Adjustable Jaw for Gaff on tapered Mast

Discussion in 'Boat Design' started by Daniel M, Sep 23, 2021.

  1. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    For reasons too long to post I am trying to put a gaff on a tapered steel mast. The taper is significant enough that I would like to come up with a solution for it.

    My first idea is to use a steel jaw with a pin hinge built in that would allow it to be cinched relatively tight or at least closer to the mast when set but flex open to be lowered or raised. I would leave a static set of parrels on it at the largest diameter of the mast as a "safety" and allow the cinch lines to be completely slack when hoisting.

    This is just a brainstorm that I want to run by a more experienced crowd before I sink to much time into it. We are talking about a 78' main and 67' mizzen COR-TEN steel, halyards will be run internal and maybe the cinch lines as well to keep them out of the way. Both will have over 1000 sq.ft. of sail area so they need to be robust.

    The dimensions are accurate but don't get hung up on the design, I just drew up something quick instead of trying to explain it all (it is much more clear in 3D, sorry). Materials and the fine points of how to do it will come later, for now I just am trying to figure out IF it will work. Or if there is already a better way that I don't know about.
    Jaw01.png
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Congrats on your first post Daniel.

    Re the taper, is the total difference (over the height of travel of the gaff jaws) the 4" between the 14" and 18" shown on your sketch?

    I am just thinking that if the gaff will always effectively be in compression against the mast, does it really matter if your parrel lines (adjusted for the 18" diameter lower down) are a bit 'looser' (for want of a better expression) when the gaff is hoisted to where the mast diameter is 14"?

    Is the luff of the sail laced with rope to the mast between the foot and the gaff jaw, or are there hoops around the mast to which the luff of the sail is attached?
    They should also help to keep the gaff jaws snug with the mast (?)

    Are you building a new schooner, or retro-fitting an existing vessel?
    We would be interested in the rest of the project as well - if you can post any photos, they would be most welcome, and help to give an overall understanding of your project.
     
    Last edited: Sep 23, 2021
  3. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    Thank you!

    Yes, it is a total difference of 4" in diameter from the gooseneck to where the gaff is set.

    That is the real question, is looser ok? I really don't know, all (3) gaffs I have sailed have had a parallel mast, and they were also a lot smaller so my knowledge is very limited. I have been known to over think things on occasion, that is why I am seeking a second opinion!

    I am leaning toward lacing and am fine with them being a bit looser, the sail will loose some shape but that is ok.

    It's a retrofit, a big one. I am converting a 128' lugger to a topsail schooner.
    Starboard Stern.jpg Sailplan.png
     
  4. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Thanks for the update - that certainly looks like a very interesting project.

    What is the history of the lugger? Did she used to carry cargo under sail in Alaska?
    And is this your intention after you finish your re-fit, or will you just be carrying passengers?

    Did you design the rig shown in your second photo?
    I am just wondering if you will get a lot of weather helm with the main mast so far aft?
     
  5. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    It was designed as a go-anywhere research vessel, now the family and I plan to treat her like a traveling second home (leave her parked in Juneau and mainly sail her in the summers). Eventually we plan to cruise full time.

    Yes the sail plan is mine, the weather helm was a concern of mine due to the mast placement of the masts as well, from the initial calcs I did it looks like the CE will be about 11' forward of the CLR, which I think will manageable, she has an enormous skeg which should help. I am meeting with a naval architect tomorrow in fact to go over the sail plan.
     
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  6. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    Option 2 is a traditional fixed jaw using the angle to take up the fore and aft slack and just live with the 2" gap on either side. This is far simpler and has less lines. Just wondering if that slop will be a pain or an issue in light wind conditions. Again ignore the specifics of the design just a rough sketch up to give you the idea.
    Jaw02-Gap.png Jaw02.png
     
  7. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    That looks like the Pharma which was tied up in Eureka, California for many years. It is good to hear it is finally being used.
    Bolger/Hartog Sailing Research Vessel https://www.boatdesign.net/threads/bolger-hartog-sailing-research-vessel.59585/#post-820417
    Eureka California Unique Boat - What Is It? http://forum.woodenboat.com/showthread.php?150643-Eureka-California-Unique-Boat-What-Is-It

    The fixed jaw is more durable and less likely to cause problems than a more complex design. For cruising in Alaska I know how I would value durability and reliability.

