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Wishbooms and straight sprit booms


Tom Lathrop

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I don't see bending a bird's mouth spar as being practical at all. A lamination of spruce or similar seems the most practical. Aside from the problems bending the bird's mouth at all I would think a shape that has an increasing radius curve as you move aft, much like the shape the sail in light winds would be the most practical and laminating to a form seems like the only practical way to control this. I am seriously considering wish bones for my next build.

Would using hard wood outer laminations over softer lighter woods for the rest be particularly stronger? Spruce box and bird's mouth masts are quite strong, but a different composition.

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That's sure a subjective statement. No one in that thread can even agree on the main thrust of it.

Tom's main thrust maybe.??

Which might be more hassle than it's worth except for when you are racing....

I definitely notice my starboard tack is better than port tack.

I can identify with Tom i wanting a wishbone sprit when racing. But I would not want one every time I go out.

I do like the simplicity and ease of rigging the straight sprits.

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Tom's main thrust maybe.??

Which might be more hassle than it's worth except for when you are racing....

I definitely notice my starboard tack is better than port tack.

I can identify with Tom i wanting a wishbone sprit when racing. But I would not want one every time I go out.

I do like the simplicity and ease of rigging the straight sprits.

That's right Ray. It's a discussion, not a conclusion and that is the point. My thinking is the same as yours and, as I said, it depends on your sailing objective whether the added complexity of a wishboom is justified. At Rock Hall, I would have liked to have had them. On the other hand, I would have also have liked more sail area because most of these boats are underpowered for racing against one design boats as I was.

Most one design racers of the same size have from 25 to 60 per cent greater sail area. Its a matter of design objective and I think Graham opts for a more wholesome rig which is highly suitable for the uses most of us put these boats to. He built a turbo rig for his Spindrift 10 with lots more mast height and sail area for some competitions. For instance look at the big difference in sail area of a CS17 and a similar sized Thistle. That is 119 sq ft of the CS versus 191 sq ft plus a 220 sq ft spinnaker. Even my Windmill which is smaller than Lapwing has 119 sq ft sail area. I may never race Lapwing again but still may build the wishbooms for my own interest. While this may seem off topic, it isn't because I am discussing performance enhancement.

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Well, if Tom's main thrust is the topic, which was never clear until now, then we are talking about when or if to use a wish bone on a B & B design. I hope to be building one, probably the Lapwing in a year or 2. I am considering building wish bones from the start. Even though I will probably never race I think I would find the distorted sail shape of the conventional sprit too much to bear. Actually, if I wanted to race, I would build a one-design boat. It is the only racing I have ever really enjoyed. I could enjoy racing bath tubs if we could all agree to the brand and style.

Even when I am day sailing, and even when sailing in a casual mood I find it hard not to try for good sail shape and trim. Trying for that most efficient trim and movement is just too much of what sailing is for me to not do so. I will probably build conventional sprits as well. But unless they (conventional sprits) are just way easier to rig, or I can't build a wish bone I am happy with, I get the feeling I will only be happy with a wish bone. Until I actually have them too compare I can only speculate.

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Well, if Tom's main thrust is the topic, which was never clear until now, then we are talking about when or if to use a wish bone on a B & B design. I hope to be building one, probably the Lapwing in a year or 2. I am considering building wish bones from the start. Even though I will probably never race I think I would find the distorted sail shape of the conventional sprit too much to bear. Actually, if I wanted to race, I would build a one-design boat. It is the only racing I have ever really enjoyed. I could enjoy racing bath tubs if we could all agree to the brand and style.

Even when I am day sailing, and even when sailing in a casual mood I find it hard not to try for good sail shape and trim. Trying for that most efficient trim and movement is just too much of what sailing is for me to not do so. I will probably build conventional sprits as well. But unless they (conventional sprits) are just way easier to rig, or I can't build a wish bone I am happy with, I get the feeling I will only be happy with a wish bone. Until I actually have them too compare I can only speculate.

Is there actually a definitive shape/arc for a wishbone boom? I have seen numerous ones on different craft, and there doesn't seem to be any consistency. Is there a "formula" for length of boom in relation to the width of it's shape?

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I've used and have designs for half wishbone setups that I find much better then a conventional straight sprit boom. This said, Graham had it right, as his intent was home built, typically by novices and a simple snotter on a straight, conventional boom is much easier. I know others that use the "fixed" half wishbone boom like mine, Eric Sponberg in St. Augustine is one of note, with great success. In light air it's a much better arrangement, but once the winds pickup a little, I see little advantage of one over the other. A full wishbone to me, is more weight then necessary in small craft, though in larger craft not as much an issue. Lastly the fixed half wishbone requires a rotating mast.

I wouldn't consider a dramatically bent birdsmouth spar, at least not in the sizes typical of a small dayboat. My booms are laminated from white spruce and it's possible to leave enough room internally for the outhaul, making a much cleaner setup. I build them flat on a layout table and clamp the laminate stack horizontally, to blocks screwed to the table. Using spacers on a few of the internal layers, makes a fine little hollow for the outhaul.

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I have laminated a few beams on a table so I think I understand your description.

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Using spacers on a few of the internal layers, makes a fine little hollow for the outhaul.

