Scaling issue

[Michail]

I am building a sailboat loosely based on an existing design, Souriceau, in a female mold from single skin fiberglass.

The original design is hard chined dingy-like 4.75 m. x 2.2 m keeled boat.

I have changed the hard chines to round shapes.

I also initially marginally scaled it up using a scaling formula (6 % length, 3.5 % width, 3.5 % depth, to 5.05 m. x 2.27 m.)

I have already cut out the transversal stations (14 in total).

Now the problem… when I was about to mount the mold, it occurred to me that if I stretch it lengthwise by another 5 % (to about 5.28 m., by changing the separation between stations from 40 cm. to 42 cm.) I might get a much better layout in the bow section (basically adding the option of sleeping there, since the two berths in the original design are very close to the entrance and will get wet, nothing pleasant for the cold weather we have in Chile)…

When I made the change of the hard chines to round shapes, the static stability based on Delftship calculations improved marginally (transversal moment of inertia from 0.736 m4 to 0.799 m4, and metacentric height from 0.765 m to 0.823 m). (The shapes were slightly “fuller”).

QUESTION:
If I stretch the total length by 11.3 %, and leave the width and depth at +3.5 % and considering the fact that the round shapes give a slightly better form stability, am I making fuss about nothing or do I still run the risk of having some stability problems?

According to the scaling formula, my width/depth is about 4.5 % narrower than it should be if it were scaled up correctly.

As it stands now, if I do this extra stretch, my idea is to compensate it with a slightly heavier and longer keel.

[Petros]

I think the only way you would ever know the difference is you built two exactly the same except for that difference, and sailed them side by side. And even than it will be hard to tell. One might have certain advantages in some conditions over the other, but I would estimate that is about it. Small changes like you are making usually make only small changes in its behavior. A narrow hull tends to cut nicely though large chop but will heel a bit more in the same beam wind, a wider hull will not heel as much but will pound more in heavy seas. You have not done anything too unusual. Consider that for many centuries hulls were just "eye balled" for shape.

I doubt you will have any problems, these factors have to get way out of balance before you develop any problems. You will have slightly different behavior over your original design, and none of it will be bad, just a bit different. You can always adjust the ballast as you noted for fine tuning it to your liking. You can also adjust the rudder size if necessary, though I would be surprised if that would be necessary.

You have done more than most in analyzing it in Deftship, so at least you have an idea the changes will affect the stablity.

Sounds like a nice size pocket cruiser you are building. Do you have any drawings of it to post?

Good luck

[Michail]

I post my "last" working design from Delftship and the original designs.

Thanks for the answer. I was really worried about this stretch, since I am not an expert at all...

[erik818]

My thoughts on this issue after having done some calculations on similar problems are:
- If you increase the length with 5%, you have to increase the displacement with 5% if you want it to rest on the same waterline.
- Gz (righting arm) will remain the same, so the righting torque will increase with 5% because you have increased displacement.
- If you increase the ballast with more than 5% when you increase the displacement, the CoG wil be lower and you will gain additional stability.
- Hull length has increase 5% so you will probably see a very minor increase in speed, although you have increased displacement with the same sail plan.

I have no good opinions on other aspects; I am also an amateur at designing boats.
Erik