Free!Ship Design Help 5.5 Planning Hull

[RoBoT6779]

Hello fellow boat enthusiasts,

My first post so apologies in advance if i've missed the right protocol for this site. (let me know, and i'll do my best).
I'm an aluminium boat builder by trade and have built boats from 4.5m up to 36m commercial catamarans.

I have built a few boats for myself and family members over the years (trailerable fishing boats 5 to 7.5 m) and designed them myself by 'eye'. By eye I mean, marking out the plate with battens using my educated guess, (pullup method bottom plates first) and bit of trial and error. They have all been successful, but a little time consuming.

I have always wanted to design similar boats using 3D software, but the ducks have just never fallen in place to get the knowledge. I have 2D AutoCAD experience having drawn up construction plans for 24m catamarans (from lines plans).

My goal is to CNC cut a Kit. So that construction time is cut down. (I have a young son and he is time hungry!!! but I want to take him fishing).

I have discovered FREE!SHIP and what a beauty that little program is. I've got my design and I can develop the plates, export a lines plan, I've even printed out and made a scale model 3:1.

Now here is my problem.
My frames are all straight!!! So I'm only talking about the bottom developed plates here (but the sides are the same).
Now as a planning hull, I expect the last third (transom end) to have flat frames (by flat, I mean a straight line from Keel to Chine, the boat has about 17 degrees deadrise at the transom). But I want the design to be somewhat of a conical development (so I can still use a pull up method of build). Frames from midships FRD should have a small amount of curve in them (as does my model when I pull up the shape) and the very frd frames may end up fairly straight over a short distance.
Is this a constraint of FREE!SHIP that the development is based on steel. A lot stiffer median. Is there a plug in for FREE!SHIP that handles Aluminium characteristics better. Or do I need to take measurements from my model and add more control points.
I noticed that on my developed plates the station lines had a large amount of curve. With straight lines in the lines plan I would expect these lines to be relatively straight especially at the transom, where it is really just a triangle.

I want the design to match what the Aluminium (5mm Plate) will do once Keel, Chine, Sheer, Stem and deadrise sizes are established. I don't want to pull the ally to the frames. The Ally WILL have some curve from Keel to Chine when the sheet lays in a relaxed state.
If anyone can understand what i'm talking about I'd love to hear you comments.

I've attached a pic of R'n'R the last boat I built by eye. It is similar to the design I'm working on.

 

[Mr Efficiency]

I am mystified as to how you created the hull in the picture ( looks like a good shape to me ) but are having a problem with this one, given they are of a similar genre. As for the conical development of steel Vs alloy, I doubt there would be a significant difference between mild steel and tempered aluminium plate. Is the problem a matter of wanting an accurate curve on the station so the plate is in intimate contact all along the section without any forcing ?

 

[RoBoT6779]

Yep Mr Efficiency,

Thanks for the response.

I'm mystified how I can build them blindfolded with no plans yet can't create a set of plans to match what I build. Having sold all of the boats I can't just take extra measurements and add the control points.

You have hit the nail on the head of not wanting to force any plate. Ally is my friend and I like to work with her nicely. HA. I pride myself on the ability to create fair hulls which require less than 2 mm fairing for paint.

Normally I would create the shape, then fill in the hull with stringers and frames and so on, matching them to the shape created (although the AFT 2 or 3 i would feather to straight if required). The ally has its natural shape and this keeps the plates fair.

So yes It would be nice if possible to create the design with the lay of the plate more. And I agree The difference between steel and ally would be minimal on this type of project.

 

[Mr Efficiency]

Ok, are you saying the transom bottom section the program has created is not straight ? That could be because you have a warped plane aft, rather than a constant vee, and consequently not strictly developable with straight sections.

