Engineering a mast

Discussion in 'Sailboats' started by jarcher, May 4, 2011.

  1. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    Hi All...

    I am looking for information about calculating the appropriate rig/mast dimentions for a sailboat. I have done some reading about "moment of inertia" and such, but have not been able to find a method of figuring what MoI a particular mast needs based on the boats dimensions, displacement and so on.

    Thanks very much for any tips...

  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    In short, it doesn't work that way. Area Moment of Inertia, or Second Moment of Area might range over an order of magnitude if based solely on the hull parameters. I designed a little skiff which could cartwheel up a beach in the surf without damage to its rig. Its mast was a bit stouter than most. As a practical matter, I just turn the problem over to the folks I get the spars from. If you are building to a design, it darn well ought to tell you about the spars. At the bleeding edge, and even some distance from the bleeding edge, it is a very involved process that requires a lot of specialized knowledge of fittings and available catalog parts. Skene's Elements of Yacht Design is an old but good reference even if you are not planning on a wood mast.
  3. Dirteater
    Joined: Oct 2010
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    Dirteater Senior Member

    Hi jarcher,

    Not to long ago I asked a similar question.:)
    search "lug sail rigging info" in this forum.

    It offer great suggestions on "sail" and such info.

    note: It covers all rigging, not just lug.

    certainly worth your while, :D
  4. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    Thank you for the information everyone. I did call a few riggers and I get very different answers depending upon who I speak with. I realize this is not an exact science. I'm just trying to figure out what ths lightest spar I can use safely is. Sounds simpler than it is, I know...
  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    call a whole bunch of em, My gosh, you're in Rhode Island. Riggers on 1 foot centers there. You could take a poll in the Wal Mart parkinglot. Seriously, if you're actually going to buy something in the next 30 days; go ahead and call six of them. Something will click with one of them and you go from there. The lightest you can go will be the most expensive to buy and the most tedious to live with, but the cheapest won't be the heaviest, if thats any consolation. Is this thing a class boat? Have you checked for an owners association? Some one might have one laying around, or at least be able to refer you to a rigger with experience with your boat.
  6. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    I have two issues. The first is that my mast is very heavy and big, and the second is that I have an annoying baby stay I would like to eliminate. The boat has no chain plates forward of the mast, so I would need to add some. I figure I can do this in stagesl first replace the mast, later eliminate the baby stay.

    No, its not really a class boat. Its a Scampi 30, made in Newport, RI. So its a class boat in Sweden, where there are a bunch of them, but they are not so common here. Someone in Sweden translated the class rules into English for me:

    "The boat must be equipped with a mast of extruded aluminum. Mast
    extrusion shall have a moment of inertia of longitudinal min. 310 cm4 and
    transverse min. 153 cm4."

    I have not yet figured out how to convert cm4 to imperial. This MoI is very similar to a US Spars / Z-Spars Z301.

    My boat has a mast with a Kenyon 5072 section, which is pretty big. It measures 7.4 x 5.08 inches and has a wall thickness of .156 inches. The MoI is 21.30 / 11.50. It weighs 3.9 lb/foot, which is a total of nearly 150 pounds for my 38 ft 3 in mast, and thats just the tube.

    As a result, I have issues with pitching, and I can't bend the mast by tensioning the back stay. So its hard to depower the main without reefing.

    I have contacted a bunch of riggers, and I have received one solid quote, one estimate, and I'm still waiting for the rest to get back to me. Getting them all into a WalMart parcking lot together is a nice idea, but in practice its hard to get many of them to return phone calls. Of course, this is their busy time of year. Maybe in a few months things will be different. Still, its going to be at least $5,000, which is a lot for me to spend right now, especially on a 32 year old boat. So I have not been pushing the riggers.

    There are two places I know of which destroy boats. They hang on to the rigs and sell them. I plan to visit them this week to see what they have, but in order to do that I need an idea of what I can use.

    So, I started by comparing my boat to similar boats. Its a masthead rig that weighs about 7,500 pounds. The numbers look similar to a J/29 masthead rig. That uses a Kenyon 4060 section. A friend of mine has a boat which is very similar to mine, and he uses a Kenyon 3656 with two sets of spreaders.

