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Thread: Semiscale Footy Catboat

  1. #11
    Join Date
    Mar 2008
    Location
    genova, Italy
    Posts
    216

    Default First hull

    Hi Rick,

    even if I have been a bit busy today, work on this project is under progress:

    I have done a first version of a computer model for the hull, ( see attachment ) in order to match good numbers with a nice shape.

    Details are still to be tuned, but we have now a good starting point.

    I have been also thinking a lot about building method, and outfitting arrangement

    I will do my best to improve the project during next weekend.

    ( Most probably I will build one of them too.. )

    have a nice week end !

    Flavio
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  2. #12
    Join Date
    Apr 2006
    Location
    Sheboygan, Wisconsin
    Posts
    579

    Default Re: Semiscale Footy Catboat

    Spiking Flavio's interest is rather like hitting the jackpot isn't it

  3. #13
    Join Date
    Mar 2008
    Location
    genova, Italy
    Posts
    216

    Default design history - sketches and screenshots

    Hi Rick, hi Graham...

    I have been working on the nice concept of this model during week end, and these are the results :

    - I have done several freehand sketches just for fun, and to plan how to fit rc inside "hold" ( see below )
    One of them is almost a real drawing, two of pdf files are showing freehand cartoons made during long boring phone calls

    - On the computer keyboard I have refined hull shape, adding also rudder, keel and deckhouse to the shell ( as well a tentative gaff mainsail ).

    - Numbers ( displacement, and centers ) are in good agreement with preliminary investigations.

    I will show "downlodable" plans of bottom, topsides, deck, and transom soon ( within this week ) so to made possible building a cardboard hull to appreciate a real size 3d hull shape.

    Before starting cutting wood, I am still working on longitudinal position of keel and rig, in order to achieve as far as possible - a good longitudinal balance -

    Most probably my kitchen table workshop will be busy during next weekend :-)

    have a fair wind ...

    Flavio

    Attached Thumbnails Attached Thumbnails Click image for larger version

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    Attached Files Attached Files

  4. #14

    Default Re: Semiscale Footy Catboat

    Quote Originally Posted by Graham Pugwash View Post
    Spiking Flavio's interest is rather like hitting the jackpot isn't it
    You've got that right!!! I was hoping for a couple of suggestions (maybe a good keel location, or vague rudder sizing)... Flavio is definately exceeding my expectations by a country mile!

    Flavio -- Love the cartoons, and the digital boat as well, she sure is pretty! I love how the rudder flows into the curves of the boat.

    -Rick

  5. #15
    Join Date
    Mar 2008
    Location
    genova, Italy
    Posts
    216

    Default looking for balance

    Hi Rick,

    happy to see that you like my "engineering" effort.

    Today I have optimized longitudinal balance, moving a bit to the stern fin keel area, and changing here and there boom and gaff lenght as well mast rake

    Longitudinal center of sail area is now a bit ( 12 % of lenght ) forward of lateral plan .
    - This distance is also known as "lead" -

    I have satisfied with this number both due to the fact that I used same value on a very pretty ( and very good perfomer ) 10" freesaling catboat model years ago, as well due to the fact that in accordance to a sound theory sail center is above a point " almost" on the center of finkeel side area ( hull and rudder not included )

    Hull volumes are also quite balanced due to a quite narrow stern ( in comparison to other catboats- at least ), in order to avoid a bow down trim when the boat is heeled ( according computer ouput , pitch angle should be less than 0.5 at an heel angle around 25 )

    All considered, I feel confident that it should be possible to have a balanced model yacht , even if catboat are usually considered poor performers form this point of view.

    Next time I will make a first evaluation of stability : heel angle versus wind speed, in order to increase/ decrease sail area, and/or ballast weight

    Meantime I have started to select a straight and dry plank of balsa

    ... see you later

    flavio

    this a refined version of lateral profile ( deckhouse has been changed a bit too )
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  6. #16
    Join Date
    Mar 2008
    Location
    genova, Italy
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    216

    Default Stability

    Today I have done basic calculations about stability performances.

    In other words, it means a lot of number chomping to check if :

    1 ) we have too much sail area ( or not enough ballast weight ) -overpowered boat

    2 ) we have too much ballast ( or not enough sail area ) - underpowered boat

    Usually it would be advisable to modify both parameters ( canvas and lead ) in order to have a nice balance.
    In this case, I feel that it would be nice to avoid a too big mainsail in order to save a "semiscale" look.

    To put it simple , optimization will be based on a proper calculation of ballast weight instead of changing sail area

    There are a lot of good solutions, depending on your average weather condition, so as first conservative approach I have designed a boat that will sail in a comfortable manner up to 10 kts, and capable to witstand something more.

