"Designing" a double-handed racing dinghy

My tentative approach too, with this rig and yours specifications :

** I have chosen L 5,50 m x B 1,60 m for the basic dimensions, i.e. more length and less beam than usually to favor the displacement mode by light winds while keeping it light and able to go planing

** design displacement D 277 kg : 127 kg for the dinghy fully equiped (it is roughly the weight of a 5o5) + 150 kg for 2 sailors

** SA upwind 16 m2 leading to SA/D^(2/3) ~ 38 , equivalent to the one of a 5o5 at D 307 kg (127 + Sailors 180)

** a hull shape of type « flat and shoulders » , the flat zone being defined where the deadrise is < 2° : the flat part can favor the planing upright, the rounded shoulder gives a lower wetted surface when heel at ~ 10°-15° by ligh winds.

** With an estimation of the intrinsic stability, i.e. the initial stability when upright (>> giving a moderate one, the beam waterline being 1,11 m for this displacement) and the dynamic stability up heeling up to 25° (>> giving a good one)

** With an estimation of the sail carrying power : the dinghy (with one sailor at trapeze), when sailinig upwind, can be sustained upright up to ~ 11,7 knots of wind without flapping mainsail. In case of sudden gust, with the mainsail fully flapped but the jib kept cleated, the wind sustained can be 20 Knots.

** With an estimation of the speed with the VPP application : with wind force 2- little 3 (6 to 8 knots) >>> ~ 4,1 to 5,1 Knots when upwind , ~ 5,1 to 6,2 Knots when beam reaching.

** + the option hull with chines / 5 panels also approached : giving a bit more wetted surface but also a better intrinsic stability, so less faster by light winds (3,9 to 4,9 when upwind, 4,9 to 6,0 when beam reaching) but a bit more tolerant to gusts.

Here attached the report (pdf) + 2 Gene-Hull files (for the 2 versions) + the VPP appli.

 

@Dolfiman thank you for providing the spreadsheets!

Here are the offsets from the chined spreadsheet imported via a macro into Rhino3D with sails from Sailcut CAD.

 

An awful lot can be achieved now. When I built a carbon rig for my IC that was a little bit too soft it was almost impossible to run out of righting moment upwind. Its all in having the spreaders working right, fully battened sails and a decent amount of roach etc. Its a subject you hear a lot of very opinionated nonsense on, but its the critical area to get right for gusty conditions. I used a second hand carbon mast for a lighter weight boat and reinforced it low down to get a mast that was very stiff up to the spreaders, pretty stiff up to the hounds and quite soft above. It was very effective. Unfortunately I aged out of the boat. But a rig like that on a hull on the lines of a Flying Dutchman would make a pretty good lake boat.

An FD shaped and sized hull, but slimmed down forward for a finer entry would be an interesting proposition for your requirement, but the FD rig is hopeless. Ultimately there are two main solutions for a fast hull. One is short fat and planing, and accept mediocre performance in sub planing conditions, and the other is long and thin and low wetted area, which will tend to a lower top speed than the planing demon, but be way superior at low speeds. There was some mention of the RS400 above. That's a Morrison design. I don't generally like his boats, but have to admit he is a genius at getting low wetted area. I've tried using his designs as a model, but it seems as soon as I tweak one line in a CAD system the wetted area goes up 10% and I can never get it back down! His RS400 is a clever compromise in getting sufficiently low wetted area in a basically shortish fattish boat that the low speed performnce is surprisingly good, but it does pay a bit of a penalty in outright top speed.

 

Maybe I should have called the thread "designing a hull" to highlight the focus on the hull

I actually started to build a quarter scale model of said hull to get a better feel of proportions and details, but there are too many other things going on right now even to finish that one.

Dolfiman, can you tell where the crew members would need to sit to achieve the assumed fore/aft trim without any forces in the rigg?

Re wetted area: My impression is, that a shorter hull yields a lower wetted area for a given displacement. I doubt it would be usefull to decrease the waterline width any further on a long hull and when shortening the hull the displacement can be easily increased by slightly more draft - the current one of 13 cm is very small.

Another aspect is the design of the cockpit floor. I am a big fan of truly selfdraining cockpits (maybe I am capsizing to often?). But on a long hull this means quite a height difference from transom to mast step. So either the crew sits very "low" above the cockpit floor or freeboard and therefore weight needs to go up considerably.

I wonder if there is some kind of compromise with an flat aft tank ( so it is still possible to crawl in from behind when in the water) and some kind of central buoancy tank à là Hadron H2 plus a kind of (electric?) pump. Self bailer would require consistent speed for some time after the capsize which is not realistic given the typical wind conditions.

I would like to prevent side tanks to avoid the hull floating to high and the daggerboard being to high above the water after a capsize. Also I would like to be able to route jib sheets or control lines under the side decks.

