Masts and spars

Discussion in 'Boatbuilding' started by Manie B, May 13, 2008.

  1. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 116, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Hi guys
    would a epoxy and glass tube / pipe,
    wrapped / laid up around say a 100mm = 4" pvc pipe
    make a good mast.

    Reason for asking is that it could be laid up so that it is thicker at the bottom than the top, and you could lay it up to any thickness required.

    Or does a birdsmouth wooden mast simply work better

    still talking about a gaff rig
     
  2. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hallo Manie, moet jy nou flippen staan en tegnies rook oor goeters hier ;)

    A mast is not just a round piece of material standing upright on a boat. There are many things that has to be considered when it is put together, like are there rigging, wiring etc going through the inside of the mast, how is the sail hoisted, the size of the sail, the length of the mast, the fittings attached to it and how it is mounted to the boat (like glue or screw :rolleyes:)

    It can be made from fiberglass, but it is not just simply a matter of fiberglassing a PVC pipe and bobs you uncle you have a mast. The direction of the glass layers is important, and so is the mast diameter for the length an so on.

    A mast does not have to be thicker or thinner in any one place... unless you want it to bend like a fishing rod. Masts just look like that since you take the picture from below and since you're nearer to it it looks thicker... same with high buildings, they are built narrower towards the top :D Trees that became masts in their afterlife were narrower at the top, but only because it was difficult to turn the tree upside down so the thickness would sag towards the ground again.

    Allu masts cannot be extruded from thick to thin what I know off. You will have to use different extrusions that could fit into one another to make it thinner towards the top.

    Are you putting a mast on your wooden boat ? An umbrella would work just as well :D
     
  3. deepsix
    Joined: Dec 2007
    Posts: 124
    Likes: 7, Points: 0, Legacy Rep: 94
    Location: SA

    deepsix Senior Member

    Hi Mannie, I dont have anwers to your questions, but here are some pretty cool ideas I have come across.

    I was just looking through the "building a mast" thread on the catsailor.com forum. There is some interesting stuff about strip planked masts with ply, glass and carbon. It seems much simpler that the birdsmouth method.

    Building a mast Catsailor forum
    Gust Spar Gallert

    [​IMG]
    [​IMG]
    [​IMG]

    Dudley Dix also has some interesting things to say about steel masts for gaff rigs. After all if you can build good boats out of steel why not a mast?
    Dudley Dix Steel Gaff Rig

    Dont discount aluminium based on the prices of professional riggers. I had a discussion about building a mast with a friend who is a professional rigger, it turns out that the actual extrusion is a small percentage of the final cost. Tapering, building sheave boxes, aerofoil spreaders, splicing, T-balls, backstay crane, gooseneck, anodising. All of these things take time and contribute to the cost of the mast. If you buy the extrusion from hulletts and do all the work yourself, and simplify everything, a conventional aluminium mast may be viable.

    Fanie

    Fractionally rigged masts are tapered, usually by cutting and welding or they are spun tapered. It is to make the mast bend correctly and also to lower the
    CG of the mast.

    A variable wall thickness would be ideal due to increasing compression loads. Also you will see some masts are sleeved around the gooseneck to increase the wall thickness to handle the kicker/vang loads.

    EDIT : Sorry I forgot its for a gaff rig, most is not applicable
     
  4. TeddyDiver
    Joined: Dec 2007
    Posts: 2,578
    Likes: 120, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

     
  5. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    And also to reduce the mast mass momentum when the rig pitches - which could be substantial in rough seas. On the other hand, the leverage of the mast would prevent pitch-poling in rougher water.

    Ok, I know that masts are tapered, but not when you wrap glass around a PVC pipe.

    And one more thing, isn't it ideal when a mast does not bend ant at all ?
     