    I have used specialized naval architecture softwares, as well several homemade excel spreadsheets, in order to plot the diagram below

    calculations are based on following data :

    boat displacement 480 gr ( 17.3 oz )
    sail area 860 cm2 ( 133 sq inches )
    deck underwater heel angle is close to 24 - 26


    Diagram most important informations are that :

    heel angle at 10 kts is 20
    wind speed when deck becomes wet is 12 knots ( more or less )

    Most probably storm rig should be used when wind is over 15 kts

    These numbers are good enough to start now thinking about building method....

    fair wind to everybody :-)

    flavio
    Attached Thumbnails Attached Thumbnails Click image for larger version

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  7. #17
    Join Date
    Apr 2008
    Location
    South Florida, USA
    Posts
    167

    Default Re: Semiscale Footy Catboat - stability

    Hi Flavio,

    Do your computer programs and calculations offer any help or predictions with nosediving and submarining when on a run (a very common Footy trait), and how to alleviate this tendency?

    I like the aesthetics of your catboat so far. Would you go so far as to say it's a catboat Sharpie?

    Regards,
    Bill Nielsen
    Oakland Park (Ft. Lauderdale), FL USA

  8. #18
    Join Date
    Mar 2008
    Location
    genova, Italy
    Posts
    216

    Default downwind nosediving

    Hi Bill,

    two or three things about downwind nightmares :

    1 - broadly speaking

    I feel confident that this model should be able to sail safely downwind without troubles up to 15 knots of true wind speed ( and much more using storm rig ).

    a -because gaff rig is lower in comparison to a modern rig ahving same area ( and in turn capsizing lever is reduced )

    b -because due to this old style sheer bow freeboard is almost double than deck height above water at midship

    c -because bow sections are v-shaped adding dynamic lift to the bow

    2 - calculations

    it is possible to make calculations aimed to assess pitch angle to wind speed relationship, even if this calculation are based on many assumptions, the results are that Brando should have a nose down angle between 8 and 9 sailing at approx 1.5 knots under a 15 knots breeze

    Enclosed computer pictures show the situation better than many words

    Calculation are not able to show if the boat is still under control, but the rudder is still 70% underwater, and triangular shaped old fashion skeg should improve directional stability.

    Conclusions :

    computer simulations are not able to give a perfect assesment of downwind performances, but number are reliable enough to support our hopes to be far form the danger of unwanted underwater scuba sailing

    But the proof of the pie is in the eating.. ( isn't it ? )

    Flavio

    PS ...yes this model is definitely a sharpie catboat.
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  9. #19
    Join Date
    Mar 2008
    Location
    genova, Italy
    Posts
    216

    Default Brando Building started

    One good new : I have started to convert balsa into sawdust

    One bad new : I have problems to download digital pictures about building progress ( frankly speaking, I have lost the connecting cable )

    Awaiting better pictures I have attached below a freehand sketch showing most interesting points :

    1 _ Footy Control Unit ( patent pending :-) )

    This time I have started building starting from inside.
    All electronic stuff has been installed on almost square ( 105 mm x 95 ) < 4" x 4" > plywood block.

    Servos are screwed to the block, batteries and receiver are held in place using "office quality" rubberbands, on the lower part

    As first approach I used an old and heavy Hitec receiver ( 30 gr ) and 4 AA Nicad batteries.

    If needed, future "upgrading" improvements to rx and batteries are easy simple and quickly

    FCU will be stowed inside deckhouse, with the ply plank almost at same deck level

    2 - Hull
    Hull has been divided into three compartments : fwd and aft there are two enclosed spaces ( to be filled with ping pong balls or foam if needed )

    Center space is between two transverse bulkheads, and is big enough to store the abovementioned advanced "FCU"

    Fin keel can be inserted ( an removed ) into a purposely designed socket on the bottom of the hold.

    3 - Materials
    - topsides, deck, and bottom 1/16" ( 1.5 mm ) balsa
    - stern transom two layers of balsa
    - fin keel "socket" thin ply
    - bulkeheads 1.5 mm balsa ( again )

    Even if I am still far for launching, "true" wooden hull shape looks nice and very "classic"

    More soon !

    Flavio
    Attached Files Attached Files

  10. #20
    Join Date
    Mar 2008
    Location
    genova, Italy
    Posts
    216

    Default semiscale catboat footy - first pictures

    ok, now I have been able to found the cable to connect my digital camera to computer.

    These are first images of hull and removable FCU ( footy control unit )

    Flavio
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