    I was hoping I could find a used section in good shape, figure out what kind of spreaders to use and where to put them, and get a machinist to make a compatible mast step. I can probably reuse my head stay and back stay, which are rods, but the shrouds are pretty heavy.

    So that's the scoop...
  7. Dean Smith

    Dean Smith Previous Member

    send ALL the boat dimensions inc disp and ballast ratio
  8. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    Okay, but I'm not sure what the ballast ratio is...

    Here are the numbers:

    Sail dimensions as measured by my sail maker are as follows:

    I= 38.85 ft (as noted below, the mast is actually shorter than this)
    J= 11.85 ft
    P= 33.4 ft
    E= 9.80 ft

    Hull numbers from Solna marketing data:

    Length overall: 29' 9"
    Length waterline: 22' 9"
    Beam: 9' 9"
    Waterline beam: 7' 10"
    Ballast: 2,640 pounds
    Displacement: 6,600 pounds
    Draft: 5ft

    Now, when the guys launch the boat, I ask them how much it weighs. They
    tell me about 7,500 pounds. I know a big Yanmar engine has been added to
    the boat, but its probably only about 350 pounds. I can't explain the
    difference between the data sheet and the scale. Hm, maybe its the massive
    mast I have ;-)

    The length and beam numbers are accurate.

    Chainplate Information:

    This is a masthead rig boat.

    The boat has four shroud attachment points. The two big ones are inline
    with the mast and each one is exactly 42 inches from the center of the
    mast step. These can each accommodate three shrouds.

    There are two much smaller chainplates for lowers. These are 18 inches aft
    of the big ones, and 1 inch inboard. They can accommodate one shroud.

    There are no shroud attachment points forward of the mast step, but there
    is a baby stay attachment point 4 feet forward of the mast step. I
    currently have the baby stay removed because I don't think I need it with
    my phone pole. It gets in the way and the mast is too stiff to allow any
    pre-bend. So far 25+ knot winds have not been a problem. No pumping I can

    Boom Measurements:

    Goose neck position:

    34 ft 2 in down from main halyard sheave at mast head

    3 ft 9 inches up from deck

    Boom length: 128 inches (10.6 feet)

    Existing Mast:

    Height is 38 ft 3 in

    From the class rules (in Swedish):

    "The boat must be equipped with a mast of extruded aluminum. Mast
    extrusion shall have a moment of inertia of longitudinal min. 310 cm4 and
    transverse min. 153 cm4."


    Luff of main: 33.7 feet = 33 ft 8.4 in
  9. Paul B

    Paul B Previous Member

    You have not said whether you plan to use a single spreader rig or a double spreader rig.

    You have the Chainplate Width. You need this and the righting moment of the boat to calculate the required moments. For a Half tonner of that era you could use 400 ft-lbs.

    The 310 x 153 is roughly the same as an 7.5 x 3.7 in^4 section. Many single spreader Half Tonners from that era used a section that was about 8.0 x 3.5 in^4.

    I think the Kenyon 3656 might be a bit whippy for your purpose. With double spreaders and checkstays it could be OK. The 6040 size might be a good choice, a bit overkill but still bendy. Your current rig is way overkill.

    In a modern rig I would tend to not use the babystay or the aft lowers.

    If you are serious about purchasing a new rig you should contact Hall Spars there in RI.
  10. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    Paul, thanks very much.

    Well every other Scampi I have ever seen a picture of, or heard about, or spolen to the owner of, has a double spreader rig. However, a standard Scampi rig is not an option for me because my existing rig is taller and I have bought a bunch of sails that fit it. so i guess that does not really matter.

    I was under the impression that a less sturdy mast needed more spreaders, while a strong mast needs fewer. I have been looking at other boats while walking around marinas, and I have noticed that masts that taper severly have 3 or 4 spreaders, while thicker masts have 2 or 1. For example, a Dayler DB2 at my marina has 4 sets of spreaders and a very thin mast at the top.

