help with understanding longitudinal stringer /farming

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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Kevin Morin
Posts: 636
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: help with understanding longitudinal stringer /farming

Postby Kevin Morin » Sat Oct 12, 2013 5:28 pm

Darrell, I can't say I've used the Dewalt tool line much, its less costly but so less durable I generally don't buy those handheld tools much. I do like their large bench mounted tools but they seem much more durable and precise than the hand held group. The Bosch is much more precise so my remarks will be less accurate without that tool in use.

The Vixen File, is commonly used to cut the edges of a jigsaw cut edge. http://www.amazon.com/Simonds-Flexible- ... vixen+file this very coarse cut (single cut) file is exclusively pulled along cuts to leave a pure aluminum cut edge without the may vertical scores left by a jigsaw cut.

Jig Saw cutting aluminum is always followed by the Vixen or a sander to clean up the cut 'jaggies' left by the coarse teeth (6-8 per inch) and their reciprocating action. The band saw cut is likewise always cleaned by a belt sander or the Vixen so the edge left for welding does not have 'buried' lubricants and other contaminants that will float out into the molten puddle while fusing the cut seam edges of hull panels of parts.

Even machine (NC) cut edges should be dressed by the Vixen or a power sander, depending on the radius of the curves and the tooling, before welding or the cut edge extra oxide build-up will make high quality welding more difficult to obtain.

If you use either saw, jig or rotary blade go to http://www.tapesolutionsinc.com/uhmwtape.html or someplace similar and buy some plastic film 'slick' tape to put on the base of the tools so the sawn chips of metal do not 'gouge' and drag under the tool. A couple of strips of this tape on the base of the tool will allow the skill or jig saw to slide more evenly and make fewer marks on the parent metal.

The chips can, and they do if not attended, get wedged under a steel tool base plate and the parent metal. Then a chip gets dragged along by the tools cutting movement and the operator can miss the cut due to drag and 'fits and starts' as the chips catch and release.

If you will be doing layout by hand, instead of having NC cut hull panels, then use a long extrusion to layout the loft points.

Image
here an aluminum extrusion is run though a series of points in a typical cambered frame created by the 'expansion of an arc' to make the layout for a cambered frame.

when you mark the cut, make sure the extrsusion batten is fair by viewing it low angle and from one end, then after adjusting for 'fair' make the initial mark along the side with a wide black 'Magic Marker' (tm) so there is a background for the real cut line mark.

Next take a metal scribe (sharpened Tungsten works fine for me) and make a very carefully scribed line on top of the black 'background' marker line, so the metal is scribed through the black ink. This line should be the real cut line for the tool/blade edge.

now when you remove the layout and fairing batten extrusion (lofting tool) you can see the very bright shiny and clearly visible scribed line. This line will be along one side of the black ink background mark.
By placing a shop light (quartz Halogen ) work lamp in front of you but off to one side, the reflection of the scribed line will be very clear as the contrast to the black back ground mark is high; silver to black.

Regardless of you're able to skill saw or jig saw this line, as long as you leave the original line you'll be able to Vixen file the final cut edge BACK to the original scribe and therefore be able to build within a 1/32" fair hull.

I use safety glasses with magnifiers under the face shield for this work; so I can see the cut well in the brightly lit work halogen lite. So even if the jig saw may move slower than the skill saw, the very controllable cut of the latter saw is worth the extra few minutes to get high quality edges when the final joints are tacked and eventually welded.

cheers,
Kevin Morin
Kevin Morin

North
Posts: 269
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

Re: help with understanding longitudinal stringer /farming

Postby North » Tue Oct 15, 2013 3:08 pm

Great advice Kevin - thanks again! I had read, in other posts about using batten type material as you describe, but honestly, had forgotten about it. I likely would not have it remembered before marking and cutting them. Hope to accomplish these setps in the next few weeks.

North
Posts: 269
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

Re: help with understanding longitudinal stringer /farming

Postby North » Thu Oct 17, 2013 7:34 am

Kevin - based on a 23' open CC build (may stretch to 25' per plans), with 1/8" hull plating and 1/4" frames (minimum frame width calls for 4" - I may do 5") - Do you think I should go with 2" x 3/16" flatbar for longitudinal stringers or would 1 1/2" x 3/16" flatbar seem sufficient?
If I use 2" then it will be the same as my frame top flanges (per plan) but will be harder to bend approaching the bow. If 1 1/2" is definitely sufficient for stringers, I will used this and order 2" for the frame flanges.
There are 8 frame stations plus transom so frames are spaced about 2' 7" apart.
Longitudinal stringers/ bars are average of 8" apart on bottom, down to 5" apart near chines. About 10" apart on hull sides.

