Jib Physics & "the slot effect"

With the jib rigging I set up, you lose the ability to partially furl the jib. I'm not sure if that is much of a loss or not. Those with roller furling jibs would be bettter be suited to answer that. For me, an all or none approach to use of the jib works well and has the advantages of no stays so that my fishing is not compromised. Same can be said of the reacher except I will have to find a way to gather the back stay when bringing in fish. I would think that would be doable. We'll see.

On the topic of the jib size, mine is only 21 sq ft but if I wouldn't mind having an extra smaller, sliver of a jib for winder days. I hate to have to down the jib on days with the wind over 17 mph. I have no desire to go bigger, thats for sure. You don't want a jib over 100% (extenting aft of the mast) because it is limited to mild winds and I am guessing would probably chafe the main after a number of tacks.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

The smallest footed jib sail that I found was from a Hobie Getaway. According to http://www.sailritesails.com, it's foot is just under 5 feet and has an area of about 36 square feet. I was thinking about using it partially furled when in jib mode. Then in light air, I could let it all out and it would act more as a genoa. It chafing the main would definitely be a concern, though. How do boats generally tack with genoa type sails? Do they bring them in, tack, then release on the other side, or do they just let the sail brush the mast?

On the other hand, I would probably be better suited with a "sliver of a jib". I would be afraid of getting massive lee helm with anything too big up there.

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-- Chad | 2014 TI

I don't have the experience to tell you if chafing would be an issue or how someone with a Genoa might tack differently. I think you are wise testing a jib or two before buying because it sounds like you have to be careful with jib size on a TI.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

Chad, usually, the headsail just rubs on anything in the way during a tack. But seriously, if you are only talking about a tiny overlap of one or two feet, you won't even notice! On yachts with headsail overlaps of more than five feet of so, hotshot racing crews will leave the old sheet still tight until the bow just reaches dead to wind, before suddenly releasing it and pulling in the new sheet. This has two benefits... the momentary back pressure on the headsail helps swing the bow across the wind (reducing the angle needed on the rudder, thus reducing drag in the water), and secondly, ensuring that the sail zaps across to the new tack, quickly maximising the time the headsail is working efficiently.

As an example, I was once in a tacking duel with a sistership (25 foot Ron Holland "quarter tonner"), and my crew got so adept in timing sail trimming on tacks, that they no longer needed to use winche handles, as they were able to smoothly move the sail to the other tack, keeping the sail working all the way through. It was amazing to see us gain at least a boat length on each tack, and we sailed upwind right through our rival (I won't talk about the other points of sail where he thrashed us though )

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker

Thanks for that, Tony. I get the feeling that adding a jib adds a whole other level to the fun factor!

I would definitely like to try a few jibs before I buy, is there any good way to go about doing this? Do sail makers or anybody else typically have a few extra sails on hand that they'd lend out for testing?

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-- Chad | 2014 TI

I did some more reading last night. In the process, I am learning that there is not one right answer to solutions as regards sailing. Everyone has different ‘needs’. The process made me think about what my needs are.

Downwind: I don’t think the Reacher is what I am looking for. With the stock sail line redirected toward the front aka corner with a pulley, I have been quite fairly happy with downwind performance. If/when I get the genoa (learned the difference between it and a jib last night) setup, I run a line to the front aka corner on the opposite side of the boat, I am guessing I will not be lacking for sail area, getting plenty of forward push.

On a reach on light wind days: I want more sail area. My 45sqft headsail will provide that. In heavier winds, I HOPE to be able to furl the genoa to different (maybe even ‘sliver sail’ sizes), adjusting to the wind of that time of day. I realize there is loss of efficiency, but in plenty of wind, and not racing, I’m not too worried.

On a close-haul: I want to be able to point more into the wind. Without pedaling, on a gps I think I can run at around 50 degrees into the wind trying to maintain the sail at just tighter than luffing. With peddling, maybe a 5 degree improvement, depending on wind speed. In stronger winds I use the haka, so no peddling. If I can improve that by 5-10 degrees, I will be a happy guy. From what I have read, a genoa (as compared to a jib) may reduce the pointability into the wind. However, I am hoping it increases pointability as compared to having only a mainsail.

On the matter of ‘slot effect’, the source of all knowledge has information under ‘History’: https://en.wikipedia.org/wiki/Genoa_(sail) (A little confusing, I think)
https://en.wikipedia.org/wiki/Forces_on_sails under ‘Sail Interactions’ has clearer information. And the 1981 Arvel Gentry paper that is the source of both notations is here: http://ljjensen.net/Maritimt/A%20Review ... Theory.pdf

It is a bit of a stretch for me, but with some processing time and rereading… maybe helpful. Enjoy!

...mark...

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...mark...