I can not picture this part about the spacers which sounds like a neat little trick. Could you elaborate.

I had contemplated asking the dumb question. Can you have half a wishbone? Who would have thunk.

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Yes, Joe - I built a half wishbone for Dickie Newsome's CS20 -way back. One of Graham's - "I think that would be a good idea what do you think can you knock one up". Dickie still uses it. You need a line to hold the belly up - it being asymmetrical - the gravity makes it sag otherwise. Personally, I can do without them. The sail sure sets pretty but the halfbone hurts my eye. I would sooner have the full job if I had to have one at all. PeterP

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The half wishbone does need a lift, if it's rigged on a snotter. This is why I use a fixed mount, which eliminates the need for the lift and the snotter. In fact, my setup uses a stop at the bottom of the bearing block, mounted on the mast, which prevents the boom from dropping to the deck if the halyard is released, which can do away with a topping lift, if desired. I've found a topping lift has more uses, than just keeping the boom off the deck when the sail is doused or reefed, so I install one anyway.

The hollow curved boom laminate is a fairly simple thing. The last I made was a 7 layer laminate of 5/16" thick 2" tall pieces (II:::II is what it looked like from the end). The center 3 pieces where ripped into small width strips and spaced with a dowel covered in clear plastic packaging tape during the assembly. This kept them separated and permitted the epoxy to ooze out, around the dowel and form a nice little fillet/tunnel for the outhaul line to live. I used to make the booms in two halves, so I could remove the dowel, but I've now discovered an air chuck and blast of compressed air will usually free the dowel, without disassemble, especially if gripped on one end with a vice and you pull on the boom, so now they're glued as one single laminate. The hollow isn't to save weight, which isn't really much anyway (maybe 20% to 30%), but to offer a place for the outhaul, without rigging up turning blocks or fair leads down the length of a curved boom.

The bearing block on the mast, which holds the boom both vertically and prevents it from flopping over is usually HDPE. Eric Sponberg uses a sweet cast bronze fitting, which I'd love to get a set of drawings for, but it does the same thing. I don't know if his setup has a stop, but I suspect it does. The bearing block makes a flat spot on the side of the mast and a through bolt (with a compression tube inside the mast) passes through the mast, bearing block and boom, in an athwartship location. The HDPE block is riveted to the aluminum mast, though a wooden mast could live with screws. The boom uses a fairly thick epoxy bearing for the bolt and on one, I used a HDPE bearing in the boom, which was the slipperiest arrangement of all. Wear is minimal as the HDPE is self lubricating. It is important that the mast be permitted to rotate easily, so more HDPE bushings or bearings are used at the heel and the partners. It's a simple, easily used arrangement and now that the bugs are worked out, very handy, with no boom/sail chafe or bad tack to live with.

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What about using a goose neck fitting for the half wish bone? Would this be strong enough to resist the twisting/sagging forces yet be an effective pivot that would make a rotating mast unnecessary? If it slides in a track one could adjust the angles for different wind conditions much like snotter block location adjustments do.

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I think a gooseneck defeats the main thrust (pun intended) of a sprit and/or wishbone, and that is control of sail shape via a snotter (single trim thing) v. a half dozen lines to control sail shape = dowhaul, outhaul, boom vang etc. Course I haven't quite figured out how to rig a wishbone snotter. I like the idea of a wishbone and can build one, but I remain a bit fuzzy on the rigging details.

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I think the whole beauty of a sprit boom, whether straight or wishboom, is simplicity. The half wishboom and the gooseneck type kind of defeats the intent. Paul's gooseneck is novel and elegant but I don't think I would go that way.

Hmm - wouldn't it be cool if one designed a mast fitting (details to be left as an exercise for the reader ;-) ) that allowed the half boom to be rotated at anchor so that the curve is up to be used as a ridge for a roomy cockpit tent?

Mike

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I like the fixed sprit boom because it does away with the snotter, mast banging, chafe and bad sail set on one tack. As with all decisions in yacht design, there are compromises to be accepted and the fixed sprit has them as well.

Outhaul tension can be brought back to the cockpit if desired with the fixed boom, so I see no advantage to the snotter arrangement. A built in boom stop can be incorporated into the fixed boom, eliminating the topping lift, which is one less string to pull. If using a half wishbone, you remove chafe and the bad tack issue too.

On the other side of the coin, it does add a small amount of complexity to the boom arrangement, though I feel pulling a pin (instead of using a bolt and nut) and freeing the boom from the bearing is about as easy as releasing the snotter, when you want to drop or remove the boom. The curved boom does have a higher turning moment created when eased out in light air, just from the weight of the boom, but this is a very incrementally higher difference between the two arrangements.

All in all, I like the fixed boom, especially in very light air, when I can get to windward on both tacks, while the conventional straight boom sprit has difficulty on it's fouled side. There's also something very cool about a well set sail, running from near deck level to masttop, unrestricted by a boom crease, that I really enjoy.

In use, both setups are very simple, handy and quick to employ. A novice builder would have little difficulty making a mast bearing for a fixed boom, though the curved boom certainly could be a challenge, though not a very big one if they think about it a bit, decidedly more so then a straight boom.

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