If this is duplicated, my apologies.The database seems to have been doing some fairly large scale updating when I first created this and got its knickers in a twist.
Very stupid question.
Once upon a time, sliding rigs were popular in short-ended boats like marbleheads and 36Rs with submarining problems. The whole rig was moved bodily aft on the run.
So exactly what were they supposed to do - and how?
A.
Adjustment of the rig on the run was a secondary issue at best. The primary purpose was to enable the sails to be set for optimum thrust on the beat (remember, you got 3 points for winning a beat and only 2 for winning a run), and the CE/CLR relation to be adjusted for varying wind conditions. I us it, and it works.
Sliding rigs, or their relative “adjustable rigs” were popular on X class boats, which had long overhangs and no diving problems. An adjustable rig was a sliding rig with a fixed backstay, which enabled a traditional hatch to be used. The disadvantage was you had to reset the backstay tension when adjusting the rig.
A long dissertation on all this, written in Gus Lassel’s trademark orotund style, can be found at:
http://www.swcp.com/usvmyg/freesail/lassvane.htm
Cheers,
Earl
That is very interesting. As you may have gathered, my practical experience of model yachts is tiny - but I have been interested in them off and on for a long time.
I used to have a copy (lost in a fire) of Priest & Lewis’ book. In cae their fame hasn’t crossed the Atlantic, they were undoubtedly Britain’s two best designers of competitive models (A, 10, Marblehead) for quite a long time after the war. Priest’s most famous design was probably the A-class Highlander.
As I remember it, they suggested that sliding rigs were largely to prevent bow burying. At first sight this is arrant nonsense. If I have a propulsive force and a drag force both acting horizontally, the tripping couple is the same no matter where I put the rig - whatever intuition may say.
Since Priest and Lewis had great practical experience and were obviously not fools, there has to be more to this than meets the eye - or nothing at all. Reaction to spanwise flow acoss the rig as the boat heels? Possibly - but at first sight ONLY when it heels?
Any ideas?
A.
I think you will find that this idea relates to ‘vane’ steering boats, not R/C. If it was done, then it was to deal with a design issue (fine forward sections) in the boats (submarineing on the run). Submarineing was happening as the boats were overloaded with sail (they used spinakers) on the run and this was ‘English’ sailing were the winds are frequently a lot stronger than we see in North America.
The practice is contary to ‘big’ boat practice, for example the full-sized Soling. Where the rig is allowed to go way forward on the run. This is done to reduce the amount of steering input (using the rudder slows the boat).
John
Well, I checked, and yes, either Priest or Lewis actually said that the sliding rig was to counteract submarining (pg 46). All I can say in their defense is that there was little if any experience with sliding rigs in the UK.
hiljoball’s comment about winds is worth noting. North American sailing is on a variety of venues, many far inland, and in much lighter and more variable air conditions than the UK (which, I read somewhere is supposedly the windiest nation on earth). You can see the effect of this in the weights of free-sailing M class boats. “Rip Tide,” at 18 lbs, was considered a real heavyweight in the US, whereas “Witchcraft” at 22 was on the light to middle end of the scale for a UK boat. This disparity gave US competitors real problems in designing and tuning for the YA Cup, as there were only three venues (Seattle, Berkeley, and Boston – all, alas gone now) where US skippers encountered anything like the conditions at Fleetwood or Gosport.
Cheers,
Earl
Interesting reading , if you wish to have again the Priest&Lewis book please go there : http://www.antiqbook.com/books/search.phtml
My copy is standing in my library since 1964 and I keep it. Agree was a very good book, naturallly for the price you have to pay today ! Is an Antique !
Claudio
Thank you very much Claudio.
I take your poiint about stronger winds Earle but I think that sliding rigs were used in this country at at least a moderate extent. Is this clear from Priest & Lewis?
If they were, did they reveal virtues hitherto unexpected in the US. I met Priest briefly when I was a child and he came over to me as a formidably clever guy (he was a member of the save [full size] sailing club as my father).
I also seem to remember that he was a professional naval architect working at Cammel Laird in Birkenhead (for US readers, the people who brought you tye CSS Alabama!). A crass mistake in basic nechanics therefore seems unlikely.
I am of course picking your collective brains for a reason. I have a half-built Footy of somewhat unusual design. I am very worried about submarining and wonder if mounting the rig a long way aft will help. I know that experimental data suggests that the spanwise component of flow over the rig is not very large - but it is all from (or been normalised to) full-size applications. I wonder whether the steeper wind gradient close to the surface might have an effect?
Over to you if you are still interested.
A.
If you set the rig further back in the boat, you will find that it will not sail well to weather or reaching.
To sail well, the boat has to be in balance. The rig needs to be adjusted so that the boat can sail to weather, hands off, and gently round up over about 10 - 20 boat lengths. If you set the rig aft of that point, you will have lee helm. This is ‘SLOW’.
It is better to have the boat beat and reach well and risk the occasional dive on a dead run.
Remember that the sliding rigs were used on vane steering boats. In other words, at the end of the beat, they were taken out of the water, the rig was adjusted manually, and the boat put back in the water for the run.
John
I agree wholeheartedly abut the balance bit. But we also move the fin (i.e. CLR) aft. And for good measure LCB and LCF so that we do not get heel-based imbalance and the thing does not pitch like a seesaw.
A
Angus,
you may be interested to see my last baby in the tread "rudder position’ - page 3
Claudio
On a Footy sized boat it would seem possible to have the entire rig move aft as the sheet is eased.
Food for thought.
Subject to weight (!!) and the practicality of getting enough travel to be useful, that is a honey of an idea!
A.
Here are some comparative pictures of running under “flattie” spinnaker with the rig full forward and well aft, and a still air picture for reference. Moving the rig aft clearly inhibits the diving tendency. It seems there is more of a downforce that we may imagine, and moving the rig aft places this closer to, or behind, the LCF.
Cheers,
Earl
Earl that is most interesting and highly informative. Unless the pictures are freaks (momentary atypical conditions) they do indeed seem to indicate that the rig is developing considerable downforce. Can you give me the moment to trim 1 cm or the general dimensions of the hull, rig concerned. If so we can have a go at figuring out how big the downforce is!
A.
Here’s the lines plan, rather grossly compressed. 36" from fore bulkhead to stern. Main is 16x48, jib is 11x48, spinnaker ditto. Mast was 12" from fore bulkhead racked forward, 17" racked aft. Displacement is 10.9 lbs.
The boat is a copy of Ted Houk’s 1949 M Class “Rip Tide,” modified to fit the 36 inch Restricted rules. A long discussion of the whys and wherefores is in the “Hull Design” thread.
Cheers,
Earl
Well, lets try the attachment again, this forum doesn’t like tiffs.
Cheers,
Earl
Getting figures accurate enough to be much use off that will take some effort but it can be done. Watch this space - but don’t hold your breath!
A.
The lines drawing was was just to give you an idea of the shape; I wouldn’t recommend spending time trying to analzye it. PM me if you’d like me to mail a CD with the full size drawings on it.
Brett has put the original Rip Tide into Hullform. If you can get a copy of the .hud file from him, reduce the beam to 8.5 inches and the section spacing to 3.6, you’ll have El Viejo modulo the reduced freeboard to get in the 36R box (which shouldn’t matter for this discussion). Isn’t this fun?
Cheers,
Earl
Not sure whether I can read a HullForm file - I use Prolines. But we’ll give it a whirl. If not, we’ll try the CD. Presumably that means back to the days of graph paper and counting squares!
A.
I have Riptides hull sections in DWG (AutoCad) also if that is any help Angus.