Simple 22ft hull

[Nojjan]

Hi all

I thought I would offer up a 22ft hull for anyone who wants it (see attached picture, Rhino and ACAD files). It is simple and rather straight, 16deg deadrise, outboard hull with developable surfaces (plywood or alu). It still needs some chine flats, maybe 75mm [3in] but otherwise it should be okay up to 30 knots...I think. Could be built under the same instructions as the Tolman jumbo.

I would appreciate if someone with experience of this size and type of design would comment on it. Every type of constructive criticism is welcome.

Regards / N

[Houndie]

Hello

I am also working on a 22ft. hull. It is a setnet skiff so its beam is greater and deadrise down around 8 degrees. I looked at you surfaces and they look nice. I have been struggling with lofting the bottom plate. Like your skiff my keel and chine lines are also straight which I think is making the loft difficult. I am curious how you lofted your surfaces. Were you able to loft using the "develable function when lofting? I looked at the curvature on the stations, buttocks, and waterlines they look nice. I have not been able to accomplish such a nice loft.

Thanks Jeff

[Nojjan]

Hi Houndie

It is not my skill but rather that Rhino (that I used for this hull) is really good at making developable surfaces. I made an effort to use low degree curves to create the surfaces thereby it is easier to keep them fair. Also the surfaces are of degree 1x4 (using 2x5 control points) which is key to make them easily developable. If the surfaces are made by degree 2x4 it is more difficult to succeed. The most usable command in Rhino for these type of surfaces is "_EdgeSrf".

I hope these tips help you. BR / N

[Houndie]

Hey Nojjan

Thanks for the data. I am going to give it a try and hope for the best. Have you been using the Gaussian Surface Curvature Graph as a reference when your fairing. Or perhaps a dynamic stations, buttocks, waterline like RhinoMarine. One of the problems I am having is when I start fairing a surface it will no longer unroll.

Thanks, Jeff

[rambat]

Houndie check your version of Rhino, they had a bug un-rolling surfaces until release 3, SR3c. Also, you cannot fair control points on the surface membrane stretched between two curves without making it non-developable. Not sure if that is what you intended. The best way to process a developable plate hull is to fair the sheer, and chine (or chine to keel), loft a surface between the two and leave it alone. I have seen many designers try to get flair into the bow of a developable hull since they appear to bow out. Its just due to the bow shape being a portion of a cone, which is single curvature and the only surface it will accept to create flat patterns. I think Rhino can "average edges" if the surface is not truly single curvature, be careful as those won't meet at the edges when templated full scale.

[Houndie]

Hello Rambat

I have attached my chine and keel. I have spent hours trying to get a good loft between these curves. I have rebuilt the curves numerous times with no success. I have tried the Devsrf plug in etc. with no success. Any pointers would be appreciated. My ultimate goal would be to get a surface using the develable option in Rhino. I am not an experienced designer and think that is the problem. I just don't see the cone.

Thanks Jeff

[rambat]

Jeff, I did a quick loft and un-rolled your bottom plate, see if your version of Rhino reads the attached file. Saying its a cone is a little simplistic, see the graphic where I projected the side shell past the chine and sheer of my deeper vee hull. The developable shell plate is of a varying conic shape, like a sheet of tin and it rolls "conically" increasingly tighter toward the bow. When I did this edge extension to yours it twisted which is ok as long as those lay lines don't twist or intersect within the section of the plate you are trying to un-roll.

[Houndie]

Hey Rambat

I think I see what your are saying. I have been able to unroll similar surfaces. They have , however, been very dirty. I ended up trying the process that Nojjan recommend and I got really nice looking lines and curves. When I put a graph on the stations, buttocks, waterlines they are the best I have seen on my computer. The only curve I am a little apprehensive about is the forefoot in profile, it is a little boxy. I think with a little time I could make it look better. I am also concerned because the surface unrolls .03% bigger. If you have time it would be great if you could check the surface that I created. It that would be much appreciated. Nojjan I would feel the same if you took a look at it too. The technique seems to work well, am I missing something.

Thank you very much, Jeff

[Nojjan]

Hi again

I think the "Skiff 1" is good step in the right direction. The boxyness of the forefoot is just a matter of time and effort to fix.

I sometimes make use of help lines/curves to stay in control of for instance the taper of the chine. That way I can maintain nice curvature=fairness but still warp the hull to some extent.

The increase in size when you develop a surface should not be a problem but I am no expert on this. The deviation should be absorbed by most manufacturing process tolerance, I would guess even if you used a CNC machine.

[rambat]

I agree, practice adjusting the control lines or edges is the single best way to gain lofting/lines experience. The bow waterlines could be slimmed by moving the forefoot aft or a more gradual turn up from the keel. The expanding plate question is a good one, I have done a lot of large aluminum hulls and do not provide shell plate geometry. I have found its often a little shy of a perfect or weld-able fit. I did a smaller skiff that was built only by welding the shell plate first, the frames "fit" and installed next. I would like to hear others opinions about how much luck they have had with building from a computer shell expansion. Also, what program was used if successful.

I less suspect the accuracy of the computer expanded plates as much as the nature of hand building hulls without stiff jigs. Such assembly precludes the attainment of the tolerance of the NC cut parts during field assembly.
Generating unfolding "plate" for GRP and cored boat designs is less risky since any deviation will be accommodated with the filler used for the joint bonding laminate.