"Designing" a double-handed racing dinghy

Hi everyone,

I would like to "design" (I am aware this will be a far cry from what a professional dinghy designer would do/achieve) a dinghy for my situation mostly to educate myself and to dream about sailing during the off-season.

If it will be build at all at some point in time in the future, it will be a lousy DIY-job for the same reason - I cannot pay professionals to design and build a one-off.

The main theme is to come up with something to sail on a small lake with very variable wind. Days with +/- 45 ° shifts and strengths varying between force 0 and force 3 (force 1-2 in average) or between force 2 and force 6 (force 3-4 in average) are common. The dinghy should not be optimized for top speed or a single speed range, but deliver a kind of consistent performance across the windspeed range.

The simple solution a.k.a. "putting lead under it" is out as it should be possible to launch from a beach and provide some "responsiveness".

On the plus side the lake is consistently > 2 m deep, no waves, only minor chop and no powerboat wake.

Some other constraints:
Width < 1,90 m for road transport
Combined crew weight 150 kgs with an very agile crew, 200 kgs with less agile crews.


What are design aspects you would look for? Thank you!

 

Welcome to the Forum TS.
Let's turn this around slightly - there are so many established dinghy designs out there - which ones are you attracted to for using as a reference vessel for your design?

Do you like any of the two handed dinghies from RS Sailing?
RS SAILING - The world's largest sailing dinghy manufacturer. https://www.rssailing.com/

Or the Topper Taz? There are 7 other different Topper designs as well in the Topaz link here.
Topaz Taz | Topper Sailboats. https://toppersailboats.com/topaz-taz/

Or perhaps a Pico from Performance Sailcraft?
Pico https://europe.performancesailcraft.com/collections/pico-boats/products/ocean-play-pico

Or a Laser Vago (which replaced the Laser II)?
Laser Vago https://www.sailcenter.com/en/shop-by-boat/laser/laser-vago/boats/laser-vago-bt12001.html

 

What materials are you familiar with/able to obtain/considering for a build?

Stitch and glue with marine plywood might be nice for a first build.

I would focus less on theoretical performance and pick a completed design from a designer with a size and material set that suits, and that stirs your heart a little, enough to keep you motivated to complete it.

Macht's gut!

 

Vago is a really good design for a give-or-take 130 kg crew. We actually sailed one with 200 kgs between us and it did not sink... but it was very, very slow...

 

Danke!

 

Bitte!

 

Take a look at the Goat Island Skiff. It is a simple build, with good performance. The design is straight forward and you can use the GIF among other similar boats for a starting place. That said, your best bet is to buy a set of plans for an established design and go from there. You will save money, and very likely save some disappointment, if you insist on designing the boat yourself. In addition, the resale value of a proven design is much superior to one that you have designed yourself.......................... If you are an experienced and proven small boat designer, then ignore the previous comments.

 

Well, if I would be a proven small boat designer, I would not ask here

I appreciate your concerns, but please be assured that my primary goal is learning during the design process, not becoming a DIY boat builder. If I would build the resulting design at all, I would scavenge foils, rigg and sails from our current dinghy. Any remaining material costs for the hull would be "tuition fees" a.k.a. write off, I bought and sold enough dinghies to understand that aspect

While I admire all the clever ideas that went into the GIS to make it easy to build and relatively fast, it does not really qualify as "doublehanded racing dinghy" in my mind.

 

Windmill Class Association Home http://windmillclass.myfleet.org/
The Windmill was originally designed for home builders. It was intended to be the next step up from the Optimus. Plans can be obtained for a very reasonable price.
Building a Wooden Windmill http://windmillclass.myfleet.org/static/woody.html
Building a Wooden Windmill http://windmillclass.myfleet.org/static/woody.html#Plans

-Will

 

I like the look of the Windmill and the relatively low weight. But according to that webpage recommended crew weight and sail area would be rather small for a 200 kg crew.

 

Simply all the basics.
What about you, what would you look for?

