Compound curve advice

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johnmcshea
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Joined: Fri Jun 12, 2020 11:44 pm
Location: Devon UK.
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Compound curve advice

#1

Post by johnmcshea »

Hello,

I'm new to this forum, I'm also new to aluminium boat construction despite being a professional boatbuilder for over 20 years my training and field has always been within wooden boats, so as far as your community goes I'm both a newbie..... and not.

I'm hoping to get a little advice if possible with relation to a hull I have been developing. Ordinarily In my world this type of hull would be cold moulded/plywood construction to produce a monocoque form. The process of laminating the many planks would allow compound curves to be formed and at times the use of pulling flat plywood sheets into place would also allow compound curves. If you take the smaller Riva/Chriscraft type boats they have both bottoms and decks made from flat sheets of plywood that are compound curved forms that have been pulled into place (under quite some stress) by fastenings and glue whilst having cold moulded topsides which are made with very little stress though time consuming.

I'm exploring the option of constructing a hull similar to the one described above but using aluminium instead of timber. I am of the belief (correct me if I'm wrong) that aluminium boats of this size are generally constructed in flat planes rather than compound curves for the obvious reason of simplicity. The compound curves however are important to my design and I would like to find the best way of forming them correctly.

To illustrate the requirements for a bow section an approximation would be a 5m radius curve (waterline) with a flair curve of 2m radius (body mould section) currently planning to be working with 4mm 5083. If any one has any advice from in house solutions to specialist forming options I would be grateful for your insight.

If you are interested to find out about my background:

www.johnmcshea.co.uk
www.devoncraftsman.com
www.instagram/johnjmcshea

Thanks in advance for any replies,

John.

kmorin
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Re: Compound curve advice

#2

Post by kmorin »

Notice: long reply.
johnmcshea wrote:
Sat Jun 13, 2020 9:06 am
I'm hoping to get a little advice if possible with relation to a hull I have been developing
John, when I first read this line I thought you were being tongue in cheek - generally plate aluminum hull forms are "developed surfaces" therefore confined to flat planes, cylinders and cones; all geometric shapes that can be formed (cold) by hand cut from flat sheets- hence "developed'.

However, after reading the post I think the first item of reply is to "welcome you to the Forum," and offer thanks for some boat building discussion material! Not everyone here is concerned with building welded metal boats, but most everyone is interested in the finished product ; so if you're going to do a Riva Aquarama Special in welded aluminum? I think that will contend with the most popular thread on the site!! (It will for me!)

So let's go back a bit and review. (keel up) The bottoms of most of the 'mahogany' run-a-bouts were not compound in their bottom (chine to keel to chine) they were flat sheet's pulled up and were therefore; flat in section view, or at best convex in section. Therefore, until Carlos Riva came up with his huge plywood forms (steel sections that held the bottom halves until the glue and pressing dried ) the lines of these hulls didn't, except rarely (Ditchburn in Canada comes to mind) actually have conCAVE body sections of the bottoms and those were exclusively reserved to the forward 1/3 of the bottom in order to obtain a sharper angle of entry in the bow-forefoot regions. This would require compound forming in metal.

Next, the topsides. Aft of the master station all the runabouts were convex- they had tumble-home and that is simply a rolling cylinder or conic section and can be formed cold. This is confirmed by the 40's and 50's (even later) riveted runabouts that feature this hull element. There are several antique riveted hull threads here that will display the 'cold formed' after topsides' sections - so this area of the "Riva" hull type will not require compound forming either.

All the deck sections on this entire class of "Riva"/Chriscraft/Hacker boats were conic or cylindrical so, again, no compound forming required there for a classic re-work of this group of designs.

Now to the areas were the going will get much less easily done. #1 if you're going to attempt a hollow Body Section forefoot, below the chine: you will need to compound form the sheets and as near as I know (?) that will also require 'planking' the areas with 'hour glass' shaped panels?

The tool used is an English Wheel, where the sheet/plate is thinned in the middle and left original (green) at the edges of the blanks. This process involves pushing the sheet back and forth between the rollers and creating the compound curvature your lines plan requires.

