Anonymous wrote:

William wrote:

While I have precisely zero boat building experience

Over the last forty or so years that I've been involved in cruising under junk rig, I've lost count of the number of people I've met, new to sailing and wanting to take up the cruising lifestyle, who have zero boat building experience and have great plans to build a 30+ft boat. I have felt obliged to quote the old saying: "build the first boat for your enemy, the second boat for your friend - and the third boat for yourself". Or "build a dinghy, to get some notion of the use of the materials and methods - it could act as the tender to the eventual big boat; then build a 20 - 25ft weekender, to learn more about handling a boat under sail, and what you really want from your dream boat; then you'll know enough to embark on the major boatbuilding project that a 30+ft boat is bound to be".

That remains as true as ever it was. It was certainly true for me. After two sailing dinghies, a dayboat and three smaller cruisers, I finally knew enough to work with a professional designer and get my "ultimate cruiser" designed and built. It takes longer, but cuts down the chances of  failure. There are quite enough stalled and abandoned projects around.

David Thatcher: Unless you are an experienced boat builder with some design knowledge, fiddling with an existing design can lead to problems and you are probably better off to find a design which suits your needs, goodness knows there are enough good boat designs out there. Just adding layers of fiberglass cloth either inside or outside may not necessarily provide the extra strength required, or desired in terms of design rigidity, for example normal sheathing cloth may add a little bit of strength, but there are much more sophisticated fiberglass laminates available such as double bias, and unidirectional cloth, it all depends what type of stiffening is required. The same amount of extra strength may be achieved by just going up in plywood thickness, from say 6mm to 9mm. Fiberglassing the interior of a boat can be a lot of work depending on the construction type. Generally internal fiberglassing is used as part of a designed composite hull, such as glass/cedar core/glass, or glass/foam/glass.

While I have precisely zero boat building experience I have done quite a bit of structural design, so I have a good idea of what I do not know in this particular niche, but I am competent enough to read a designer's intentions in his plans. Also, I am aware of the different types of fiberglass cloth available but I kept the detail in the initial post intentionally brief. The plan(s) I am looking hardest at using tick the 'Must Have' and many of the "Nice to Have" boxes, once I adjust for headroom. 

David Thatcher: Remember also that building a boat is a great deal of work and difficult to fit in around other things in life. Look at Annie Hill's beautiful build and the time taken to achieve a quality and well constructed vessel, and I myself am currently building a 6 meter bridge-deck sailing catamaran which I am fitting in around every thing else in life. It has been a complete year from the start of the project to get the hulls finished, and I have now begun on the bridgedeck structure, which I am sure will take another year, so that is two years for a relatively simple plywood catamaran, and I am a reasonably experienced boat builder. 

Since I am a cranky selfish bastard with little to no social life the time requirements of the build don't bother me. As to difficulty: Like the great sage Jeremy Clarkson says, "How hard can it be?";-)

Tony Brown: Going up a thickness in the plywood may well be a sound idea. Personally, I'd sheath the ply in glass cloth anyway (inside and out, if the design permits), irrespective of what the designer says. The rudder could be changed along the lines of Arne's suggestions with chord and end plates. Beyond that, it's almost impossible to address the structural strength questions (backbone) without knowledge of the particular design.

Yes, boat building takes time, but that shouldn't put any dedicated person off. You've just gotta keep at it!

I couldn't agree more, and thank for the tip on Arne's rudder. I will look into that.

David Webb: the general consensus among designers is that any change more than 10% up or down in the design results in a new design.

Duly noted. 10% would suffice, and initially I had coincidentally looked at precisely that but then figured "in for a penny, in for a pound" and so doubled it. 

Thanks to everyone for their replies. You have all given me good info. I have some other possible design ideas I may bounce off you once I nail down the precise design I will use, and examine the full plans. 

the general consensus among designers is that any change more than 10% up or down in the design results in a new design. I suggest that you try to find a design closer to the size that you want, and go with that. Changing the design by the approximately 15% you are suggesting could mean that the whole design is compromised and has to be changed. Much easier to find a design that is closer to the size you actually want. Alternatively design from scratch the boat you want and have a Naval Architect put it on paper and do all the calculations to get things right.

Anonymous wrote:

I am looking at modifying an existing plywood design and am interested in input from those of you with building/design experience.

The design I am looking at is 32' LOA, and I am considering adding 5-6" between each of 10 stations, and adding 4" to the cabin height. I understand that this will affect rigidity and hull strength so I intend to add 2-3 layers of fiberglass cloth both inside and outside of the hull skin (the current design only calls for epoxy). I can calculate the change in displacement and thereby modify the keel ballast accordingly. I do not intend to alter the bow profile nor the beam. I do know that the internal dimensions will change, and can work with that. The JR sail plan has to be done for the design anyway.

What other, unforeseen, issues should I be looking for? Will I need to potentially enlarge the rudder? Does the backbone need to be strengthened, or will the fiberglass addition be sufficient? etc. 

Incidentally, the design is a fairly robust one, not a high-strung racer.

Thanks

Unless you are an experienced boat builder with some design knowledge, fiddling with an existing design can lead to problems and you are probably better off to find a design which suits your needs, goodness knows there are enough good boat designs out there. Just adding layers of fiberglass cloth either inside or outside may not necessarily provide the extra strength required, or desired in terms of design rigidity, for example normal sheathing cloth may add a little bit of strength, but there are much more sophisticated fiberglass laminates available such as double bias, and unidirectional cloth, it all depends what type of stiffening is required. The same amount of extra strength may be achieved by just going up in plywood thickness, from say 6mm to 9mm. Fiberglassing the interior of a boat can be a lot of work depending on the construction type. Generally internal fiberglassing is used as part of a designed composite hull, such as glass/cedar core/glass, or glass/foam/glass.

So tread warily and do a lot of research about what you really need, and are able to build in terms of money and time investment, and skill level. Remember also that building a boat is a great deal of work and difficult to fit in around other things in life. Look at Annie Hill's beautiful build and the time taken to achieve a quality and well constructed vessel, and I myself am currently building a 6 meter bridge-deck sailing catamaran which I am fitting in around every thing else in life. It has been a complete year from the start of the project to get the hulls finished, and I have now begun on the bridgedeck structure, which I am sure will take another year, so that is two years for a relatively simple plywood catamaran, and I am a reasonably experienced boat builder.