 

From my current experience I would see the following (often conflicting) aspects for a design:

Light winds:

• The transom should be clear of the water so crew weight needs to be close to or forward of the center of buoyancy. In some designs this means that the forward hand needs to be in front of the mast. In a gust the crew needs to move back again, which takes too long when starting in front of the mast => The center of buoyancy needs to be well behind the shrouds to allow the crew to stay behind the shrouds => Either the hull shape needs to be "special" or the hull needs to be relatively long.

• The wetted area should be as small as possible => Short, "roundish" hull

Light puffs

• I believe that it is better to let the boat heel over while the puff lasts than messing up the flow across the sails by easing the mainsheet or by moving crew weight around too slowly. This should not distort the flow of the water along the hull too much and I do not like it when wedge shaped boats with an extremely wide transom heel over forward/sidewards instead of only sidewards.

Medium winds

• a narrow angle of entry at the bow is supposed to reduce resistance => Narrow hull, long hull, or max width far aft.
• a long waterline is fast => long hull, small/no overhangs

Strong gusts

• In general they will last only for a few seconds, so the theoretical possible speed does not have considerable affect on average speed in a club race etc. However I believe it is important that the boat can accelerate and turn the "extra wind power" into more boat speed, e.g. to still be able so gybe safely. Some dinghies struggle to accelerate in a puff, my impression is that they lack areas in the underside of the hull that generate sufficient dynamic lift.
• A low weight helps => short hull and/or low cross section to reduce hull volume
• In any case I do not like water sloshing into my face ILCA-style when hiking, so some chines in the rear half would be good.
• A relatively big main with offboom-sheeting for fast reactions combined with a relatively small jib that can stay cleated seems to work better than a large, overlapping jib that is big enough to capsize the boat on its own.

 

A lot of your requirements are about the rig, rather than the hull. Most of the "Book of plans" type dinghies have pretty awful rigs. So you need to devote a good bit of thought to that. Unfortunately a carbon fibre spar will meet your needs far better than a metal one. What you need is a rig that will twist off at the top and dump power almost on its own without a great deal of intervention from the sailor. Small changes in things like the angle of the spreaders can make huge differences. Bethwaite's first two books are in my opinion the only sound published writing on this subject, much that is written is hopelessly outdated. Bethwaite's second book is a good source for you on dinghy design generally. He can be very Sydney focussed, and you have to allow for that, but there is a lot of good information there. Also have a good read of this web site. SailCraftblog https://sailcraftblog.wordpress.com/

If high speed offwind performance is not a big priority you might wish to look at Canoe derivatives (its not widely advertised, but the Finn dinghy is a canoe derivative!), the various European lake boat classes, and the UK's Thames Raters. All these share a common theme of a long and reasonably narrow hull that's easily driven, so is very good in light to medium winds, but are less effective at flat out planing. length is generally going to be your friend if you want to carry weight. I think the traditional German classes would have a lot to offer you in concept, I would start by looking very carefully at them.

 

I tried a Finn-Dinghy and it felt like it would hit a "speed limiter" when I tried to accelerate in a gust on a beam reach... that's why I want at least some capability to transition to higher speeds. The
long pre-WWII "Rennjollen" are also visually appealing

I`ll throw in "something to hate", and yes, it does look pretty ugly:

 

I tried two drafts as "book ends", 295 kg would be e.g. 150 kg crew weight and a 145 kg boat, 388 kg would be a 200 kg crew and a 188 kg boat:

At 295 kg displacement:
Waterline beam: 1,05 m
Wetted surface area: 4,4 m2
Center of buoyancy: 2,77 m from transom
Draft: 0,13 m


At 388 kg overall displacement:
Waterline beam: 1,14 m
Wetted surface area: 5,1 m2
Center of buoyancy: 2,73 m from transom
Draft: 0,15 m
Entrance Angle 13,3°

Actually if it the boat would really weigh in at 188 kg sailing ready, it would be really hard to launch in our place...