<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by michaelb
keel weight is 1.265kg (just measured on my trusty digital scales), keel length is 800mm, arm lenght is 80mm, cant angle is actualy around 35degrees
… so if i have any idea on maths …<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Ah. OK… Here we go, and there is an exam at the end (smile).
Your lever ratio of 10 (lever arm 80, keel 800) makes the 1.265 kg bulb look like 12.65 kg to the winch working with the lever. So you are asking your winch to move an apparent 12.65 kg.
To start, let’s work with an effective cant angle of 90, for the times when the boat is blown horizonal and you still want to cant the keel out. Next, let’s take the stall torque of your HB 725 of around 14.5 kg-cm, and decide that we are only going to ask this winch to deliver around 25% of this rating. If you drive it any harder, you have less margin for friction losses, less speed when canting, and so on. So we want to work something out that has the winch driving with around 4 kg-cm of torque.
The question becomes, what kind of arm do you need to move 12.65 kg with 4 kg-cm of torque? Answer, 12.65 / 4 = 31.625 mm. If you put a drum on your winch and ask it to cant the keel with a line attached to the top of the keel lever arm, your drum needs to have a radius of around 32 mm. That’s a rather large drum.
No problem, you can now rig up a 2:1 purchase – wire a pulley block to your keel lever arm, and have the winch wind the line that runs through the block. Ignoring friction losses, your drum radius can now be 16 mm. That’s better, 'cos of course a 32 mm dia drum is a pretty standard size.
Final reality check, let’s calculate the length of line that the winch must wind, and the number of drum turns needed, to cant the keel 35 degrees with a 2:1 purchase and a 32 mm dia drum.
We asume you’ve positioned the winch so that it can reel in the keel lever arm fairly directly. The distance the keel lever arm travels when rotated 35 degrees is given by
Travel = 2 * lever radius * sin (angle / 2)
= 2 * 80 * sin (17.5)
= 48.1
We have a 2:1 purchase, so we are going to run around 96 mm of line.
A 32 mm dia drum will take 96 / (pi * dia) = 96 / (3.14 * 32) = 0.96 turns – call it 1 turn. Great. One turn cants the keel 35 degrees port, one turn cants it starboard.
Lester Gilbert
http://www.iomclass.org/
http://www.onemetre.net/