sprit rig and the wishbone

[dale]

Thought I posted this last night but...

I've always wondered about the effeciency of the sprit rig when sailing on the tack that presses the sail against the sprit.  Is that needless worry?

Has anyone thought about making the sprit like a wishbone like the builders of the Nonsuch do?  Could probably get a nice curve with laminated plywood or perhaps steam bent.

[ScottWidmier]

Dale, I decided to do a search on sprit rig online and came across some interesting sights.  The first one is an essay by Michael Storer on traditional rigs where he states that a lug or sprit rig are more efficient than a gaff unless the gaff is equipped with a gaff vang.  Of course, I don't know if he means the traditional four-sided sprit sail or a leg of mutton or sprit boom sail.  I hate how the terms for these two different types of sails used interchangably.  http://www.storerboatplans.com/Faq/tradrigperformance.html

A few other articles on sprit rig.

http://www.boatbuildercentral.com/howto/sprit_rig.php

http://slidercat.com/blog/wordpress/?p=52

Ray Aldridge came across a very interesting test comparing traditional rigs to Bermuda on the same type of boat hull (catamaran).  http://slidercat.com/blog/wordpress/?page_id=57 .  What this test found was the sprit being 30 percent more efficient to windward to the bermuda.  Reading this article pushes me towards a four sided sprit to be honest.

So, I begin to wonder why the sprit got replaced by the Gaff rig and some information in the sailmaker's apprentice as well as this article: http://www.la84foundation.org/SportsLibrary/Outing/Volume_24/outXXIV02/outXXIV02n.pdfhttp://www.la84foundation.org/SportsLibrary/Outing/Volume_24/outXXIV02/outXXIV02n.pdf may explain why.  It seems like for anything but a small boat the rig becomes unhandy with the length and weight of the sprit as well as reefing the sail.  May not be a big concern with my 12' boat though.

I may have to experiment with both a sprit and a gaff sail for this boat.

[PAR]

Michael Store is referring to the sprit rig, not a rig with sprit boom, such as used on many B&B designs.

The article you list (slidercat.com) was printed back in the 70's, I think in Sail Magazine and it's still as ridiculous a read it was when I first saw it. These "trials" have been attempted from time to time, with a more recent one done by WoodenBoat Magazine (I think), in which the crab claw rig was better over all. This one in particular is full of flaws, 30% better oh please. If this was true, every Americas Cup entry since would have eaten the penalty for not using a Bermudian main, just to reap the benefits of that 30% and hoisted the broom long before '83.

Simply put, if you had a fleet of sprit boomed rigged boats, all very identical, in every regard. Each having an expert skipper and all sailing on the same race course as well as the skippers can muster. The boat with a wishbone boom would do very marginally better in moderate wind strengths and noticeably better in very light wind conditions. In fresh winds, there may be a measurable difference between a wishbone and conventional sprit boom in terms of boat speed, but it would be a small fraction of a knot.

In very light air, when every ounce of energy of boat momentum must be maximized for boat speed, the sprit fouled side will be noticeably slower then a wishbone equipped boat. Then again a fart that moves you off one of your cheeks, will also have adversely affect on boat speed too. In these conditions, real sailors shine and actually make way to the mark, while others just complain about the lack of wind. Sail shape is very important under these light air conditions so the wishbone has a decided advantage, which disappears as the wind strength increases.

[ScottWidmier]

That 30 percent did hit me as sensational and does raise a lot of questions if true.  Just raises the point of how hard it is to compare across rigs and get some real data!  I guess the answer is the best rig is the one that fits your design and sailing style.  BTW, isn't the three-sided sprit rig that Michael uses on his PDR racer a leg-of-mutton or sprit boom rig?  A sprit rig is the four-sided rig.

[dale]

As lake sailors, we often sail in light wind conditions.  Maybe it would be worth exploring a wishbone boom.

[Peter HK]

My brother had the experience of racing his Bolger Gypsy against another identical in every way except the sprit boom was set on the opposite side i.e. opposite "good" tacks. The other skipper was also a very experienced small boat sailor. They were within a boatlength or 2 for several races and no difference could be detected on either tack.

