Making a Little Moby into a Bigger Moby?

[David Virtue]

I've had a set of plans for Little Moby for years and I was wondering if it would be possible or even a good idea to enlarge the boat to about 17'-18'. How would it be to say just add 20% to all the major dimensions to come up with a bigger boat?? I once had a 20' sailboat built by Paul Rollins and we just enlarged a 18' sailboat plan designed by Bud Macintosh. It seemed to work out just fine. Thanks

[Keith Wilson]

Why not just stretch it? It'd make a very nice relatively narrow 20' boat, and would need little if any more power. Old-time builders did this all the time, and Phil Bolger has made three versions of his Shivaree/Fisherman's Skiff outboard boat in different lengths.

[John E Hardiman]

Not a hard and fast rule, but a good structrual rule of thumb is up scaling <10% is kinda ok if you add a little to the panels and stiffeners, >10% redo all the structure and structural calculations.

To compare approximate changes in structural loads.

panel weights, shear load: up 10%=+21%, up 20%=+44%
displacement, moment load: up 10%=+33%, up 20%=+73%

Note that upscaling the size of the panels/stiffeners the same amount would not work unless you maintain frame spacing in order to prevent corresponding loss of stiffness due to increased panel length

[Alexander Shramenko]

In the book "Principles of Yacht Design" by Lars Larsson and Rolf E. Eliasson
there is a table of scaling factors - p.12, Fig 2.1. Proportions versus size(Barkla) .

It described on p.11: "The scaling factors shown in the figure produce boats
of similar behaviour regarding performance and 'feel' when scaled in either
direction from base model".

Also on p.91: " It may be shown that, everything else being scaled properly,
beam should be scaled as (length) in power 2/3…”

So, if you want “just add 20%” to the length, beam will be wider
(1.2) in power 2/3 = 1.129, or 12.9% .

[ 05-08-2004, 12:25 AM: Message edited by: Alexander Shramenko ]

[Jamaica Mike]

I believe that Little Moby is a smaller version of a larger (18') design by the the same designer - Witholz. It is in the Wooden Boat plans book.

JM

[John E Hardiman]

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[John E Hardiman]

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