Spray Rails in Rhino

[Willallison]

I'd be interested to hear how you guys add spray rails to a hull bottom in Rhino.... or any other curve/surface that must be constrained to lie on another surface, for that matter...
I've used a couple of different methods, but neither is what I'd call elegant...

[Eric W. Sponberg]

I think the easiest way, and how I would do it in ProSurf, would be to create the basic bottom first, and then add the spray rails by creating two new surfaces for the spray rail bottom and the outboard side such that they penetrate (intersect) the basic bottom surface. Then use the basic bottom surface as the cutting entity to trim off the excess of the spray rail surfaces.

By doing it this way, you create completely independent and fair surfaces for all entities in their precise positions. Let the edges fall where they may at the surface-surface intersections.

Eric

[Willallison]

Thanks Eric - that's essentially how I've done it previously. But it makes getting a rail of say, constant width, difficult to achieve - especially on non-monohedron hulls. It may well be the best way, but it's always worth asking others if they have a better one

[Sean Herron]

Hello...

I do not call them spray rails - perhaps you are talking about same - but for lift strakes - here you go - go to FRONT or STATIONS (bow on) - lay in some diagonals - project curve to hull surface - extrude curve planar down - extrude same horizontal - trim to each other and there you be...

See http://www.boatdesign.net/gallery/sh...t//ppuser/3673 ...

You could do same in plan - or do above - then project to Cplane - straighten out the aft runs and then re project to the bottom...

I think I have some progress pics - nope deleted them - sorry...

Hope that made some sense - do play with straight line diagonals and projection to surface...

SH.

[Willallison]

Thanks Sean. The best method I've come up with is similar to the one you've described, though a little more complicated. The way you go about it ensures a fair curve for the rails, but doesn't give the shape that I desire - looking at your body plan view (front) the rails are a straight line, mimicking, as you would expect, the diagonal from which they are derived. I prefer a curve, similar to the shape of your chine.
The way I do this is to draw a planar curve in the front view, extrude, then intersect with the bottom. I then extrude vertically down the required distance to intersect a second extrude curve that provides a taper to the rail. I then extrude horizontally and trim to the bottom.
Unfortunately this is quite a messy way of going about things and doesn't provide curves / surfaces that are constrained to the bottom - so any time you want to alter the bottom shape at some time in the future you have to go thru the whole process again.
I think this is a clear case of where software can rule the design - rather than the designer. Sean's method is clearly simpler than mine, but doesn't yield what I would consider to be the desired shape (let's not get into the argument about best spray rail shape - I think that would open a can of worms as big as the "do multihulls pane" argument! )

Incidentally, Sean, you are right - they are often call lifting strakes, but I think this is a bit of a misnomer. The additional lift created by conventional rails is minimal. Their primary function is to separate the spray from the bottom, reducing wetted surface. They also contribute to dynamic stability

[Sean Herron]

Hello...

I believe the greatest utility in Rhino is the means to fiddle and do whatever you can think of - like choosing the best hammer for the job...

I am mimmicking Don Aronows Cigarette 01 - at least as best as I can...

I dropped that hull a while ago...

I suppose you could fiddle with duplicate curve edge from surface - off the chine and then project to Cplane and leave them there - so they are there to project to a changed bottom...

All kind of answers - get two rails projected to the hull - then build curves at the transom - do a two rail sweep...

If you project one rail - you could build at the transom what you want - then run a one rail sweep and extend surfaces and trim to hull...

Thinking...

SH.

[Willallison]

Yes - I absolutely agree - alas I'm only converse in the art of wielding a sledge hammer, when quite often I'm sure I'd be better off with a ball-pean

Don't get me wrong - I wasn't taking a swipe at your rail shape - as I said there's any number of combinations that have proven to be effective.
Aahhh - Cigarette 01.... I've been searching for a model of that boat for years...an absolute classic

My favourite program - Multisurf (which is way to $$ for me) has the ability to constrain curves / surfaces to one another. I've found it to be very useful...

[Tim B]

Ok, Slightly different hull-shape and method.

Extract an iso-curve where you want your spray-rail.

Split/trim the spray rail to length.

At the aft-end, draw a horizontal line outboard, then up (well past the hull bottom) to give your profile.

Put a point at the forward end of the curve.

Sweep between the forward point and aft section.

Split the surface with the hull to get rid of the excess.

Perfect spray-rails every time. NB. the hull in the attachment is a single surface. The chine-flat is produced using control-point weighting.

Cheers,

Tim B.

[Willallison]

Thanks Tim
Now this I like the sound of! I do, of course, have a couple of questions!

Help describes adding an iso-curve as follows:
edit/control point/insert knot, select surface. This adds an additional "row" to what I would call the "surface mesh".
How do you then "extract" that curve before trimming the ends?
Also, I assume this process relies on having a surface which produces iso-curves (or is derived from them) that are the desired shape. In the following pic the iso-curves are "wavy" - wouldn't this produce equally wavy additional iso-curves and hence spray rails?

[Tim B]

Will,

The command you're after is "ExtractIsocurve" it is on the "Curve->From Objects" menu.

Yes, you do need surfaces with smooth isocurves. Most smooth and fair hulls have smooth isocurves.

Cheers,

Tim B.

[Willallison]

I have to say, this last post by Tim had me somewhat concerned, as the model I posted above is now 32ft of CNC cut frames....
Thankfully, with planking already started, the hull has proved to be remarkably fair, despite the somewhat irregular isocurves apparent in the pic you see above...

[Martijn_vE]

Iso curves are curves on the surface with equal parameter values and are determined by the way the controlpoints are distributed (and the knot vectors of course). It is indeed easier to fair a surface by (more or less) equally distributing the conrol points. This automatically results in smooth running parameter curves. On the other hand it is very well possible to produce a surface that is very smooth, has a good curvature distribution but whose iso curves appear not fair.

Also, placing sprayrails on an isocurve seems like a serious modeling limitation. You want to be able to determine its shape based on other design requirements instead of making it dependent on where you have put your control points.

The best way to determine if a surface is fair is by looking at the curvature disribution (whether Gaussian, mean or other). If these are ok then just forget about our iso curves.

[aleutka29]

Originally Posted by Willallison

My favourite program - Multisurf (which is way to $$ for me) has the ability to constrain curves / surfaces to one another. I've found it to be very useful...

MultiSurf Solo is priced to be very much in line with Rhino. I'm sure Rhino Marine would be more expensive. There are no limitations on the number of surfaces which can exist in a model as in the past with MultiSurf LT.

In MultiSurf the path of the spray rail would be placed on the bottom as a curve on a surface. Whether drawn in as a spline or projected as described above, that would be no matter.

A relative Curve would be made from this curve to create the vertical surface. The shape of this curve can be further enhanced with the use of a graph to determine the rate of offset from the hull curve.

This relative curve would now be projected on to the hull and the surface between these two curves would be the bottom of the strake.

The point here is the whole thing is controlled by one curve on the surface and will update when the hull is changed.

[Willallison]

Thanks Aleutka29 - I'm not familiar with Multisurf Solo - I'll certainly check it out. I have used MS LT in the past and that's pretty much how I'd have done it in there too.