The topsides will require a similar, if less drastic forming exercise. It is my understanding that the combination of the 'hour glass' panels' outlines and the cold rolling will allow the panels to be formed so that: the final used hull plank can be cut from within each of the 'Wheeled' rough pieces. The intersections will be butt welded seams that are vaguely "sectional" to the hull.

There is a builder in Louisiana, USA that does this type of topsides forming (I've not seen them take time to build a hollow section forefoot?) The family name is Gravois (sp?) and they have shown true flared topsides which are compound curves you've referenced: as opposed to flammed (conic developed surfaces from flat stock) topsides as is 99% the case with boats we'd expect to see here.

One last note; I did see, in the Seattle boat harbor, years ago, a 58' seiner that has been planked from 'chine' to sheer at nearly 45deg to the waterline? These planks weren't all that wide and were 'twisted' into a compound curve of the bow. I noted this with interest at the time but didn't bother explore it further. So another approach to a curved compound shape would be to explore 'planking' the surfaces so strips of hull plate were able to form like you were planking a compound 45 deg ply planked hull? Never tried personally.

I'm not sure I'm much help? I've not been able to find an owner willing to fund my construction of a true 'flared' hull. The Riva shape is completely exotic as it is as much sculptural as it is dedicated to being a "bikini freighter" it has, unfortunately, no purpose in the serious offshore fishing pastime of the vast majority of our readers on this Forum.

We'd all probably agree, these cellulose and goop, shiny skiffs are some pretty hulls but.... taking time to have those design elements in a serious offshore saltwater fishing hull isn't the highest priority in the welded plate aluminum hull market.

I'm more than willing, I'm enthused, to post in regard your ideas, shapes and metal boat building learning experiences. I've built in welded aluminum since the 1970's and hope you will post the lines of your current draft(s) and any further questions you think I might help explore?

Again, welcome to the AAB.com Forum, and thanks for posting. Hope to have helped you and our readers to consider some of the design and building challenges you face?

cheers,
Kevin Morin
Kenai, AK
kmorin

johnmcshea
Posts: 2
Joined: Fri Jun 12, 2020 11:44 pm
Location: Devon UK.
Contact:

Re: Compound curve advice

#3

Post by johnmcshea »

Thank you very much Kevin for taking the time and thought to reply it's really appreciated. I hope I can contribute to the forum in return from my own particular pool of knowledge being an outsider.

If I may ask a few really quite remedial questions....

If you take a relatively simple barrel section or conical section. Would you, as a fabricator use the frame sections to pull the hull plank into shape or would it be necessary to pre form the shape and then attach without stresses? (I can sense bewildered shaking of heads) please bear in mind that as a wooden boatbuilder springing into place is commonplace and generally comes before carving to fit, just to justify my perspective.

What would be a recognised process for this more simple method of forming?

The intermittent welding techniques used in framing, I understand that this is to reduce the risk of warping the structure; my question is that is just to reduce heat stress during the fabrication process or is it important to have those intermittent welds for expansion during the life of the hull?

Expansion/contraction due to ambient temperature once hull is complete, is this negligible? Does it cause any particular problems, particularly if other materials are to be used alongside, for example timber?

Lastly am I right in thinking that Hull builders use mig over tig for the reason that for the required penetration Mig allows quicker movement and therefore less concentrated heating? Or is it simply a question of speed efficiency for commercial/time purposes?

Grateful for any replies, they're not wasted!


J.

kmorin
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Re: Compound curve advice

#4

Post by kmorin »

John Mac, you're welcome to any time or remarks here, the purpose of the site is to focus on welded aluminum boats so that's what we do here. Glad you've dropped by to post.
johnmcshea wrote:
Sat Jun 13, 2020 6:37 pm
If I may ask a few really quite remedial questions....
The only questions not all that welcomed are those where someone is working on their wooden or plasti-boats and want to know what we think about some or another step? So, we'll try to reply to any question in our experience - or try to point you to some source of info?

