Hull shape I cannot get right. Help?

Hull only for cncmodel1.3dm (1.2 MB)

8 years ago i spent a lot of time trying to get the hull correct on the cad model and gave up.
The shape at the waterline where the rudder post enters the boat would not behave or join
So the boat was built with frame templates from the cad surface and all frames aft of the heel of the keel were made slightly oversize and then faired by hand.
The result was as good as i had hoped and the cad files lay dormant for a long time
(see Kerenza cornish lugger of facebook for the actual build)
Now I have a CNC router and hope to find help to correct the file so I can make a half model.
Can anyone please help, advise, or sort it?


How much different is the shape “as built” from the existing CAD model?

The CAD model shows some reverse curvature in the hull side above the small “transom” while the photo suggests the hull side above the small transom does not have any reverse curvature. Instead it looks like the is a slight kink the hull side ahead of the transom along the plank seam. That would make a significant difference in modeling the hull aft of the transom/rudder post.

Thank you for your interest.
The difference is quite significant.
The small transom is much smaller on the actual boat. For scale the oak post is 6" wide so the picture shows the small transom only about 18" wide where the counter surface meets it.
The drawing has it around 38"
I dont need a high degree of accuracy for a half model but as is it looks dreadful.
I have used Rhino for 20 years with a great deal of success. . The entire boat is built from Rhino files and with zero paper lofting, but ive no training in cad. . Ive just blundered about and learnt most of what i need. .
I simply cannot get the hang of pushing surfaces around in any local detail with any constraints. The results when I have tried have been amusing but mad. lol. . Apart from this one shape, very little need for it so far.)
The light blue line is technically a boot line so its lower edge is the waterline which is zero in the drawing


A nice hull, very pretty. Congratulations.

My thought is that the hull sides should be split below the 10th strake into two surfaces.

Without such a surface break you are going to have a corner that cannot be represented in a surface.

I have no idea how to do this!
I have searched and searched several times over several years and just spent several days trawling through tutorials with zero success.
I cannot make the small “transom” at the waterline smaller without totally messing up the hull shape .

I too have found that information on how to do real hull forms is lacking. Every example I have found on how to do them on Rhino has been overly simplified to avoid problems like you describe that occur in nearly any real hull form.

I may be doing them wrong but I have to someone show me a “right” way that works.

Going by my trial by fire, I would split the hull as in this picture and do three surfaces:

  1. The area above the line.
  2. The triangle at the stern below the line
  3. The hull below the line.

As you say, there is no @#$@ way to get a single surface that will go around that corner without screwing everything up.

You are going to have to create multiple tangent surfaces for this to work.

have you tried subd for this type of modeling problem? The new Tools in v7 may be just the ticket…Hull only for cncmodel1_kfix.3dm (1.4 MB)

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Hi Russell, (very nice hull)

Here is a method which gets close…
Create some construction lines, trimming surfaces for reference then untrim the hull side and you can see there is a sizeable gap between the transom edge and the surface.


If that surface is manipulated with the control points to closer match the transom, you should be able to fix without splitting the surface. Dragging the points beyond the Cline will reduce the width of the rudder post face to closer match to the photo.

Then create some construction curves to split the aft hull, using intersects with the trimming surfs and the red extracted isocurve - the original hull used as a guide to locate the isocurve.

Then blend the under side at the upper part of the split and fill the rudder face with an EdgeSurf and extend/trim to suit and split the resulting surfaces with each other.

I’ve uploaded the file in V5 if you wanr to check it out. I’ve imported @theoutside SubD surface in there too for comparison.

Hull only for cncmodel (BM).3dm (1.9 MB)

This is a quick fix but you could spend some time and get the adjustments just right.

You can see here that the original in purple just needed a bit more manipulation

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The subs is interesting in that the control points can still be edited.


This looks so close to what i need
Alas I only have Rhino 5 and cannot open your file.
My entire income has been wiped out this year so I don’t see a route to upgrading atm
I don’t have a clue what subd is?
Ironic that I am so out of my depth with a problem that is mostly above the waterline :slight_smile:
But it has cheered me up no end to know it is possible!
And bolstered the attitude I try to keep a firm grip on which is "The only way to guarantee failure is to not bother trying "

This is very helpful indeed.
I dont understand a lot of it but it is great as guidance to what i need to learn!

