Scaling Down a Sharpie

Hello All

I wanted to try and scale down the lines of the classic 27' New Haven Sharpie to 17 or 18 feet.

I understand I could use a ruler and scale down the offsets. Just wondering of there are other considerations to be made when scaling down like that? Like increasing freeboard?

Any thoughts would be appreciated. Thanks FM

 

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Hi FM, wlecome to the forum . .

There are full old plans, and instructions for building, of a 15' (New Haven, not sure) Sharpie in the Yachting Magazine issues of December 1930 & January 1931.

And a lot of other info there . .

Good Luck !

 

@Angélique, why do you want him to look for information about a 15' if what he wants to build is a 17 or 18 feet?.
@FromMystic, you're right, probably the only thing you have to touch up is the freeboard.

 

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I don't want him to look at it, he can look at it if he wants to, I especially mentioned 15' so he can skip it if he doesn't like that, I posted it for the chance he's interested in the related info there as it also gives scantlings and instructions for building near his prefered size, BTW why do you ^TANSL want to know this . . .

 

There are plenty of sharpie designs that are shorter. Take a look at Reuel Parker's The Sharpie Book for starters. These boats are fussy size-wise, on account that about a million of them were made over a 50 year period, so they pretty much got optimized for the materials and economics of the time. You can't just scale down a vessel by that much. It won't work properly. The hull won't develop enough righting moment, but the crew will weigh more proportionally. So as boats shrink, they generally get a bit heavier and beamier when going from 27' to 15'. All the basic design ratios change. You need to start with a boat much closer to your desired size.

 

The simple answer for your scaling question is no, you can't just scale it down. Scaling causes some unforeseen issues that prevent anything more than a 10 - 15% reduction, before major issues crop up, such as stability, wetted area, volumetric concerns, etc. Your proposed reduction is way past reasonable. This said, you can disproportionately scale her down, though she then becomes something somewhat different than the previous 27' sharpie. A proportionate scaling would result in a very skinny boat, that simply wouldn't be able to hold up a her rig, let alone a press in it. If the beam was decreased disproportionately, say holding it to 6', you might have something that will sail.

The results will not be desirable, though it will be a scaling. You're much better off finding a skiff of similar proportions and rig. In other words, there's no such thing as a small sharpie. Once they get reduced in size they become skiff hull forms, because the hull has to be grossly distorted to make it functional.

What specific things about the New Haven do you want to see in this 17' boat?

 

There is what is called affine transformation, which the designers have always used to obtain shapes starting from those of a similar ship. I speak only of scaling the forms, I do not speak about scantlings, weights, propulsion engine, rigging, nor about many other things, only of forms.
I see no reason why the OP can not use this procedure.

 

Perhaps because 15' plus 15% = 17.25'

 

Sure and more revealing yet, 15' plus 20% = 18'

 

A 27' sharpie is proportionally narrower than a 17'. The ratio of crew to displacement is very different between the two sizes too. Considering that there are many free line plans for sharpies of the size you are looking for, it would be easier to use them rather than trying to modify one. Chapelle has a few designs too. He took the lines from working sharpies, which will have a rather low freeboard and a fair amount of sheer. Increasing the freeboard a bit won't change the handling of the boat.

 

Could someone please explain what this means: "I wanted to try and scale down the lines of the classic 27' New Haven Sharpie to 17 or 18 feet."
Perhaps it means: "Tell me where I can find the body lines of a sharpie of 15' ". Or maybe not.

 

My apologies for being too cryptic.
The 15% was by reference to PAR's quotation of the rule of thumb about changing the scale by not more than 10 to 15% (which applies to decreases and increases).

Normally I would not have bothered to reply but I am concerned about anything that might discourage Angelique from posting in her usual helpful manner on a whole range of subjects. There is good reason why she has the highest rep/posts ratio on the site.

On a lighter note regarding "Affine Transformations" they speak of little else in the local pubs.

 

latestarter, your intentions are very commendable. Thank you. Sorry Angélique if I have prevented you from expressing yourself in your usual helpful manner.
Regarding the "affine transformation", maybe the translation is very literal and it sounds like something stupid (I do not know) but I'm sure that boat designers know what I mean.
By the way, I do not know any rules of thumb that limit the scaling of forms, nor can I think of the need to limit the scales. Maybe there's something I'm misunderstanding.

 

Thank you for your reply.
Affine transformation is the correct translation see wikipedia https://en.wikipedia.org/wiki/Affine_transformation it just struck me that it is beyond the knowledge of nearly everyone.
I did read an explanation why the rule of thumb works, if I come across it I shall provide a link. It had something to do with different aspects of a design that change in a direct ratio or the square or the cube of the ratio.

 

There are many aspects of boat design that are not directly scalable, such as the weight of the hull, the size of the anchor, the maximum number of people on board, ... many things. It is not about 10 or 15%, you just can not apply a scale factor. But for shapes, there is no limit. Why should it be? I do not see why.