Any difference between the "good" and "bad" tacks seems to to be very small.

I wonder if the extra weight of a wishbone sprit or sprit boom would negate any advantage from improved flow?

Cheers

Peter HK

[Hirilonde]

I wonder if the extra weight of a wishbone sprit or sprit boom would negate any advantage from improved flow?

If you designed the wishbone to be the same strength as the sprit boom it would be made of 2 pieces that were individually much lighter and totaled little if more weight.  I believe Graham is of the opinion it isn't worth it though, that the difference in performance is negligible. The beauty of the sprit boom is in its simplicity.  

I have no significant experience to draw any conclusions either way.  When it comes my time for me to build my cat ketch, I don't think I will experiment with a wish bone unless someone can present compelling evidence that it is worth it.  I will certainly read the posts of anyone who tries it though  

[dale]

this has been discussed before.  Tom Lathrop made this comment in one of the threads listed below.

"It's easy to make a case based on aerodynamics for better performance with a wishbone or a half wishbone boom but, in practice, the difference is minimal. We have never seen any measurable performance increase using the more complicated rig. We have sailed both rigs on identical Bay River Skiffs on numerous occasions and the simple sprit boom rigs have always been faster. Not to say that the sprit boom rig is superior, but even a small difference in skipper ability overshadows any perceived performance improvement.

"Given that the straight sprit is far simpler to build and rig and no significant improvement has been seen using the curved spar, my choice is for the simple rig every time. On larger boats, the story might be different."

[Peter HK]

Dave said

If you designed the wishbone to be the same strength as the sprit boom it would be made of 2 pieces that were individually much lighter and totaled little if more weight

It is simply not possible to build a wishbone boom as light as a simple sprit boom using the same technology. The differences in performance between them seem insignificant from the aerodynamic point of view- I was simply pointing out a possible negative- extra weight in the rig.

It certainly wouldn't be a huge difference in weight...but that was the point, a little difference in aerodynamic efficiency negated by a little difference in mass of the rig.

By the way as someone with a lot of experience with wishbone rigs on larger vessels- they are often underbuilt for the the torsional stresses involved- I often had to repair the wishbone sprit boom on my 33ft trimaran, yet it seemed to be of appropriate dimensions and to the experienced designer's specifications.

Cheers

Peter HK

[Tom Lathrop]

Yes, These articles that seek to prove that some other sail type is superior to the marconi/bermudian rig show from time to time.  For the most part they are BS.  

It has been demonstrated many times that an elliptical foil (Spitfire wing) has the highest lift/drag performance.  A gaff or gunter rig can be made to approximate this shape better than the normal marconi, at least on paper.  Some people have built some complicated mast/sail combinations to form an elliptical shape.  Even for racing where complications are readily tolerated for performance increase, the marconi, or large roach battened marconi is the dominant choice.  I like the rectangular sprit, boomless rig least of all.  Maybe Oyster will not read this

Graham chose the sprit boom rig for many reasons, with most of them leading to simplicity of rigging at minimal or no loss in performance.  Low cost, ease of handling and superior maneuverability are great plus factors also.  In the 15 years since the experiences quoted by Dale, I have seen no reason to change those thoughts.

Even though I chose battened sails with mast track for Lapwing, I miss the simplicity aof the battenless, roll-up sails on my Bay River Skiff, Loon.  The choice was made because I now need an easy reefing method.  

[PAR]

Tom, the elliptical plan form so often touted as the most efficient actually isn't as good as you might think. Everyone talks about the Spitfire as if it was a revolution. It wasn't and in fact got it's elliptical plan, because they needed more room for fuel tanks in the wings, not aerodynamic logic. One other British aircraft (a 30's vintage bomber) and the P-47 also had an elliptical plan wing and neither of these aircraft could be remotely suggested they out handled anything.

The reality is the elliptical plan does offer lower induced tip drag, which sounds nice. Along with the reduced tip drag is also reduced tip lift with this plan form. Tests have repeatedly born out that the use of a square tip and rectangular plan forms may increase tip drag, but the additional lift from the increased area at the tip and span end, offset the parasitic losses. This is especially true if you utilize foil sections to benefit in these portions of the wing.

