Hull Design

Design Notebook/Diary

10/9/1997

This is what's made the boat the shape it is. Lines as originally drawn. Excludes wings. The International Moth route of ultra light and narrow non-planing hull (this was before foils became universal!) seems to me to be something of an offshoot from the mainstream. A more accessible boat has a reasonable amount of static stability - at least with a crew on board - and rather more dynamic stability. In order to gain good performance at the current state of the art this means a planing boat. The low wetted area shapes seen on the low powered UK development classes, the National 12 and the Merlin, are fine for their particular uses, but my preference is for a boat that compromises top end performance less, even if it takes a penalty in light and moderate airs. Without the modest sail area limits that these classes have wetted area should be less of an issue. The huge sail areas and multiple rigs seen on skiffs are probably impractical in European conditions in an all round boat. This means that the low rise of floor hull seen on recent 18s and UK rules Cherubs (and offshoots like the 49er) is preferred to the high rise of floor route followed by the 12s and 16s. Julian Bethwaite's claims for minimal displacement "hump" on recent 18s and the 49er are most interesting for light to moderate airs performance, but there's very little information on how this is achieved, so one can only look at his boats and see what factors one thinks might be important. Unfortunately to the casual eye there is little about the 49er hull shape that looks radically different to the B14, which is not, I believe, claimed to have the "hump free" performance. The main boats that have influenced me in looking at the hull shape are:-


All in all this leads to a fairly narrow waterline beam boat, distinctly wedge shaped, and with a great deal of beam above the waterline in order to be able to carry an ample rig in order to reduce the performance loss in light airs. Its also got very little rocker, (unlike the Bethwaite boats, which are relatively deep chested for this type of boat) which is likely to boost performance in flat water and planing conditions but pay a penalty in some wave conditions. It could also be a nosediver, although all the extra length compared to a Cherub will help some! One factor is important. As this is not meant to be a mass market boat there is no need to design it with ease of handling as a high priority. The Cherub Class motto currently reads "Its no fun if its easy", and I willingly subscribe to this. Compared to the mass market boats like the Topper Blaze this boat will undoubtedly be twitchy, idiosyncratic, and a lot less tolerant of poor boathandling. On the other hand I hope it will provide me with years of enjoyment in learning how to get the best out of it, it will certainly be a great deal easier on my aching back muscles on the shore (the projected bare hull weight is around 50kg) and I hope it will be just a little bit quicker. I just hope I can get my boathandling up to scratch!

17/9/1997

Having gone to look at the jig I had - almost inevitably - a fair number of worries and second thoughts. No matter how many drawings or balsa wood models you make, the real thing always looks different. I think the main worry is the height of the wings above the water, because I fear I'll spend too much time with them hitting the water. This is mainly a boathandling/practice issue of course. General Arrangement Sketch. Showing Wings and basic interior layout. We considered angling up the wings somewhat to gain greater clearance, but the problem is that ideally there should be no concavity in the line from shroud anchorage to shroud anchorage, but if the height of the wing at the gunwhale is increased the foredeck shape changes, the... If the worst comes to the worst I shall just have to get out the angle grinder in a couple of year's time and chop em off and build new ones... Inside the boat the false floor will run up to the bow, with a separate bow tank above it. There will probably be a deck behind the mast to the back of the daggerboard case, partly to support the daggerboard, and partly to provide something to sit on in drifting conditions.

9/10/1997

False Floor height etc..

