Scaling up a design

[Nels]

Who can offer advice on scaling a design up - specifically Joel White's Flatfish (which is a big Haven)? I have read conflicting methods. One school of thought says one can simply lengthen a design a bit (maybe 10%-15%)and not worry about the other dimensions. The other school of thought says to multiply all dimensions by a common factor (for example 1.15 to add 15% to the length of a design). Can anyone offer me some help here? I would like to expand the Flatfish to about 22' - enough to have a small cabin for weekending.

[PVanderwaart]

I would not be in favor of stretching a Flatfish. I think the longer hull would probably be OK, but you have to work out the ballast, cabin, centerboard, and larger rig. There a number of variants of this Herreshoff-type hull that you might use instead. The most obvious is the LF Herreshoff Prudence (or H-23). I don't know if draft is an issue; Prudence draws 3'.

LFH's design #43 is shown in Sensible Cruising Designs, and described as the ancestor of the H-23. To my eye, it's not as refined in appearance, but it is a centerboard boat with a cabin for two.

[Canoeyawl]

One problem (of many) with scaling up a design is that the volume of the hull increases by the cube and the sail area only increases by the square. You have to mind the S/A to Displacement Ratio also the Displacement to Length ratio and the draft or LWL. On a small boat with just a length change these may prove inconsequential but the Calculations are always a good exercise to see how much you may change the character of the craft.
I have seen these numbers worked up for a small change on a larger boat, (a Yankee One Design – 30’6” l.o.a. 6’6” beam, one of my favorites!) including wetted area, resistance, sail area, speed calculations, righting moments, heeling moments, etc. and the process was more complex than one would assume. The results seemed to prove disappointing as the boat was never built and a larger design was chosen…
I would be leery about messing with a Joel White design without the blessings of a N.A. unless the change was very small indeed.

[mmd]

This topic has been thrashed here over and over and over again. I don't reccommend doing it unless you have the technical knowledge and/or experience to fully understand the consequences of the changes you are about to make. Unless you can - by calculation or dint of experience - accurately predict the hydrostatic and hydrodynamic effects of altering the volume and mass of the hull, you are making guesses in the dark on a subject that may well cost you a large sum of money for a boat that performs less than satisfactorally. It can certainly be done, but it is not an exercise of merely factoring up dimensions. Granted, my opinion on this subject is formed by the constraints of my profession where my clients expect me to get it right the first time. I would reccommend either selecting another design more suited to your needs and desires, or investing the time & resources required to design the changes properly. However, to give the devil his due, if you enlarge the boat by 10% or less only longitudinally (maybe 3 - 5% transversally) you won't get into too much difficulty ... probably. But be prepared to do reballasting and sail & rig modifications after the initial launching to fine-tune the design modifications.

[Gary E]

I think you are talking about a sailboat so this may not mean anything.

When Post built their first 40' Sportfish in glass, the charter gang loved it but found the cockpit lacking in room. So, the folowing winter a lot of 40's were stretched to 44' and 46's. That works so well the factory soon started building the longer models.

[mmd]

True, Gary. The issues involved in enlarging a planing hull are considerably different than those of a displacement-speed curvacious sailing hull.

[Rick Tyler]

Nels is talking about taking a design that is already 20'9" and expanding it to 22 feet. Personally, I can't imagine why making it a foot longer would would mean that a cabin is now practical (Nels' goal), but I also doubt that scaling up this design by less than 5% is going to be a big deal.

If I were Nels, I would look for a design that met my needs -- including the weekending cabin. If he has his heart set on a Flatfish 22, I'd say go for it. It's not a substantial change. If you made a few mistakes building a 21-footer, it might come out 22 feet long even without the builder meaning it.

[John Turpin]

I'm dealing with this issue now. My 2006 project is a 16' Paul Fisher gunter yawl that I'm scaling up by 10%. I'm working with Mr. Fisher on this and he's going to verify my calculations. My greatest concern is accurately sizing centerboard size/weight and ballast.

[Carlsboats]

Having built a boat very similar to the Flatfish, I am not sure you will get much of a usable weekender cabin space by going up a foot or two in LOA. You could do it, holding beam constant and respacing the building molds, and you should get a slightly faster boat with a bit more stability, but at 22' LOA the limits down below would still be severe. You will not have even decent sitting headroom unless you push up the cuddy cabin overhead, and that will make her look boxy.
Also, getting in and out of the cabin is going to be a chore, with the CB case in the way.
My thought is that you would be better off starting with a different design entirely.

[johnw]

Check out "Skene's Elements of Yacht Design," and you'll find a section called the laws of mechanical similitude which discusses exactly this problem. Rescaling has certainly been done. I've heard of someone making a new yardstick that is larger by the percentage they are enlarging the plans, thereby eliminating the need to come up with a new table of offsets. Herreshoff Manufacturing did a cabin version of the Fish Class called the Marlin, and the Fish is what the Flatfish is based on. However, the Fish is deeper than the Flatfish, so it might be hard to come up with sitting headroom. Why not check with the Herreshoff Museum, you might be able to get plans for the Marlin.

[Rick Tyler]

Originally posted by johnw:
Check out "Skene's Elements of Yacht Design," and you'll find a section called the laws of mechanical similitudeThere is also a section on this in The Nature of Boats by Dave Gerr.