    I guess the short answer is that I thought it depended upon the mast and rig, and I have no objection to using two spreaders if it means I can use a lighter mast.

    I'm sorry, I am not sure what you mean. Is 400 ft-lbs a measurement of the righting moment?

    Thank you for the metric to imperial conversion!

    Yeah I was thinking this as I looked at the dimensions and moments of those spars. A friend with a very similar boat has the Kenyon 3656 and so far, so good. I was also thinking the 6040 was a little bigger than I wanted. If it is still bendy then I guess that's good. The J/29 masthead has a tapered 6040.

    I actually looked at a Kenyon 3656 today on a mast rack. I kept thinking it looked pretty darn thin, but I loved how light it was.

    How can I safely eliminate the baby stay? If I just don't use it, won't the mast be able to bow aft and snap? People have told me my mast will "pump" if I don't use it.

    It really depends upon the budget. I do have a call in to Hall. I'll try them again tomorrow. But I think they mostly are doing carbon now.

    Thank you!
  11. Paul B

    Paul B Previous Member

    I have seen Scampis with single spreader rigs. In fact, I don't think I have ever seen a double spreader rig on one that was original. Of course I haven't seen that many Scampis in the last 30 years. Perhaps they did go to the double spreader rig in the Mk III production. With the wide CPW you mention it sounds like it would be for a single spreader rig originally.

    Yes, going with a smaller mast will require more spreaders. But it isn't quite so simple. You can always add spreaders to make a lighter section strong enough athwartships. However, this does not help the longitudinal momemts. In many 3 spreader rigs with conventional spar shapes there is an internal stiffener on the nose (and sometimes at the back) to shore up the moments. I don't think you want to play with this on an old Scampi.

    The Dehler DB2 would have 3 sets of spreaders and a set of jumpers. Those boats had very whippy rigs and the one out here lost at least 2 rigs in less than 2 years of competition when they first arrived.

    Yes, that would be an estimate of the RM.

    What type of boat does your friend have? Yes, the 6040 is a bit of overkill. The good thing is it is a very available tube. Kenyon had it, Hall had an almost exact copy, as did Forespar. It was used on the J29, Capri 30, Santana 3030, and many other production boats. It would probably work out as a single spreader rig for your boat.

    The J29 you mention doesn't have a babystay. Neither do most rigs designed after 1980. None of these boats have the masts inverting and snapping. The babystay on most racing rigs was there to help bend the old, too-stiff masts. It wasn't there to stop the rigs from pumping.

    I also think Hall is mostly working in Carbon these days. I just wonder if they are still doing some tin rigs for production customers and perhaps they have something between 6040 and 5636, or maybe they have a suggestion for using something they have in stock.
  12. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    I see you're in California. The Scampis I have looked at were here on the east coast, built in Newport RI in the late 70s. They are all essentially MK-IV boats built from 1977 to 1980ish. I don't have the exact dates, but what happened is that someone out here noticed the success of this boat in Sweden and decided to try to produce and sell them here. They made a deal, brought the molds over and built some boats.

    I guess it didn't go too well for them, as they are long gone. I apologize if you already knew that. I have never taken a close look at pre-1977 Scampis, so maybe that's the difference.

    That is very interesting, thanks for the info. No I want to keep it simple. I suppose I should then look for a section that meets or exceeds the suggested MoI in the longitudinal direction since that can not be stiffened easily. If I am guessing right, Ixx is longitudinal while Iyy is side to side?

    I always forget, I'll ask him again tomorrow. It is *heavily* modified. He got a practically destroyed boat and pretty much rebuilt it, inside and out. He even modified the stern. He spent several years doing it all. But the thing I recall is that its similar in overall length, max beam and displacement. I don't know about length at waterline or chainplate width, I'll ask him about that.

    One rigger told me the 6040 is "marginal" as a single spreader rig but solid as a double spreader rig. I noticed that the J/29 masthead rigs are double spreader, using the same spreaders as the lighter 3656 (the Kenyon SP3 spreaders). Also, the J/29 mast is tapered at the top,m I assume to reduce weight aloft. That rigger had some software he put all the dimensions into. I have no idea what it was, and googling around has not revealed anything along those lines.

