Rotating or fixed mast?

Discussion in 'Multihulls' started by aussiebushman, May 6, 2010.

  1. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    I'm sure I'll get a lot of flack about this, but it is a genuine question. I'm building a Trinardo 6.2m Tri using the rig, hulls and sails from a mark 4 A Class with a purpose-built new centre hull. The A Class mast is of course, a rotating alloy foil shape with the ring at the head for tensioning the luff.

    The issues are:
    • I have no experience of rotating rigs - only fixed ones
    • I will be cruising only - no interest in racing
    • Most sailing will be single handed
    • Setting up the rig when launching will also be single handed, with my intention of using a gin pole and winch to raise and lower the mast. This will be much easier to do if the mast can be coupled to a hinge joint at the mast base before raising it and the luff tensioned with a simple halyard and clutch system
    • There will be plenty of times when sail reduction will be essential and though I understand the principle of spilling wind using the rotating rig, does not dispensing with the existing tensioning arrangement, adding a couple of reef points to the sail with a single line reefing system provide greater safety and convenience for single handed action?
    Advice from cruising sailors and/or designers will be very welcome.

    Regards

    Alan
     
  2. catsketcher
    Joined: Mar 2006
    Posts: 1,298
    Likes: 143, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Don't go the A

    Gday Alan

    My little 600kg 6 metre cat 4.4m wide will probably have about the same righting moment as your tri - maybe mine has less than a Trinado as they are wider. I have a 16ft skiff rig on it and have bought a broken Etchell mast as a replacement. The 16 stick is in no way capable of handling the loads from a stable boat.

    I don't know the moments of your stick but a typical rotating A class stick will be less able to handle load than a well trussed skiff rig and I predict it will wobble like crazy and then break in about 15 knots. I don't even like th idea of you submerging A class hulls. A's are well known as being lightly built.

    I would urge you to buy an old Hobie 18 and use the hulls and rig from this boat. A surf boat tri was built with A class parts 20 years ago and was very fragile. Better to change now than to do it after you have built the boat.

    As for the rig - rotating is done on the small cats so the mast should stay rotating to help hold it up.

    cheers

    Phil
     
  3. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    G'Day Phil

    Thanks for your advice. If I had been able to find an H18 at a decent price I would have bought that instead of the A Class. However, in view of your comments, I'll keep looking and a delay in finding one will not stop me from progressing on the centre hull and beam structures. (The latter are nearing completion)
    I'm not sure you have addressed my concerns about rotating versus a fixed mast, in particular regarding the hinging and reefing. Further comments appreciated

    Cheers

    Alan
     
  4. Gary Baigent
    Joined: Jul 2005
    Posts: 2,995
    Likes: 116, Points: 63, Legacy Rep: 509
    Location: auckland nz

    Gary Baigent Senior Member

    Aussiebushman. you don't spill the wind with a rotating mast, you semi stall the rig by flattening the mast, that is pulling the spanner central.
    You could probably use the A Class rig but you'd have to considerably beef it up, first by sanding or etching it, laying on one layer of epoxy fibreglass, then a few layers of uni directional carbon fibre - tricky but it is possible. Some will say no, no to this - but it will work. They freak out thinking the carbon is taking all the loads and if it breaks, then the mast will quickly fold. It is BS. A stiffened spar is a stiffened spar, end of story. Yes, I have done this and the mast is still up after 15 years. Having said that, a heavier section mast like a F18's would be a better bet.
    The same beefing up would have to be done to the A Class hulls, more glass/carbon and more weight ... but it could be done.
    A rotating mast is superior to a fixed one because it provides a clean wind flow from mast to sail, less drag plus you have the ability to flatten or make fuller your main for different wind and sea conditions by differing angles of rotation. Go for a sail on one, fix the mast fore and aft, then ease it round - the boat will immediately accelerate. Reefing is no different than a fixed rig. You need simple but effective mast spanner controls.
     
  5. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    Thanks Gary

    That cheers me up because although it was under a couple of grand for the complete A Class, I have already done a lot of work setting up the frames for the centre hull where the spacing of the beams and the relative hull widths are critical dimensions. I have also al,ost finished fabricating the new beams and these have obviously been designed specifically for the dimensions of the A Class hulls and I would be very unhappy about having to make new ones.

    The A Class hulls need repair work (also started) and your suggestion of epoxy/glassing them is something I had considered already, because of the skimpy way they are built - essentially just gelcoat over thin ply with a few stringers and some foam stiffeners. As to weight, the epoxy and glass will add some, but I'm also seriously considering filling the hulls (or at least the forward and aft sections) with pour-in urethane foam. This will greatly stiffen them and make them a lot safer in the event of hitting a log or whatever.

