Changing dimensions/effect on stability

[BlueBell]

Don,

Is there not an existing, proven plan in the dimensions you seek?
A design with proven stability and seaworthiness?
Wouldn't a replica be more authentic than a recreation?

Sorry to rock your boat but these questions I find pressing.

BB

 

[Don McDermott]

There was a thread on scaling and ballast calculations in 2014. It might be interesting to compare the factors in the spreadsheet attached to that post to your scaled design. Using that spreadsheet, the new beam would be 13 feet. No advice, just information.

I looked at the equations to see what the scale factors looked like some years ago. The equations are unnecessarily complex as written. That seems to be the consequence of using Excel to set up a problem where a pencil and paper would have saved a lot of time. We don't always benefit from labor saving devices. Once you work out the math, Excel crunches numbers nicely.

Beam: (C9/($C$7^0.7)*($E$7^0.7)) reduces to C9/($E$7/$C$7)^0.7

($C$7/$E$7) is the scale factor (new length) / (old length). Call it F

=C9/($C$7/$E$7)^0.7 ==> C9/F^.07
(scaled down beam = original beam/F^0.7)

48/58 = F = 1.21
F^0.7 = 1.14
New beam = 15/1.14 = 13.14

This is the best representation of the equations in the spreadsheet that I can do without tabs. All of the C values and E7 are inputs, the rest of the Es are calculated using the equations to the right.
_____________________Input ___Output
Length_________________C7 ____ E7
Sail area ________________C8 ____ E8 = (C8/($C$7^1.85)*($E$7^1.85))
Beam _________________ C9 ____ E9 = (C9/($C$7^0.7)*($E$7^0.7))
Depth _________________ C10 ____ E10 = (C10/($C$7^0.7)*($E$7^0.7))
Freeboard ______________ C11 ____ E11 = (C11/($C$7^0.7)*($E$7^0.7))
Mast height _____________ C12 ____ E12 = (C12/($C$7^1.4)*($E$7^1.4))
Sectional area ____________C13 ____ E13 = (C13/($C$7^1.4)*($E$7^1.4))
Wetted area _____________C14 ____ E14 = (C14/($C$7^1.7)*($E$7^1.7))
Lateral area hull __________ C15 ____ E15 = (C15/($C$7^1.7)*($E$7^1.7))
Lateral area keel __________C16 ____ E16 = (C16/($C$7^1.4)*($E$7^1.4))
Volume hull _____________ C17 ____ E17 = (C17/($C$7^2.4)*($E$7^2.4))
Volume keel _____________ C1 ____ E18 = C18/($C$7^2.1)*($E$7^2.1)
Displacement tot __________C19 ____ E19 = (C19/($C$7^2.38)*($E$7^2.38))
Wetted area tot ___________ C20 ____ E20 = (C20/($C$7^1.63)*($E$7^1.63))
Sail area / wetted area _______C21 ____ E21 = (C21/($C$7^0.22)*($E$7^0.22))
Sail area / displacement tot ___ C22 ____ E22 = (C22/($C$7^0.26)*($E$7^0.26))

Dave, thank you for all of this, very helpful to have!

 

[Dave G 9N]

Don,

Is there not an existing, proven plan in the dimensions you seek?
A design with proven stability and seaworthiness?
Wouldn't a replica be more authentic than a recreation?

Sorry to rock your boat but these questions I find pressing.

BB

There are 8 wherries still in use, 2 are cargo. The type died out around 1900 so unless someone took an interest in the early 20th century, the 3 0r 4 lines plans available on the internet might be all there is. There is a book Black Sailed Traders and a few others. The LOA might not have varied much, so a direct comparison to a 48 ft boat may not be an option.

The reason I brought up the bugeyes is that they had similar hull lines driven by similar physical circumstances but at a later time and there might be more information available, certainly more lines drawings. The bugeye was the stepchild of over fishing, local building traditions and regulations. Dredging was prohibited in the fishery until around 1880 when dredging under sail was permitted in waters over 15 feet deep. The growing scarcity of oysters drove an increase in vessel size and the need to make money hauling freight in shallower water in the off season drove the shallow draft. So we have convergent evolution of hull shape for hauling freight in shallow water under sail along with a range of LOA and beam to look at. The rig was driven by bridges in Norfolk, but not in the Chesapeake. The deeper water over the oyster beds made room for the centerboard. Granted it is an indirect comparison.

