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View Full Version : GT 80 Wingmast--SYDI's largest free-standing wingmast to date.



PD Staff
09-02-2010, 12:28 PM
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In the Spring of 2009, the builder of the GT80 sloop, Francis Bruyninckx of A&B Yachts in Belgium, contacted SYDI on the recommendation of his designer, Arthur Peltzer, to consider designing a free-standing carbon fiber wingmast for this design. We quickly reached a deal and design work began immediately. The mast was to be designed as totally free-standing, although there would be running backstays to balance the tension of the jibstay in heavier air. There would also be a second headsail, a code zero, and equipment for a spinnaker and storm trysail were added after the project was well under way.

Mr. Bruyninckx wanted to build the mast himself, and we decided that wet impregnation in epoxy resin and cured under vacuum bag would be the most appropriate method of construction for him. Post-cure at elevated temperature will be necessary to bring strength and stiffness values up to maximum levels. This will be a big, heavy mast, and so overhead handling cranes will be necessary during lay-up. My tasks were to do comlete engineering and design for two tubes--the round-section shorter stubmast, and the larger elliptical-section wingmast; draw and specify the laminate schedules; design and draw all the structural details for all specific parts such as the masthead, stay attacments, and boom tiller control; produce full-size patterns for the mast sections and bearings; engineer, design and draw the bearings, and finally, calculate all the weights. It was a big job, and this is the largest free-standing wingmast SYDI has designed to date. What follows is a description of some of the details that we had to create.

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Fig. 2. GT80 Final Sail Plan, original by Peltzer Design, and modified by SYDI.

Right off the bat, we had to answer some very critical questions--how were the bearings to be built, and who was going to build them? They have to be made of metal--perhaps aluminum or stainless steel, but the shear size of the mast--about 400 mm diameter at the top of the stubmast, and about 450 mm diameter at deck level--really limited the types of structural hardware we could use, specifically pipe, tube, or rolled plate. I approached two well-known European bearing manufacturers who had experience in large rudder bearings for yachts--one manufacturer did not want to touch them, and the other was just on vacation so much and did not respond in a timely manner that we lost patience. We were delayed a number of months just trying to deal with that. Ultimately, we decided it might be best that A&B Yachts build the bearings which would be quite easy to cast in silicon bronze. Bronze is fairly high up on the galvanic scale, so, in relation to carbon fiber, which is very noble and way at the top of the galvanic scale, bronze could last quite well. It is very strong and stiff, between aluminum and stainless steel with a commensurate density between the other two. Ultimately, however, Mr. Bruyninckx suggested that Holland Spar in The Netherlands be the bearing fabricator, and they suggested machining the bearing bodies out of large aluminum billets of 6082-T6 alloy, in which they have particularly good experience. This proves acceptable for ease of construction, strength, and low cost, provided that good insulation be made between the aluminum and the carbon fiber.

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Fig. 3. Lower bearing between wingmast and stubmast. Note that the bearing incorporates both journal and thrust rollers, and that all the running rigging and electrical wiring come down through the center of the stubmast.

Detailed drawings were done of all the parts of each bearing. These were drawn at full scale (actually, all my drawings are drawn at full scale and printed at smaller scale) so that they could be used readily as machining patterns. All rollers are the same size, made out of 316 stainless steel. The bearings, of course, and the inside and outside diameters of the mast all had to be sized accurately and properly so that first, they could simply be assembled, and second, so that the wingmast could slip directly over the upper bearing and down onto the lower bearing without interference. This was all done completely in 2D AutoCad drawings in various projections and overlay checks.

Another problem that had to be solved was how to run all the running rigging, electrical wiring, and lightning ground. All of these things were going to run inside PVC pipes inside the wingmast and then down through and out the bottom of the stubmast. The main downhaul, outhaul, and two reefing lines are going to pass through openings in the side wall of the wingmast and join the others going down through the center of the stubmast. All running rigging then runs back through the bottom and sides of the hull to the cockpit. At first, there were just two headsail halyards and two main halyards, one of which was a spare. Part way through the design, a change was made to include two masthead spinnaker halyards and a storm trysail halyard. The PVC pipes all had to be reduced in diameter to make room for extra pipes carrying the extra halyards. These pipes, too, can't be totally fixed inside the mast, either, because the mast bends side-to-side and fore-and-aft, and these pipes will have to go along for the ride. They have to be allowed to slip slightly in relation to the mast structure, but they also have to carry their own weight. This is done by using the coupling joints which are just bigger in diameter than the holes in the horizontal stations that the pipes pass through inside the mast. So about every 3.5 meters of mast height, there are conduit hanger stations where all the couplings are located on the upper side of the station. These can be seen in the drawing below.