    Many boats with gaff rigs have tapered mast, including the Marshall catboats. I don't know of any which use an adjustable jaw for the gaff.
    Are you aware of any larger, gaff rigged vessels with lacing rather than mast hoops? Which is more reliable and less likely to hang up when dousing sail?
     
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  8. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Neither option will work reliably in heavy weather conditions, there is to much slop in the system. The only way to take up that much taper would be to use a closed ring with spring loaded articulated shoes, wich will result in a heavy monstrosity that needs frequent service.
    Fortunately there is another way, put a track on the mast and use a car for the gaff. Appropiate size track and cars are commonly used on superyachts, you either buy or engineer something similar. Added bonus is that you can now redesign the rigging and decrease mast size.

    The real question is if the vessel can stand up to the additional sail area. CoE/CLR is a no brainer if you use lee/bilgeboards, you just move them around until you get the desired balance.
     
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  9. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    Good spot Mr. Cockey, I figured someone would recognize her! She has sat for a while but is still in great shape. I was not aware that the Marshall catboats had a tapered mast, I will certainly see if I can learn more, don't need to fix a problem that doesn't need fixed! I may do hoops, I just figured one of the newer strong, slick line materials (thinking Amsteel) laced forth and back would do the trick and if not I can go to hoops without being out much money. I plan to mess around locally on her for the next 5 years to get some of the kinks out before going any distance.

    Rumars, that is kind of what I was afraid of and am trying to address. I actually had thought of a track for the sail but figured it would add complication, never dawned on me to track the gaff. Heck, weld a 3" I beam to it, put a car on it and you are done! Like you point out it also gives me far more rigging options as well. Tensile wise the the masts can more than handle it, there is not a thing on this boat that has not been over engineered! These things are 20" diameter (I forget the thickness off the top of my head but when I but it I remember it was substantial) COR-TEN steel, originally designed to be free standing with tabernacle that allowed them to be lowered hydraulically. I was planning to give that up and go ahead and stay them. I can't backstay the mizzenmast but it was designed for 2,400 sq. ft. and I am looking at 2,100. With shrouds and forestays to support them it should be structurally sound but that is what I will be paying a naval architect to review. You are absolutely right about the lee-boards, they have not been fabricated yet so I can tweek them if necessary to balance the plan. The big question is righting arm, I have a couple of ideas for that but before I get crazy I will see what the experts say.

    Thank you gentlemen, I like the track idea and will see what I come up with, it also solves the lacing vs hoop dilemma as well. I appreciate the input.
     
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  10. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    A track on the mast which needs to take the loads of the heel of a gaff needs to be much more substantial than a track which is only used for the luff of a sail.

    If you have not done so you may want to have a compotent and knowledgeable naval architect do a heel test of the boat and calculate stability curves. Then evaluate how much wind is reasonable with various combinations of your sail plan. A sail plan for a boat that size designed on the basis of "it looks about right" can result in a boat which is used as a powerboat, or which sits at the dock.

    Do you and your crew have experience working aloft and handling top sails? Or will the top sails be used mostly for show and rarely set?
     
  11. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    For a track I am literally talking about using a structural steel I-beam welded to the mast. I will size it for the load which will involve taking into account the twisting force that will be on it but that can be fairly easily modeled. I might oversize it if necessary to match it's flange thickness to the masts thickness to make the welding easier. It will take me a bit to run the numbers to see if all of this is viable but I like the general idea.