 

As regard the sailors position : I investigated 2 positions of their centre of gravity, i.e. 150 kg at either 2,10 m (~ 40% Lw) or 2,70 m (+ 0,6 m forward) : this last position leads to a nose down trim of -0,58°, a bit less wetted surface and no transom immersion. See document V1 attached.

As regard a shorter beamy version with less wetted surface : I investigated this issue with two more versions L 4,6 m x 1,9 m and also L 4,6 m x B 1,6 m. Rationale :
These 2 versions are designed with the same rig & sailplan SA16 m2 as the L 5,5 m x B 1,6 m previous one, same daggerboard, same rudder, same weights of these items + same mass units (kg/m2) for the hull and the deck. The max beam of 1,6 m or 1,9 m is longitudinally positioned at the same position relative to the length, i.e. at X 30% of the waterline. The general shape of the hull is similar, the topsides are just more or less open and the hull body draft (Tc in the data output) is adjusted in oder to have design displacement = weight (light boat + 150 kg/2 sailors).
At this stage there are still an infinity of solutions, so the process needs an aditionnal criteria to make the versions consistenly comparable : I choose to keep the same initial stability, i.e. the GM1° when the sailors are at X ~ 40 % Lw and Z 0,65 m is kept at 0,56 cm as it is for the L 5,50 x B1,60 m version. Rationale : intrinsic stability is the major criteria used by the builders to split their offer between the racing dinghies for skilled sailors and the family ones for beginners, I think it is the fair criteria for this comparison.
See the V2 document attached, the main trends of the performance are :
** Up to 6 Knots of wind, the 3 versions are very close with a slight advantage for the L 4,6 m x 1,6 m version : the lower wetted surface makes the difference
** From wind 7 to 11 Knots, a net advantage for the L 5,5 m x 1,6 m version when upwind : the lengthy waterline makes the difference
** From 12 Knots of wind, net advantage of the L 4,6 m x 1,9 m version when upwind to beam reaching : the sail carrying power makes the difference

 

Typo mistake above : …. is kept at 0,56 m … of course, not 0,56 cm.

 

It does indeed, and the lowest wetted area is a fully immersed half sphere. But, important as wetted area is, in all put the lightest of conditions its secondary to form drag and wave making. The half sphere doesn't slip through the water very well as one may observe on any navigation buoy in tide! One of the most surprising observations I made from my one off singlehander was that it, a boat with a fine entry and straight waterlines forward, barely kicked up any bow wave at all, whereas older style designs definitely do. This isn't just a gain at high displacement speeds, its a gain at almost all speeds, because energy isn't being lost to wavemaking.

 

Dolfiman,

thank you very much for providing the comparisons!

I would have expected that the different waterline widths+lengths cause a difference long before hull speed is reached but that does not seem to be the case.

The 5,5 m version seems to be long enough to have the crew cause "bow down" trim while still sitting aft of the shrouds.

Your chosen GM1°-Value of 0,56 m, what kind of "behaviour" does it "mean"? Can you maybe compare it to some dinghy types?

I am still wondering about waterline widths. Do you happen to know how close D55, D49 and R12 in your example file match their "inspiration" in regard to waterline width?

 

try to find plans
Cherub (dinghy) - Wikipedia https://en.wikipedia.org/wiki/Cherub_(dinghy)

 

Actually, I was a bit frustrated by the claims of the dinghy builders about the stability, for example RS Sailing shows a kind of scale with 6 levels for the "stability" but without any more explanation (to my knowledge) : RS700 has 1 (we understand it is very low), RS Aero has 3, and RS Toura has 6 (said "stability" in its presentation). Do I did a specific investigation of the subject (link here below) and proposed a scale for :
--- the initial stability, corresponding to the feeling when on board the dinghy, you (your body weight) are not exactly centered.
--- the dynamic stability, corresponding to the righting moment that the hull can provide when you go heeling up to say 25°, to help you delay or react before the capsize.
I called these two the intrinic stability due to the hull shape, to not be confused with the sail carrying power which is the capacity of the crew weight extension windward to balance the heeling moment of the sails aera and height, when sailing upright meaning the hull shape has no contribution there.
My investigation : About dinghy intrinsic stability : proposition for a standard assessment | Boat Design Net
Proposed scales within specified conditions are objective and allow comparisons of various design, but what you can dispute are the names I gave for each level :
0 < GM1° in the specified conditions < 0,3 m >>> poor stability
0,3 m < GM1° < 0,6 m >>> moderate stability
0,6 m < GM1° < 0,9 m >>> good stability
0,9 m < GM1° >>> very good stability
I had not the opportunity to check with sailors if this makes sense.

About the waterline widths of my examples "inspired by" existing dinghies like 5o5, 49er or Renjolle : for each , I try to collect plans, photos, etc ... , and do best for a similar model but I never had any direct information, moreover this is very dependant of the crew weight on board.

 

If I find them, what are you proposing to do with them?

A cherub is pretty much the opposite of what I would like to end up with...