Your first question brings up three separate groups of metal boat builders' practices. #1 those who use frames (ring frames/x-verse frames/Body Section frames) to create the form of the hull and then plate/plank/sheath that form.

#2 those don't use frames but form the boat from either a "builders' plate model" and use temporary section former's (not continuous from sheer to sheet but just in each panel) and tack up their entire hull - then, post tack-up, fitting bulkheads and whatever framing is left- along with decks, bulwarks, sheer clamps (guard decks in the Pacific NW).

#3 those who do as you mentioned- roll, or 'form' (I'm not including compound forms here) the plate/sheet and either attach to #1 or incorporate in #2.

As a way of generalizing, if the hull surface panel will be made of 1/8", (0.125"), 3mm up to about 3/16", (0.187") 4 to 5mm material then cold forming deep curves is much more manageable by pulling with nylon strap winches, long pipe clamps and the like. So, either of the first two methods works fine to build in these materials.

Above this beginning at 1/4", (0.25") 6mm and heavier material will not be easily hand tool pulled into very steep curves, cones, or cylinders; therefore forming tools like sheet rolls or press braking is much more common in these heavier scantlings. But I will note; these materials aren't as common in hulls less than 24' LOA.

Say you were planning a 20'/6-ish meter LOA with a 4mm bottom- you could frame and pull the sheets to the frame and longs using a variety of methods. (#1 above) Also you could just cut the outline and pull the chines together and tack the keel/forefoot/stem sections edge to edge as the come to touch. This would be #2 as a building method, where there's little need to even consider pre-forming a V bottom UNLESS you plan a hollow body section, as mentioned previously?

Last, to remark about is that some builders loft and cut, tack and form up the frames- drape sheets over- pulling the sheets to the frames and longs and tacking. Then they batton off the curves of intersection of the plates- at chines and sheer, for example, and saw these curves in preparation to adding the next hull panel.

not sure if this exactly addresses your question but hope I've included some useful remarks about common practice here?

Regarding welding of welded aluminum boats. TIG is most often used for a single pass finished weld where appearance is important or there are more than one or two pieces of metal being welded such as a topsides, transom, engine mounts and similar intersections. TIG is usually not used for hull seams, however some builders to use it there as well.

MIG welding's deposition rate in aluminum is about the highest welding speed in handheld or manual welding. Therefore MIG will provide the least NET heat imparted to the material. As the weld progresses, any method or welding, the parent metal expands- so the longer the weld takes to put on the hull the more the parent metal will expand.

"For each and every expansion; there shall be a contraction of equal or greater movement" This paraphrase of the underlying physics of welding is particularly true of sheet aluminum. There are two areas of sheet aluminum parts in welding terminology: one, the edges; two, the "field".

These two areas of any marine aluminum hull components form the basic break down of welds on the hull. The long seams, hull panels' intersections at the keel, chines, sheer and any other long-wise panel EDGE behave in one manner and the cross seams and some of the field welds of longitudinals are the second topic of discussion.

This is somewhat involved and there are many methods but suffice to say stitching longs to the hull panels- or even xverse ring frames to the hull (not discussing sealing bulkheads) is usually to provide full strength while minimizing hull panel distortion and MIG is very often used to add these welds in the least amount of time possible.

Once welded, correctly, the entire hull is one piece of metal so it expands and contracts as one piece- the effect of weld type or weld layout is more involved with heat distortion minimization than temperature changes of the hull after construction.

Yes, there are instances of combined materials' differing rates of expansion and contraction where they can become separated over time. Worth discussing but without details sections to examine the proposed installation there are few Rules of Thumb to apply.

Mig's speed, along with the relatively lesser skill level required, is the reason most welded aluminum hulls are welded with this method- or at least mostly welded using MIG.

Cheers,
Kevin Morin
Kenai, AK
kmorin

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
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Re: Compound curve advice

#5

Post by kmorin »

John, sent a site PM to you this Sun. afternoon.

Cheers,
Kevin Morin
Kenai, AK
kmorin

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