Keel displacement?

[LTDboatdesign]

I am in the process of calculating a the weight of my Keel on a boat I am designing. I got to thinking.... I want the keel to be about 30% of the displacement. But if I was to go with 30% of the hull Displacement(which is what i have calculated). After it was installed I would have less than 30%, due to the added volume of the keel. Because I have design a rather full keel it could add a considerable amount of displacement to the boat.
I am wondering if keel weight is generally calculated including its own volume of if its just left out? Or maybe i am completely missing something?[

Thanks for the Help

Cody

[alan white]

You are missing something. The hull including all appendages may be considered an envelope within which certain percentages of the overall weight are distributed unevenly with the keel ballast at the very bottom.
On average, the hull below the water will weigh the same as sea water with most of the volume being far lighter than water and the ballast (if lead, e.g.) weighing much more than water.
Therefore create the hull form first with a target displacement (using the midsection area), and then calculate the total weight of the ballast material (usu. lead) at your desired percentage of the total and finally calculate the volume in cu. ft. required to hold lead that weighs 30% of the overall displacement.
Lead weighs 709 lbs per cubic foot.

[Perm Stress]

Originally Posted by LTDboatdesign

I am in the process of calculating a the weight of my Keel on a boat I am designing. I got to thinking.... I want the keel to be about 30% of the displacement. But if I was to go with 30% of the hull Displacement(which is what i have calculated). After it was installed I would have less than 30%, due to the added volume of the keel. Because I have design a rather full keel it could add a considerable amount of displacement to the boat.
I am wondering if keel weight is generally calculated including its own volume of if its just left out? Or maybe i am completely missing something?[

Thanks for the Help

Cody

That 30% you talk about is 30% of total WEIGHT of boat; unless there is close prototype or reasonably detailed weight estimate calculation, all the talk about keel % is really pointless.

Now, if a boat you design has integral keel like J-class, or Folkboat, R-class, International metre class, Scerry cruiser, or similar, keel volume is included in volume displacement; than required part of keel volume is given over to ballast. And that's all.

In any case, 30% of ballast mean 30% of weight, irrespective of hull form as such.

[DCockey]

As Perm Stress mentions, the ratio which is usually considered is the weight of the ballast to the total weight of the boat including the ballast. Total weight of the boat equals the weight of the water displaced by the boat. So include the displacement of the keel.

[LTDboatdesign]

Thanks guys, thats what i suspected, but its good to hear it from someone else, on the downside, my calculations with the keel volume added are now showing a considerably heavier boat. Oh well, won't make that mistake twice.

Thanks for the Help

Cody

[Perm Stress]

Did you calculate weights?

hull, rig, deck hardware, interior if any?

Displacement is determined by weight of all the stuff on-board, not nice lines drawing.