Designing the Lightest Nesting Dinghy in the World

[benjy1966]

Ahoy All,

I wrote this article for a yachting magazine some time ago and I thought you might enjoy the thought process behind designing a lightweight nesting dinghy:

http://www.ventspleen.com/?p=209

Cheers

Benjy

www.woodenwidget.com

[messabout]

Nice work Benjy.

[ancient kayaker]

Very neat. My main interests are wood boats, lightweight boats, miniature sailboats, skin-on-frame boats, and nesting boats, so this hits a lot of buttons for me. I must confess I would have been concerned to eliminate or perhaps disguise the step, but it seems to work fine.

[benjy1966]

Ahoy Terry,

If you can suggest any way to lose or disguise the step and still keep the volume and the nesting capability, I'm all ears! But as you say, in use the step is quickly forgotten, it's only the way it looks that seems odd. It really doesn't seem to impact on performance at all.

Cheers

Benjy

[michael pierzga]

Nice boat ! The sun shines thru....

a tender on a small craft is always a challenge. A few years ago I met a cruiser who built his own thirty foot cruising boat. A craftsman designer.

He custom built a two piece , glass foam core, ...bow and stern bolt together, tender .
The aft section of the two piece tender was molded to the inside dimensions of the sailboats cockpit. Since a small 5 foot box is not a good row boat, tender, , he took this molded cockpit profile section then added...bolted on... a pram like bow section. When the tender was in use it was probably 8 or 9 feet long and a good shape .

To stow in the tender in the cockpit he separated the cockpit shaped stern section from its bow...lowered into the cockpit...fastened securely , then took the bow section and placed it over the mid cabintop hatch, fastened on top of a purpose built frame....this pram bow now functioned as a spray dodger for the hatch.


Very clever , functional and attractive,

[benjy1966]

Ahoy Michael,

Thanks. I love the way it lights up. Very handy when navigating at night. Just put a light inside and it glows like a Chinese lantern!

That boat you saw sounds very interesting. Amazing the lengths people will go to to find the right dinghy for their boat. And why not, having a good dinghy can certainly improve the whole cruising experience.

A few years ago I designed the Deckster

http://www.woodenwidget.com/decksterinfo.htm

Mainly because I wanted to try the Hobie Mirage Drive in it but also so that I could try stowing it around the mast on the cabin top where it wouldn't be in the way on the deck using a removable bow section that could take the mirage drive or a keel. It worked but was too short and too heavy plus it made the mother yacht look awful and boxy. It was the design that finally made me realise that I didn't ever want a heavy dinghy which then led to the Stasha.

Cheers,

Benjy

[michael pierzga]

Yah,Great idea !!... the mast cut out , very practical. The trick to a dual purpose...part cabin top , part dingy is in the initial design phase of the vessels super structure. With thought a nice dingy could be designed to store on the cabin top for sailing...acting as a doger ...then launch the dodger tender and head to the beach for beer !!
'
In the end, if the customer wants a rib, water ski tender , hes out'ta luck...but for a general purpose...get that baby in the water fast..the little deckster is great.

[ancient kayaker]

Originally Posted by benjy1966

Ahoy Terry,

If you can suggest any way to lose or disguise the step and still keep the volume and the nesting capability, I'm all ears! But as you say, in use the step is quickly forgotten, it's only the way it looks that seems odd. It really doesn't seem to impact on performance at all.

Cheers

Benjy

I looked at this problem a few years back but got sidetracked by other boat design challenges. from what I can remember there are several ways around the problem. The usual one is to put the joint some distance from the broadest part of the hull; that usually leads to a 3-piece boat and is my personal preference as it also reduces stress on the joints.

Another is to make the "cut" where there is considerable flare in the sheers so the upper part can just sit on as well as in the lower part. This works particularly well on shallow hulls like rowing shells.

