Homebuilt sailing dinghy for children

Dear Forum,

I'm designing and building a little dinghy for my boys, and I would like to share some ideas and pictures with you.

The boat should be
- suitable to teach children how to sail
- fun to sail for one kid, and allow to have a second kid on board
- faster than an Opti (our sailing club is on a river, sailing is only possible when VMG > current)
- lightweight
- self bailing
- shorter than 2.51 m (no need to pay taxes, no license plate, no periodic check of riverworthiness)
- fitting through the doors of our basement
- built from easily obtainable materials
- not necessarily a one-design racing class
- not necessarily very fast to build

In addition it should have a real bow and it must be decorated with a parrot (wishes of my son).

With kind regards

Ara

Suggest you take a good look at John Spencer's Firebug.

http://www.firebug.co.nz/

If by real bow you mean a vertical stem, and not a pram bow, that's a big ask on a practical 8ft boat.

Design

The dinghy was drawn using FREE!ship Plus, Version 3.4. The shape was chosen in a way to allow building with "stitch and glue" from 3 mm and 4 mm plywood sandwich (poplar/epoxy/carbon).


@ gggGuest: [FONT=&quot]The Firebug, but also the Northbridge Junior, Thomas Nano 2.6 (http://microskiff.blogspot.ch/) or the OpenBic ha[FONT=&quot]v[/FONT]e been used as inspiration. [/FONT]
[FONT=&quot]As you can see on the attached lines, I had to cheat a bit with the bow. The boat officially ends at 2.50 m with a blunt bow, and maybe I will add a removable styrofoam fender to add some centimeters of length.[/FONT]


With kind regards

Ara

Preparing the wooden cores

The "Develop Plates" function of FREE!ship Plus was used to get the coordinates for cutting the plywood. 3 mm poplar was easily cut with a Stanley knife, and then the two halves were stuck together with double sided adhesive tape and clamps for sanding.
Once symmetric, the halves were stitched together with wire and glued with epoxy and microballoons.

With kind regards

Ara

You should check out some of Gavin Atkin's work.

He has two designs that might be of interest: his 'Flying Mouse' and his 'Eek!' designs.

Ara said:

The dinghy was drawn using FREE!ship Plus, Version 3.4. The shape was chosen in a way to allow building with "stitch and glue" from 3 mm and 4 mm plywood sandwich (poplar/epoxy/carbon).


@ gggGuest: [FONT=&quot]The Firebug, but also the Northbridge Junior, Thomas Nano 2.6 (http://microskiff.blogspot.ch/) or the OpenBic ha[FONT=&quot]v[/FONT]e been used as inspiration. [/FONT]
[FONT=&quot]As you can see on the attached lines, I had to cheat a bit with the bow. The boat officially ends at 2.50 m with a blunt bow, and maybe I will add a removable styrofoam fender to add some centimeters of length.[/FONT]


With kind regards

Ara

Click to expand...

Nice work! I like the fact that the ends are pretty balanced, given the size and style of boat. Most top designers these days have realised the problems of the extreme wedge-type hull, as seen in the Bic and Thomas Nano.

Weight

Most of the requirements specification above is somehow linked to weight.

Learning to sail on a lightweight boat should be easier, because it reacts immediately to gusts, body movements, tiller movements, waves, changes in sheet tension, etc. Especially kids seem to profit from the direct link between cause and effect.

Lighter boats are of course more fun to sail. However, I wanted at least the cockpit floor to be sufficiently strong for a second kid or even an adult (e.g. myself, at 70 kg).

Boatspeed is also linked to weight. And since I am not a trained naval architect, weight reduction is one of the obvious ways to go: I do not know what underwater shape causes the least drag, so I try to get as little volume and surface as possible under water.

Handling of the boat inside my basement boatyard, transporting it on top of the car, and bringing it from the storage rack at the sailing club to the water is all easier with a lightweight boat.

The "Layer properties" dialog of FREE!Ship Plus allowed to estimate the final boat weight for different combinations of plywoods and laminates. The specific weight of poplar (approx. 300 kg/m^3), biaxial carbon (200 g/m^2), glass (80 g/m^2) plus epoxy gave a target for the hull somewhere between 17.5 kg (very optimistic, half the weight of an Optimist) and 20 kg (realistic).

A balance was used to record all the pieces added to the hull. Trials with small pieces of carbon/plywood/carbon sandwiches showed promising mechanical properties. However, the hardware store grade plywood is quite porous, and it soaked up more epoxy than expected. At least it should be waterproofed by this treatment. Building from a little heavier marine grade plywood - if available - would probably give a similar result.

The attached pictures show the main bulkhead and the sides ready for lamination. Smaller pieces up to the size of the bulkheads were laminated in vacuum bags using a pump salvaged from an old refrigerator, larger pieces were done without vacuum.

@sharpii2: Thank you for the hint to Flying Mouse and Eek. Gavin Atkin's construction is probably a lot more reasonable than my attempt here: faster to build, cheaper, using less of the toxic epoxy and finally as much fun to sail...
@CT249: Thank you for the encouraging comment!

With kind regards

Ara

Whilst weight is a big contribution to ofwind speed, I think you will find hull length and displacement speed more of a problem upwind. At 2.5 meters this will significantly restrict the speed unless you plan on offwind all the time....