With the exception of the Spitfire, the P-47 and the few other aircraft in WWII that employed these shapes, none of the subsequent designs, especially ones well known for increased performance, bothered using elliptical shapes, but used rectangular plans with square tips instead. Darn near all performance designs since also have adopted square tips and rectangular forms as well. The Spitfire was a lovely aircraft once it got enough power to handle the BF-109, but it needs to sit down and stop taking credit for stuff it really shouldn't be accredited with.

[Tom Lathrop]

Paul,

I think you misinterpreted what I said, or meant.  I don't credit the Spitfire with either inventing the elliptical planform or being the end-all of efficient wings.  The P51 was much better in many regards.  In retrospect, I should not have mentioned the Spit at all.  I only used it because everyone would recognize it.  You should not be so down on the Jug though.  At altitude it was well respected by even the FW190 which I consider a close match to the P51 as a fighter.  Aircraft operate in a couple fewer dimensions than sailboats.  One medium rather than two and they don't have to tack so they can have things like winglets, tip fences and other flow control devices that emphasize the asymmetric wing sections.

It is pretty clear to me that just about all high performance rudders use elliptical forms though as well as many elliptical fin keels.  Even though current AC boats have nearly square tips, they are pretty close to elliptical in overall form.  Nearly all fully battened racing mains also approach the elliptical form.  I'm certainly not an aerodynamics expert but it does appear that the elliptical form is good in unstable conditions like that found on a sailboat on the water.

I didn't mean to place so much emphasis on that part of the previous post, just observations relative to the simple sail we use..

[PAR]

I wasn't picking on 'ya Tom, I just get tired of the Spitfire analogy, which was a production run compromise for tankage, not aerodynamics and it took several generations of Spitfire to get to one that could dance with the "big boys". If the Germans hadn't switched tactics and started bombing cities, they would have shot them all down in another month or two.

Don't even start me up on the FW-190, P-51 debate. If you compare both "D" versions the 190 is close, but except for roll rate gets beat by the Mustang in every regard.

The Jug was a pig, but a really well built one. At altitude it did a fair bit better (it could dive), but literally every ship in WWII could slice and dice a Jug. The big question was did you have enough ammo to bring one down, as they could absorb huge amounts of damage. It was probably the premier ground attack machine of the era in it's defense.

I'm not seeing elliptical plans in appendages in recent designs, though there was seemingly a fad there for a while. What I'm seeing is straight leading and trailing edges with a radiused lower leading edge corner on most designs. I also see more attempts at getting increased area at the masthead then any effort to make the plan elliptical.

It would appear we're more then boaters Tom.

[Tom Lathrop]

Maybe we boath (pun intended) spend too much time watching the Military Channel.  The arguments of superiority of one fighter or another goes on forever.  One of my dreams would be to get a ride in a dual P51.  Adherents of the Spit would be looking for your hide although I somewhat agree with you.  Roll rate is a pretty big deal in a close in dogfight.  In all these planes, a main issue was whether the other guy could be made to fight in your best arena. 

Got my first flight out of a pasture in a J3 in 1943.  The hottest ride ever was in an A26 in 1948 when on summer training with the CAP Cadets.  I get to Oshkosh now and again for renewal.  Latest venture into the aircraft realm was building a propeller for a friend's Pietenpol homebuilt. Still, boats are number one with me.

[Ken_Potts]

  Hey Tom,

  If you really want to ride in a two-seat P-51 I might be able to help.  Before you jump on that offer I should probably mention that the operating costs of that particular aircraft are a bit pricey ($3500/hour-ish) and the owner will probably want that covered.

  Who's Piet were you working on?

  Sorry about the hijack, guys.  In order to get back on topic I'll add that the day I realized I was becoming a really good sailor is the day I noticed the difference in sailing qualities from one tack to another on my straight-sprit-boom CS17.  I was feeling really good about myself then I found that the starboard lockers had a bunch of water in them...