    I mentioned in a different post that the centre hull is being built right-way up directly onto the trailer for a whole lot of reasons, not the least of which is orientation. Pictures are attached

    Cheers

    Alan
     

    Attached Files:

  6. Gary Baigent
    Joined: Jul 2005
    Posts: 2,995
    Likes: 116, Points: 63, Legacy Rep: 509
    Location: auckland nz

    Gary Baigent Senior Member

    Alan, that looks a very substantial main hull and it will be heavier than I envisioned .... and therefore the beefing up of A Class hulls, beams and mast will be essential, essential. Also you will have to curve the beams down at the outer ends because your A Class floats are very low wooded, low freeboard and if the beams run straight to the float decks, your boat, when sailing, is going to heel over like a monohull. You want the float bottoms to just kiss, or be slightly above the water when the tri is at rest afloat.
    Don't fill the main hull, or the floats, with urethane foam - better to have bulkheads with accessible hatches so that the interiors can be aired, rot otherwise.
    I hope that setup on your trailer is not the trimaran's overall beam .... because if it is the boat is way too narrow ... and would be very dangerous to sail. Hope I'm reading this wrong. Email me if you want some help: coxcreek@slingshot.co.nz
     
  7. catsketcher
    Joined: Mar 2006
    Posts: 1,298
    Likes: 143, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Main hull hydrodynamics

    I don't mean to rain on your parade but looking at the main hull I wonder if you would be better pulling the whole thing down and starting again. The main hull looks very wide and a low flare hull is very tricky to design. Unless you get the weight exactly right then the low flare hull is going to get fat very quickly.

    I did this very problem with my little cat. I underestimated the weight and although the boat is nice it does need some foam on the hulls for cruising. I had built boats before, like you, and just got it wrong.

    My advice would be

    -reverse engineer a similar sized boat - F22 or similar - use this as a minimum weight. The main hull looks (on a very rough viewing) like it has too little flotation for this weight.
    -ditch the A class stuff. You will be way over load and weight so make your own floats - floats are easy to make.
    -If you are immovable in going ahead - redraw the lines and pull the flare up more - low flare is what I use in my little cats but as I said before it is much much harder to design for than flare higher up. If you have a slight problem with a higher flare hull shape no worries but with a low flare the problem gets serious.

    I only say this because I wish someone had come in one night and split my hulls down the centre when I was at your stage. I would have less work to do now

    cheers

    Phil
     
  8. jamez
    Joined: Feb 2007
    Posts: 506
    Likes: 28, Points: 28, Legacy Rep: 231
    Location: Auckland, New Zealand

    jamez Senior Member

    I looked at doing this a few years ago, and gave at least one designer a bad moment by enquiring about building one of his creations with Tornado floats and rig:D . That said, A few people have knocked together lightweight beach tri's that have worked ok with existing cat hulls and rigs.

    I can think of two longer boats built with Tornado floats that ended up going to larger purpose designed floats (one is the oft mentioned Bieker/Brown Trinado, the other Dragon a 25'racing tri here). Even the Tremolino, probably the most numerous donor tri, has had a bunch of bigger disp (than the original H16) floats designed for it.

    Alien is IMO the type of boat that might successfully utilise strengthened A cat components. If you join the Multimarine forum on Yahoo there is quite a bit of discussion and a load of pics about this boat.

    As an example the Scarab 650 gives an idea of the relative proportions required in a 'bigger' more powerful boat with cabin and accomodation. http://www.teamscarab.com.au/scarab650/design.html

    You might also check with Chris Ostlind who has designed/conceptualised a number of lightweight tris around 18 - 22 foot
     

    Attached Files:

  9. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    Hi Guys

    Greatly appreciate your input and Phil - don't think I am dismissing your suggestion of starting again but I have already looked hard at the Scarab - fabulous boat, also the Catri (my favourite, but too costly as would be a F22 clone), plus a few other design options from Kurt Hughes, Dudley Dix and Kristofferson etc. It is just that I'm semi retired, so labour is not an issue, money is. I have quite a lot of boat building experience and think there are practical solutions that will work with what I have started.

    I need to explain a couple of ways in which the pictures give a wrong impression:

    • The frames seen in the picture are mostly temporary building frames, to be removed after strip planking. Only the transom, three main bulkheads and the stem assembly will remain, I'll tape the others when epoxy-bonding 9mm Paulownia strips to be covered inside and out with 300 gsm double diagonal glass cloth and epoxy, the combination of which will bring the build weight about halfway between foam and WRC. Interior fitout will be mostly in foam panels
    • The outer hulls as shown in the pictures are there for alignment purposes only (since removed fror repair during the hull-building process). The lowest point of each ama is a few mm above the DWL of the new centre hull. The existing catamaran beams will now be telescopic. They have been cut in half and extended with sliding glass-wrapped alloy sections to make the final overall boat beam 4.6 metres, yet slide in to a trailer width of 2.5 metres. New upper beams will make up a triangulated structure. They are hinged at the centre hull and lock down onto the lower beams at the amas to provide rigidity and strength. The ends will be rounded off at the outboard ends
    I accept that extra bouyancy chambers inside the amas might be better than foam fill. Glassing around the hulls with a light cloth and epoxy should provide sufficient additional strength.