Another trick depends on whether you are most concerned with looks or performance: if performance, then the step can be left above the waterline and smoothed out below the waterline where the hull is narrower and easier to fit inside another part. In essence the lower part of the hull is widened to accommodate the upper part but only above the waterline.

The intention to make it a nesting boat has to be considered when designing the lines as there is an interaction. I'm not saying it's easy, the challenges are unique to this field, but if any of the above gives you an idea ...

[benjy1966]

Ahoy Terry,

Flare in the sides is out because a keel needs mounting on one side when sailing, that's one of the main reason the sides are upright.

Not quite sure I understood the nest suggestion because the underside of the Stasha is not stepped. The step is only at the sides.

In practice the small (1") step really seems to make very little difference and from most angles it's hard to see anyway. I just couldn't see any way around it. The advantages of putting the back in the front and not the other way around seemed to so far outweigh this single (visual) disadvantage.

Thanks for your thoughts

[ancient kayaker]

In that case you have already done what can be done by the sound of it. The only thing left would be to hide it with side decks but that would add weight and complexity, and is not in the spirit of the design. I think it is a great little boat and hard to improve.

Just for interest, I am investigating a new (to me) construction method for a partially strip-built hull in which laminated half-ribs are attached to a ply bottom, the curve of the bilge is constructed with cedar strips, and the sheer planks are ply. It has been pointed out to me that it can also be built as SOF if the ribs are attached to the underside of the ply bottom and the usual stringers added. The ply becomes the floorboards, however it is not removable. If you build another SOF that is an idea to consider; it has the benefit that no building form is required. Still working on it ...

[Jeremy Harris]

I quite like the idea of a SOF boat with ply garboards. I'm familiar with the general fabric and frame construction of lightweight SOF, from building aircraft, and know just how tough it can be. My only concern with building a very light boat (for a very specific one-off task) has been getting the ability to take point loads right at the bottom of the hull. Ply garboards would fix this at a stroke.


Benjy,

I just love the nesting dinghy design. If I still had a yacht it's definitely be on my list of boats to build. There's something intrinsically attractive about lightweight SOF boats.

Jeremy

[SamSam]

One thing I always wonder about craft like that is it's durability. I wouldn't think it would have any abrasion resistance, and the least brush up against something sharp would slice a big hole in it. Loading it with hard objects would be a little risky.

[benjy1966]

The Stasha was born of a need for a very light dinghy. Even adding the thin 4mm ply floors adds a whole 2 kilos, or 20% of the weight of the hull! Adding ply to the design would add much more weight.

Using a water based varnish which does not soak into the skin allows the Dacron to retain it's anti tear properties. I abused my Stasha all summer and never put a hole in it despite some pretty rough treatment including running up the beach on the surf with two of us in the boat.

If you were really worried about the vulnerability of the skin, it could be covered with two layers. That way the kevlar string could also be hidden between the two layers of cloth. That would only add about 600 grams but a plywood bottom (even a minimal 4 mm thick) would add about 4 kilos. I'm just guessing here.

It's amazing how quickly a good design can be spoiled by adding too much weight. I calculated that even one coat of varnish added about 200 grams!

I've been thinking of ways to reduce weight not add it. For example by making the bulkheads in two layers of 5mm ply instead of 1 layer of 10mm. That way parts could be cut out to reduce weight. Maybe the bolts and joining pieces could be Titanium. It would be interesting to see how light it could be made.

[benjy1966]

Oh yeah. I carry a roll of duct tape aboard at all times for a quick repair if needed but that Dacron cloth is incredibly tough and I haven't needed it yet. I stowed the keel in it badly one time and for a few days it was really pressing at the cloth but despite the sharp end, it did not puncture it but it did stretch it quite badly. A quick pass with a hot iron and it pulled out. It really is a lot tougher than you think.

[ancient kayaker]

Originally Posted by benjy1966

... It's amazing how quickly a good design can be spoiled by adding too much weight. I calculated that even one coat of varnish added about 200 grams! ...

Oh wow! - and I thought I was obsessed with weight ...