Scaleing up a plan ???

I recently built a “Footy” [cool little boat]I want to double the size and make a “2 footy”. My queation is about the keel/weight. Can I do something about not doubleing the draft? the pond has rather shallow edges. Can I shorten the keel and add more weight, I realize that the depth has to deal with the righting effort.What would be a good starting point to reduce it to? Thanks for the advice. JK

I think you may be in luck because when you sacle up a boat, its displacement already goes up, if it is to float to the same scale waterline, by I think the Cube Root or some huge factor like that. Do I sound vague enough? Anyway, if you keep the keel the same depth as the footy, it should work well at the necessary scaled up displacement in two footy size. You can of course adjust the sail plan to the new displacement, needless to say.

Like those humungus Jboat models that have the same scale keel depth as the really humungus full scale Jboats, and they sail quite respectably. I’ve seen them many times.

Others could answer better about how keel depth affects stability than I could, but in my experience it isn’t as big a factor as I thought it would be.

Great Idea the 2 Footy!


A good place to start would be to make the keel as long as the pond will allow. The weight of the ballast can go up, but not so much as to bring the boat to much below her waterline. Just know that the boat will perform poorly with this kind of modification.

John, just to note here, the RC J boats are not scale depth at all. Check the rules on this and you?ll see that up to two inches of draft are allowable over scale.

I asume the total weight of your original Footy is approx 500g…so when you build this same boat at twice the size it will need to weigh 4000g to sit on the same waterline.
Might be a little sluggish with this displacment/length ratio :slight_smile:

Hi Jake,
You shouldn’t need to double the keel depth, but it’s always better to do a little wading in the shallows rather than to sacrifice performance when the wind pipes up. A simple method which will give a similar amount of stiffness is to consider the keel bulb & sail area to be at both ends of the playground “see-saw”. The forces exerted by each are the “righting moment” & “heeling moment” respectively. The idea is to balance the 2 forces. Moment is expressed in units of distance X mass, in this case inch-ounces may work well. Figure out what the area your new sail plan will be & compare it to the Footy sail plan as a % increase. The “moment arm” (distance) of the sail plan will also be greater because the center of effort(CE) of the new sail plan will be higher. The CE is roughly at the centroid of the combined area of the both sails. Multiply the new distance times the previously figured % area increase. This will tell you how much to increase the righting moment. Since volume (ie mass) will increase as a cube function (as compared to the sail area increasing as a square function) the moment arm (keel fin length) will not need to be doubled. Your final length will be determined by how much mass you decide to put in your keel bulb in order to achieve the same balance. Who knows, maybe you can find a souce of depleted uranium to use instead of lead…:wink:
Hope this helps.
Bill K
ODOM #217
MultiOne #9
K-1 Ice/Land Boat

Thank for the infro guy’s I’ll look into that uranium, I may have some under the bed.[:-boggled]
Here’s a pix of the "real"Footy

Download Attachment: foot1.jpg