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Fig. 4. The conduit hanger station is at 24.5 M. The conduit arrangements are seen in the section views at right. The rest of the detail shows how the headstays and halyards are arranged and rigged.

We had to be careful, too, that all these PVC pipes did not interfere with each other, and that they were perfectly straight from top to bottom when the mast was not loaded. This was all done in 2D in AutoCad with various projections to ensure there was no interference. These projections also served to accurately create the full size patterns for every station. The geometry of each pattern with its proper cutouts and openings ensures that all the conduits will be perfectly straight, as desired. Note, too, that the largest conduit is for the electrical wiring harness, and its mate opposite is solely for the lightning ground wire which, therefore, is well isolated from everything else inside the mast.


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Fig. 5. The principle of construction had to be drawn for both the wingmast, shown here, and for the stubmast.

The technique of building the mast follows the process I developed over 25 years ago using wood-epoxy (miracle fiber "W") as a mandrel, and then laying up the carbon fiber over that. The wood remains captive in the mast and serves as internal reinforcement for all the hardware bits that are installed in and attached to the mast. The drawing above describes this principle briefly. In addition, the drawing shows the principle of carbon fiber layup, with layers of 0/90 cloth, +/-45 double-bias fabric, and unidirectional tapes. All UDR tapes are left constant width--there is no trimming. The overlap of the strips of UDR over each other, different in amount over the length of the tapered mast, is essential to keeping the process as simple as possible, and for the calculated strength and stiffness of the mast. How much carbon fiber is needed is worked out in a spreadsheet where the engineering of the strength and stiffness at multiple stations along the mast are calculated. Overall deflection of the mast at the design load is also determined. Once the spreadsheet values for the wall thicknesses at every station are determined, the spreadsheet continues with calculating the number of strips of UDR and the number of layers of off-axis fibers that are necessary for the layup. Once those values are known, they have to be converted by hand to a laminate schedule drawing which is then used by the shop to actually lay up the mast. Complete diagrams and instructions are drawn out accordingly.


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Fig. 6. The principle of construction for the mandrel also had to be drawn, again for both the wingmast shown here, and for the stubmast. Every station was copied and drawn out for full-size patterns to make construction as easy as possible.

Even with this amount of detail, still further details had to show how to build the wood-epoxy mandrel, as shown in the figure above. The master reference line for the wingmast is its trailing edge, which is perfectly straight, and also perfect for proper shaping of the mainsail luff. Patterns are to be lofted in parts, transfered to plywood layered with carbon fiber fabric eash side, and cut out and glued together as shown. There is a transverse shear web that runs the full length of the wingmast, made similarly to the stations with carbon fiber and plywood. The leading and trailing edge timbers are marine plywood, glued to the stations and shaped accordingly. The side skins are thin marine plywood glued to the wood and carbon fiber framework. The conduits for the running rigging, electrical wiring, and lightning ground are installed as the stations and mandrel go together. Internal reinforcing in the form of wood blocking is also installed so that there is good structural support for the bearings, gooseneck, and standing rigging attachments. Once entirely assembled, the wood-epoxy mandrel looks pretty much like a completed mast without the hardware. So, in the end, a lot of planning has to happen and be drawn out in detailed plans so that the mast can be built properly. Remember, I am here in Florida, writing in English, and the plans have to be read and understood easily by a crew in The Netherlands and/or Belgium.


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Fig. 7. The GT80 wingmast will probably start construction in late 2010 or in 2011. SYDI will likely get involved in the design and engingeering of the Park Avenue boom and the chainplates for the stays and running backstays.

For more information, updates, and images of this project (renderings here courtesy of Arthur Peltzer), you may contact Arthur Peltzer's website at Peltzer Design, and Francis Bruyninckx's website at A&B Yachts. If you would like to discuss this or a similar project with SYDI, you may use the Contact Us link below.



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Paul Kotzebue
09-02-2010, 02:05 PM
Way cool. It's nice to see such craftsmanship in design and engineering.