The 7ft10in hull
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Hulls is a great program. It has given me hours of fun and helped to deepen my understanding of boats, and before going any further, I would like to thank its developer, Gregg Carlson, for making it freely available.
This tutorial takes you through part of a design process for a general purpose 11ft stem dinghy in order to illustrate Hulls' features.
To my mind, the first step is to check that the hull you are interested in has not already been drawn and in the Hulls format, *.hul. You may be surprised at what you can find in what Gregg calls his inspiration list, which includes a soft-chine plywood Laser-alike, and a Munroe Egret, which goes by the filename of 'Derby'. There's also my own small collection at http://home.clara.net/gmatkin/design.htm .
If it has already been done, you could save yourself hours of needless effort - unless of course, like me, you enjoy drawing boat hulls!
If it doesn't already exist, you are very likely to find something similar, which you can use as a starting off point - putting new numbers into an existing and similar offset table is far easier than starting from scratch, at least to begin with.
By 'similar', I mean something with the same number of chines, and the same sort of bows and stern. For example, if you wished to draw an 11ft dinghy with a flat bottom and two chines, you might wish to start with one of my drawings '7ft10in.hul' (pictured above), which you should be able to pull down easily from my Web pages area using the following URL:http://home.clara.net/gmatkin/drawings.htm
This file is a good start, even though it's shorter and fatter than the shape we are setting out to develop here.
Having used Hulls to open this file in the usual way, give the file a new name and save it straightaway to avoid the possibility that you might over-write the original. When I did it for the purposes of writing this alternative tutor, I called mine 11ft.hul.
The next task is to extend the drawing to 11ft, or 132 inches. This is made easy in Hulls using the Calculations menu:
Calculations>Rescale>Length Overall (Change All)
Enter the 132in figure for 11 feet, and RETURN. The result will look exactly the same as before, for while the given dimensions of the drawing have increased, the proportions have not changed in any way and the program fits it into the available screen space as before.
We probably need now to change the proportions of our dinghy, as there are tried and tested rules governing beam and freeboard. At this point, I turn to some graphs published in a useful little book, 'How to Design a Boat' by John Teale and published by Adlard Coles Nautical. (John Teale is also a regular and interesting contributor to the superb British magazine Classic Boat.)
According to Mr Teale, a reasonable beam for this dink would be about 4ft 6in or so, presuming a waterline length of just under 11ft. So, accordingly, now we make the beam of our dink 54in. Go to the Calculations menu again:
Calculations>Rescale>Maximum Beam Only>
And enter the figure 27in to produce a total maximum beam of 54in. You'll notice that this makes the planned dinghy a little less beamy.
Now it's time to think of that bows freeboard figure. Here Mr Teale suggests a figure of 1ft7in or so. But this time we can't simply rescale the maximum height, as the whole thing turns on how much water the boat is displacing. My guess is that this boat will weigh about 100lb (this is based purely and simply on another boat of similar stitch and tape construction, but you can check the numbers later) and with, say, three adults weighing 200lb each, we quickly come to a huge-looking displacement figure of 700lb.
Clicking on the top lefthand view will give you a view along the bows to stern axis of the boat. Using the Calculations menu again, enter 700lbs:
You will observe that the water level rises to just kiss the chine. In order to keep wavemaking and general fuss to a minimum when travelling through water, I have read that it is wise to make sure the waterline does not rise above the chine at the design load. I think the writer was referring to hard chine boats in particular, but I suspect following the same rule won't do any harm here.
We can't go ahead and adjust the bows freeboard yet, as I can see a problem that will materially affect the displacement results we've just been looking at: it is that the lower part of the transom is under water, and will cause wasteful turbulence as it moves.
So I would start to work on fixing that by looking again at the stern-on view. Select the blue line denoting the stern frame, left click on the lowest chine of the hull and pull it upwards to the 700lb waterline.
(Try not to double click, as Hulls will then ask you to type in new X, Y, Z co-ordinates. If this happens DO NOT click cancel - this makes the software think you mean that X, Y and Z are ALL zero and instantly turn your sensible drawing into nonsense. If it happens, go back to your last saved version. I believe that Gregg will put this minor problem right shortly, if he has not already done so by the time you read this.)
Also raise the point at the left of the screen, where the centre line of the boat meets the flat bottom of the hull. the bottom must curve front to back, but as plywood does not take kindly to being forced to take compound 'tortured' curves, let's keep things simple by keeping the bottom flat from side to side.
