Thread: Help I made a boo boo

A mistake, get over it. Boat building is all about problem solving, you'll have more opportunity to solve other "issues" during the project. The extra stringer idea sounds like the wise bet. It keeps dad saying "I told you so" and you can move on to your next encounter with problem solving unimpeded.

I second that not because I have any idea about a fix but because father knows best. Don't forget that laddie!

Does anyone have an answer that involves some sort of structural/technical reason to go one way or the other rather than 'father knows best'?

I was really hoping boatmik might have an answer here.

You have a few choices, which you've described. The rip out and do it over approach, would likely be the ideal solution, though not entirely necessary strength wise. The forget about it and move on approach will likely mean some localized weakness in the area, though you could hope for the best. Lastly is the recommendation of your father, which solves the localized strength issue, lets him be the "one that knows best" and you don't have to rip out the whole stringer.

One other option, which is what I'd probably do is to scarf in a repair, directly on the stringer. This will make it just like the plans and you will not run into a "conical development" issue, which is slight, but possible with the extra diagonal stringer approach. The scarfed in repair restores the original intentions of the stringer design and under putty and paint, you'll never know it was there. Use a 6:1 scarf ratio at least, maybe 8:1 if the area has much twist and curve (hard to tell from the photo).

Howdy,

What is the distance between where it is and where it should be?

What is the ply thickness - I cant remember whether a Hartley is 6mm or 9mm in that area.

If it is not too far away from where it is supposed to it might be OK as it is - the bottom panel is starting to have a fair bit of curvature in that area - that's why the stingers suddenly become diagonal. I suspect the framework in that are is more about having something to attach the ply to in the building process rather than something that is strictly necessary for structural strength.

Answer the two above and let me have a think.

Also are the other diagonal pieces parallel to each other or radiate out - this will tell us the way the ply is bending. A good way to show it is if you would put the others in place - even with masking tape and take a photo from further back so we can see the full setup approximately how it would be when it is all together even with the mistake.

I quite strongly suspect it is not very important, but in the worst case PAR is on the money. Between us all we will work out a good way.

MIK

It is 200m sternward (is that a word?) from where it is supposed to be at the keelson end. The chine end is where it is supposed to be.

I have taken 2 more pics, one with just the diagonal stringer at fault and one with the other two in place where they are supposed to be. As I said I would probably move the middle one slightly if I dont put an extra one in to minimise the gap.

Howdy,

It all looks like it will be well supported regardless of the change/error once the other diagonal stringers go in.

I don't think there is a lot to worry about.

The only caution I would make is when you do the dry fit of the plywood in that area just to make sure that there is neither a gap or the stringer is distorting the ply in that area. If there is a gap fill it with extra glue when you bed it down and don't overtighten the screws that go into the middle of the stringer.

If there is a high point it will need to be planed down a touch, which is the point of doing a dry fit anyhow. Best wishes, and keep up the good work! Michael

Thanks Mik, aside from aesthetics are there any side effects of putting in an extra stringer?

Is there an issue with it becoming too stiff?

There are a few reasons for the use of diagonal stringers in that area of the hull. First is a matter of practicality, the longitudinal stringers start running out of room and second the nature of the hull shape, which is conically developed in this area. What this simply means is, the forefoot shape on each side of the centerline is made from a section of a cone, a single axis curve. Sheet goods (plywood, sheet metal, etc.) can only bend along one curve axis at a time, so the shapes used are cylindrical or conical in nature.

The diagonal stringers on your Hartley align with the curve axis in this area of the boat. Yep, it's a dead straight line, if placed along the axis. This boat was designed by hand and had it's "plate development" preformed by hand (a tedious operation BTW) so the axis was well understood by Hartley. In the 1950's through the early 70's this was a common method to make building easier and place the reinforcement directly under the common axis of the curve in the immediate area.

Previous era designs would have the longitudinal stringers, gradually "funnel" (decreasing space between them) down into the forefoot, until eventually they'd end on the chine and/or stem in mass. The problem with this approach is the amount of twist that you need to place on each stringer and the "landing" area getting saturated with closely spaced fasteners.

Anyway these are the general engineering principles, employed at the time Hartley penned this boat. Plywood was still relatively new (actually not, relatively trusted is a better word) and stringer methods where carried over from strip plank and carvel building methods.

It wasn't long (about 10 to 12 years) after WW II that engineers started to figure out plywood's inherent qualities and support methods were revised (enter developed and tortured planking methods). This changed the stringer arrangements commonly seen to what you have in your Hartley.

Since then, we've found even more things about plywood and the redundant framing (stringers, cleats, floors, frames, etc.) have been further reduced, both in dimension and count. With the acceptance of epoxy and taped seam concepts, the same reduction (in many cases total elimination) of framing structure has taken place (started in the 1970's and advanced through today).

In short of this historical overview, in the 1950's the stringer arrangement on your boat was considered slightly light weight, though certainly not racer category light (dimensional sizes) in the scantlings department. By today's standards, the scantlings in your boat are considered well over the top in regard to size and number of structural elements. So, your concerns about strength and unsupported areas is valid, but only in regard to historical accuracy. The strength of your boat will not be an issue, regardless of the repair option you elect to employ for this particular problem. An extra stringer will stiffen up the area slightly, measurably, but not enough to be a concern.

When PAR and I agree, which is most of the time, I feel very confident.

In a modern stitch and glue boat of this size and skin shape there would be almost nothing in that area of the boat and the curvature and the gluing around the perimeter alone would be enough to support the skin without any diagonal stringers at all.

Hartley was designing for non gap filling glues and amateur boatbuilders - so he had to consider fastenings at every point - in case the glue let go or never quite bonded in the first place. With epoxy you don't need all the extra timberwork to screw things into because the glue is so reliable and will fill up any conceivable gap with full strength - even for amateurs.

So you could probably almost eliminate the diagonal stringers completely with no bad result at all. They may assist with the actual bending and control of the ply to make the planking of the bow end a bit easier - and being a bit out will not affect this process. That's why I needed the photo to see that it didn't affect the geometry of the conical development too much.

You are safe to just put the remaining two in.

You don't need our permission or not to put in an extra stringer, but the above might give some background as to why it is an unimportant question to us.

Best wishes
Michael.