Help on first sailboat design (exercise)

You can use a bulb at the keel for more righting moment. An advantage is that is increases stability without increasing displacement.

 

No, that is not correct.

 

Thanks Banjansailor, really similar boat to the brief I posted!!!
I adjusted the sail plan on the basis of the one of Expannie (bigger overlap for the jib and smaller main).
For sure the hull I drafted so far is not good and I think you are spot on on all the points (and I . Despite its imperfection I 'd like to move on with the design process to "complete a full round" of all the steps once before going back to improve each of them. I'll re-work it on the base of Expannie.

Thanks Gonzo! Can I have a full keel with a bulb? my worries would be that the wetted surface will increase too much. I was thinking at extending the keel "down" to allow space for more ballast if needed.

 

So I still have the problem of verifying if I need a ballast (and how much).

To understand where the G should be, since going through the force on the sails at a certain angle and wind condition would require a sails model that is a bit to complex for this first "loop" I took a stability diagram of a well know seaworthy sailboat, the Contessa 32, and try to locate the G on my hull to match the same GZ.

Had a first go at it and to have the same GZ my G should be really low.

In the analysis thought it is not clear to me if for the angle at which the GZ is calculated, the assumptions is to have the boat in "equilibrium", i.e. with the submerged part of the hull that equals the boat displacement (or if we assume to let the boat roll on its total hull Volume Centorid).

 

Precisely that is the mission of the bulb, place a weight, large, as low as possible. That means an increase in displacement, naturally since a weight is added, but the value of the GZ (righting arm) increases. But it is not necessary to place a bulb, it may be enough to place strips of lead, for example, in the lower part of the keel.
You should optimize the size and shape of that bulb or that added ballast, to achieve the desired increase of the GZ with the minimum increase in the wetted surface and with the minimum increase in displacement, also ensuring that the longitudinal position of the CoG does not I changed too much. Like everything in shipbuilding, it is a compromise between several effects opposite each other.

 

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Thanks Gonzo! Can I have a full keel with a bulb? my worries would be that the wetted surface will increase too much. I was thinking at extending the keel "down" to allow space for more ballast if needed.

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A bit of extra wetted surface is not going to matter much in a high displacement, full keel hull. In fact, because you need less ballast to have the same righting moment, it may end up being a wash.

 

Ok clear, so now I really need to know the weights distribution of my boat to be able to move one, meaning scantlings and weights of the interior.
For the scantlings definition I since I assume the boat to be "built" in wood I thought about going with the Skenes Elements of Yacht Design scantlings rules for a first round.

 

Skene's, Herreshoff and Gerr's methods give similar scantlings.

 

For a first pass, very preliminary, it might be enough although I recommend any other more current procedure. Especially in the second pass, when you need to reduce weights, reduce scantlings, current regulations may be more appropriate, they are more professional but they will give you better results. The formulas to calculate boats structures have evolved a lot in the last 20 years.

 

I think your VCG (Vertical Center of Gravity) placement in this sketch is way too optimistically low. More likely than not, it will be a few centimeters above or below the DWL. As for ballast, yes, you will certainly need some. More likely than not it will be about one-third of the loaded displacement of the boat, give or take. I usually start my preliminary calculations using this number. When doing so, I make sure to count the keel deadwood weight a part of the ballast. The deeper the VCG of the keel and the lighter the hull and rig structure, the less ballast you are likely to need. This is the major reason Gonzo recommended a bulb keel. But the bulb of the keel must oriented parallel to the DWL if it is not to produce unacceptable drag. Heavy items which permanently dwell below the DWL, such as the boat's engine, can also be counted as ballast.

GZ is typically calculated with the boat sitting level at its DWL. The closer the GZ is to the VCG the less stable the boat is. If the GZ ends up lower than the VCG, the boat is unstable and will capsize.

 

Once again guys thanks a lot for your messages!!

I'm giving a first go at Herreshoff and Nevins Scanlting Rules (that I've found in the Appendix B of Details of Classic Boat Construction: The Hull) and calculating all the various variables from Hereshoff tables. However I am stuck on a small (I'm sure stupid) detail:

For the calculation of the Keel in the Nevins's Rules I find 2 data, one for molding and one for siding. Are "sided" and "molded" two construction processes for the keel (I can't find a clear explanation online) ?

Assuming those are parameters for the longitudinal thickness of the keel, to dimension it completely, shouldn't one need also a parameter for it's "width"?

 

Sided is the dimension sideways. Molded in the dimension perpendicular to the planking,in the keel more or less vertical.

 

Thanks Gonzo!