Sailing skiff to sportsboat conversion

Discussion in 'Boat Design' started by hambamble, Jun 25, 2013.

  1. hambamble
    Joined: Jul 2012
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    Location: Gold Coast, Australia

    hambamble Junior Member

    Hi everyone. I just bought an old 16 foot skiff. Carbon/kevlar hull, weighs in about 90kg, its an older design so the hull is a bit wider than newer designs. Apparently the owner before the bloke i bought it from put a bulb on the bottom and sailed it as a keelboat, he apparently reinforced the hull round the daggerboard so its much stronger. I am tempted to do a proper conversion and turn it into a little sportsboat. I was originally going to build from scratch, but that's a bit cost prohibitive at the moment.

    Basically I am trying to figure out what would be required. I think at the least it would just be adding a bulb, and moving up from there is lengthening the hull, adding some freeboard, moving the rig/daggerboard, particularly if i lengthen the hull. I like the idea of adding some wings to get some weight out wide. I am limited to about 6m length total so it still fits in the garage, the bowsprit bit sticks about 2 foot in front of the bow, so that means extension to about 18 feet before i have to start chopping things off the bow.

    My main concern is that i don't want to go to all the effort and cost and have built a dog of a boat. I'd rather keep it as an old skiff that's fun but not competitive. I don't expect miracles, but the goal is something fun that keeps up with the fleet and that i can take non-sailing friends out without capsizing.
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    those are a lot of changes, and sounds like a lot of work. If you add the extra length do you think you would need the extra free-board? You presumably would be adding some displacement capacity with the extra length, that is what I would try first. add the length, and than perhaps the wings and see if that gets you want you want.

    If you are not adding more sail area, why would you need the wings? Or do you just want to make it more comfortable to hike out? Add some equvalent deck space perhaps?

    Post pictures, sounds like an interesting boat. You guys down under have all kinds of interesting and fun sailboats available.
     
  3. hambamble
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    hambamble Junior Member

    I'll post some photos tonight when I get home after work. The boat is meant to be sailed 3 up with 2 crew on trapeze, so the wings would mean I could sail without the trapeze, or trapeze off the wings and sail in stronger breeze. Adding length is easy compared to freeboard but I feel if I'm doing one its easiest to do both at the same time.
     
  4. hambamble
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    hambamble Junior Member

    Wow, sorry it took so long to get photos up. Its been foul weather and the photos weren't coming out that well.
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  5. Skyak
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    Skyak Senior Member

    Adding weight takes directly away from performance. The question is how much performance do you give up and what do you get for it. My thoughts are that you are not going to get to a boat that will right itself without dumping it's crew out of the cockpit. At best you can add enough bulb so the hull rights itself and you crawl back in -any more and you start to get problems with the rig being undersized.

    As an alternative suggestion, why not go the other way and invest in sail reefing capabilities that you could tweek to the crew talent and conditions of the moment?
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    It appears you already have a "sport boat" if a bit dated.
     
  7. hambamble
    Joined: Jul 2012
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    hambamble Junior Member

    Skyak, Its not a bad thought. These boats originally have several rigs and the crew would pick the appropriate one for the days racing. I only have the No. 2 rig, but many of the old alloy spars are cheap due to a recent move to carbon. I have a few of the bigger jibs, and a kite, and second hand sails are easily found.

    The current mast has a fibreglass tip that bends when loaded up, and the sail is cut to fit this bend. The problem is that I don't think that reefing it would keep a nice sail shape due to the way they're cut.

    The other component is that I can't right the boat by myself if i capsize. It would be good to have enough lead on the bottom to keep her self righting, even if it means loosing some speed.

    According to the Australian trailerable yacht and sportboat rules, minimum length is 4.6m (15 feet) so no extension is necessary. Ballast is based on CBH (class based handicap) and depending on what I get, and assuming a 'ready to sail' hull weight of say 130 kg, and assuming the handicap over .8 (which other converted skiffs tend to be), I would require about a 70kg bulb.

    So basically, within the rules, this is what i can do:

    bulb of about 70kg
    extend hull to any length
    wings out to 3.5m (about 800mm per side)
    Maximum draft 2.5m

    70kg should be HEAPS of weight to keep her upright, particularly if i use a deeper keel, say 1.5m or so. Now i'm going insane thinking about what the best thing to do is. Might have to do a cardboard mock up to see how the extra length looks.

    Another option is to do a detachable sugar scoop on the stern. I see no reason to do it, but it would be a cool project, and I love design challenges. Just need to make sure it can handle the rudder loads.
     
  8. Skyak
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    Skyak Senior Member

    If you are modifying the design to a rule I can't comment intelligently.

    If you need a bulb for righting single-handed, that sounds like a very good investment (fun/cost). My only input here is go with a very deep low aspect keel and a proportionally lighter bulb -70Kg sounds excessive. Your boat needs to be sailed flat for planing performance so the bulb contributes nothing to high performance sailing. You might use wings to fit the single hand weight to the boat designed for two.