You could use the trial version of V7. It gives you 90 days to practice.—Mark


The fundamental difficulty in modeling the boat as built is the hull surface is not smooth. The planks above and below the seam highlighted in yellow are not tangent at the top corner and the tansom, and the kink runs forward for part of the length of the hull. @miano and I have identified this problem in previous posts. This means that the surface cannot be modeled using a single NURBS surface without using multiknots (or stacked control points, not a good idea), a SubD model would need to be configured properly to model the kink. The hull shape as built can be modeled with several NURBS surfaces. @BrianM post shows how to achieve a nice looking single surface, but one with a different shape from Kerenza as built.


Let me have another goes at it and try to put all the pieces together.

You need to divide the hull up into multiple surfaces. The stern suggests that this should be 5 surfaces (not including the transom).

The analysis here is to use the planking as the guide. Think about each outer plank face as a separate surface.

If you start at the sheer strake, you find that next plank down shares a tangent edge between the tow and that the two end edges at the stern form a tangent curve. Therefore they can be merged into one surface for simplicity.

You can see the same for the top eight strakes.

But between the 8th and 9th strakes, they share a common tangent edge but their ends are not tangent. Therefore a new surface starts there Two planking strakes (9 and 10) form that surface (#2).

Moving down to the lower edge of strake 10. You have surfaces that are tangent to it but not to each other. Therefore you have three potential choices:

  1. Make strakes 9-10(2) and the lower strakes a single surface, trim, and make the triangle a separate surface.
  2. Make the triangle (4) and strake 9-10 (2) a single surface, trim the bottom and have the lower strakes form another surface (3).
  3. Make all three separate surfaces.

Trying Option 1, you find that if you try to extend the aft edge such that it will form a continuous curve with the end of strakes 9-10 will give you a weird curve. Maybe you can get it to work but it will be a lot of trouble.

Trying Option 2, you find that this certainly works at the stern. You can just extend your frame curves for strakes 9-10 to the centerline. This might cause problems at the bow but in this area it works.

Trying Option 3, you find this works even if there are problems at the bow.

Continuing down, strake 11 has to start a new surface. Moving to strake 12, it shares a common tangent edge with strake 11 and the ends form a tangent curve. They can be merges as one surface. Continuing to the keel, all the remaining strakes appear to be mergeable in the same way.

This analysis just takes into account the stern. You would need to redo it, taking taking the forwards into account the same way as the aft edges.

This shape, as represented by the stern, should be doable in Rhino without a lot of problems. If you could do it with planks, you can do it in Rhino.

Alternatives when documenting/modeling a boat:

  • As designed

  • As built

  • As currently exists (most applicable for older boats)

  • As someone thinks the boat should have been designed and/or built

  • As can be modeled using a particular methodology

Depending on the use of the model any of these alternatives may be appropriate.

For Kerenza there is a major difference between as designed and as built which Rusell, the original poster and builder has previously mentioned.

I really appreciate everybody’s input on this.
It seems to me this is an exceptionally difficult shape to model and I’m simply not going to get it anything like good enough without a LOT of learning.
I am being evicted from my workshop on 1st of Jan and somewhat desperately need this project carved to help demonstrate what the machine can do to aid my securing a new workshop locally.
Alas the file from BrianM has the smaller transom much better but lost some of its shape amidships and for the intended project the problem is too great.
This is my first test cut (from my original file) to prove viability. . The plywood is too garish and the next will be along the same lines but more subtle. So the contours of the hull overall need to be fair

I have no idea what this work would cost but I am willing to pay someone to sort the hull shape if i can afford it?

Hi Russell,

Here is a more careful attempt, following @miano guidelines.

Play around with ‘MoveUVN’ to adjust the shape using the control points. The red sections shown in the image are lines from your original hull to use as a guide.

The main transom looks different in the photo, so the aft end probably needs some work.

Good luck - don’t want to lose the workshop…

Hull only for cncmodel (BM2).3dm (1.5 MB)