will perservere.
Regards to all :zbeer:
Bob Boisvert (AKA: FLABOB)
Titusville, Florida, USA.
Mine involves a carbon US1M which I had just finished fitting out, when it fell off the scale, and off the edge of the table, with a 4 lb keel attached. Shattering deck beams on impact.
My favorite however is a local club member who finished his Soling, and as he triumphantly showed it off in the living room, he unceremoniously had the rig taken out by the ceiling fan.
For allo of you who weren’t holding your breath (and anyone who was) I am reliably informed that the heavyweight wooden version of ES has now been sailing in far more wind than is normal in Florida with very satisfactory results: she didn’t even sink!
The launch and first race of the much lighter plastic Easter Saturday is scheduled for Saturday floral or otherwise!
Inow know what a tom-cat feels like sitting in the vet’s waiting room.
I have designed boats for fairly complete strangers before, but every time I’ve been there at the launch - or more or less. Sitting here waiting for a message from Florida is distinctly trying.
Incidentally, for those who haven’t rumbled yet - Juan Ponce de Leon * discovered Florida on Easter Saturday, which in the old Spanish Church calendar was La Fiesta Florida - hence Florida. Hence also Easter Saturday. Come to think of it, I suppose that was a day of waiting as well!
A. :graduate::zbeer::zbeer::scared::scared:
Yes, Martin,I know there’s an accent on Leon, but that’s the leasst of my worries!
Ponce de León may have discovered it, but Pedro Menéndez first colonised it, naming the first township San Agustín, patron saint of his home town, Avilés, where I also happen to live.
