Frames and Stringers

Dave Gers Book "Boat strength" has strip planked boats with frames and Hardchine boats with stringers. there is a bit in the book which seems to say a stringer is required at the turn of the bilge in a displacement strip planked boat. My impression is this stringer is optional but if it were added would go a long way to strengthening the boat like the diagonal straps recommended for the bigger boats.
My overall impression is planing boats have stringers and bulkheads but don't have frames, and strip planked displacement boats have frames and possibly one stringer at the turn of the bilge. Have I got this right. Pictures of boat internals would help. mik

 

You can put frames or stringers, both, or whatever you want. Depending on what you put, how elements are supported, you will have to put plates of varying thickness or ribs of varying module.
The problem of recipe books is that they do not explain the concepts of the structure and therefore the non-expert reader does not know what he does, does not know how analyze problems and, therefore, address them.
The best thing you can do is copy the structure of a similar boat and start playing with the separation of the constituent elements and their scantlings to get the structure of minimum weight.
These days there are software, very easy to use, allowing you to make calculations quickly and easily. In the time it takes you to read that book, you can learn to work with one of those programs.

 

Hi Mik

Interesting observation there. Tansl point is very valid but its worth noting that stringer on chines is not just a structural add on, but a clever way of making building easier as well.

One challenge for hard chines boats is accuracy of line for appearance sake, and with all the planing and shape adjustment that may be needed, having some meat there is handy.

Also, it provides a faired, view-able edge to work to during the build.

Then there is the need to get something solid to screw plywood or planks down on to, for some materials that makes it a popular method.



As Tansl implied - rather than being a desirable feature of itself, it is just one option for a hull designer out of many.

In actual fact, with stitch and glue, there are many boats with no chine logs at all, so it is easy to see that its the overall calculations that rule the process.



Just to hijack the thread a bit here, a question for Tansl - do you do enough design work on smaller (say from 20 to 30 feet) plywood boats to be able to know if chine logs are still as popular as they used to be, or are we seeing mostly glass re-inforced chines these days ?

 

Hi rwatson,
I do not think I fully understood what you say but I seem to be pretty much agree with you on many things. Maybe not respond therefore exactly to your questions but I will say that the chine, if it has the proper angle (I can not remember the minimum / maximum value required) and reinforce it properly, it can be used as longitudinal reinforcement, avoiding putting an inner reinforcement. Similarly they can be used the rails spary
I never designed the structure for a plywood boat, so I can not comment about it. Similarly they can be used the spray rails.
I insist that a good designer can make the structure as he likes better, as long as he calculates it correctly. Now there are boats for their special features can best solve their problems with a transverse, longitudinal, structure or an "egg box" structure.

 

He is saying that laying the chine log first produces a very important reference line. It is easier to correct it instead of strip planking a whole side and then finding out it is not fair or follows the design lines.

 

Thanks, Gonzo, is now clear. I thought rwatson spoke of resisting elements.

 

The choice of frames, stringers, etc in wood construction is directly tied to the choice of "planking" system used.

Hulls with traditional longitudinal planks generally need frames to hold the planks in position relative to each other to maintain the shape. The frames may also serve as transverse molds to establish the shape of the hull as it is planked. Some methods of traditional construction use separate molds to establish the shape and the frames are installed after planking is complete.

Strip planking needs transverse molds to establish the shape during construction. Frames and bulkheads may be used as molds though there is not the need for frames to hold the individual strips in position relative to each other because the strips are directly fastened/glued together.

Large panel plywood planking may be built over a system of molds/frames/bulkheads and/or stringers, or just bent into shape until the edges meet as in some forms of "stitch and glue" construction.

Diagonal planking (not applied over a strip planked layer) typically uses a system of longitudinal stringers rather than transverse molds to establish the shape during construction. Frequently the stringers are left in place as a permanent part of the hull.
Photo from the B&B Yacht Design website http://www.bandbyachtdesigns.com/ocracoke.htm

 

Thanks Dave a picture is worth a thousand words
Frames and stringers must add a little weight bit but Ocracoke is hardly what I'd call heavy. It looks and performs like a little PT boat, 150HP should give about 46kts