I know you had given me advice in earlier post re: sizes in bar vs smaller angle, but thought it best to ask specifically, now that I have given you spacing, etc. If you do feel smaller angle (or tee) would be better, of course, fire away!
Darrell

Kevin Morin
Posts: 636
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: help with understanding longitudinal stringer /farming

Postby Kevin Morin » Thu Oct 17, 2013 10:04 am

North, I think 3/16"x1-1/2" 6061-T6 will provide adequate hull stiffness for the framing you describe. I might consider changing the bottom plate thickness to 3/16" in the after half, that is from approximately amidships aft to the transom as this is the 'running' surface and is shaped with less curvature than the forward half. So, keeping this area with less shape stress fair and clean is easier to do with slightly thicker metal than 1/8".

Since most flat bar, unless ordered in longer lengths, is delivered in 12' piece; it's worth considering deeper bar -2"x1/4" aft the amidships station on the bottom for the same reasons. I don't see the need to move to angle on a hull this small, and the spacing transversely of the longs sounds pretty close so, again, I can't see the need to convert them to angles.

Welding them into a bevel is much more effective in their attachment to the hull than a open fillet, so I do recommend planning the entire hull panels' weld sequence and beveling the longs outer edges to accommodate that plan. For example, if you plan to add 2 or 3" weld on one side, then skip 4-6 and repeat on the other side, (I'm not saying that is the pattern this is an example), then I'd prep the welds with respect each beveled section that will accommodate the welds, and make sure the layout took into account the frame's locations and staggered from the longs on either side as well.

The bevel allows you to put the weld 'in the field' on the inside of the hull panel with the minimal heat of contraction due to the shorter 'legs' as the bevels help bury the root and narrow the face reducing the area of contraction. Short welds, with beveled allowance/prep, well cleaned before welding and applied 'hot and fast' in a drag-style (non-patterned bead; not torch orientation) in the 'race track' method of crater elimination- will allow the full structural connection with the minimum of shape distortion to the hull.

I'd also use 5086 alloy material in the bottom and sides if at all possible- 1/8" hulls are more difficult to keep fair through weld-out than heavier materials because the welding needs to be proportionally smaller than in heavier materiel. Scaling MIG down, for thinner metals, requires more preparation, speed of deposition and actually requires a higher welding amperage than traveling slower, and the overall process requires more weld skill and experience.

For these reasons, I usually encourage first time builders to the 3/16" designs over 1/8", not that it can't be done, but the welding is much more critical while the newer builder is in the learning phases. No one ever practices enough before the begin their build, so they often face a bigger challenge building their first welded boat in thinner material.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

North
Posts: 269
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

Re: help with understanding longitudinal stringer /farming

Postby North » Thu Oct 17, 2013 11:45 am

Thanks Kevin. Understand your recommendation for bevelling the filet welds. I did this in practicing forsome bend tests.
By the way, have to practice more, but also plan on bringing in a local experienced aluminum welder, to help me set up - settings, techniques, etc once I have the frames cut and tacked together.

I can get the flatbar in 20 ft lengths, in the 1 1/2" x 3/16 size, as well as larger.

I will be using 5086 for the hull. I have wondered about the 1/8" hull thickness and the possibility of going to 3/16", for ease of welding and possible more durability.
My "problem", unless another supplier comes in with better pricing, is that I have asked for, and can get 5' x 20' x 1/8" or 3/16" sheets, but they will be cut from a coil (not locally) and there is a big setup fee. So there is a huge penalty to get one sheet of one thickness vs another thickness. If it is spread over 4 sheets of the same thickness, at least it is more palatable.
That means that it will not be economical to order 2 sheets of 1/8" and 2 sheets of 3/16", vs 4 sheets of either. And, I woudl think it would be overkill to use 3/16" for the complete hull (bottom and sides) as well as deck.

