Scaling Down a Sharpie

[BertKu]

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.

I agree with you, another problem is the fact that if the plans specify for 6 mm or 8 or 9 mm plywood , where the hick can one then get 6 mm less 10% or 15% or whatever percentage plywood or any material which is specified by the plans. It is just a nightmare and also one either makes the boat too light or too heavy by deviating from the specifications. Bert

 

[TANSL]

It is a very big mistake to apply the same scale to the thickness of the hull as to the shapes of the boat, or apply a factor that depends on the dimensions of the model vessel and the current boat. Scantlings depend on many variables that have nothing to do with the general scale applied in the transformation. To do a serious, professional and safe work, there is no choice but to recalculate the scantlings, among many other things.

 

[DCockey]

John Gardner included a "New Haven" style 18 foot sharpie in one of his books, possibly Building Classic Small Craft. The Mystic Seaport store should have his books in stock.

 

[messabout]

Ruel Parkers book; The New Cold Molded Boatbuilding , is loaded with Sharpie designs of many sizes.

According to custom or Perhaps local parlance, when a sharpie gets smaller it becomes a Flattie. The design characteristics are the same , the name merely changes according to whim.

There is a nice little sharpie/flattie/skiff/ rowing, sailing boat living on a trailer in my back yard. It is a 16 footer that satisfies my needs very nicely.

 

[PAR]

Again, there is no such thing as a small sharpie, though there are lots of designs that have sharpie aestedics, yet a very distorted hull form. A flattie is a term that was used in the mid 1800's to describe what happens to a sharpie, if made smaller than practical. These were characterized by a considerably wider beam/length ratio and a sloop rig, rather than the typical cat ketch/schooner seen on the sharpie. A small sharpie would be 25' and flat bottom, while a flattie would have a modest "V" in her deadrise aft, which help her handling at low speeds and loaded up. Flatties typically had a transom stern, while the New Haven the elliptical treatment. Flatties typically were built bigger than sharpies, some 50' and 60' long. These larger sizes often were referred to as something else, such as a bateau and skipjack, though their heritage was clearly visible.

What the OP needs to do is establish what he likes about the 27' New Haven and see if he can incorporate these elements into a 17 - 18' replication. The addition of an elliptical stern isn't much of a problem, if fitted to a modern transom stern skiff. Other than her stern and a few occupation typical elements, just about any flat bottom skiff could be made to look like a New Haven. Personally, I'd prefer to have a modern V bottom skiff hull form, with the topside resembling the New Haven, to take advantage of the types evolution.

 

[philSweet]

Exactly. A dead flat bottom sailing skiff at 17' isn't something to aspire to. In order to hold the rig up, the boat needs to be proportionately beamier and somewhat heavier. The Newhaven sharpie's signature ability was that it could carry about 8 times it's own dry weight in payload, which is why it was cheap. It was material minimizing in the extreme. The average 17' recreational skiff doesn't need to perform well over an 8-fold load range. A 2- or 3-fold load range is enough. That means a vee bottom can be exploited to accommodate the extra heft yielding better performance and economy.

The earliest was probably the 15' Cricket. Alas there are none in existence and no known lines. It was a popular boat with individual fleets with a hundred boats. A later 16' type commissioned by the St. Pete. Y.C. appeared in Rudder in 1919. They used prys when racing.

Later came the 16' X-Boat class (1932). They are still an active class.

http://sailboatdata.com/viewrecord.asp?class_id=4554

I think these show what needs to happen to a sharpie for it to function well at 16'.

 

[Angélique]

There is what is called affine transformation, which the designers have always used to obtain shapes starting from those of a similar ship.

My dearest TANSL,

It would be extraordinary nice of you if you were a bit less extraordinary vague there, and so it would be extraordinary kind of you if you were so extraordinary nice to reveal just a few of the, on this by you mentioned manner, successfully 37% in length scaled down sharpies, as this is the topic at hand here. And please forgive me, but I also humbly ask for the unveiling of just a few the fabulous designers who were able to do so, who you called ‘‘the designers’’ there, but who are unfortunately unknown by me yet, as I would like to start a new subject thread on these fine forums to glorify their marvelous achievements, and spread their great knowledge even further, to enhance future boat design by those who don't have gained this great technical knowledge yet, of which you are a dedicated preacher.

Just for the little chance these simple questions will put you in your underwear, or worse, I would like to apologize to you in advance, because since the time of the above publication of your hand, contained in the above quote, the main purpose of my life is only to gain just a little of the great technical knowledge you have preached there, but just haven't revealed the facts of it completely and clearly to me yet, but just to give me the asked for examples will help me extraordinary to put up my poor understanding of this matter, as I surely will study the kindly given examples carefully.

As can be supposed by anyone seeking for technical knowledge, my thanks to you will be eternal, if you would be so extraordinary kind to reveal what is asked for in this post.

