Excuse me for this long post, but I’m not be able to make it shorter
Urca Footy Conception and consideration of the Footy construction weight
When I decided to make the Urca Footy Project for the competition, I preferred not considering the problems of wild duck and not go too far with the formulas because they are often wrong especially in little little scaled boat.
So I started from a situation of status, that is about what I was able to make based on my experiences.
First of all, the weight aspect, so I took into consideration the minimum weight of the structure + rc equipment to make a Footy and this is in short the results.
RC System
2 xServos (rudder + sail). min 25, max 50 grams
1x receiver min 5, max 10 grams
1 x accu min 25 max 50 grams
Cables-switch-various min 5 max 15 grams
The sum of minimum is 60 grams, the maximum sum of 105 grams.
For my boat I chose servants known that I would guarantee a good reliability but is also very light.
A HS55 (rudder) and HS225 (Sail) weighing 8 +28 grams: 36 grams total
For the receiver I used an old Jeti 5ch from about 10g.
For batteries I used a 350 mAh Lipo 2s by an integrated voltage regulator LM7805 total weight of about 30 grams, have not made the switch to mass saving, cables, sealing bags and various are about 5 grams
Servo horn sails and rudder control rod by a total of 4 grams
The total weight of the radio turns out to be: 36 +10 +30 +5 = about 85 grams
The structure
having already made monocoque hull (hull + deck integrated) for RG65 weighing less than 80 grams, I made an estimation for the footy and I assumed that 35 grams of the hull + deck mocoque are a goal achievable. This objective has been achieved without the use of molds by laminating the hull + deck monocoque with 2 layers of glass cloth 80 and one from 27 on a master of extruded polystyrene to be loosen as you can see on my site here:
http://www.progetto-urca.com/urca/progettando/URCA_Footy/costruendo/index.htm
After lamination, the hull was sanded and weight ready to paint is about 35 grams.
For the rig from 14 dm2 I assumed a weight of about 20 grams (then obtained) by analogy to rig of similar dimensions made for RG65.
The fin and the complete rudder + axis, by analogy with other similar carbon-made Rohacell that I realized, it was estimated at 30 grams total.
The result could be more light, but considering that the hull “wall” is about 0.15 mm, the boat should always handle by the fin and then … better a little bit stronger
The internal wiring, the supports of the servos, the mast step and glue to hold everything together are about 6 grams
The glossy painting of hull and appendages makes about 10 grams
The closing of hatches + transparent plastic adhesive tape is 4 grams.
Total structure ready to sail: 105 grams
This results in weight over all without bulb of 190 grams.
I know that doing the hull in depron could recover about 15-20 grams, 10 grams less from fin and rudder, servos + battery saving in weight could be another 20, but to descend much more should I entrust myself to not electronic so reliable under fading.
So the minimum weight reasonably achievable I think is around 135-140 grams and to have a ratio of 0.6 between bulb weight and total weight is needed a bulb of 220 grams and a total of 350-360 grams, but I think that it is really the limit with all the construction and boat a little 'fragile.
So I considered my estimated 190 grams for the structure+ electronics as a good compromise between low weight and reliability, also because the races are won always arriving at the end and, hoping not to jinx, I can say that so far, three years, I have never once stayed at the mark for reliability issues ... so I guess it was a good choice. At this point, in order to have a good relationship bulb weight / total weight of at least 0.6 it follows that the bulb must weigh at least 300 grams and so I did.
So Urca Footy V2 weighs just under 500 grams with 300 grams of bulb
Of course, since the draft of footy with geometries about standard is fixed for all and righting moment derived from a ratio 0.6 is comparable between the various boats it follows that more weight in the bulb means more sail area
So, being the weight of the bulb the engine of the boat with the sails do not consider derogated fall below 0.6 in this ratio on a racing boat “all weather”.
Then I looked at the data I found and about the weights of the boats and I deduced that the maximum acceptable for a competitive Footy all weather was around 500-550 grams.
This confirmed my assumptions and assume that that boat with bulb weight ratio / total weight worst than 0.6 were not even to be taken into account and that they should not in any case exceed 200 grams of structure + RC less than accept from starting to be heavily penalized at the project level.
This is very important expecially in a boat like the Footy that has a number of sets of sails free and which in practice is always taken to the limit of the heel, and then uses the rightening of weight even in the condition of air weak.
Appearance of hydrodynamic stability
since with a boat always at the edge of the sail area and mass ridiculous to rise from 30 to 60 degrees of heel is a constant continuous, rather, a continuous oscillation, especially in lakes with obstacles around which disrupt the breeze, I considered of fundamental importance the greater directional stability as possible in order to stay on course and govern even with more than 50 degrees of heel.
This may seem OT, but I think the greater inertia of the boat will also help maintain the direction and the damping of oscillations, so I think that a heavy boat 500 grams may have a small advantage over a more than 350 due to its inertia.
Hydrodynamic and aerodynamic appearance sails appendices
To consider the balance between heeling moment and righting moment, I prefer not to use formulas that consider the aerodynamic lift or other “pure” because all models and footy are especially slow and small, and move in a flow much more disturbed than any other vessel… so using a linear formula to solve a non linear problem is quite always a big mistakes.
Therefore, as with the other models, to test the lateral stability I use a parametric system for simplified formulas with coefficient balance report here attached.
Formula heeling moment: MSB = (*) x (Sup tot) x VV2 x Ca x cos (f)
Formula righting moment: MRA = (d ') x (mass bulb) x sin (f)
where:
- (*)is the experimental coefficient of the resulting sails for the “real boats” which was about the book and I made a correction on the basis of experimental observations on models: now I use 0.07 which gives plausible results.
-VV2 is the wind speed squared
-Ca is the distance between sailing center and center of lateral efforts(in height, of course): the resultant of the side forces in first approx could be placed on the center of gravity of the fin - (F) is the angle of heel
-D 'is the distance between the water line and the center of gravity / axis of the bulb.
Then, with my calculation program in excel (where there are all geometric data of the boat) I enter the coefficient (*) obtained experimentally by observations at sea heeling the boat to the wind and refined during the tests.
This allows me to predict with a good approximation of how much sail I can take sailing upwind with the boat that I have all the data.
From tests done, my footy with 8-9 knots sails without problems holding the rig of 15 dm2, then the graphs posted are slightly pessimistic.
I think the streamlined appearance as well as hydrodynamic are the two major components that can make a tangible advantage to boats “heavy”.
Assuming that a boat with 300 grams of bulb will have a larger and more high sails, the aerodynamic advantage will depend on two factors in parallel:
- an higher Reynolds number of operation of the sail because, being equal all other factors will have an average chord probably greater
- the average of the sail area farthest from the water, then exposed by a flow a little less noise and less “slowed down” by the effect of the gradient.
The greater height of the sail plan will be a big advantage in days of very light or almost absent conditions and will not be so disadvantageous in the days of strong wind because if the boat heels coping well with more than 45 degrees of hell and keeping the control without act on the rudder, in this case will benefit from the effect of the gradient. During the blast skidding will sail very close to the water surface where the wind will be much less than what we perceive as half a meter or 1 meter in height.
Considering the gradient also explains very well as a well-balanced footy can bring a surface of sails larger than that habitually used in equal righting moment from any other sailing model.
At the same time, large sails also require a largest fin that will have greater wet surface but also the advantage of a higher Reynolds number of exercise and the ability to stall less, especially at very low speed.
continue in next post