Here is an example of the prices and large fee seen in the 1 sheet examples:

1875” x 72” x 240” 5083 alum sheet -$2374.75/piece qty1

.125” x 48” x 240” 5086 alum sheet -$590.00/sheet x qty 5 sheet -$2950.00/lot

.1875” x 60” x 240” alum sheet -$1070.00/piece qty 1

Similar thing in 5052, as that is what they quoted originally:


1/8” x 5’ x 20’ alum 5052 sheet -$533.75/sheet price based on buying qty of 4 sheets.
.19” x 5’ x 20’ alum 5052 sheet -$1102.50/sheet price based on buying qty of 1 sheet

I will likely start by just ordering 1 sheet 5' x 12' x 1/4" (5052) for the frames (they will all nest in this, plus a small piece of leftover I can pickup while at the supplier) as well as the flatbar for the longitudinal stringers and frame flanges.
This will keep me busy for a couple of months, and allow the shape to develop.
I will then use a large roll of construction paper ,or another method to do rough templates of the hull bottom, sides and likely deck - so I can see how they will nest together and what sizes and how many sheets I need to order (and if they are 1/8", 3/16" or a mix of the two)
Of course, any further advice from Kevin ,or anyone is appreciated.
Darrell

North
Posts: 269
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

Re: help with understanding longitudinal stringer /farming

Postby North » Tue Dec 17, 2013 7:06 pm

Well, after spending some time testing out the new to me (used) Cummins 4BT it seems to run well, so after degreasing it some more, I will take off some parts, clean, prime and repaint later this winter (Actually my father-in-law says he will do it, so even better.) Found a 1:2 Velvet drive at a great price as well.
Now, after being sidetracked a bit, I finally cut out my 1st three frames (in 2 pieces each) last weekend and will cut out the remaining frames this weekend.
My new worm drive saw works great, except wehn I hit staples or something else hard, which was under the sheet. I had lifted it a bit and placed a couple of wood blocks under, so my blade wouldn't hit any nails on the packaging pallet it came on, but still hit something. Ruined my first Freud Diablo 60 tooth fine/thin blade, which had worked well. Had a couple of 40 tooth Freuds as well, that i had picked up on sale. Put one on and it seemed to work as well.
Jig saw also worked well, and used wd40 to keep the blade lubricated and cool.
Once the two half frames were cut out, I would hand file (bodyfill file) it to a nice edge, then use double sided tape to attach it back to back with the other half - and use router table and a flush trim blade to make them identical. I was happy with it, but am olnly using a relatively cheap wood bit. As I have more to do, I broke down and orded a couple of spiral flush cut bits on-line, one from Amana, a 51520 - AMANA TOOL ALUMINUM FLUSH TRIM 1/4 D DOWN and and one from Whiteside, a RFTD5125 Whiteside 1/2" SPIRAL FLUSH TRIM W 1-1/4" CEL . Also got a cheaper, standard straight cut wood flush cut as it was much cheaper, as a backup 2715 Whiteside Flush trim/template combo 7/8"cd x 1/2" shk x 1-1/2" cel.

After I cut out the rest of the frame halves, and router them to be indentical, I will start welding them together, and then welding on an overlapping gusset, where they join, per the plans. Plans also show 2" x 3/16" flatbar welded as a 90 degree flange on top of each frame's bottom frame section. It doesn't show this on the side frame portion, or if it does, i have overlooked it.
If anyone could tell me if it is common to have a flange only on the top of the bottom frame, and not on the sides, I would feel better! As teh gusset will be on by then I assume I will just lay the flat bar down, tacking on place along bottom frame, and gusset as well, onto the other side, as the gusset will be higher than the top edge of the frame, where they meet in the middle???? Or, I could keep the flange on the side opposite the gusset, and then the flange would lay down righ ton the bottom frame, as it would not touch the gusset. Thoughts or advice????

I would also like to hear any tricks to keeping the frames flat or true, when welding them together, or when welding the flatbar on to form the top flange. Not siue how much it will want to warp with the heat, but I woudl think alot....

Then I will get some scrap steel to build the building form/ jig. I am thinking of using two 20 ft pieces of something (pipe, angle, etc) along with angle for bracing, and to keep it a couple of feet off the floor. I would like to use steel, rather than wood, so I can build some attachment point at each end, that will allow me to adjust height of a pick up point on each end. The idea is that I could use the backhoe or a floor jack to lift the weight at each end, off the floor, onto this point, find close to the center of gravity, and therefore be able to flip or rotate the boat much like a chicken on a rotisserie. I will try to do as much welding flat as possible, especially as I will be using spray transfer - I have only praticed flat, but have read that is does not work well out of position ie vertical or upside down.
Any pratical tips would be appreciated!


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