All else being equal a low false floor means low centre of gravity in light airs, a longer mast foot and thus more overlap available to "reef" the mast length, and a bigger gap between the daggerboard support and the floor to make a more comfortable seated position in light airs whilst a higher false floor means less of a height difference between wings and floor and thus easier to get across the boat etc., but of course less height under the boom/and or a higher rig...
Halo (my Cherub)'s false floor is 185mm above hog in the centre and is convex - i.e. goes *down* towards the chine, dipping probably about 30mm - 40mm (too much hassle to measure it in the tanks!). At the transom its 63mm to the inner skin, so probably 72/73 overall as the hull is 8mm foam, and at the chine its probably around 25mm, i.e. 2 layers of foam thickness. This can be regarded as the bare minimum to make a boat self drain because I know the boat is only very barely self draining with 28 stone on board! (and not forgetting that there are side tanks too).
However this level of buoyancy, combined with an open transom, means that the stern sinks tacking/gybing in light air (fine for stopping quickly when misjudging approach to a jetty but not much use for anything else) and so we need more buoyancy aft - say minimum 100mm at the centre and with a concave floor so that it goes up towards the chines. There'll also be a solid transom with a smallish scupper in the middle, so that water doesn't come in tacking... Perhaps the scupper should be an inverted triangle so that very little water can come in if the transom is immersed slightly, but there's still plenty of width higher up for draining large volumes post capsize. Given that level of aft buoyancy, the next step is to calculate what maximum nose down trim might be so that the boat self drains at any trim angle. This was a question of feeding numbers into Hullform, but to cut things short a bit it ends up with 310mm and the bow and 265m at the centreboard
The floor probably wants to be flattish for about a foot each side of the centreline, otherwise the depth under the boom is more or less wasted, as far as under boom clearance is concerned (at least middle to aft end of cockpit, but nearer the sides rise up is probably a good thing judging by my estimate at how the ergonomics of Andy Paterson's 1997 Cherub Nationals winner must work... This boat has a very clean and open layout which is a big influence on the layout.

Midsection This midsection shows the two favourite options for the false floor, either running right up to the flare or stopping about half way up the topsides. They both have their pros and cons on paper at this stage, but things will be a lot clearer when we start looking at large pieces of cardboard made up as mock-ups inside the shell.
The daggerboard is supported by a small "tunnel" as it needs 300mm height for proper support. Its going to be in a sort of "cassette" that can be slid along the boat in order to change daggerboard positions. I have, painfully, to admit that for the first time in my life I've taken an idea from an International 14. In International Moths the case is typically approx. half foot behind the mast position, with the modern raked una-rig, and this is as good a starting point as any as the first rig will be fairly Moth like, and its unlikely any rig will need a board further aft than this. The slot will have about 500mm forward movement, which ought to be sufficient for experimenting with self tacking jibs and so on.

23/9/1997

Starting to think a little about the wings and about fitting out the boat. The wings will, I think, be solid, glass over 8mm divinycell foam with carbon round the edges and on the beams/legs that attach them to the hull. They'll be hinged in some way, if for no other reason than it will be a lot easier to handle the boat off the water and trail it if its 3 feet narrower. I haven't decided whether to put some buoyancy in the end of the wings. On the one hand it will add a useful bit of extra stability for dubious boathandling, but on the other hand it means that the wings have to be fixed to cope with upwards loads, it adds weight, and it might make the "waterstart" technique for capsize recovery a little more (even more?) awkward. The waterstart is a technique the International Moths have evolved since they started taking the shrouds out towards the end of the wings. With the shrouds there its impossible to climb round the front of the wing as you pull the boat up, and so, if I understand correctly, the technique is to pull the boat up and grab the wing as the boat comes up and pull it over towards you. As the wing starts sinking to windward and the boat starts falling over on top of you, slide onto the wing, still floating in the water, sheet in, and up comes the wing with you as well. Sounds easy, doesn't it? I have this feeling that, unlike any boat I've ever owned before, I am going to have to devote some time to capsize practice!

Gear wise it will be pretty simple - the standard kicker/cunningham/outhaul set, shockcord tidies under the wings/flares like the Moths have, no standing rigging adjustment under way, and a centre mainsheet on a bridle going right up to the boom, finishing up on the boom. There won't be a main jammer, just a ratchet block on the boom. In the current boat I've completely stopped using a main jammer, simply because sheeting from the boom is so much more direct and the extra turn doesn't help at all. Its also nice to be able to tug on the mainsheet to be able to relieve ageing stomach muscles - because of the bridle you can't really oversheet the boom very much because as soon as you do it becomes a 1:1 mainsheet system!

19/10/1997

You'll notice this page gets added to even though the boat is under way. I suppose its a case of "let the dog see the rabbit" - when I did my last boat, a Cherub, I planned all sorts of stuff in great detail before the builder started on the shell, and when I actually got the shell in the front room to finish I changed my mind about nearly everything!