Then click on the side view. To be frank, we've produced an ugly underwater shape, and so will have to think about raising the bottom at the third frame as well. To compensate for the loss in bouyancy, and given that we need this dinghy to carry 700lbs efficiently, I would then widen the body of the hull at both the stern and the third frame slightly. Boats intended for use with an outboard and/or for carrying large loads have to have wide sterns in any case.
Now let's have a look at that bows freeboard figure. Again from the stern view, I find that in my drawing the water level is at 7.5in, while the overall maximum height (at the bows) is 32.3in (though by now your drawing will be different, and increasingly all your own!).
Subtracting one from the other I get a freeboard figure of 24.8in, whereas from John Teale's graphs I'm really looking for about 19in. Now, you might choose to have a particularly high bows - wherever I go I see boats that shamelessly break Mr Teale's rules, and there are popular designs that have high bows in order to cope with areas of rough water. However, I respect Mr Teale's judgement and I think I'll follow the book and rescale the maximum height. I'll do this by bringing the maximum height down by 5.8in (that is, the old freeboard figure less the new one). Obviously this will rescale every vertical dimension in the boat, but let's do it anyway and see how we feel about it. As you might now expect it's:
Calculations>Rescale>Maximum height only>
In my drawing, I insert a new maximum height of 26.4, and then look again at the displacement situation by asking the software to recalculate the waterline at 700lbs. The good news is that I'm still happy about the overall form this boat in many ways, but the bad news is that I've still got a stern that's in the water, and the waterline is now above the level of the chine. What should I do now?
There are at least three choices that I can see:
I may consider that these things don't matter too much, and do nothing. That's ok, so long as I'm not looking for this boat to row like a Thames skiff.
In this case, I choose the latter option (Have I made the right choice? Who knows?) and find that widening the boat by 6in overall reduces the problems I'm having with the chine and the transom. Remember that to widen the boat by 6in, you have to add only 3in to the Rescale Beam figure.
I also widen the flat bottom slightly at each frame from bows to stern. The displacement calculation says that I've still got some transom and chine in the water, but by now I'm prepared to rationalise and say that it probably won't carry three 200lb adults all that often, after all.
Now look at the side and plan views. The side view now shows me a sheer that I don't care for too much. Knowing that a sweet sheer can usually be obtained by making sure that the sheerline is straight for the first third of its length, comes to a minimum a little abaft the halfway mark, flattens out for a bit and then rises cutely up to the stern. Fiddle with it for a while until it looks good to you, but remember that a little dinghy like this will need to have a health amount of freeboard amidships.
In the case of my drawing I'm also a little unhappy about the centre of bouyancy, which is just about halfway from stem to stern, and I know that John Teale says it should be a little aft of this point. This time, I click on the plan view and move the second, third and fourth frames slightly towards the stern. This also will make the bows section far easier to make because the ply will not have to curve so sharply.
I also further widen the flat bottom and the adjacent chines to produce fair set of curves, and look again at the way the bottom curves. I have read Uffa Fox, and understand that to give this boat a fair chance of planing (with an outboard, obviously!), I have make sure that the deepest part of its 'chest' is about a third from the bows. I do this by deepening the second frame slightly. This is probably best done in the stern to stem view. Do look at it from the other views too, to ensure that the curves continue to be fair from each angle.
Now it's time to look again at displacement. What do I see? There is still a trace of transom in the water, Dammit!
Now, the stern of this boat has a cute slope. By squaring it off, I might just bring the transom out of the water. This time, let's use the offsets table to fix the problem. I go to the Data menu, and click on Offsets Table. This does not look like a traditional offset's table, but it's not too hard to follow - just remember that X is half the beam at the point you are looking at, Y is the height and Z is the distance from the stem.
In this case, I can straighten out the stern by making sure the Z co-ordinates in the stern are all 132in, and then hitting the 'Done' button. From looking at the side view, I can see that a small tweak will bring the transom out of the water altogether, and still produce a fair curve. So I move both the flat bottom at the stern and the next chine up slightly. This is getting better. Now the displacement curve tells me that the waterline rises to the very bottom of the transom, and that there's only a tiny amount to the uppermost chine in the water. Great. But now the centre of bouyancy has edged forward again. Hah! - that means more tweaking of the beams in the frames in the after part of the boat. So I do it. I never said this was a simple exercise!
At this point my curiosity leads me to look at the stability of this boat. But here I must issue a small warning - leave the Auto-spline Chines set to ON before you do this, as on my PC poor old Hulls goes completely crackers and crashes without letting you save. I have lost a few drawings that way.
Given its form, this hull should be highly stable, and I can find out exactly how stable by going to Heel on the Calculation menu. This tells me that at 15 degrees of heel, and with its weight evenly distributed about the boat, my little vessel would have an impressive-sounding righting moment of 383 foot-pounds, but this is a fairly academic figure in a boat this size, as trim will depend on the passengers moving as required.