My only argument about beam is that a bunch of guys did something different with the beam under similar circumstances, and there might have been a good reason. It could be that 2 feet of beam made for a larger hold. The book I mentioned was first printed in 1941 and has a list of length, beam and depth of hold for about 600 bugeyes ranging from mostly 40 to 80 feet. The author was looking to find information on every bugeye ever built. There is a brief summary of size ranges, number of boats built and a graph showing length over time. The beam varied roughly linearly with the length, around 3.6 to 1. a 58 ft boat would have a roughly 15 foot beam and a 48 ft. boat about 13 feet. The builders in the Chesapeake would not have tended to go as low as 11 feet beam on a 48 ft boat. If they didn't do the stability calculations, at least they settled on something that was working. Average length was 50 ft from 1870 to 1885 then ramped up to over 65 ft. by 1900 while the number of boats built dropped off along with the catch. Most building stopped around 1905.

 

[Don McDermott]

There are 8 wherries still in use, 2 are cargo. The type died out around 1900 so unless someone took an interest in the early 20th century, the 3 0r 4 lines plans available on the internet might be all there is. There is a book Black Sailed Traders and a few others. The LOA might not have varied much, so a direct comparison to a 48 ft boat may not be an option.

The reason I brought up the bugeyes is that they had similar hull lines driven by similar physical circumstances but at a later time and there might be more information available, certainly more lines drawings. The bugeye was the stepchild of over fishing, local building traditions and regulations. Dredging was prohibited in the fishery until around 1880 when dredging under sail was permitted in waters over 15 feet deep. The growing scarcity of oysters drove an increase in vessel size and the need to make money hauling freight in shallower water in the off season drove the shallow draft. So we have convergent evolution of hull shape for hauling freight in shallow water under sail along with a range of LOA and beam to look at. The rig was driven by bridges in Norfolk, but not in the Chesapeake. The deeper water over the oyster beds made room for the centerboard. Granted it is an indirect comparison.

My only argument about beam is that a bunch of guys did something different with the beam under similar circumstances, and there might have been a good reason. It could be that 2 feet of beam made for a larger hold. The book I mentioned was first printed in 1941 and has a list of length, beam and depth of hold for about 600 bugeyes ranging from mostly 40 to 80 feet. The author was looking to find information on every bugeye ever built. There is a brief summary of size ranges, number of boats built and a graph showing length over time. The beam varied roughly linearly with the length, around 3.6 to 1. a 58 ft boat would have a roughly 15 foot beam and a 48 ft. boat about 13 feet. The builders in the Chesapeake would not have tended to go as low as 11 feet beam on a 48 ft boat. If they didn't do the stability calculations, at least they settled on something that was working. Average length was 50 ft from 1870 to 1885 then ramped up to over 65 ft. by 1900 while the number of boats built dropped off along with the catch. Most building stopped around 1905.

Thanks Dave. I've read Black Sailed Traders, and there are a couple other books about wherries which I also have. The LOA varied quite a bit with wherries, and a 48-footer would not have been unusual. In fact, most wherries were not nearly as large as the two cargo wherries that still exist. Unfortunately, we don't have the plans for any of the average-sized wherries, and those people who actually saw the wherries are hard to find these days! As you say, it's likely that they rarely had such a narrow beam, because they wanted to maximise cargo space. But some smaller ones had to go through a lock at Dilham and their dimensions would have been restricted to no more than 50 feet long with a beam of 12'4". I suspect they would have used every inch of that, but hard to say. Another wherry had a narrow beam to pass through Old Wayford Bridge, but I've been unable to find out how narrow that bridge was as it was torn down many, many years ago. I have spoken with several people over the last week, and the general consensus is that if I ballast her properly (when she isn't carrying cargo, that is) and ensure that the sail area is scaled down correctly, my design will be perfectly stable. I've run some calculations as well, checking COG with metacentres, and at a heel of 9 degress, the metacentre is still about three feet above the COG. So I think the design is fine. Much less stable than the original, but still very stable, because those original wherries were like rocks, they hardly heeled at all, even when unladen. I sail on one of the traders on the Broads, so I have first-hand knowledge of that! So I don't mind less stability, as long as she is still stable, which everything points to her being.