    A reefed rig is always beat by a right sized rig, but if the purpose is for accommodating novices, reef points should be worthwhile (you won't race with a novice). I have often thought about bending a tube to an airfoil profile and using it as the bottom batten at the reef -then turn it to shape the bottom of the sail. The other thing I think you need is a sail selection chart to see what you are missing. Light wind sail rigs might offer nothing to the recreational sailor (I just don't sail when the wind is light). If you are thinking of light wind skiff race rig being useful to offset keel weight you need to consider that rigs are designed to righting so your safety factor could go negative.
     
  9. Petros
    Joined: Oct 2007
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    Petros Senior Member

    the sugar scoop stern is an interesting idea: you can keep your current rudder location and covert it to a spade type rudder on a shaft. the end-plate effect of the hull extension will reduce drag and improve rudder efficiency, and keep the rudder forces off the stern extension. I would try that.

    and than add the wings and leave the freeboard where it is at.

    That looks like a fun boat to go out and play in.
     
  10. hambamble
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    hambamble Junior Member

    I wasn't really thinking about the rules with the design until the other day. Then I thought that if I am going to do a whole lot of modification, it will be more fun if i have people i can compete against, and also i think if it is designed within rules, I imagine it will increase the resale value.

    I totally agree that a 70kg bulb is heavy. Very heavy. I would think that single handed a 40kg or so bulb would suffice, particularly when you consider that if she doesn't right, i can always climb up on the keel and use my own weight to assist. The problem is not only in the weight as well, the heavier the bulb, the more structure is required in the hull to take it. A 30-40 kg bulb i think would require minimal alteration, but 70kg would require substantially more and is a much more difficult task.

    Luckily i just re-read the rules, and i was wrong in my previous statement. The about 70kg was only if I failed to meet other stability criteria. I need to go and measure and calculate what they want, so I'll figure it out later this week when i have some time, but the general gist of it is that it is self righting, and i hear that many of the registered boats are quite questionable.

    As for the spade type rudder Petros, its a good concept, but i think that setup might be a bit of a challenge. If I doubt the rudder would clear the ground when on the trailer, and I don't really fancy going for a swim each time i want to connect/disconnect the rudder. If you have any ideas about how to do it that would be great!
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A 40 kg bulb, cantilevered on a centerboard as you've described, is a lot of leverage, so don't under estimate the reinforcement you'll need, plus a reasonable safety margin.

    There are a few ways to handle this rudder down/up haul issue. The easiest is a weighted blade, which sinks willingly and a "over center" line, mounted aft of the pivot pin, used as an up haul when trailering or in shoals.

    Another approach is a buoyant blade with a line rigged on the other side (forward) of the pivot, as a down haul. Rigged to a auto-release cam cleat, a simple an effective solution.

    Both of these methods place the lanyard inside the rudder head, out of the flow, though still immersed.

    I've used a cable system successfully on bigger boats, in the same fashion, over the center style, with the added benefit of being able to use it for both up or down haul operations.
     
  12. hambamble
    Joined: Jul 2012
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    hambamble Junior Member

    These skiffs regularly see three grown men hanging on the board to right after a capsize. I appreciate that this is quite different as its more of a static than dynamic load, but i would have thought 40kg or so shouldn't require too much more support. Perhaps just extend the daggerboard case up 100mm or so, and link it all in with the framing and the mast step.

    I hadn't even thought about hinging the rudder like that. I was thinking going off the back of the sugar scoop, with a couple of tubes to reinforce, similar to a 49er.

    http://www.sail-world.com/photos_2011_2/Alt_IMG_827920111022 at 130225.jpg

    I have spoken to a few other people who have done similar conversions, and they said that your weight really needs to shift forward and aft when going up/down wind. This makes me think that a sugar scoop isn't such a good idea, as it would ideally need to support the crew weight to get your weight as far aft as you can. It would be better to do the scoop and then deck it in so that it can be better used by the crew, and then any wings can extend the full way back, and you don't need to run forward of the scoop when tacking.
     
  13. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Hey Hambamble,
    Did you ever follow through with this project? I am actually making a daysailer from a skiff of the same size but with a smaller bulb and reduced sail area to help righting moment.
     
  14. basil
    Joined: Apr 2005
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    basil Senior Member

    Hambamble,

    G'day,

    What about going in a completely different direction. Glass in a couple of aluminium tubes and bolt used catamaran hulls to each side and turn it into a very quick tri. You could easily make it trailerable by sliding floats in and out.

    Tony
    Gold Coast Qld
     

  15. Vantage475T
    Joined: Aug 2016
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    Vantage475T Vantage475T

    skif trimaran

    That is exactly what I am doing with my ISO dinghy - thread elsewhere.

    Hopefully some decent weather and tides will sync up and I can take it out for a further test shortly.
     
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