Mirror 16 Build

Discussion in 'Wooden Boat Building and Restoration' started by Sam C, Oct 8, 2021.

  1. Sam C
    Joined: Sep 2021
    Posts: 8
    Likes: 1, Points: 3
    Location: Boise, ID

    Sam C Junior Member

    I'm in the process of planning and hopefully building a Mirror 16.

    A little background: The Mirror 16 had its heyday in the 60s and 70s. It was sold in kit form to be built by the owner at home. It was "stitch and glue" construction (meaning that the panels were temporarily tied together with copper wire, the joints were fiber glassed, and then the wires were removed). The boat is very light and stiff for a wooden boat. No plans were ever released and the patterns for making the kits have disappeared (as far as I know). I have spent the last couple months putting together a set of plans that should result in a boat that will meet the class measurements for the Mirror 16. (I've attached them to this post and the old 'Mirror 16' thread). I'm hoping to build this as a second sailboat that the whole family can ride in in the coming months. This isn't my first plywood boat and I am very familiar with fiber glassing.

    I have several changes that I will implement or that I'm considering implementing to hopefully improve the design (or because I'm having to guess). These (and my reasonings for them) are as follows:
    - Change centerboard to daggerboard - to reduce the size of the hole through the boat and provide better support for the daggerboard.
    - Added 2" of width to the bench seats - to make the boat more comfortable to sit in (hopefully)
    - Added hinged cushioned seats to the benches - for comfort and to provide a sealed cover for the bench boxes.
    - Provide an option for a non-retracting rudder - more reliable easy to make.
    - Provide an alternative daggerboard design that should pop up if the water gets too shallow
    - Provide vent tubing to all of the water(air)tight tanks - to allow the pressure in the tanks to equalize without stressing the boat. (I think this may have contributed to the premature demise of many of these boats).
    - Fiber glassing hull, ribs, floor, and bulkhead on both sides (not just the joints) - to provide stiffness, strength, water proofing, and impact resistance (and hopefully not too much weight).
    - Leave a bit more web in the ribs - strength and impact resistance
    - Add (smaller) flanges to both sides of each rib against the hull and the floor - this will likely take a bit longer to build but should help the ribs (and hull) survive impacts better.
    - Add reinforcing blocks below the floor and seats - for a better connection to cleats, thwart, toe straps, mast king post, etc.
    - Fully glass the keel rib and daggerboard case - add strength to the most stressed members and prevent the leakage that ruined so many of these boats.
    - Increase the size of the chain plate blocks - spread the mainstay load out a bit more in the shear panels
    - Add an asymmetric spinnaker - unless someone is willing to measure their spinnaker pattern for me.
    - Multicolored sails - why not
    - Use modern rigging components - they are available
    - ¼" marine grade plywood (douglas fir) for all plywood panels and glass it both sides once installed - this will be heavier but more damage tolerant.
    - Scarf joints between plywood panels - stronger than butt joints and less likely to crack or rot out.
    - Aluminum extrusions for mast and boom - fairly available and I have no idea what the original mast and boom were made of.

    Hopefully my modifications won't add too much weight. The original Mirror 16 usually weighed around 260 lbs. I'm hoping I can keep mine below 320 lbs. It should have a fairly reasonable max weight of 1000 lbf.

    Thoughts, impressions, criticisms, and advice welcome.
     

    Attached Files:

    Skyak likes this.
  2. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    A little strange to glass it all. The amount of glass to make it watertight is going to add quite a bit of weight.
     
  3. Sam C
    Joined: Sep 2021
    Posts: 8
    Likes: 1, Points: 3
    Location: Boise, ID

    Sam C Junior Member

    From what I've been able to gather, the mirror 16 is fully fiber glassed on the outside and most of the joints inside are taped. Glassing the inside of the hull and the ribs will probably double the fiberglass weight. Adding glass on both sides of the floor will add more. I was guessing that it would add around 60lbs. I still need to do a full estimate of the weight of the boat, though.
     
