Plywood on Frame Modification Questions

Calusa, expanding a plan set is not usually a good idea if the original designer was a knowledgeable boat person. The plan set that you have shown suggests that the designer may not have had much experience with design discipline. ..(I am being generously polite here)

As is so often advised, post a statement of requirements (SOR) for your boat. How many occupants will it have, what speed do you wish to have available, is comfort an important consideration, Will it be launched from a trailer, do you have budget constraints, is it to be used in the flats that will limit draft, are you planning to take out to Egmont Key or other longer distance lumpy water locations.... ?????.

A 15'-7" boat that is seven feet wide is not going to handle very well, it will pound your teeth out, and it is not much, if any, safer than a boat with a more realistic length to breadth ratio. A stitch and glue design is much easier to build, weighs less, has a cleaner interior where the fish guts accumulate, and will probably not bust your budget, will last until you are tired of it and more.

 

Most designers have enough safety margin to allow 10%. It might be easiest to go two ways or 3 versus one way. One way might fail.

 

I agree with every word of messabout's post.You can't expect to make anything other than really tiny changes to a workable design for plywood.The nature of the material pretty much restricts it to conic derived surfaces and the keel and chines in particular will be arrived at by determining where those conic surfaces meet.Adding a few inches to an already chunky looking boat's sheer height will increase the windage at speed and fuel consumption will go up while speed comes down.

 

I agree with you on this but we must also consider that a freeboard that is too low, in a boat without a deck, can make the boat totally impractical. As always, it is about reaching a compromise between characteristics that oppose each other. I would deduce, first of all, the minimum freeboard necessary so that the ship does not flood when the passenger moves. Once that value has been calculated, there is no reason to increase the figure and that the windage, fuel consumption, etc. be more than necessary.

 

I'm not quite following what you're saying here Fallguy.

 

If you want to make a boat larger and the designer allows a 10% increase. The boat is increased 10% in all directions (3), or at least 2, beam and length, but never one direction typically.

Changes to the sheer are often done in s&g designs to allow for user preferences like function, easier fish catching or to make it drier or aesthetics, a sweep to the sheer that is more pleasing. Of course, these changes require forethought to provide structural support.

 

Generally speaking, I disagree. It is very likely that a normal design will allow a 10% change in one direction, in 2 or in three. In any case, however, some verification will have to be done, of course.

 

A 10% increase in three dimensions will create a boat with quite a bit more displacement. Will the scantlings be up to the job???
A 10% increase in length is often reasonable or so I've heard.
Selway Fisher Home Page (selway-fisher.com)

 

Most designers use a safety factor in designing scantlings. If a boat is not capable of going from 20 to 22 feet within its scantlings; it probably has been designed too close to failure. This is not verbatim, but a professional designer mentioned it. May Jacques Mertens rest in peace.

the reason for going in more than one direction is a boat jig or bulkhead is easier to adjust two ways; especially true of tortured panel

 

OK, let's take a look at a stitch & glue plan that I wouldn't mind trying to modify. It's a pretty simple Spira plan 14' in length. Snippets from the plan are shown below, with no modifications on them.


I may be way out of order with what comes first with what I'm looking to do but lets start with the sides. Suppose I add an inch in between each station, making this a 15' 3" boat.
And I add say, a couple of inches in height to the sides and cut both panels out.



Then widen the transom by 6 inches top and bottom width and add a couple of inches in height keeping the same side angle.
Cut that out, laminate to correct thickness.

With bottom side up, stitch the bow together and place a couple of temporary frames/spacers between the sides, and bend the sides back to the transom and attach. Adjust the temporary spacers to form a bottom shape roughly 6 inches wider than the original shown below. Then once everything is symmetrical, level and measures evenly, lay the bottom plywood on top and trace with a pencil. Cut out the bottom and stitch the whole thing together.

 

Maybe Atkin has something for you.
Simple wholesom boats without extreams.
Atkin & Co. - Boat Plans (mysticseaport.org)

 

Rater than modifying the design as indicated in post# 25,find the actual boat you want to build.Making the transom that much wider will require that several other sections will also have to be made wider in order to achieve a fair shape.By doing so you will have changed the distribution of buoyancy of the boat.Which may mean that in waves,the additional buoyancy at the stern will lift the stern more and as the boat pitches in response the freeboard at the bow will not be as great as intended-even the additional 2 inches may not be sufficient to keep water out and you won't find out until afloat.I suggest you keep looking for a design which has successfully been built that conforms to your requirements and stick to the designer's shape.He should have taken care of the factors that affect performance and the user can perhaps advise you on the behaviour of the boat.

 

If the transom is 49.5” wide and you want it 6” wider; this is a 6/49.5 % change.. 12.1%

15.25’ / 14.03 is 1.087 or an 8.7% change. This means you are making the boat wider than longer. When you modify tortured panel designs this way the torture can be your own if the panels will not conform to the jig.

The proper way to increase the plan is in two directions and the same percentage, if the designer allows it. All the designs by Mertens allow it.

The metric system is easier this way.

Also, the reason for expanding in 3 directions is if the bottom is not rockerless bottom, etc., you can be adding another problem in tortured or dev panel construction.

The fact the design is called ‘ultra lite’ means the design was already close to minimum scantlings. It is probably foolish to expand it without modifying the structure.

 

Calusa, deep down do you want to change plans just for the sake of it? If so, build some models.
Your needs are not unique, and someone has drawn plans that will meet them.
I will say for a flat bottom boat you want a narrow bow. A flat bottom with fuller bows will extract a price.

 

Fallguy - thanks again for your help in explaining the percentage thing.

The designer does allow a 10% change, and I just watched a video he made a few years ago where he specifies exactly how and what to measure in order to change the dimensions correctly. Basically it's going to be a calculation at each of the stations shown on the plan, all using percentage rather than actual measurements.
I hope I said that right ......

So, I am going to do the calculations and glue together a scale model to make sure it's going to come together correctly.
If it does I'll go ahead and built the full size boat.