 

[RoBoT6779]

No, The transom is straight and the hull is developable. The problem is that all of the stations are straight throught the boat. The design is probably fine. But if it had some shape at some of the stations I would rest at ease knowing when I build it, I won't be adding extra tension to the plate.
So for example if I develop and cut the Bottom, Side and Deck plates (sheer decks to hold shape) from my design (which I have). Then erect them right way up with only the transom in the hull to hold the deadrise (with straight lines from the keel to chine on the transom only). Not all the stations will be straight (from keel to chine, in the real world that is). They will have some shape (most being about where the console is on R'n'R ). This I know from experience and from the model I made on the weekend.

 

[Mr Efficiency]

If the hull you have created with the program is showing straight sections all the way through, then you haven't stipulated it has to be developable, surely ? Because there will certainly be curvature to them if the shape is developable.

 

[Mr Efficiency]

I've heard about people building plate boats without any plans to speak of, I was a bit sceptical, but apparently it happens !

 

[RoBoT6779]

In my developable design, all of the frames will have a straight edge from keel to chine. ??? I guess this is buildable but not without extra work (for my method).

But yes small plate boats up to about 8m can be built quite easily. Although you need to be careful when you get over 6.5m and start adding big cabins etc. From a non boatbuilder looking in, it may seem hard to imagine how it's done, but probably the same way I look at people who can draw up a 'bloody plate boat' Ha. Once you have the know how things are easy. Don't get me wrong the first 10 I built (for a company) had templates for the bottom and sides. We would change little bits and pieces to customise the boat to the client. That experience sure helps a lot. But once the shape is established the rest is a walk in the park, with a welder in hand, and a meat axe.

I think I might be grabbing my scale MDF model this afternoon and measuring out some areas of (my) concern, adding some control points and see what happens.

 

[Mr Efficiency]

I can only think you are manipulating the program without specifying your hard chine hull has to be developable, there should be obvious convexity, particularly at the turn of the forefoot.

 

[RoBoT6779]

Ok, I have created a bottom surface and a side surface. I have control points along the keel, chine and sheer only. You are right I have not specified that the hull has to be developable!!! How would I do that??? I have simply created a shape then developed the surfaces. Am I missing a step (I hope so). Come to think of it, in the first instance of creating my hull I used the crease tool to create my chine. Is this the wrong method? I have noticed at closer inspection that the exported lines plan stations don't exactly match up to the chine line.

 

[DCockey]

Frames will usually be curved for a developable surface. Exceptions can be areas of the bottom with constant deadrise, and of the side with constant flare (deadrise and flare measured perpendicular to the centerplane of the boat). This agrees with RoBoT6779's expectations:

.....
Now as a planning hull, I expect the last third (transom end) to have flat frames (by flat, I mean a straight line from Keel to Chine, the boat has about 17 degrees deadrise at the transom). But I want the design to be somewhat of a conical development (so I can still use a pull up method of build). Frames from midships FRD should have a small amount of curve in them (as does my model when I pull up the shape) and the very frd frames may end up fairly straight over a short distance.

If the deadrise is constant then the frames could be straight.

 

[RoBoT6779]

The design has 17 degrees at the transom. About 24 degrees amidships and approx 40 at one of the last frd bottom stations. So a variable deadrise.

 

[Mr Efficiency]

And you are certain that you have elected to make the rendering developable ? Something is wrong, it has not followed that instruction, you are definitely not going to have straight sections with what you describe. I dare say it is just showing the default mode, a straight sectioned hard chine boat.

 

[DCockey]

I agree with Mr E. A bottom with that variation in deadrise won't have straight sections/frames.

 

[RoBoT6779]

Well I not sure what you mean by 'elected to make the rendering developable'. I have used FREE!SHIP for approx 6 hrs using a manual I found on the net to work out what i know (very little i think). I made the surfaces I wanted to develop into layers and simply made sure they are developable by looking at the gaussian curvature and developability check. Then used the develop plates tool. It sounds like I have missed a key step somewhere, by not stipulating the type of design or parameters the program is to run???