UK-Cherub Class

Get Your Heart Racing


Designing Cherubs

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Boat design is a serious business which is not to be undertaken lightly. After all, even professionals get it badly wrong a lot of the time.

Designing your own Cherub

Disclaimer This document is compiled by an amateur, not a professional. It has been compiled in good faith, but almost certainly contains errors and inaccuracies. Its also very much based on how this amateur goes about drawing boats. A trained professional will doubtless approach things differently. If you feel unable to take responsibility for your own actions and errors without resorting to the legal profession then you are advised not to read it, let alone build anything based on information here.
I should also point out that I've never built a Cherub to any of my paper designs*, although I have built a one-off singlehander.

One of the aims of the Cherub Class has always been to provide a platform for people to design their own boats. Quite a few careers have started with designing Cherubs and related boats. John Spencer was the first, the original Cherub being the start of his career, but Russ Bowler, Iain Murray, various Bethwaites and the UK's Andy Paterson all had Cherubs at or near the start of careers that have included significant designs in other classes. There are quite a few ways of going about designing boats, and those of you with appropriate training will be able to make your own choices. Apart from the obvious choices of naval architecture and mechanical engineering quite a few successful designers had trained as civil architects.

Some thoughts on design extremes.

People's first boats tend to be extreme. Obviously you want to go with your ideas, and there's no point in doing what Uffa Fox did, which was not to put his radical ideas in practice until his third 14 footer, the legendary groundbreaker 'Avenger'. On the other hand you don't really want to end up with a boat that is way out on a limb and only effective in some conditions (as I rather fear I did with my one off single hander). Off hand though I can only think of one first boat in recent years which was an unqualified success - the Italian Bistro. So its probably worth considering designing both your radical new idea and a moderate version of it, and take a long hard look at both before deciding which one to build.

Balance is Everything

Julian Bethwaite will tell you that the most important thing he does is to travel with his eyes open and his mouth shut. You'll also find that you start drawing you don't really know what a boat looks like. It sounds ridiculous until you try and draw a familiar boat like a Laser from memory… Look at the whole picture. Think about where you will be sailing the boat, how heavy you are, whether you are brilliant boat handlers who can manage a tricky boat or whether you'd be faster in a stabler platform. Think about the rig you are going to put on. The rig needs to match the hull needs to match the crew needs to match the foils needs to match the rig…

Start with an Idea

The most important thing is to be familiar with Cherubs, to have sailed in the class and observed how different boats behave, and how you see the benefits of different shapes. Its also helpful to have a starting point - “What I'd like is a boat a bit like an Italian Bistro, but finer at the bow, narrower and with the beam further aft”. Maybe you get a sketch like this:

Sketch

Make Sketches

From there I suggest you arm yourself with a drawing of your starting point and a sketch pad and start sketching ideas. It's important to consider the boat as a whole, not just as sections, profile etc. In order to do this I find it useful to draw waterline and to a lesser extent buttock sections through the boat. You also should have a good idea about how you want the waterlines and buttock lines to look, because this is how the water is flowing past your boat, which in the end is what is most important. I usually start by drawing sections, and then work out the waterlines from them, and then modify the waterlines to nearer what I think I want and alter the sections to suit. Through a few dozen pages of sketches you are getting a fairly good idea about what your boat will look like.

You'll find as you go through the sketches that your ideas will change, especially as you start converting sections into waterlines and vice versa and get a real feel for how the different factors inter-relate to make a complete boat. Once you get past the initial sketches graph paper starts making a lot of sense for this business of matching sections to waterlines and back again. It sounds like a lot of work - and it is. You need to be thorough though - you are going to be building this boat for six months or more and maybe sailing it for five years. It's a long time to be saying to yourself that you wish you'd thought more about the transom. You want to be able to visualise the underwater shape of the complete boat before you get past this stage.

Detailed Scale Drawings

At this stage most of us will want to turn to the computer and a drawing package. To a large extent it probably doesn't matter what you use, but the Australian Package Hullform, from Blue Peter Software, has been used for at least one Nationals winning Cherub. What you are using the package for is basically as a tool to do some of the donkeywork of mathematical calculations and of fairing lines and sections. Simon Roberts and Dave Roe both use their own custom computer drawing systems for boats, which they've steadily evolved to suit their particular requirements. The Italian Bistro was actually designed on a high-end programmable calculator. Alternatively - and especially if you have the required technical capability - you may prefer to carry on with pencil and paper, but perhaps move to larger scale drawings and a proper drawing board. Julian Bethwaite is certainly one top designer who's a firm advocate of staying clear of the computer. Perhaps at this stage - or better still while you were sketching - you first come across a nasty surprise when you discover that some of your most prized ideas won't work together. When I drew my first Cherub I wanted a flat midsection with lots of turn up near the chines, a V section bow, and straight waterlines in the bow with no lumps and hollows. I discovered you can't get that. With straight waterlines a flat mid section gives you a flat-bottomed bow. So you have to start making compromises. Perhaps three quarters of the art of sailboat design is the art of selecting the best compromises!

I don't propose to produce a manual to using Hullform, but note that Cherubs are quite tricky boats to put into a hull design package. You will need to get into the program thoroughly and use some of the subtler features to get the best out of it. This especially applies when drawing the deck line and snout area, where most Cherubs have angles and straight lines that the default settings of the program don't handle well. I also advise you to get into a regular regime of saving new copies of your drawing so that you have old versions to look at and maybe go back to if a particular idea doesn't work out. Its also good to get a version of your master design - and maybe one or two others - into the package so that you have a starting point for looking at the figures that it produces for you.

The major advantage of using a computer is that the process of fairing the sections can be automated, and also that you can do any number of “what if” variations on a theme and evaluate them. However a significant disadvantage is that it is all too easy to make a change in one place that affects another part of the hull through the fairing process, which you may miss. Another factor is that changes that look quite small on a computer screen can turn out to be some inches on the finished boat.

Models

I have ony ever heard of one Cherub design being tank tested, and you're not going to have the facilities yourself (unless you're studying Ship Science at Southampton, in which case I imagine reading this article is unnecessary). However I still find it useful to build a balsa wood and cardboard model of my design, just so that I can look at it from different directions and get a real feel for what the finished boat will look like. I'd especially (bitter experience here folks) make sure you build a model of your final design. I didn't for the single hander, and I suspect that if I had I might have started getting worried about just how flat it looked. As it was I didn't really get concerned until I saw the building jig - at which point it was a bit late! The intermediate design that I did build the model of had a lot more rocker…

Go For It

You don't really know how satisfying it is to have a boat that you have designed yourself until you get in it and sail it. Even though my singlehander is less than a complete success I don't regret doing it at all. And maybe next time I'll get it right!

Starting Points

I hope to have a few designs available electronically to give a starting point.

There is a 1994 1994 design with flares and snout added to 97 rules and a 97 rules boat which I've sketched out, loosely based on a Bistro as outlined above. Its not remotely “ready to build” and almost certainly not quick, so be warned!

Coming later should be Dave Roe's Pasta Frenzy design - when I can make an electronic version.

Jim Champ, 2000

*Though I wish I'd got round to building my 1978 Cherub design- when I look at the drawings it bears a distinct resemblance to the highly successful mid 80s Kiwi design Tasman Express!


tech/designing.txt · Last modified: 2013/06/25 15:55 (external edit)