  4. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    The original boats were never glassed on the outside,only along the chines and other seams.There was one for sale on the UK ebay site a few days ago incidentally.I would caution about adding too much extra strength and weight as this will seriously slow down what was quite a fast boat.The analogy I like to use is a hammer-the impact when you use a heavy one does more damage than a tap from a light one.The boat needs to be just strong enough and as designed won't be too challenging to handle ashore.Adding 100lbs will only make the job harder.Its a shame that the Jack Holt plans weren't carefully archived as he was probably the man responsible for more small boats than any other individual.With an archive,they could have gone on being built for many years to come.
     
  5. Sam C
    Joined: Sep 2021
    Posts: 8
    Likes: 1, Points: 3
    Location: Boise, ID

    Sam C Junior Member

    Yeah, I wish more of his plans had been preserved, too. It would have made this process a lot easier. I missed that the outsides weren't glassed in the instructions. I'd had seen some pictures in other threads where the outside was glassed and had assumed that was typical.

    I guess I also should have started with my list of requirements.
    - Family cruiser boat (i.e. comfortably fit up to 5 people of various sizes and their stuff and still be fun to sail)
    - Fit on the trailer with the Hobie cat (weigh less than 500lbs, 6ft beam, 16ft long, mast less than 22ft long)
    - Look awesome
    - Fairly quick to set up
    - Easy to maintain
    - Home built

    You got me thinking, so I ran some numbers. My drawings are in CAD, so it's not difficult to get the areas of different parts.
    Here's how everything shakes out:
    - Plywood areas - 290sf (everything), 233sf (everything except seats)
    - Plywood weight - 192lbs (¼" doug fir) vs. 160lb (original, unless the original 5mm ply weighed more than 0.5lb/sf) I'm somewhat stuck with the ¼" (6mm) ply where I live.
    - Fiberglass - 80lbs (everything both sides), 65lb (everything except seats, both sides), 50lb (finish coat visible surfaces, wet out hidden) vs. 20lb (taped joints only and smear extra resin in tanks)
    Assuming that the 260lb figure (listed in several places) is a fully rigged weight, that would mean that all of the hardwood trim/framing, rigging, sails, mast/boom, etc. weigh an additional 100lbs.
    Depending on what I do (and how accurate my assumptions are) my build will likely be somewhere between 30lb and 100lb heavier than the original (290lb to 360lb or 10% to 40% increase in all up weight). The stated payload capacity for the original Mirror 16 was 1000lbs. I'm planning on reducing the payload capacity of my boat down to 900lb to account for the possible extra weight of my boat.

    I understand (and completely agree) that additional weight in a collision situation increases the energy involved and ends up causing additional damage. I've also had design situations where strengthening something ends up increasing the forces and it still fails regardless of the strengthening. The options there are to go the other way or try something different. I'm not certain that this is one of those cases. I do intend to run some impact calcs on a few configurations just to see if the increase in strength and durability is worth the extra weight or see if there is a more optimal option. I also understand the effects of weight on the speed of a boat like this. Sadly, I have several factors working against me for the speed to matter (mainly my size, location, and other boat). Competitive sailing isn't a thing where I live and the odds of me meeting another Mirror 16 owner/builder in person is getting closer to 0 every day (unless people start building them again).
     
  6. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    I can't remember ever seeing a payload capacity stated for the Mirror 16,I also wouldn't advise trying to fit 5 people in one.In fact a couple and two sub teenage children would be quite enough and then only on a calm day.I found one of the few online images of one and would invite you to consider where the jib sheets would be while you are sailing as this restricts the crew space a bit.The boat has rather a lot of sail area and you don't just loll around while it wafts along.Just mentally choreograph the process of tacking when the helmsman and crew have to move from the windward side deck,across the boat and take up position on the other side deck.Where will the other occupants move to that isn't in their path or within the arc of the tiller?In anything much above a force 2,you will be required to use crew weight to keep the boat level.By the time the wind reaches force 4 it will be planing most of the time and will be demanding quite a bit of physical effort to sail unless you reef.Which sort of defeats the purpose of choosing a design with good planing performance.The Mirror 16 isn't by performance standards a tippy boat and is well mannered according to those who have sailed them.

