Simplified powercat alloy framing?

Discussion in 'Metal Boat Building' started by skypoke, Feb 26, 2005.

  1. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    First let me say that I do have some experience with this as I am now completing construction of an 8 meter alloy powercat from a NZ designer. We had all parts plasma cut and they have fit remarkably well. The boat is reasonably fair and very attractive. Though this was our first ground up build i'm determined to build again, and larger.

    This boat was designed to code and uses conventional framing and plating. Plate thicknesses range from 3 to 6MM, transverse frames are somewhere around 30" apart, three watertight bulkheads. Dealing with the framing was easily more involved than the plating.

    Here's my question...I see that many builders of alloy craft in the US have gotten away from the conventional transverse frame/stringer arrangement and are using just a couple transverse frames or bulkheads with longitudinal framing welded to the hull plating. Although I had initially assumed they necessarily used heavier plating, looking at the specs for Moose boats catamarans I see that they are calling out the same thicknesses I used. (These boats appear to be built with this style construction.) I am sure this would be a far faster and simpler method of construction. I don't care a bit about certification to any standard but do, of course, care greatly that construction techniques and design are legitimately strong.

    Is anyone aware of any design guidelines for this type const.? Any designers working with this? Any recommendations on converting a conventional design to a longitudinally framed style? I'd appreciate any info. Here's a picture of my boat project:
     

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  2. D'ARTOIS
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    D'ARTOIS Senior Member

    That goes for any length below 20 mtrs - above they have to apply to certification authorities otherwise you will get problems with the insurance cy's;

    sofar you talk about changes in alu constructions, they are mostly due to the intro of "Alustar" a plain copy of an ex-USSR military type of alloy that allows less reinforcements - for the remaining, a builder can go as far as he wants as llong as the constructions remains as a whole. The AlMg6 is denser and therfore stronger and less elastic. Specifically for the hull it is a good material, in your case of design the superstructure is less important and can be made of the old AlMg4,5mn
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Skypoke,

    In general, longitudinal framing is more structurally efficient than transverse framing, particularly for larger vessels, because the longitutinal frames contribute to overall "hull girder strength", whereas transverse frames do not.

    On my aluminum open 60 sailboat design Project Amazon, the owner requested transverse framing because the way he was going to make up the frames, it was much easier to loft, fabricate, and set up the frames either in flat plate or angle bar. We did have longitudinal girders in the design, but not frames.

    When I did the first of the steel and aluminum Moloka'i Strait motoryachts, we started with transverse framing, but because of the compound curvature of the hull, we had to add longitudinal stiffeners full length to help hold the plates together as we bent them into place. This lead to switching to longitudinal framing on all subsequent Moloka'i Strait designs. For those hulls, it is easier and quicker to build with longitudinal frames, wider spaced transverse web frames, and even more widely spaced longitudinal girders.

    At the request of the builders, they wanted the transverse web frame spacing in multiple units of 16", which is common household stud framing, so that we could make effective use of standard 4'x8' sheets of plywood and panelling when finishing the interior and thereby reduce waste cuts to a minimum. So all hull, deck, bulkhead, and deckhouse longitudinals are spaced at 16", and the transverse frames are spaced at 32". Girders fall in place of a frame, so in some multiples of 16". Such are the practicalities of shipbuilding!

    One other thing I should mention is that all of our longitudinal frames are simple flatbar for ease of painting. An angle bar or T-frame is more structurally efficient than a flatbar. But on a small size frame (less than 3" high) the back side of the angle or T is nearly impossible to paint reliably. And you need the paint to prevent rust. Steel vessels rust from the inside out, so you have to be particularly careful about this. By eliminating the cross leg of an angle or T, you eliminate the hidden surface, so you don't have to worry about rust collecting in unreachable areas. When the frames get big enough to inspect and paint behind, then you can go back to an angle or a T.

    Our transverse web frames and longitudinal girders are mostly Ts, sometimes Ls, because these are tall enough to get behind with paint.

    Eric
     
  4. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    Mr. Sponberg,

    Thanks for your informative reply. I've been looking a bit more into this, I see some level of controversy surrounds what seems to be referred to as a "frameless" boat.