  6. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    I made my tri's mast from allu the microlite guys use, they sleeve into one another so I have 4 layers of allu, and it is still not stiff enough on 6 meters and that's on a small 12m sail. I can only imagine what the big(er) cat's 50m^2 sail forces are going to be in a strong wind. I'm sure the cat is going to plane ;)
     
  7. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 116, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Hi Fanie
    thanks for your reply

    i am so frikken depressed - your post ABSOLUTELLY lifted my spirits thanks:D

    i am unfortunately getting ahead of myself here
    bending moments still appear in a mast - even if stayed
    compression at base - and big time at the boom goose neck
    top sway under the cap shrouds
    so well no fine
    how does a epoxy fibreglass tube perform

    has anybody tried
    i have read all i could on bird mouth wood etc.
    aluminium WAY too expensive
    and yes size does matter
    rather a fishing rod than a umbrella:D

    still on the gaff rig
    38 / 42 sq main
    24 sq jib
    72 sq genaker

    SURPRISE SURPRISE SURPRISE

    5.7 KNOTS SPEED AT 6 KNOTS WIND - LOADED 4811 KG
    CATEGORY BBBBBBBB

    starting to become a very nice cat - heavy displacement - good speed in light winds - afordable - easy to build - hard chine boat - epoxy on ply - small motors

    delftship also very good results
    sailcad similar good results
    days of manual calcs also good
     

    Attached Files:

  8. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 116, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Geez i type soooo slooow that by the time i get back to the thread there are more posts:D

    what i meant by thick and thin is
    its like wrapping insulation tape around a pipe
    first layer all the way up
    second to 3/4
    third to 1/2
    fourth to 1/4
    catch my drift:idea:
     
  9. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 116, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Very interesting to read about the steel mast
    surprising:D
    goes to show hey
    if you dont ask you will never know
    thanks
     
  10. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 474, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A 'glass mast has been done before, though it has limited returns. First, it's generally more costly then more convention means, second, 'glass isn't particularly strong, unless you use epoxy and high modulus fabrics (which is why regular GRP lay-ups are so heavy) and then there's the flexural and fatigue rates, which are lower then most of the other spar materials.

    Steel is a good spar material and was the first after wooden, used by old Capt. Nat in the 1880's on an A/C defender if memory serves me. Of course it's only practical on certain hull forms and those with substantial size.

    A lot depends on size and intended use. Gaffers, tend to be heavier, with higher D/L's and slower roll moments, which is easy on the spars. In this regard an excessively heavy mast could be tolerated, but why, when better suited materials can be incorporated, with less cost and effort.
     
  11. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hi Par, the problem with some materials is that you have to sell the finished boat to obtain the mast :mad:

    I'm sure with a proper design and carefully laid glass one could build a mast suitable for our purposes. I know Manie plans a cat similar in size to mine, and it's not going to be a racer so it may well work...
     
  12. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 116, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Very interesting to read that a man like Dudley Dix has done the homework.

    Nothing wrong with a steel mast for a gaff rig.

    As he has desribed it - it is virtually the same size as i would require.

    Cheap and i could most definately build it myself
    thanks gents:D
     
  13. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 170, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Which boat is this for Manie ?
     
  14. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 116, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member


  15. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 140, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    You can build a fiberglass mast but it will have some drawbacks. Fiberglass, while less expensive, has a low elastic modulus meaning it is not very stiff. To solve that it becomes heavy. It is not especially strong either, also adding weight. It also does not stand up to sunlight very well so it will not last as long (this is true of carbon/epoxy mast too, they need lots of paint). A carbon epoxy mast would be light and stiff, but also more costly and fragile. I have read making a composite mast is not that difficult to make in a home shop.

    You also might consider a laminated wood mast, it can be made lighter by making it hollow. It is relatively inexpensive building it up from smaller pieces, durable and you only need wood working skills, and lots of clamps (you will have to build an assembly jig).

    I have also considered what it would be like to build up an aluminum mast similar to the way an aluminum airplane wing is made; ribs, stiffening stringers, and a thin skin all riveted together. It requires a pretty detailed structural analysis, but it too can be built with simple tools in an assembly jig. Though labor intensive, it should be less expensive in materials and much lighter than a standard extruded alum mast. It would be very strong and light. I may build one someday for the ocean going cat I will be building in my next life.
     
Loading...
Similar Threads
  1. Mikey2
    Replies:
    36
    Views:
    11,891
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.