 

I hope I'm not too late.
Designing your own boats is super fun.
I focus on several things.
Low Displacement Length Ratio is #1.
Because you want to sail with a fairly heavy crew, you should make the boat longer than you would normally want.
You make up for wetted surface with reduced wave making and a softer transition to higher speeds. SA/WS can be solved on the SA side.
Waterline beam of 1200mm is usually very comfortable, and a remarkable number of traditional racing dinghies are clustered around this waterline beam.
You can go narrower and you will be faster and more tippy. For a two person dinghy, 1000mm is probably as thin as you want to get.
This drives you to make the rig light, Vanishing stability at 11degrees is a bit frantic.
Max Wl beam should be about 2/3 aft and forward waterlines can be more or less straight. Shoulders are out of style. You don't want to get stuck behind the bow wave, In order to keep the boat trimmed properly, it is often better to make the bow smaller than the stern bigger because too much buttock curvature aft isn't fast. Julian Bethwaite and I have a perpetual pissing match about many things, but his hull shapes are pretty much right on, with the caveat that their drawn waterline displacement is about 20% less than their actual displacement. This is so they are on their drawn lines when planing. This is a skiff thing because they have multiple rigs and have really big big rigs.
SHC

 

You are not too late and very welcome

4,8 m hull length would be way more convinient for storage, transport and so on. But the the dinghies that rule our club races are already 5 m long - and are singlehanders!

My current "design's" waterline width is 1,05 m with 295 kg displacement and 1,14 with 388 kg displacement, so it would fall between the values you mention. I do not want to go too extreme, but on the other hand I do not want to be too close to existing "wide" designs like the German H-Jolle (Die H-Jolle – Deutsche H-Jollen Vereinigung e.V., unfortunately only in German).

My impression is that pushing the max waterline beam 2/3 aft results in a very "fat" transom, which I would like to avoid. I guess I am naive enough to hope that it is less critical for a relatively long hull.

What are "shoulders" and how do they relate to the hull being able to climb the bow wave? I was kind of hoping that crew moving back somewhat + the flat area in the middle of the hull generating some dynamic lift would help rising the bow.

Some people claim Bethwait's 29er was also drawn for reduced displacement despite featuring only one rigg, because it would be always windy in front of his home. From my own experience with ca. 145 kg crew it is ballistic when the wind is up, but it really sucks when not. (And feels like it will sink at 160 kg crew weight )

This "design" needs to properly work for racing with the 150 kg crew down to zero speed/no dynamic lift. For the 200 kg crew it would be ok if it sits somewhat lower with a wider waterline, more stability and less speed potential. Not aiming at a self draining cockpit would also create some "robustness" there before the "sinking feeling" starts.

 

Best not to have much of a bow wave to climb. Shoulders basically means some of the beam carried forward with convex waterlines, as opposed to the waterlines being pretty much straight until they taper off near max beam. You might find it easier in your head to draw the size of boat from max beam to stern to carry the weight you want, and then draw out a longer bow with straight waterlines so as to get the max beam point at two thirds.

AIUI Julian finds that the reduced displacement is a good compromise for his boats, and its hard to argue very hard with results. He and I sailed much the same boats in our teens where the tendency was to have more weight in the boat than would hit the marks when static. His boats are typically longer and slimmer than the development boats that they can claim descent from, and that is without doubt the right way to go. And IME although long fine bowed boats may not be faster than more traditional shapes in the very light stuff, they do feel nicer to sail, and that's probably more important.

 

Thank you for the explanation!

If I understand it correctly, hulls like 470, 505, 1980 int 14s etc. got "shoulders" e.g. to counter the "bow down" forces of the symmetric spinnaker.

You could say I am redesigning the dinghy that would be the donor for rigg and foils, is capable of supporting 200 kg crew weight, but is only 14 foot long. Stretching out the bow by about 90 cms to decrease the angle of entry, but also stretching out the rear by 90 cms to get a clean water release at the transom without foredeck acrobatics which result if the bow is streched out but the mast stays in its position relative to midlength. (And to avoid capsizing over the transom at 200 kg crew weight again...)

To me it seems pretty easy to argue about Bethwaite's results. Looking at the 49er it is freakin' fast when the wind is up and crew weight hits the optimum righting moment for the rigg/widths. But crews desperately trying to loose weight for Quingdao to get down to the weight closer to matching the static bouyancy was no advertisement for the class. So it all depends on what conditions the designer envisions. 29er and 49er seem to be more designed for Sydney Harbour/Lake Garda/Hyeres/San Francisco Bay which is fair enough.

It was however quite refreshing when the X0-dinghy was designed acknowlodging the fact that for most people with real lives the typical conditions on there actual home sailing waters (rivers, puddles like ours...) are very different.

That said having testsailed it the X0 dinghy would be a solution to my "requirements" but unfortunately it did not get to the point where it would be commercially available, anyway.