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- Boats Sailboats Help! How to calculate weight/volume of a keel bulb?
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| I was thinking of an external leadbulb of 12.8 kg on a keel fin of about 35cm length. Profile for both could be a NACA 15 % - just my first idea. The hulldraft of the galeon is 15cm and if possible I don´t want to go beyond 50cm draft overall. 12.8 kg because that´s 40% of the overall displacement. It still may take some time untill I know the weight of the ship itself - as I´m still in the early stages of the build. But right now 40% external ballast sounds fair to me and the remaining necessary ballast could be placed inside of the hull. Please correct me if I´m wrong. That´s the rough concept I have so far. Any qualified suggestions and - most appreciated - formula(s) + + precise calculation(s) would be very welcome. Also corrections to this concept would be of help if that appears to be necessary. | |
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| Some thoughts: 1. High windage hulls, like yours, will need a lot of keel area if you plan to sail to windward. My BottleBaltimore has lots of windage due to lots of exposed hull. The boat needed a much bigger keel than I expected. She is somewhat under-canvased, which hurts (slow speed => less lift off the keel => poor performance to windward; solved by making a large keel => less wingloading => slow speed ok). I express sailing physics in terms of airplanes, both because I am a pilot, and because the physics of sails and keels is lift-oriented. 2. Interior ballast has almost no righting moment, due to the short lever arm. So don't depend on that weight to keep your boat upright in the wind. Ballast considerations are primarily for control of heel. If you find that your boat needs more ballast than you desire, deepen the keel to get greater righting moment for the same or less ballast. A deep keel will also dampen roll-rate, giving a more graceful and realistic motion to your ship. 3. Be sure you can reduce sail area easily at the pond, otherwise you will find yourself quite limited in the range of acceptable sailing winds. Once a hull heels past 45deg, rudder effectiveness diminishes greatly. Keel lift no longer resists the drift to leeward. Plus you risk shipping water and foundering...been there done that :-) Adjust sail area to match the wind, keeping the boat's heel under control, and you can sail in anything from a zephr to a scale gale.. been there done that :-). 4. Model squareriggers move more slowly than racing sloops. The keels that work for sloops will not work for model squareriggers. Airfoils on the keel are nice, but not necessary. Keel area is more important. BottleBaltimore fin keel. | |
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COMMENTS
Keel Bulb Construction. Hi all. Just sharing another keel bulb build. Going with a 590gm bulb for a 750 sized yacht with a max sail area of 2226cm2, Fin length of 345mm and a NACA 0064 bulb profile with thickness to chord ratio of 12%.
Float the hull, preferably with gear inside close to its final position, and put the bulb in. Move the bulb fore and aft until you get the hull to float on the waterline that you want. That is your bulb position.
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To weight 12.8 kg, your cylinder must measure 1127 cubic centimeter (12800 / 11.35), for a diameter of 8cm, this gives a length of 22.54 cm. To have a better approach, you can decompose the volume as two cones, a short in front and a largest to the rear.
I finally received my new IOM fin, bulb and rudder today! It was shipped from Australia from Radio Sailing Shop. So far they look great, but to be honest, I'm not entirely sure what I'm looking at. The IOM fin is long, thin, carbon-fiber, and very light weight. It only weighs 101 grams!
Bancroft 550mm Sportsail Sailboat 250g Keel Bulb Ballast. $ 15 69. Buy in monthly payments with Affirm on orders over $50. Learn more. In stock. Only 2 left, Order soon!