When we actually put some cardboard in the hull to look at options for the false floor it became quite obvious that a scoop deck from the flares right across was the way to go. It looks neat and effective, and will be the best structurally. The wings will overlap the hull slightly, so they won't need reinforcement at the inner end, but they will be substantially reinforced and thickened and radiused at the end for strength comfort and a little buoyancy.
Its interesting to speculate on how quick the boat might be. It will be slower overall than Clive Everest's RS600, because the RS600, which is actually a very small boat, will be longer, narrower on the waterline, and have considerably less aerodynamic drag from its much smaller wings. The hull shape is quite different from the RS, however, and it wouldn't be altogether surprising if there weren't the odd wind direction/strength combination when this one has a slight edge over the RS. Clive's hull has more rocker, so all else being equal should be better upwind in waves, but on the other hand my rig will be based on some developments that have been made since the RS600 was designed, and, once (if?) it is fully developed there may be some small advantages there. Inevitably, though, it all really comes down to sailing ability, and I have no doubt at all that Clive in an RS600 would handily beat me in the CH++ in any conditions, but then again Clive in the CH++ would probably beat me in the RS600!

How Successful was The Design Process Overall? (June 1999)

In other words what went right and what went wrong!
I suppose the top aim was to get a boat I could readily handle off the water. I can declare that one well and truly hit. The boat and trailer combination feels as if it is half the weight of its predecessor and I can readily pull the boat up the launching ramps at the club single handed.
Reaching performance in light to moderate airs appears outstanding. With the apparent wind forward of the beam it just goes and goes. At 12 knots the stern wave system is about half what it is at 6 knots, and smaller than any other dinghy I've ever sailed (but then I haven't sailed a Moth!).
Upwind the boat planes in around 8 knots of wind at about the same pointing angle that most conventional single handers achieve their best upwind performance at, so that is satisfactory. It's tricky to get the boat at the optimum pointing angle - Graham Caws' sail is astonishingly close winded and you can point very high indeed - and stop almost completely while doing so! It seems fastest to sail with the boom sheeted about 18inches off the centreline at the transom, which feels a very long way out indeed, but like this the boat is pointing with Enterprises and the like and staying nicely on the plane.
Broad reaching/running I'm less enthused with. There's no doubt at all that the boat is happiest with the apparent wind forward of the beam. Sailing deep downwind in light airs may prove to be a bit of an achilles heel, and will need a bit of work to sort out when and how far to gybe downwind etc.. How much of a problem this is will only be visible when I get the boat handling up to a state where I can race the boat seriously. There is no doubt at all that playing the wind shifts and getting the angles right will be crucial to race winning. Once the wind gets up enough to get planing properly its less of a problem, but I think it seems to plane on a beat earlier than it does on a dead run!

Major Review after some Two Years...

Two years on I'm rather less satisfied with the hull shape than I was initially. To an extent I guess this is inevitable. There are a couple of particulars. The first is that I wish that I'd seen an RS300 and known how successful and accessible it would be before I built this boat, and the second is that I wish that I'd set out the rules for a lighter crew weight.

The first is just one of those things. The 300 is very much narrower tham my boat, and had I known that many typical club sailors would be happy sailing it I would have gone narrower on this boat.

The second is related really. I always knew that the chances of other people taking up a new development class in the late 1990s was minimal. However in spite of that I used a rule draft as an outline for the design that would carry plenty of weight - much more than I actually weigh. If other people had built boats I guess I would have been happy with this, but as - predictably - they haven't then I'm actually sailing with more weight carrying ability than I need. This is great for carrying passengers, advanced training etc, but not part of the real need for the boat.

These both lead to the same conclusion - the boat is too wide at the waterline. A narrower waterline would be less wetted area, less form drag and especially less drag in the critical high displacement - slow planing speed regime. It probably also means something of a planing "hump" : if you read Bethwaite you'll know what I mean. Whether it has one of these I don't know - I really should get round to doing some tow tests.

Another factor is the very low rocker. This is low to such an extent that the boat has to be travelling very fast before the bow lifts significantly from the water. Consequently this also means that you have to get a long way beyond displacement sailing before wetted area reduces very much. Is this significant? I'm starting to wonder - its noticeable that my Cherub lifts the bow much sooner, and that boat is a lot better upwind in planing conditions even though the sail carrying power should be marginally lower and the rig is quite a bit smaller. There is more discussion about design issues on the PlusPlus Mk 2 page.

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