    I have the stability curves done by two navel architects (Bolger himself and then later by Hartog). I am meeting a third tomorrow and he will also do his own stability analysis, unfortunately until the leeboards are fitted a heel test will not be of much use but I need the stability study to finalize the design of the leeboards and all of that plays into the design of the sail plan, thus I am paying for another naval architect (which pains me to no end because I love figuring things out myself but this is not one of the times I am willing to learn the hard way!).

    2 yes, the rest no, but I plan to have the top sails self furling. To be honest they will probably be mostly for show. If they get used more than half a dozen times in a year I would be surprised, but when the conditions are right I can't wait for her to stretch her legs.
     
  12. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I am having difficulty in visualising how you will do this.
    Can you post a sketch illustrating your thoughts here?
    Will the sail have 'C' shaped sliders that slide up along the flange?
    If yes, then I can see lots of potential for sliders to stick / jam.
    Not to mention wearing the paint off your steel beam, which will then rust happily.

    With tracks, I think you basically have two options - one would be to have 'T' shaped sliders attached to the sail, which slide within a channel attached to the mast - but there would probably be a lot of friction here.
    The other would be to have cars (preferably roller bearing) sliding along a track attached to the mast.
    And for sails this size, you really want to reduce the effects of friction as much as possible, as they will be heavy beasts to hoist, even with little friction in the system.

    Will your third naval architect do an inclining experiment on the vessel, or has this been done already?
    If you are not too sure about inclining experiments, here is a link to a post I wrote last year about them.
    The same basic principles would apply to your boat.
    Adding a second story to a displacement cruiser trawler https://www.boatdesign.net/threads/adding-a-second-story-to-a-displacement-cruiser-trawler.63909/page-2#post-876470
     
  13. Daniel M
    Joined: Jul 2018
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    Location: Alaska

    Daniel M Junior Member

    Here is a very, very rough idea of what I thinking about, the car for the gaff is completely the wrong size but it gives you an idea. The traveler for the sail will probably get tweeked quite a bit when I start experimenting with it. Before I fab them I will probably make wood mock ups to see if there is jamming or any other unforeseen issues.

    For the traveler the wheels will be some form of plastic (UHMWPE?) on the gaff car probably they most likely will have to be steel, not sure if there is a way around that but I will certainly look. I plan to use Corten steel for the gaff car and the I-beam, same as the mast, whenever possible I try to use the same metals. The travelers will be aluminium.

    There still is a whole lot to figure out, for instance I would like to use the slope of the I-beam flange to help the cars center them selves a bit, but that might not be practical and I may switch to an H-beam. Is it worth riveting a teflon wear strip to the inside of the H-beam? My initial thoughts are that it would be overly complicated and not last very long and not worth the effort but it certainly would reduce the friction! I am not completely sold on the wheel placements. Then there is the fact that until I do the stress analysis to size the I-beam I really can't finalize anything.

    Inclining will need to be re-accomplished (actually I plan to do it 2 more times because this sucker holds so much fuel I went to know the difference when she is dry). Also again no lee boards, no rigging aloft, I will be adding an electric motor, battery bank, solar panels, back hydraulic motor and I whole lot of other stuff. I do not want a best case, I want to know the full range. This is still in the early phase of the redesign but I want to keep my eye the overall effect. Everything effects everything else that is why I am starting with the sail plan and ending with hanging the leeboards. She is also designed to pump seawater ballast so that will help quite a bit.

    Jaw03.png
     
  14. bajansailor
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    bajansailor Marine Surveyor

    Your proposal above might work if the I beam is fabricated in stainless steel, rather than mild (or even corten) steel.

    But how are you going to allow the gaff to oscillate - will it be connected to the saddle with a universal joint?
     

  15. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    I assume you have the lines from Bolger and Hartog and the shape of the boat as built is close to the lines. A naval architect should be able to take the lines, measure the freeboard at several locations and provide a good estimate of the weight of the boat and fore/aft CG location. The heel/inclining test is then used to establish the CG height. The effect of adding leeboards on the weight and CG location can be calculated. With the lines, weight and CG location curves the stability can be calculated.
     
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