    Re the rig, Sheerline Spars in Sydney are unbelievably helpful and there is a stronger second hand mast available than would certainly overcome the problem you have all raised about the strength of the A Class rig. If Sheerline will accept the A Class mast as part-exchange for a heavier rig it will be a lot less work than wrapping the latter in carbon fibre as suggested by Gary - though that is certainly a worthwhile option as a fallback
    I think I have now sold the Catamaran (see www.mainproject.info) so will shortly be able to devote more time to the tri project

    Cheers

    Alan
     
  10. CatBuilder

    CatBuilder Previous Member

    Not getting into the actual boat, but sticking with the rotating mast question, I would not get one. If you are a cruiser, your main goal at sea will be to keep things simple. The rotating mast adds another level of complexity, IMO.
     
  11. Gary Baigent
    Joined: Jul 2005
    Posts: 2,995
    Likes: 116, Points: 63, Legacy Rep: 509
    Location: auckland nz

    Gary Baigent Senior Member

    Alan, good to see you're looking at alternative masts. One point: if you should ever reinforce an alloy mast with carbon, it is absolutely essential that the first layer onto the roughened alloy, is FIBREGLASS. If you don't do that, electrolysis will commence fizzing from the carbon contact ... and pretty soon your mast will be useless. A glass/epoxy layer breaks the connection.
     
  12. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    I have no input on the mast. Just want to say good on you for doing this. I would like to make my own similar tri. I am thinking of starting with a Nacra 5.8 or similar to have stronger everything and more buoyancy. I would probably use an F22 or something as a guide to designing vaka lines. I would keep the rotating mast though.

    Do you have a dedicated thread or blog on this boat? Any design drawings? Please tell us more about your boat.

    Dennis.
     
  13. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    Trinardo developments

    There have been some interruptions over the last few weeks in dealing with a buyer (I hope) for the 30' catamaran, including 2 16 hour workdays this week on the slips. Hopefully, I can now get back to the Tri project.

    Kjell - the designer has added some important advice about the Vaka flare. His design was intended for closed waters and minimal wave action around Stockholm and he suggests that more flare might be good for the conditions we have here in Eastern Australia where the waves can reach several metres with a heavy swell.

    One option would be to start again with a different design but I have already pointed out the need to keep this on a "shoestring" budget even if it means performance compromise so I'm more inclined to stay with what I have done so far with some essential mods. Therefore, I propose to leave the lines as drawn at the aft end of the hull and reshape the forward section to give more lift. I would do this by bonding the first few strip planks at the aft stations, then packing out the temporary forward frames to retain a fair curve

    Though I have plenty of boat building experience, I do not have the design skills to make the changes to the lines and welcome advice about how much additional flare to add. Attached are the lines drawings of the forward stations.

    Thanks to all. Alan
     

    Attached Files:

  14. catsketcher
    Joined: Mar 2006
    Posts: 1,298
    Likes: 143, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Gday Alan

    I can't really help here but my worry was that the flare was too low at the back and the middle from what I see. I am not sure that adding more flare up front would help. What you have to decide is
    -will there be enough freeboard from the waterline to the point where the flare begins to fatten? My guess is that you need at least 300mm of pretty vertical surface. The immersion rate of such a small tri is really low so putting two people and some gear on it will sink it really quickly. If you haven't enough vertical room above the waterline then you will be dragging flare around.
    - has the designer (and you) got the weight right? If it is too heavy (most boats are) then this flare will be dragging around in chop and light winds. In enough breeze the main hull will rise.
    I can't really see the whole picture with just the forward sections so can't advise. Flare up front on a tri is very problematic. Sure you get more volume but you also get more tripping moment. That is why the A class and huge Irens tris have upside down bows. Flare up front can be bad as it stops the boat fast with increased drag. The sections you posted look okay though.

    Fattening up the front of the boat only will have ramifications with trim as well.

    Get the design as you want Alan - don't worry about the cash involved in some daggy sheets of chipboard. You should have the design absolutely perfect before you start. I like Chris White's advice "Everything you can't throw overboard should be first rate"

    Can we see some sisterships?

    cheers

    Phil
     

  15. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    G'Day Phil - and others who are helping me here

    I already said that I do not understand the flare issue so I'm happy to accept advice from those who do. Here are several of the mid/aft station drawings and getting more flare between the waterline and the knuckle does not seem to be difficult, with any changes to get fair lines towards the bow pretty minimal. Your comments will be most welcome.

    I have looked again at the lines for the Catri 24 which has much greater volume and is optimised for hydofoils. This boat appeals greatly and I have contacted the designers for the plan costs, but I can forsee major expense building it, so modifying the Trinardo design continues to be my best option - with the full consent and assistance of Kjell who designed it.

    Re mast, Kjell pointed me to a similar builder who had trouble due to an undersized mast and this reminded me that the rig on my 30' cat has a mast made up in two sections with a reinforcing sleeve in the middle and a strong pair of diamond spreaders. Sheerline Spars here agree that this solution will work even in high winds on the tri so this seems the way to go - giving a strong mast in perfect column at minimal extra cost

    Cheers

    Alan
     

    Attached Files:

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.