Good luck with it, I used to sail on the tidal Thames and anything much under 4 meters was too slow for working upwind and against the tide, though if there was enough wind things like a Mirror could just do it.

Hull construction

The transom and bow pieces were coated with carbon and glass. Then the still uncoated bottom of the hull was aligned on the floor. The bottom was still very floppy and fragile at this stage, and it was placed on blocks of styrofoam to ensure the correct scoop/rocker line and V-shape. Transom and bow were stitched to the bottom and glued with epoxy.

On the bottom of the sides the laminate was trimmed not exactly flush with the plywood, but with about 3 mm of excess. This protrusion served then as a guide to align and stitch the sides onto the bottom.

The main bulkhead between cockpit and foredeck was prepared for the later installation of the cockpit floor by adding a U-shaped row of plywood pieces.

Hull weight by now was 5.56 kg.

The next step was coating the inside with carbon (200 g/m^2). I did not use an additional layer of glass on the inside, because it can stay rough and does not need sanding.

With kind regards

Ara

Upwind / Downwind

SukiSolo said:

Whilst weight is a big contribution to ofwind speed, I think you will find hull length and displacement speed more of a problem upwind. At 2.5 meters this will significantly restrict the speed unless you plan on offwind all the time....

Good luck with it, I used to sail on the tidal Thames and anything much under 4 meters was too slow for working upwind and against the tide, though if there was enough wind things like a Mirror could just do it.

Click to expand...

Hello SukiSolo,

I forgot to mention about our sailing area in the first post. The river is flowing from east to west, and luckily the westerly winds are prevailing here in central Europe. So downwind performance is more important than upwind performance. Children will surely first learn to sail in westerly winds.

Nevertheless I hope that at least in autumn it will also be possible to sail upwind against the current. We are sailing above the dam of an electrical power plant, and after a dry summer the amount of water flowing down the river is much smaller than in spring, when floods of rain and melting snow prevent any sailing.

With kind regards

Ara

Try a 'Sabot'. quick to build, sails fairly well, good for 2 kids, and you can row it. Been around for years and a good training boat.

Ara said:

The transom and bow pieces were coated with carbon and glass. Then the still uncoated bottom of the hull was aligned on the floor. The bottom was still very floppy and fragile at this stage, and it was placed on blocks of styrofoam to ensure the correct scoop/rocker line and V-shape. Transom and bow were stitched to the bottom and glued with epoxy.

On the bottom of the sides the laminate was trimmed not exactly flush with the plywood, but with about 3 mm of excess. This protrusion served then as a guide to align and stitch the sides onto the bottom.

The main bulkhead between cockpit and foredeck was prepared for the later installation of the cockpit floor by adding a U-shaped row of plywood pieces.

Hull weight by now was 5.56 kg.

The next step was coating the inside with carbon (200 g/m^2). I did not use an additional layer of glass on the inside, because it can stay rough and does not need sanding.

With kind regards

Ara

Click to expand...

===================
Looks good ,Ara!

Daggerboard and -case

Three more bulkheads made from 3mm plywood, coated on both sides with glass (80 g/m^2), were added to support the cockpit floor. Drainage holes were left open in the corners between bulkheads, hull sides and hull bottom.

For the daggerboard I chose the classical, reliable NACA0010. The core was cut from styrofoam (30 kg/m^3) using a hot wire. The wire was heated by a 12 V car battery charger and pulled by the gravity powered "interception theorem" mechanism. The laminate layup is on each side 4 layers of carbon (200 g/m^2), with fiber orientation alternating +-45° and +-90°. Lamination under vacuum was done in two steps. After curing of the first side, a groove was cut into the core to take up a spar made from two layers of 2 mm birch plywood. The daggerboard weighs 1.06 kg. For safety reasons the trailing edge consisting of 8 layers of carbon was not sharpened razor-like, but left at a thickness of about 1.7 mm.

The top of the daggerboard was then used as a template to laminate a case. A slot for this case was cut into the bottom of the hull, and the hull was placed on chairs and leveled. The daggerboard case was glued into place, while the vertical alignment of the board was checked with a plumb bob.

With kind regards

Ara

Buoyancy

I planned to build two separate, closed compartments below the foredeck and below the cockpit floor. Both should be big enough to keep the boat floating, and I do not expect that both will severely leak at the same time.

Nevertheless I wanted to add some solid buoyancy, which is also used to take up some loads from the cockpit floor and the mast step on the foredeck. 75 liters of styrofoam (15 kg/m^3) were glued in between the bulkheads and in the foreship using polyurethane foam. Blocks of styrofoam were also placed around the daggerboard case. The daggerboard was put inside the case during application of the PU, to make sure the expanding PU would not squeeze the case too much.

In between the bulkheads the styrofoam was cut to the correct height using the hot wire. Little strips of stainless steel foil helped to avoid burning of the bulkheads and to provide a slippery surface for the wire.

With kind regards

Ara

Rudder gudgeons

"It's easy, you take a block of polyoxymethylene and you just chip away everything that doesn’t look like rudder gudgeons" (Michelangelo).

With kind regards

Ara