    I write software for a living, so maybe once I learn more about this I'll whip something up. I just ordered a few books that I hope will teach me a bit more.

    Wow, that's great to hear! So many people have been trying to tell me about the "pumping" and disastrous and spectacular failure I would suffer if I take that baby stay out. Of course I took it out ages ago with the existing mast and have not had the slightest issue. I was just worried about not putting it back after a switch to a lighter rig.

    But, without either a baby stay or chainplates forward of the mast, I assume I won't be able to create any prebend? Is prebend all that important when I can just tension the back stay?

    Someone else told me that Hall might have some Kenyon 6040 left from having bought out the Kenyon stock when they got out of masts. I know RigRite has a bunch of Kenyon stuff and they are literally 15 minutes down the highway. They quoted me $2,499 for a 38 foot 3656 tube only!

    I have not been pushing Hall because, frankly, I am expecting the quote to be very high and I don't want to waste their time. I really don't want to put a brand new $5,000 to $9000 into a mast for a 32 year old, 7,500 pound IOR boat.

    So I am looking for a used but good tube I can retrofit on to my boat. An Evelyn 32 mast looks like a good bet for the 3656 tube while J/29MH is good for a 6040. I found an Evelyn 32 mast on Craigs list, but it was damaged and had been repaired with a sleeve. I'm not too excited about that but if it was repaired correctly it should be okay. The repair is in the gooseneck area, and it may prevent me from using the spar because it blocks the bottom 15 inches of sail track groove right above the gooseneck..

    If you had to choose, what would you suggest, the overkill 6040 or the undersize 3656? My priorities are weight reduction (especially aloft), an increase in flexibility and the elimination of the baby stay. I'll be doing almost entirely inshore racing, except that I'll probably take the boat as far as Block Island (about 13 miles south of the Narragansett Bay). I doubt I'll ever do anything like Newport to Bermuda in this boat. I am thinking these parameters favor the lighter section?

    Thanks very much Paul, this information is fantastic!
  13. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    400 ft-lbs doesn't sound correct for the righting moment for a 29', three ton boat. That's one 200 pound person standing 2 ft off center. Is there a zero missing?
  14. Paul B

    Paul B Previous Member

    No zero missing.

    That is the estimated RM@1degree.
    The estimated RM@30degrees would therefore be around 13,500 ft-lbs.

  15. Paul B

    Paul B Previous Member


    The 6040 is probably marginal as a single spreader rig (I have not run any numbers). Just a quick guess makes me think the lowers would be right at about 10 degrees with that CPW and a single spreader.

    The 5636 looks marginal for a double spreader rig, given that you are deck stepped.

    A rig design program is fairly simple, if you have good reference material. It can be done on an Excel spreadsheet in a couple of hours. The problem is getting the good reference material. Most sparbuilders have various safety factors and other variables they can play with when looking at a particular boat and the posible use of the boat. You won't find that info in any books. But you will be able to build a basic spreadsheet.

    I've sailed on many boats that started life with babystays and then removed them (SC27, Olson30, IOR boats, etc). When those boats were originallt built the masts were beefy things and the sails were quite stretchy. You needed to be able to induce a lot of bend and a babystay was very helpful. Now sail materials don't stretch as much, so less mast bend is needed as you go through the range. Therefore babystays are no longer as useful.

    Prebend is important. Having a deck stepped mast is problematic. However, you can wedge the mast step (like a J22) to help induce prebend. Be sure to discuss this with your rigger.

    If the E32 rig is about the right length (I think it is) and the repair is done well that might be a great choice if the price is right. The E32 is a bit lighter than your boat but they stack the rail with 7 bodies or so and that really adds to the RM. So if it is working for them it could be OK for you. Just be sure to keep paying your insurance premiums.

    If you are using a bolt rope mainsail (no slugs) then the 15" of missing track shouldn't be a problem.

    When it comes time for resale some potential buyers might shy away from the boat if the rig looks too noodly.
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