    I have a feeling that the figure of 1000lbs may relate to the volume of the built in buoyancy.Which is intended to keep the boat float after a capsize and is rather a lot more than the weight of the hull plus crew.Which could be partly accounted for by the double bottom.Not only does this shed spray quickly and effectively,it was a feature to allow camp cruising as you could spread a sleeping bag on the flat surface with some certainty that it would stay dry. In essence I think you ought to regard the design as a racing dinghy for 2 people that can also be used for other things in the right conditions.I believe there is a group on Facebook for those interested in Mirror 16's but since I don't do Facebook I know nothing about it.

    [​IMG]
     
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  7. Sam C
    Joined: Sep 2021
    Posts: 8
    Likes: 1, Points: 3
    Location: Boise, ID

    Sam C Junior Member

    I ended up finding the 1000lb capacity in this archived article about the boat. I get about 24cf of volume in the floor tanks which would amount to 1500lbs of freshwater buoyancy. Subtracting the weight of the boat and an extra margin (so water isn't constantly coming into the boat through the slot) and 1000lb sounds about right.

    I know about the sail area (slightly more than my Hobie cat for a boat with a narrower beam and no trapeze). It sounds exciting in a good breeze. I'm planning on putting a furler on the jib and making the sail with the reef points like the original. I've been debating about whether to make a second "family friendly" mainsail that is somewhere around 75sf for more docile sailing when the wind might be a bit much. I haven't decided for sure and I probably have at least a year before I'll get to that.

    In other news, I put together another prototype in cardstock. I exported the plywood panel shapes as dxf files and was able to use my wife's CNC cutter to cut out a 12:1 scale cardstock model. I taped everything together and noticed that the bulkhead shape doesn't quite work for cardstock (or likely plywood). The bottom hull panels do not want to bend as sharp as the bulkhead profile is shaped. I will relax that curve and repost the plans. Luckily, that change does not look like it will affect the rib shapes.
     

    Attached Files:

  8. wet feet
    Joined: Nov 2004
    Posts: 741
    Likes: 133, Points: 43, Legacy Rep: 124
    Location: East Anglia,England

    wet feet Senior Member

    Congratulations on tracking down that article.The way I read the description of the buoyancy suggests that the amount of air in the tanks would give 1000lbs of buoyancy,which is a lot,particularly for a boat that weighs 260lbs.After all,the wood of the hull will have positive buoyancy in any case.Using a density of 62.5 lbs/cu.ft that would indicate 16 cu.ft of air contained within.I would urge extreme caution about adding more volume to the tanks.The reason being that after a capsize the boat would float so high that climbing on to the centreboard to right the vessel might be rendered very difficult by the height of the centreboard,should excessive buoyancy be built in.Such excessively buoyant boats tend to blow downwind faster than their tired sailors can swim and it can end unhappily.At the very least,the windage of the high floating hull will blow it downwind of the rig;which will mean that if the crew are able to attempt to right the boat they are immediately on the lee side and can do little to prevent the boat blowing over on top of them.The usual cure for this state of affairs is to have one of them swim to the bow to act as a kind of sea anchor so that the boat weathercocks as the athletic one slowly brings it up.
     
    Sam C likes this.

  9. Sam C
    Joined: Sep 2021
    Posts: 8
    Likes: 1, Points: 3
    Location: Boise, ID

    Sam C Junior Member

    Thanks for the run down on righting one of these. I've contemplated having a rope ladder stored somewhere in the boat that can be tied to the thwart and tossed over the high side of the boat to help with getting the boat righted. I would expect the tanks to be a bigger hurdle to getting it righted since they enclose almost 7cf of volume each. The seat tanks have openings in a few locations in the original documents but those openings are not likely to be below the water line even in a capsize.

    I think I have the floor in about the right location. I did have to guess but I used several photos online and in the assembly instructions (as well as measurements 31 and 35 in the class measurement form) to work out about where to put the floor. I'm pretty confident that I'm within ¼" of the original (±1 cf). (If the floor is too low, the joint between the floor panel and the bottom hull panels would stick out from under the benches near the transom and there wouldn't be room to put the aft tank stoppers in the transom. If the floor is too high there isn't enough room to fit the seats below the inwales at the transom).
     
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