Join two lofts for ship hull

Hello Guys

I’m looking some advice on a trawler hull that I’m working on here in Rhino. I only started using Rhino a week ago and the progress has been quite good so far. At the moment, I’m working on a design of a pelagic trawler that’s going to be 3d printed at 1/72nd scale as an RC model.

This evening I have completed the main loft for the upper portion of the hull (the blue section) and then lofted the lower keel bulb/bilge around the propeller shaft.

My plan is to complete the frame work for the lower section and use the curve graph to ensure that the frames are smooth, then loft it. As you can see in the screenshot I have extended the lower section up into the main part of the hull, the idea in my head was to join both together and then trim of the excess from the lower section, and then trim away the blue portion so that this leaves a cavity in the lower section of the hull.

I hope this makes sense guys, and is this possible? I thought it be best to work in two lofts rather than one, so far it’s been quite straight forward.

Any ideas or pointers would be much appreciated, but please bear in mind that I’m completely new to Rhino.

Thanks for looking


When joining two surfaces where a smooth joint is important you usually want at least one edge and preferably both edges to be untrimmed edges. You can use MatchSrf to match an two untrimmed edges, or an untrimmed edge to a trimmed edge, but not two trimmed edges. Untrimmed edges also simplify adjusting the edges and adjacent surface by moving control points.

Is the number of surface control points the minimum needed to model the shape with the desired accuracy?

That’s not the way it works in Rhino. You have the order of operations reversed. You first trim and then join. The idea is the trimming operation will create 2 edges, one edge on each surface, that are within tolerance of each other so that the join operation will work to make a single polysurface.
For the trim to work on both surfaces there needs to be a complete intersection curve that cuts all the way across both surfaces. You can check the intersection using the Intersect command.

Do you want the red surface to blend smoothly with the blue surface? Or is there supposed to be a sharp corner between the red and the blue? Looking at your picture it appears that if you trim and join these 2 surfaces you will get a sharp corner where they meet.

@m5currie When you talk about “trim and join” are you talking about physical building the model in two parts and then joining, or creating the Rhino model?

Hello Guys

Thank you for the replies earlier.

Ok, @davidcockey I was initially thinking about building the hull creating the two parts using one set of curves with as close as possible the minimum number of control points, but it seemed after a few curves there was going to be a lot of fairing required.

So I then decided that perhaps i should loft the upper hull first, get that correct then work on the lower portion and join them together. At the aft end the join should be fairly sharp from what I know and can see on the lines plan certainly at the back end. the forward end where the transition from the forward part of the bilge should be fairly smooth.

I’ve attached the file here with my curves. The bottom section still needs to be worked on, curvature graph to be applied to check fairness etc.

A few days ago I exported the upper section out of rhino into fusion 360 and it looks great, just need to construct the lower section now and see where i can go with it.

Thanks for looking guys


Finnur Fridi Aft Loft.3dm (3.0 MB)


I’ve just read your message again, I mean to design in Rhino.



Maybe good to know, there is no need to create perfect seams and join them, it may even be better to leave a slight overlap. I am finding that, as long as you keep a totally closed outside surface contour, the .stl export generator will make a proper printable object of it. You will have to tick the box “Export open objects”, and you may see a message that your file is not “suitable for rapid prototyping”, but when I do a preview in Cura and analyse the printing layers all is well and proper. I do not own a 3D printer, but do have experience in having stuff printed by a specialized company.
If you want to be certain before exporting, do a Boolean and select it to check if all parts are included.
The only caveat is that separate objects which have parallel surfaces only touching each other on the joint may not be welded together properly (as I unfortunately experienced in a complex print). Again, a slight overlap will avoided that.
(PS: Undo the boolean before you close the Rhino file, so much easier to make modifications later)

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That’s nonsense. If he wants to be a competent modeler he will learn to make good joins and good seams. If you don’t learn those basics you will spend your time with Rhino forever just muddling instead of modeling.

It would be helpful if you posted the surfaces you created. A picture of what you are trying to model would also help people make suggestions.

I assume you used loose loft to create the hull surface because a normal loft produces a very messy surface that will be very difficult to use for subsequent operations like trimming and joining.

Enclosed is a model showing how I would approach this. I extended some of your curves so that the loft will fully intersect the hull and then after cutting out a square hole in the hull used BlendSrf to connect the end that you want to smoothly join with the hull.

Ooops I forgot to post the model
ShipHull3.3dm (155.7 KB)