And please, forgive me the boldness of my questions to a man as great in technical knowledge as you are. And please, also grant me the honor to thank you humbly for your presence at these wonderful forums and the amiable bestowed dissemination of your extraordinary wisdom, to me, your humble servant, apprentice, disciple, in technical matters regarding boat design, as well as sophisticated life in general.

My thankfulness to you my excellent preceptor will be enormous, even much larger than all the words of this world can describe together, that is if you just answer my simple and humble questions as described in my above humble request.

Sincerely,

 

[messabout]

The vee'd bottom of the mosquito class was more of structural consideration than a hydro dynamic one. A well designed plain flat bottomed skiff can have superior running lines when heeled than a vee bottom. The picture in Phils' post shows a boat on its ear and jolly well not how one would choose to sail. In fact a flattie when heeled may run with less fuss than a vee bottomed equivalent. Only when heeled mind you.... A flattie will pound your eye teeth out at speed when sailed flat. One more thing, the flattie is far more easily encouraged to plane than a vee bottomed example. In which case the pounding is accepted because of the rush provided by going fast in a boat too generally regarded as primitive.

No matter... I would buy one of those Mosquitos, any time, for the $150 price as quoted in the article. My heavens, how times and prices have changed.

 

[Ilan Voyager]

I see that our dear Angelique can be demoniac when she wants. May Buddha preserve me of her wrath.
In fact affine transformations are simple operations that need only a few mathematical tools as we can see on the wiki article.
https://en.wikipedia.org/wiki/Affine_transformation
LOL
Happily there are old geometric methods with tools like spline battens combined to simple elementary arithmetic operations that can solve the problem on a boat hull. I know it's obsolete with the magic of computers. With a soft using NURBS the results can be hilarious.

 

[Angélique]

By scaling form without changing shape the - dimensions - dry weight - max displacement - stability - will all change at a different rate, and long before 37% scaling down, the ratios between those parameters will be very different from the original, and so the boat's behavior becomes different, meaning worse if the original was a well balanced design.

And also you don't scale the - wind - waves - crew - down by 37%, while a 17' and 27' sharpie style boat often sail in the same waters, how to solve that when only scaling ?

 

[TANSL]

Angélique, thank you, thank you, thank you. My limitations with the language prevent me from expressing my emotion and my gratitude for the admiration you feel for me. I love the grace you have for expressing it. You know that in me you will always have a true friend. Thank you.

 

[CT249]

Among designers of craft as far apart as racing skiffs (such as Bruce Farr) and warships (D.K. Brown) it's been noted that scaling a design down is fraught with issues and rarely successful. As Angelique notes, things like the wind, crew weight and waves don't scale down very easily.

 

[CT249]

The vee'd bottom of the mosquito class was more of structural consideration than a hydro dynamic one. A well designed plain flat bottomed skiff can have superior running lines when heeled than a vee bottom. The picture in Phils' post shows a boat on its ear and jolly well not how one would choose to sail. In fact a flattie when heeled may run with less fuss than a vee bottomed equivalent. Only when heeled mind you.... A flattie will pound your eye teeth out at speed when sailed flat. One more thing, the flattie is far more easily encouraged to plane than a vee bottomed example. In which case the pounding is accepted because of the rush provided by going fast in a boat too generally regarded as primitive.

No matter... I would buy one of those Mosquitos, any time, for the $150 price as quoted in the article. My heavens, how times and prices have changed.

Racing dinghy designs these days rarely go for the "flattie" concept because it doesn't work very well apart from boats with a specific set of ratios and dimensions, as in UK Cherubs, late-model seahugger Moths and Australian Skates. They tend to suffer from excess wetted surface area, as well as problems in chop.

PS- I think the X Boat in Phil's pic is actually being sailed exactly how you'd chose to sail when you are doing a roll tack, as they appear to be.

 

[CT249]

Exactly. A dead flat bottom sailing skiff at 17' isn't something to aspire to. In order to hold the rig up, the boat needs to be proportionately beamier and somewhat heavier. The Newhaven sharpie's signature ability was that it could carry about 8 times it's own dry weight in payload, which is why it was cheap. It was material minimizing in the extreme. The average 17' recreational skiff doesn't need to perform well over an 8-fold load range. A 2- or 3-fold load range is enough. That means a vee bottom can be exploited to accommodate the extra heft yielding better performance and economy.

The earliest was probably the 15' Cricket. Alas there are none in existence and no known lines. It was a popular boat with individual fleets with a hundred boats. A later 16' type commissioned by the St. Pete. Y.C. appeared in Rudder in 1919. They used prys when racing.

I think these show what needs to happen to a sharpie for it to function well at 16'.

Interesting info about that 16 footer - thanks

 

[TANSL]

CT249, I agree, as it could not be otherwise, that many things on a boat can not be scaled down but what the OP has said, and I think it is what some of us are talking about, is : "I wanted to try and scale down the lines of the classic 27 'New Haven Sharpie to 17 or 18 feet. " (See post #1)
That, in my opinion, can be done without any limitations but, if I am wrong, I would like someone to tell me why.