Can you resize a design without complications?

If possible means it could be built, then yes it is possible.

But let's take the 6' headroom the 30' boat has... When we scale everything up you now have a boat with 11' instead. As you mentioned it would be insanely heavy, and twice the width it should be, all sorts of problems are created by trying to do this.


On the other hand a good designer can take a 30 footer, and keep the same look, and similar concepts and design a larger boat using the smaller as a prototype. As I understand it, the maximum change is around 10%. More than this either way and you really need to redesign the boat.

 

Simply dont even think about it !!
It is possible to respace frames to change the size aa small amount but the more you do it the bigger the inherant probelms become .
Saw a 28foot Yacht that was scaled to a 36feet and was a absolute pig of a thing ! Only one out of the mould was ever made and then the moulds were scrapped . Same with power boats . For each foot added the inherant problems the boat may have can grow by 20% or more !!

 

Another option is give the boat to the US Government and you would have the values of the boat reduced to 50% in no time.

 

Or you can get married and the divorced and end up with a boat big enough to Hold in one hand Maybe but you wil have lost the bath tub as well so !!

 

JFWIW..
Here is a simple formula we used when checking things for ½ scaling our prototypes of 30 and 40 foot catamarans as we wanted the prototypes to be usable as Picnic boats ready for market upon completion, instead of a lost cause as prototyping is too expensive to throw away in my mind. The scaling worked well.
To scale displacement take the scale ratio. For example you want to reduce a 10 foot boat to 0ne foot. That is a 10 to one ratio. Raise the scale ratio to the third power. 10 x 10 x10 =1000 divide the displacement of the full sized boat by the scale ratio cubed...in this case you divide 1000 by 1000 and the quotient is one. Lets take a different case in which the one thousand number does not occur twice. That is kind of confusing, so think of a ...16 foot rowing or sailing skiff. It might displace 360 pounds when occupied by one person. Let us build a model that is scaled to one and one half inches to the foot. That would be one eighth scale. so.....8 x 8x 8 = 512....divide....360/512 = 0.703 pounds. That is all the model could weigh in order to simulate the 16 footer. If you built the model to 3 inch to the foot scale then that would be a quarter scale or 4 to 1 model 4^3 = 64 .....360/64 = 5.625 pounds. The same rule applies when using the metric system of measurement.

 

The laws of mechanical similitude apply in both directions (scaling up or down) folks. So, if you want to make a 22' boat a 24' boat, it probably will not kill you, but stray very far from little steps and you'll be "talking to Davie".

 

Scaling a boat down by a significant amount runs into 2 things that do not scale: the human body and the wind.

In the first case, the smaller boat ends up with too much heeling force because that scales to a cube law compared to the stability which - as noted above - scales to the 4th power.

In the second case, you can cut down on the number of crew but they'll object to the removal of body parts to fit them into a narrower cockpit.

However, for a certain range of boat size the disproportionate increase in heeling force is compensated by the proportionately greater weight of the crew and its ability to act as mobile ballast.

 

In the true meaning of the word "scale" yes objects can be scaled up or down but in doing so it would be common sense to use the engineering appropiate for the size of the new scale. One wouldn't use 2x4 framing for a dolls house, nor should one use the 3/8 by 3/4 in. steamed bent ribs of a 16ft. runabout for a 35ft. cabin cruiser. I realize these are extremes but even minor adjustments in scale could easily surpass the specs. on the dimensions of the materials used and the operating characteristics of the new machine. The stretch DC8 being a prime example.

A yacht is not defined by the vessel but by the care and love of her owner--

 

Again the law of mechanical similitude applies in both directions. Sam Langley sure proved this out in his attempts to scale up his flying rubber band powered models to full size. They folded up as soon as the structures where loaded. So as Bataan mentions, the scantlings too need the appropriate application of engineering.

 

I think it should be obvious what scaling will work and what won't. If you scale a typical tandem canoe 100% up or down you'll have a useless boat. If you scaled a 42' Uniflite down 100% it would be ridiculous scale the cabin and deck. But to scale the hull only and make a center council open sport fishing boat w OB power should make a good boat w much of the qualities of it's 42' parent. And if you scaled up an 18' canoe 100%, cut about 10' off the stern and installed a transom you'd have a big "Laker" freight canoe. If I was to scale up my 10' Whitehall style row boat to 30' like my Willard and incorporate the "innards", cabin and deck of the Willard I'd have a very useable boat I think but it obviously would'nt handle like the Willard. Would be considerably wider and have the characteristics of a wider boat. It would have Greater stability and a quicker motion. The WLL would be longer and the displacement less (unless it had 2 tons of ballast like the Willard) so it would be faster and go all speeds w less power. All in all i think it would be a good boat .....just a different boat than the Willard.

 

Ok, I am new but want to ask a similar question and think it best fits here. the discussion above seems to be addressing scaling of sailing vessels, but does the same apply to powerboats.

Background:
I love the design, and look of the WWII 80' Elco PT boats, but feel the full size vessel would be beyond the capabilities to operate by one or two people. My goal is to take the lines of teh PT boat and scale them down to a 40' version. I understand that beam may not scale down the same, and equipment internals will obviously change, so that is not the issue. I simple want to understand what considerations have to be made for this kind of a scaling, or if I should hire a designer to make a new design on the same lines.

 

The laws of similitude apply to lawn chairs, trash cans and powerboats too. Simply put, if there is much alteration in scale, anything but a slight change needs to be re-examined by a professional, in regard to the "lines" and volumetric considerations.

The best way to have a 80' PT boat, is to pull the styling clues from the PT and incorporate them into a 40' yacht. Some proportions will be off a little, but the general idea will be there and the boat will preform properly, with the added benefit that humans can fit inside it.