The Shipyard ( Dry Dock ): Builds & Questions > Yachts and Sail
False keels again
Will:
I'd like to pick the assembled brains of the forum and ask if my idea for constructing a false keel would work.
I hope to build a hull for a working schooner rig, just over 4 foot long. I will need a false keel, both to act as ballast and to provide righting balance. I reckon on around 20 kg of ballast. I plan to use a stainless steel bar, 1m long, x 60mm wide x 3mm think embedded into the wooden keel of the model. Basically I will construct a sandwich of wood - steel - wood. I'll then build the rest of the hull in wood. When that's finished I'll have around 35mm of steel sticking out the bottom, for the central 1m length of the hull. I can then use this to attach ballast and a fin keel, and gives me an easy way to experiment and adjust trim.
But the questions that I have are:
Would the turning force cause the steel to break the wooden structure of the hull?
Would the different thermal expansion of the steel rule out the idea?
Many thanks
Will
tigertiger:
If you know the lines of your boat and can work out the volume of the hull below the waterline, you can work out how much ballast you will need. 1 liter of water = 1kg, minus the all up weight of your model (you can guesstimate this if you don't know).
As models get longer, they need less righting force. As your weight will be in the keel, that is already low down. If you use a keel fin it only needs be short. A schooner rig is much shorter (height) than a bermuda rig on a racing yacht, and the tilting forces will be much smaller.
The sandwich should be no problem, I have seen molten lead in some plans sandwiched in the keel as ballast for sail boats. With only 1m of length and 3mm thick, I don't think expansion will be a problem.
Turning forces also no real problem.
If you pre-drill your steel bar with several equidistant holes before fixing in the wooden keel, the trimming the ballast will be easy.
tarmstro:
It would be good to know the size of the boat you will be building. Ideally you should provide plans of both hull and rig, so that people on the forum can help you do some calculations....
roycv:
Hello Tarmstro, I see you are on post number 2 so welcome.
The text says that it is 4 feet or 1.2 metres, and is a schooner and is heavy at 20+kgrms.
Are you building model yachts!
kind regards Roy
tarmstro:
--- Quote from: roycv on April 20, 2018, 09:02:39 pm ---The text says that it is 4 feet or 1.2 metres, and is a schooner and is heavy at 20+kgrms.
--- End quote ---
If the displacement is 20kg, then the ballast must be less than that. Think about these weights:
* Hull
* Rig & Sails
* Rudder
* Fin/Keel
* Ballast/Bulb
* Electronics & Controls (if any)The weight of ballast should be simply the total designed displacement minus the weight of all the rest of the parts. Sounds obvious, but that gives you the maximum weight of the ballast. Meaning that if you make all other items lighter, you can have a heavier ballast... And only then you calculate the lenght of the fin/keel, and for that you take into account the sail area and the overall height of the rig.
Example. My RG65 has a designed displacement of 1000gr. Rig is 70gr, rudder 10gr, fin 60gr, hull 120gr, electronics 150gr --> total is 410gr, which then allows for a 690gr bulb as ballast. Experience in RG65's has shown that for the bigger rigs used a 35-40cm long fin is good for a ballast of around 700gr... so my fin is 40cm long.
We really need to know what the design looks like, how it will be built (carbon, fiberglass, balsa wood, hardwood, 3D-printed?), what the rig looks like, etc.... so some kind of specific advice may be given....
In any case, 20kg for 120cm seems way to heavy. Modern Marblehead sailboats are about that size, and only weight 5kg max!
More info please!
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