Building of Aluminium Boat . Mounting of hull framing structure

[an2reir]

Hi there, we are developing a new aluminium boat 5.8 meters ; all the framing and hull components were cut by CNC . I did design the boat. The builder opted to build the boat by : A. assembling together the hull framing structure, keel longitudinal stringers and transversal frames, B. Welding together the hull bottom and transom C. Insert the assembled framing into the hull bottom.
The hull was designed in developable surfaces; the forward stations do have a very slight curvature; of about 0.7mm ; the forward transversal frames were designed with straight edges at the bottom . At the first attempt of fitting the bottom framing onto the hull bottom there appears to be a gap between frame and hull bottom plate. Should the forward transversal frames edges better be designed with more rounded outwards edges ? Does aluminium hull bottom plate 5 mm thickness typically bend more substantially in its forward area in the section view ?

 

[Mr Efficiency]

If you want the transverse sections to match the designed shape of the bottom, they must be that same shape, it is typical of the sections forward to be slightly convex, on a plate alloy boat, because it is a developed shape, and with simple, not compound curvature. A straight sectioned hull would have compound curvature. I'm not sure how you got in this pickle if working from plans.

 

[an2reir]

Hi there thank you for the feedback, I am the one who did design the boat and I did design the hull with developable surfaces. There is a slight curvature in the forward sections at stations St 2, St 3, St , St 5 that is 0. 6- 0.7 mm from a straight line at bottom of those forward stations . The distance in the mounting of the framing into the hull however appears to be substantially more.

 

[Mr Efficiency]

Do you have the other lines drawings to show ? That just looks like straight sections to me, which for my money makes for a better boat than the developed shape, but when you are using plate alloy, can't be done without divine intervention, or super expensive plant.

 

[an2reir]

Yes, I do understand, there is curvature but it is 0.7mm. I did a developable surface in Rhino and unrolled it. Non developable surface wont unroll. I will check if the Rhino software did read correctly the surface as developable . I do understand what you say is there is a possibility it should be more of a curvature. I thing however the distance in photos is coming from deviations that added up from the building of the bottom framing and from the the longitudinal stringers not yet having been beveled on the lower edge to fit correctly on the bottom

 

[Barry]

A most unusual welding and assembly process for such a small boat.

In any case, before you work at trying to fix the framing into the completed lower hull, I would take the time to try to fit the side pieces of the hull to the side of the bottom of the hull to
see what other fitment problems manifest themselves.

Do the finished outer width of the chine at various points coincide with your drawing? I would expect that the curvature may have brought the actual width in a bit and may create an
issue fitting the sides.

When we had our hull bottom and sides cut, we bolted them to a steel male jig, fit up the sides and bottom together and did the chine, keel, and transom welds and as the plates
were affixed to the jig, we knew that when we pulled the skin off the frame that the surfaces were developable. With the deformation caused by welding and of course we had to pull the sides and the bottom of the boat to the jig, ie build in some stress before welding, the shell would deform and before we even began to weld the interior longitudinal shell welds, we had to
spread the bow at about 1/4 way back to push the bottom plates outward to true up the hull. We accomplished this by installing a 1 inch ready rod which was inserted into a 1 inch pipe, and attaching this to the hull bottom from the inside and then using nuts expanded the pipe, ready rod length until the gunnels became flat.

Then we would proceed with welding the interior welds, ie hull shell welds.

You might try spreading the hull at a few locations and see if the curve disappears, but with the reversed chine already welded in place, I doubt that this will pull the curvature out of the lower plating.

Alternatively, or in combination, you may try to pull the plate up to the frames. I am not optimistic that this will work as you may get some of the rear frames to match but may create
more distortion near the bow. But an hours worth of time may be well spent.



As stated above, I would be worried now that your pre cut hull side plates will not match.

 

[an2reir]

Hi Barry


Thank you for your feedback , indeed very useful and very kind. We did check similar to your above suggestions yes the topsides do fit very well on the chines. The pre cut side plates do match perfectly. They were generated by the same Rhino procedure . It does however appear that my Rhino generated developable surface of boat bottom plates is curved only about 0.7 mm at station 4 , I will check in a number of ways if the developable surface might have been better. I am reading carefully all your message and think though all you wrote. I think there is some deformation from the hull bottom plates welding but mostly from the bottom framing assembly and welding

 

[DCockey]

Which Rhino command/proceedure did you use to create the surfaces?

Rhino will unroll surfaces which are "close enough" but not exactly developable.

A simple and "exact" way to check if a surface is developable in Rhino is to examine it with the Curvature command. At any point on a developable surface one of the displayed curvature "circles" should be a straight line. The reported minimum curvature should be zero if the surface is exactly developable.

 

[gonzo]

The problem is that the boat you designed only has developable surfaces on the aft sections. You have two options to fix the mistake. The first, and easier, is to add a section to the frames. The second, if you want the hull to have the same shape as what you designed, is to cut the bottom plate into triangular shapes and set it to the frames.

 

[DCockey]

Gonzo - how are you determining that the boat only has developable surfaces on the aft sections?

 

[an2reir]

Hi there and thank you for your comments
I did the hull in developable surfaces with the DevSrf plugin. I did check the hull surfaces with the gaussian curvature and it appeared to be acceptable. But will check again according to the above the above from D Cockey. But I think the problem is indicated by the above procedure described by Barry. I think my builder did not build a setup accurate enough to make sure the building of the hull bottom is precisely as per design drawings; namely the sheer strake curvature to be as per design drawing. If the bottom is built without welding on the keel and only sustained by the sheer strakes then the entire hull bottom will bend logitudinally and thus the curvature of the hull bottom will increase. I think the shell should only be built in a steel gig that is accurately done so that the sheer strakes are correctly curved as the drawing

 

[Mr Efficiency]

It is immediately obvious from the drawing in the third post on the thread, that this is not developable shape, and the entire bottom may be that way, looking at the drawing above. You can adjust the frames to suit the shape it has taken up "naturally", but adjusting the plating to fit the frames, is not realistic. Sorry you have got this problem, your hull shape seems OK, but you would need stretch forming to get it done in alloy.

 

[an2reir]

We did this scale model wich came out pretty good. I do uderstand Rhino will unroll surfaces that are not exactly developable; but what does that mean? does that mean the unrolled surface will be curved excessivelt when bent ion the bottom frames or or that the surface wont bend properly on the bottom frames as designed ? My bottom frames of the fwd aree slightly curved . In this case Rhino did unroll the bottom surface and the scale model did look quite good . An other consideration is that the transversal frames are not to touch and be welded on their entire lenght on the hull bottom ( will distort the hull bottom wich is 5mm ) , but only the longitudinal stringers are to totally touch and be welded on their entire lenght on the hull bottom.

 

[TANSL]

think the shell should only be built in a steel gig that is accurately done so that the sheer styrakes are correctly curved as the drawing

That is totally correct. The internal structure is what must conform the whole.
Check, removing the shell of the structure, if the measures of the hull are what they should be or if, due to the welding and a poor method of construction, the assembly has been deformed.
If the shell is not correct, it is best to cut back the plates of the hull and weld them little by little, and with the appropriate sequence, on the structure of frames and longitudinals.

 

[Mr Efficiency]

Your model conforms to straight sections ? What kind of material ? If a softer, malleable, thin alloy, that is possible. But your full size boat won't conform to those frames. It appears the stringers too, will not be touching the plating. No good padding out just the transverse frames, the stringers need to be in intimate contact too. You are going to have to pack those gaps, throughout.