Building of Aluminium Boat . Mounting of hull framing structure

I am inclined to think the surface is developable tough. The gaussian analysis here seems to indicate so. As well it does unroll also in paneling tools who is more strict on dev surfaces. My FWD hull bottom lines are not straight but slight curvature.

 

We mainly built jet boats in the 6-7 meter range (20 - 23 feet) using 6.35mm (1/4 inch) bottoms, with stringers set at 10 - 12 inch centers.
The inside stringers would be about 6 inches x 2 inch angle, prox, then the longer leg getting progressively shorter as we moved to the chine.
When we skip welded these, say weld 6 inches, skip 8 - 10 inches, we found that the stringer would camber down due to the shrinkage at the weld.


We would then have to take a temp stick and above say every second weld, apply heat to the short flange and part way down the web, let it heat up, which actually increased the longitudinal camber but when it cooled it would go back to a straight stringer. This can be a little touchy and you have to ensure that you do not apply too much heat. Looking at the depth of the web on your inside stringers, you may have quite bit of deformation caused by welding and I would give some thought to a skip welding process to reduce this effect. BUT if you skip weld, you need to
make the welds opposite each other, ie one inside then one outside the lower web at the same location

 

It isn't developable to straight frames. Not by a long margin. And the plating sitting well proud of your frames announces that eloquently. It looks like a good seaworthy design, and would ride a lot better with the straight frame shape, but that is plate alloy, you can't get it to do what it doesn't want to do, easily. Are you thinking that gap you can put your hand through, is just a fault in the way the plating was assembled ? There could be faults in that as well, but fundamentally, straight frames won't work with developed shapes conforming to your basic chine and centreline/forefoot. And the topsides will have similar issues.

 

Your illustration is of mean curvature, not Gaussian curvature.

A drawback to using Gaussian curvature to determine whether surface is close to "developable" is that it is dimensional with dimension of 1/length^2. Change the length units used in the calculation from meters to millimeters and the Gaussian curvature changes by a factor of 1,000,000 though the geometry does not change. Gaussian curvature is the product of the two principal curvatures. An exact developable surface has one principal curvature equal to zero which means the Gaussian curvature is zero. But if one principal curvature is small but not zero then the value of Gaussian curvature also depends on the value of the other principal curvature.

 

The marks left by grinding highlight the distinct convexity of the forefoot area of this alloy boat...….

 

You will just have to deepen the stringers, it seems. That should not be too difficult, but the hull shape won't be as drawn, which isn't the end of the world.

 

So if this is the case, then the stringer must hang below the transverse frame. The pictures do not illustrate this very well. So how far are the stringers away from the hull sheet.
We almost always had to push the bottom of the hull up to the stringers, not much, but then we were using 1/4 inch. A 2 x 4 piece of lumber on another fulcrum, under the sheet, and just give it a gentle push up. I would not be surprised if you can easily make contact. I would give this a try, small tack welds, start at the back of the inside stringers, move back to front.
Then to the other opposing stringer, tack stern to bow, then move outward to the next stringer and so on.

Probably take 45 minutes with two people, then take a long straight edge and get underneath to check for straightness.

Use very light tacks and if the hull is straight, go back over the tacks to make sure they will hold and stitch weld the stringer in place. Perhaps before you decide to move forward with the final
heavy duty tack welds, quickly check that the sides of the hull still match up.

 

Hi there, thank you I am reading all the above very carefully and thinking it through. I did again check now the gaussian curvature and it appears like the attached. I would be inclined to beleive it is a developable surface. The hull bottom lines are not strainght but slightly curved. But I will do like above D Cockey wrote and double check by changing the units and so forth. Thank you Barry I am reading carefully and try to learn from your kind advice. what was now happening was the builder did the hull bottom without having had made an accurately dimensioned supporting steel setup for the hull bottom as can be seen in the photo attached. Therefore he was attaching by welding points the two bottom plates one to the other and to the transom and the chines were attached by welding to the bottom plates. All this was done without welding the keel at the center. I beleive here is the error that has generated the warping of the entire bottom and did increase the curvature of the bottom plates at stations 4 , 5. . I think the correct procedure is to have the bottom fastened to an accurate setup that is cut on the outer bottom lines conours at each station; then attach the two bottom plates at ceter and following attacyh the keel. Only after that attach the chine strakes by welding points. After that check very carefully by measurents in the horizonatal transversal plan and in the vertical plan the dimensioning to correspond to the set of plans.

 

I had a look around for the lines drawing of an alloy boat like yours, could not find one, but they are there, and clearly illustrate that the station at the turn of the forefoot typically has a lot of convexity, it is very hard to avoid having that, and your lines are without it, you have about a 30 degree straight vee at that station, whereas typically this kind of boat would be around 17 degrees starting at the centreline, at that station, increasing gradually all the way to the chine, in an arc.

 

Hi , Efficiency ; yes I do understand and did take note ; the hull bottom lines at the forward section are typically more curved. I am trying to integrate al the comments and advice from above in order to determine how to now proceed to do best. first thing is to take accurate measurements and check against the drawings. Further to that, One way may be to keep the bottom framing( that includes keel) setup as is and fixed bottom up ; take apart the bottom plates and try mount the hull bottom plates on the bottom framing in the best way they will fit ; see what the differences are and where they are and identify the correct way to make fit.

 

I'd say you should check the dimensions in the vertical and horizontal of the chine and centreline at the various stations to see whether they are as per plans. No use measuring anywhere on the plating, it won't agree with the plans. I'm surprised anyone could draw up a boat that well, and not realise the lines are not going to work in alloy, or is it a design that has been used previously for other materials ?

 

Hi Mr Efficiency, the hull bottom is developable as far as Rhino can tell, I just did ckeck the topsides and there no single shadow of un developabeleness; we will make it work no worry I think there are many ways

 

Just zero chance that is developable, as per the profile you posted early on, I'd say, but that does not mean it is a lost cause, but I'd be checking that chine line as already built, for consistency with your plans dimensions, otherwise there may be more troubles.

 

Thanks Gonzo, yes I think those are the two solutions.

 

While we do not build this way, had we been given the complete cut kit, I would have built the internal framing and stiffened it for the build. Turned it upside down to form a male plug, then
placed the plate onto the aluminum frame. When the hull bottom was loosely tacked, I would have then attached the sides, tacked it all up. If the fitment was good, and you would know that the plate profiles are attached to the framing, ie the bottom is flat, then proceed with a stronger tacking process, then weld it up.