Finally, if we were interested in making this dink sail, we might like to consider adding side decks to provide a comfortable sitting out position. We can do this by adding a chine.
Again, let's use the offsets table. I use the Chn slider bar to add a fifth chine, and then hit the offsets table button. First I make the Z co-ordinates for the fifth chine in each frame the same as its sisters lower down the frame. Then I make the height (Y) of the fifth chine the same as the height of the fourth at each frame. I can add a camber later if I want to. Then I make the width (X) at each frame three inches less than the maximum beam at each frame. (Except at the bows, of course!)
I also make the rabbet line of the stem lie back into the boat to form a small deck area. If I place the final chine offset about 20in from the bows I get a result I think is beginning to look sensible - but which but makes the deck very narrow in parts. I fix this by adjusting the fifth chine at the second frame.
By this point, of course, I don't really know the detail of your dinghy, which will naturally be different to mine. However, from looking at mine, I can see that 11ft is probably too short for a real stem dinghy designed to carry a load as large as this. If I were to start again, I would probably opt instead for a conventional pram, or something with a narrow transom at the bows. From what I have read, I would also suggest that if you mean to row or tow this dink anywhere, it would be worth giving it a skeg that would help it to track reasonably well - exactly how large it should be is a matter of trial and error but, from looking at other boats of about this size, an area of about a square foot, give or take a little, should not be too far wrong. Make it run from, say, three feet from the stern and allow it to end with the stern, round it off, and attach a metal strip for protection.
Three dimensional modelling Pressing the CD button at any time produces a three-dimensional view that can be moved around using the X, Y and Z buttons at the top of the screen. Also, if your HTML browser has a VRML add-on, pressing the VRML button produces a three-dimensional object in that format. I've tried it, but it was very, very slow on my 90megahertz computer with 40megs of ram and I don't bother now, as the wireframe view that the CD button produces works well.
Developing the panels This is possibly the most impressive feature of Hulls. Go to File Save, and take a look at the dialogue box. There you will find scads of useful buttons to press, and criteria to input. Place up to eight structural frames where you like. I suggest for our 11ft dinghy, you might like to choose three or four and distribute them reasonably evenly. Some of these will handily double as walls for built-in bouyancy.
If you change your hull model and wish to nest a new set of developed chines, as you might do if, for example, you wanted to fit the panels for a whole craft onto a limited set of sheets of ply, it seems to help to save the new form, and close Hulls and then reopen it and your file to ensure you are nesting the correct panels.
DXF output facility On the dialogue form I would also click on expansions, DXF, and text. When save is pressed, these will respectively tell the software to save the developed panels in the CAD standard .dxf format, produce a .dxf image in three dimensions if you have a 3-D CAD package (which will appear as a drawing in plan and elevation in a 2-D package), and output a set of co-ordinates that can be marked out on sheets of ply prior to being faired with a batten and then cut - at least it's better than proper lofting!.Here I have to add an important warning. The co-ordinates that Hulls produces are in eights, as is traditional in boatbuilding. However, there's a bug in Hulls that produces co-ordinates that read like this 12-8. Interpret this as 13-0. One day this may be fixed; until then it isn't hard to live with, but you have to know about it!
One very important use for the .dxf output is to ensure that stern and bows transoms of complex multi-chined craft really are flat. Output the file from Hulls, check it using your CAD package, and go back and amend it if you need to. Of course, you could do the whole thing by Pythagorean geometry - if you like fiddling about with small things that can be fixed in better ways..
Buttoning on Patterns>Nesting produces a handy gadget that allows you to nest the developed sheets onto up to five 8 by 4ft ply, and Save here produces the co-ordinates you asked for earlier.
Outputting offsets I haven't tried this, but I gather it's possible to output ASCII files of offsets produced using Hulls. You install the Generic/Text Printer Driver. Then, select MY COMPUTER/ PRINTERS - then click on ADD PRINTER. Select GENERIC and when installing the driver, select the PRINT TO FILE option. To use, simply right click on the GENERIC/TEXT printer icon and select as default before firing-up Hulls and saving the offsets to the file of your choice. Of course, after exiting Hulls you will need to right click on your usual printer and restore its original default setting.
What else can Hulls do? If you've got this far, I honestly think you'll find the rest of Hulls' features fairly self-explanatory. Try them out!
Gavin Atkin, September 2001 I do not undertake to support Hulls, but I would be grateful to hear of any difficulties that arise from using this tutorial. My email address is email@example.com
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