Anyone built a Mirror dinghy?

[snowleopard]

Coo. Sounds interesting. Good luck, and let me know if any information from the original pack would be useful. I think there's a set of full size patterns in there too.

Their instructions for assembling the basic hull shape might be useful. As for the full size patters - I tried to buy a set on eBay just before I bought the wreck but they were withdrawn because the copyright holder banned them from being sold.

I would have preferred to make up the panels with both sides pre-glassed but a sandwich is too rigid to take the curve required in the bottom panels towards the bow. That means I'll have to glass the outside without benefit of a mould and there will be a few hours fairing with a long-board

The construction is very strong and light. I anticipate 1mm of glass on the inside and 1mm glass + 0.5mm kevlar on the outside.

As for the structure, I am cutting out the main mast bulkhead and plan to sail it with a una rig (standing lug). Not sure whether to keep the side tanks as I'd rather have more space for carrying gear but I may have to keep them to provide rigidity.

 

[snowleopard]

That sounds an interesting way to build a dinghy. I'm quite keen on getting a rigid tender... and I have a few sheets of PU foam left over from a building project... how light, and how durable, does this construction end up being? Does the foam mean you can build without buoyancy chambers?

Not sure exactly what you mean - is it Kingspan or something like that? Generally PU foam is bad news as it isn't strong enough. After a bit of flexing it is liable to break down and allow the skins to separate. Also if water gets into the core (almost inevitable) it will wick through and the whole thing will become waterlogged, massively increasing the weight. The correct material is PVC foam which unfortunately costs as much as best marine ply! I have recently heard of a new polypropylene foam with a surface fibre to allow the laminate to bond. Around £14 for a sheet 1200 x 1200 x 10.

The construction is tremendously light and strong, typically 1/3 or less of the weight of a solid grp boat. My cat, 12m x 7m weighs only 5 tons all up.

The foam in the sandwich is only 10 or 12mm thick so not enough buoyancy to keep the boat and crew afloat. The buoyancy would be about 800 newtons.

 

[Kelpie]

Ah OK. Nevermind! I've got some sheets of 25mm Kingspan, did wonder if it would be strong enough. I'm pretty sure it's closed-cell though- I used a chunk as a float in a water tank for months and there was absolutely no sign of any takeup.
I'm toying with building a rigid tender over the winter... something that would take a sail maybe... obvious choice is ply, but I was curious if foam+glass would work out lighter, stronger, or cheaper. Probably not!

 

[snowleopard]

Lighter yes, by a big margin. Stronger - yes. Cheaper - no chance.

But - for ease of build GRP is streets ahead. I expect about 1/5 of the labour time for building an equivalent ply dinghy.

Derek Kelsall who has pioneered a method of building round bilge hulls from flat panels claims to be able to build a 30 foot cat hull in a day with a team of 4.

 

[Kelpie]

Well sounds very interesting. A 7ft pram dinghy surely won't cost me all that much in foam/grp, will it? And it has to be light to have any chance of replacing the deflatable as a practical tender.
Do you have a link to any useful info on this sort of construction?

 

[snowleopard]

The standard method for foam sandwich building involves making a male mould out of rough timber and laying the foam over it, attached by screws or sewn with fishing line. The outside is glassed then the plug is removed and glass applied to the inside. It has the advantage that you can make it any shape you like. link. I have done 4 hulls this way.

The alternative is to use flat panels, laminated on a melamine-topped table (mine was 45 ft x 6 ft) using a vacuum bag to foce the foam into the laminate followed by the second skin applied to the top of the foam. These are then assembled over frames and/or permanent bulkheads and the seams joined with tape as per a Mirror. Link

The really clever one, developed by Derek Kelsall, is to make flat panels with selected parts of the inner skin omitted so that it bends only in certain places - that gives you a U-shaped trough. By cutting narrow darts in the skin the single curvature is then bent into a 2-way composite curve. Link

Google on Foam sandwich boat building and you'll find loads of stuff.

 

[DownWest]

Just a thought,
why not put the foam panels on the complete old hull as a jig? Then glass over.
As for old epoxy, try the heat gun. If it is epoxy, it will come off before the wood catches fire....150C? Andrew Simpson turned of the vac pump on one side of Shindig´s hull (glass over strip-plank) too early and had to pull off the resulting mess of bubbles. Faced with a massive grinding job, until someone suggested the heat gun. Un morceau de gateau.

Any body interested in a 40x20 cat? Not mine, but two lower hulls in strip/glass (smooth!) and all the bulkheads in foam sandwich. 40ft melemine table for the side panels, some rolls of glass and drums of epoxy. Set up at the moment, but could be knocked down for transport. Obvious catch, it is in France. I´m holding the key for it.
A

 

[Kelpie]

SL- any clues about where to get PVC foam or what I'm looking for? A bit of googling reveals either craft shops or ceiling tiles- is it all the same stuff or is there a 'marine' version with a couple of extra zeros at the end of the price tag?

 

[Bajansailor]

Just a thought re single skin vs foam sandwich - for a little dinghy it might be very feasible to just make the panels single skin while still keeping them very thin and having sufficient stiffness.
Especially if you can introduce some curvature wherever possible, as this stiffens up a flat (wobbly) panel considerably.

I am currently (sort of, it is an on-going project....) building an 11'4" long double chine 'dodger dinghy' which will be two part and nesting (in 6' long), with just a ring frame connecting the two halves together. The plan is for it to stow upside down over the companionway, forming a 'solid' dodger there.
The hull has two chines each side, and is made from GRP sheets laminated with a CSM / WR / CSM combination. Although in retrospect I could have made the panels just from a single layer of stitchmat of approx the same weight (cant recall the weights used offhand, they were what was available here at the time).

I think that if you are building for yourself for ultimate lightness, and realise that you cant take a flying leap into the dink from the yacht without running the risk of breaking something, then it is possible to build a hull shell very light indeed.
An excellent example of this approach taken to the max are the Geodesic Aerolite designs here - http://www.gaboats.com/construction/.
I saw a few of these dinghies and met the designer in Maine 15 years ago, and they are really quite amazing.
This link was found in the Cruising Resources website - they have an excellent link re hard dinghies here at http://cruisingresources.com/Hard_Dinghies__p6

 

[electrosys]

I'm in the process of dismantling one to use as a template for a GRP replacement.

You might find http://mirror70407.com a useful source of info, or as an aide-mémoire when slicing up the hull. There are modern build photos there as well as some original ones by Barry Bucknell.

I'm just coming to the end of a Mirror restore (due to bl##dy cr#p quality plywood) and it would have been useful to have known how the thing was put together before I applied the angle grinder (!) - but Sod's Law being what it is, I found the info a couple of days afterwards ...