how to caclulate the weight of a keel

[ashley needhamParticipant @ashleyneedham69188]

I am constructing a barge like sailing craft : it is a boxy thing 19×8 inches, very light, most of the weight will be the 6v 4Ahr battery 800g ish positioned on the hull floor in the middle.

 

The mast will be about 18 inches high (24″ above the waterline) and total sail area 144 sq ins, consisting a main sail and fore sail.

 

I have made a steel keel out of a bit of 1 1/2″ by  1/4″  by 4 inches deep.

This has some holes in the bottom so I can bolt some other bits on, lead weights etc etc for experimentation. It weighs about 300g on its own.

 

Any ideas please, is there a starting formula to use or what. Ant thoughts gratefully accepted, I do not want to nip out on its first voyage and do a Posidon imitation (turn over)

 

Thanks, In advance.

 

Ashley

[ashley needhamParticipant @ashleyneedham69188]

[Colin BishopModerator @colinbishop34627]

Ashley,

 

If you know where you want the waterline then that should dictate the actual weight of the keel allowing for the battery and fittings etc, The issue then is stability and that would be largely determined by the depth of the keel. The deeper it is the greater the  ‘righting moment’.

 

So my approach would be to put some sort of fitment on the bottom of the hull which will allow you to clip the keel on at various depths until you get the performance tou want. Then you can make it permanent. Start deep for safety and work your way up until the boat falls over.

 

Colin

[ashley needhamParticipant @ashleyneedham69188]

Colin. Sounds like a good ploy to me.  Our pond is fairly shallow, and so a shorter keel is to be preferred, but I have an adjustable system so i will experiment. Ideally the lighter the better.  We will see

 

Ashley

[ashley needhamParticipant @ashleyneedham69188]

No sweat. the bath test reveals all ! well, a bit anyway.

 

With 750g of battery, 300g of keel and 400g of lead weights on board (that is, just resting on the hull floor), the boat has a massive one inch draught, and appeared to be jolly stable to me..I may have to ballast up a bit to get the waterline right.

 

It may even be that i will simply stick or screw the flat lead rectangles to the centre underside ?

 

It did occur to me that you could make a “pocket” in the bottom of the boat to hold the battery, and this would be a keel weight in itself! however this would look silly out of the water, and also get in the way of the auxiliary prop.

 

There was a huge difference in stability on my HMS Glorious (3/4″ plank bottom) comparing having the battery ON the plank, or sitting in a cutout IN the plank, just 3/4″  height difference. Amazing.

 

Ashley.

[Geoff CropperParticipant @geoffcropper73342]

Hi,  The way I find the all up weight of my model hulls is float it in the bath, put the wife’s cooking scales in the hull, set to zero, then gently press down on the top of the scales until the hull gets to the plimson line and read off the weight on the scales.       Then weigh the superstructure, motor and battery etc and deduct that weight from the all up weight and whats left is the ballast weight.       Hope that’s some help.   Works for me.    regards  Geoff.                 Ps.       Fresh water weighs 1 gramm per cubic centimetre so multiply waterline length x beam x draught in centimetres x .5 (point 5 block co-efficient) and the result is all-up weight in gramms.    1000gramms = 1 kilo = 2.2lbs.       If you are making a rectangled shaped barge hull, then don’t bother with the .5 (point 5) then just multiply length x beam x draught in centimetres.        Geoff.

Edited By Geoff Cropper on 10/12/2010 11:30:07

[ashley needhamParticipant @ashleyneedham69188]

Geoff,  great idea. i like it, I like an empirical approach.I will make a note of this for the future.

 

The keel has by now been decided upon, it is a steel blade about 100mm deep and 250g, to which i had bolted 2x 200g lead squares.   In fact a visit to Kingston today this very morning (sat) has furnished me with two Zinc sacrificial anodes, in a slim eggy shape at 200g each, which have just replaced the crude bits of lead, and having actually JUST been painted over, looks a bit more professional.

 

We will see if it makes a difference to the performance, such that it is..

 

 

 Ashley

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