    I was wondering if it might be possible to build a 32' or so multihull utilizing only 3 bulkheads plus transom, longitudinals to be possibly the tophat style channel. This would, of course, be in aluminum alloy so the painting issues wouldn't apply. Here are some reasons I'd like to get away from the traditional framing, these are based on my experiences in building this one boat, with more experience these might not be issues at all:

    Plasma cutting of the many curved longs from plate was very wasteful. Sure, they were nested relatively closely,considering the varying curvature of the nested pieces on 30' plate, but the heat from cutting such long, skinny pieces caused them to crawl around during the cutting process resulting in considerable irregularity. I don't see much way around this other than waterjet or router cutting, both at considerably more expense. We had to remake some of these as they were too far out of spec.

    The mig welding process does not seem to lend itself well to welding these longs into the slots in transverse frames and bulks. A one inch long mig weld is most likely not going to develop a lot of strength when you consider the cold start and length of run. We're using state of the art welding equipment with advanced start features, run-in, cold start, crater etc but were unable to resolve this completely. Also, the sheer quantity of small welds and attendant surface prep was time consuming.

    We tied up a lot of time in bracing this extremely flexible framing. Even with tacking reinforcements onto faces of frames, bulks etc. there was still more warpage resulting from shrinkage than I would like. Lots of time invested in this.

    The final product with less framing would, I think, provide a cleaner looking bilge and hull sides. Less places for those troublesome items like dropped pennies to hide. Far fewer inside corners to trap whatever.

    In conclusion, I speculate that an alternate framing method, if structurally appropriate, could result in a cleaner, less expensive, easier to build craft even if thicker plating needed to be used. There's also the ease of welding benefit that goes with the thicker plate. Making up and welding a 4mm chine flat to a topsides plate, with an open corner accepting a fillet weld is somewhat challenging.

    The problem I am running into is most NAs are pretty much locked into building to the various classification societies guidelines and approved methods. They really don't seem to leave a whole lot of leeway for alternatives. But here in the relatively unrestricted USA I don't need any of these approvals for an uninspected passenger vessel. Looking at many of the NW Pacific alloy work boats and skiffs, it appears they are doing this very thing with good success.

    Chuck
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    Chuck,

    Yes, there is always a trade-off between weight, material type and cost, and complexity of framing, which also brings in the labor cost. In general, most designers and owners try to achieve the most structure for the least amount of weight, complication, and cost to build. The engineering science has directed us to longitudinal framing, primarily, as the most cost effective and structurally effective way to build boats and ships. This has evolved along with classification society rules which are written accordingly.

    Stepping outside that area, then, is to go towards the extremes--lots and lots of framing with very thin plating (which borders on the ridiculous) to quite thick plating with no framing at all, which in some cases may not be a bad idea. It turns out that in any conventionally framed boat, the weight of the plating is roughly two thirds of the structural weight--therefore approximately two thirds of the material cost. So to reduce weight and cost, so the thinking goes, you put in more frames because with more frames the plating can be thinner, which weighs less and costs less. But as you have found, the complication of construction drives up labor cost.

    So there has to be a point where everything balances. Most builders will likely fall someplace in the middle with plating that is easy enough to weld up with the minimum amount of distortion, at an acceptable weight and cost. That point is in a different location for different boatbuilders.

    There is nothing really wrong with going with a frameless method. You will have to live with fact that the boat will be considerably heavier than may be ideal, and therefore it may require more horsepower to move at the intended speed. You may be trading off engine cost and the cost of fuel for up front construction cost. Those are the considerations, and if you pout some numbers to it and did a thorough study, you could determine if a frameless boat is all worthwhile. If that is acceptable to you, then fine. An engineer should be able to justify the numbers for you. And at 32', you don't have to worry about classification society rules because classification societies don't class vessels that small, generally. And the rules were intended for much bigger vessels anyway.

    I hope that gives some insight.

    Eric
     
  6. CDBarry
    Joined: Nov 2002
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    CDBarry Senior Member

    If you use MAESTRO to optimize your structure, it will make the trade off and optimize for either minimum cost or minimum weight.
     

  7. Thunderhead19
    Joined: Sep 2003
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    Thunderhead19 Senior Member

    Eric makes some good points, I would also like to point out that Moose boats uses extensive jigging to hold everything together and keep things aligned. If I was making a one-off multihull, I'd be inclined to go with some internal framing to keep the whole project reasonably self-jigging. If you don't go the self jigging route, you'll wind up having jigging of some kind, and then throwing it away when you're done. I'm one of those irrational people who hates to see stuff being thrown out.
     
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