Enjoying a 2012 TI
w/ hakas, a stainless steel
mast topper & furling jib

Everything you described is correct. Though I'm not sure about a 45 sq ft jib (I have never tried one that big, but would think you would be ok showing it all in light winds).
My understanding of slot effect is it only works very close hauled upwind. What it does is organize and align the air going over the back side of the main making the main work harder. Meaning you can sail in lower winds than you would without the jib (without the jib in 5 mph winds the main normally just luffs and can't grab), the jib fixes that. Yea ya still have to peddle though unless you like going 2 mph.
One other cool feature of running a jib is in higher winds (over 12-14mph) you can point the boat straight into the wind, pull the main out all the way and pull it tight as you can get it. With a small jib (not sure about you really big jib). Basically you start pedaling (kinda hard) once you have a little forward speed turn off the wind (maybe 20 degrees) or until the sails catch. You can easily maintain 4-5 mph upwind (via slot effect). Of course you have to keep pedaling. Huge danger in that setting though, It's an unnatural sailing attitude, if you not paying attention or the forward motion stops you will turn sideways to the wind and capsize with that much sail out and tight (been there done that (lol)).
FE

I think the genoa/Main/duel barber hauler setup would give you all the downwind speed you could want. It also is likely to perform better upwind than a reacher/jib/main rigged boat in lighter winds say below 8 mph.

The reacher/jib/main combination I would expect to perform better in the moderate winds. Anything above moderate (17mph), all you want is the main and reefed at that.

These boats will get more exciting with these extra sails. So will the risks. I am leary with just a jib/main rig alone and have had moments where like Bob says, the boat turns in a wind gust and you feel the boat lift sideways. So be careful with more sail. It's the gusty days you have to be especially conservative with the extra sails. I dont want to think about what would happen if someone tries to use the reacher on a beam reach or higher. Would not be good.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

I have a big assymetric spinnaker (130 sq ft, bigger than the Hobie reacher and more like a parachute (what I call a puufy sail). They are really great in light winds downwind any thing from 100 degrees to 260 winds from behind. You can easily sail 1=1 in most light conditions. So if the wind is 5 you will go 5 mph. The batwing jib setup with the jib to one side and the main on the other is not quite that good, but it's not bad either.
Fe

I appreciate the feedback! Much less lonely than trying to do it alone. Still on the topic of jib physics...

Fusioneng: As I understand it correctly, you and Hobie both use the stay line as a halyard line for the spinnaker. The line is not tied or firmly attached at any point on the mast top unit. In other words the mast is free to move, but moves fore and aft only under the influence of the mainsail. Is that correct? I am guessing the forward pressure on the jib and spinnaker don’t move the mast forward because of the stay line attached to the stern and to the sail. However the same is not true for the mast moving aft when pulling on the mainsail control line. The stay line instead of tightening, loosens because there is less distance for the line to travel with the top of the mast pushed back creating less of an angle on the rope. On a sailboat this would cause the jib loosen ‘automatically’ or at least when I changed the tension of the mainsail. I am guessing I don’t want this.

I set up a small scale, low budget experiment at home using rope strung to fore and aft anchor points and a chopstick as a mast (I live in Japan). My concern above (mast moving back loosened the line) was affirmed in the experiment when the rope was not attached to the top of the chopstick mast. Of course with the rope attached to the ‘mast’, there is essentially no top mast movement forward or back.

If the rope is really attached to the mast top swivel not allowing for or aft movement, does that not negatively affect the shape of the mainsail? So I have heard rumored.

If it is not attached and allows movement at the top of the mast, the genoa tension would tighten and loosen based on the movement of the mainsail, I think, or am I wrong?

Obviously I am misunderstanding something. Physics teacher(s) needed here.

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...mark...

Enjoying a 2012 TI
w/ hakas, a stainless steel
mast topper & furling jib

I think you are greatly overthinking this. Just how far do you think the masthead moves? With the halyard/backstay free to move through pulleys at the masthead, I would doubt the tension difference in the halyard functionality would be trivial; in fact, it would seem obvious that the total tension (sum of the halyard and backstay) would remain the same, and because the masthead pulleys would enable the mast to freely move back and forth, halyard tension would hardly change. That's my take anyway!

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker

Tony: You are likely correct. My chopstick mast demonstrated the principle of physics correctly, i think, but in an exaggerated manner.

I hope next month that it gets enough above freezing to put on my drysuit and give it a whirl.

The stainless steel company offered to make the mast top swivel for me. i should have draft two drawings soon. They said probably less than a kilo, (2.2lb).

....mark...

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...mark...

Enjoying a 2012 TI
w/ hakas, a stainless steel
mast topper & furling jib

HokMark:
You are correct Hobies reacher has a single rear stay line that doubles as a halyard. One thing to note is with the reacher sail pulled up tight, the knot prevents the mast from pulling forward too far (at least that's the way mine works, and I assume Hobies reacher works). The main reason for the stay is when you are on a downwind run with a reacher the forward force on the top of the mast is extreme, and that little 1/4" stud at the base of the mast isn't designed to withstand that kind of force, (thus the need for the stay), the stay line takes most of the force, and the load on the mast is downward (not forward).
Now on upwind, the stay line isn't needed, actually because the mast is so bendy when the sail is pulled tight the mast actually bends back quite a bit (forming a curve). On my boat the rear stay line gets quite loose (this is why I was warning that the rear stay line can get tangled up in the main batton posts (happened to me and it was very scary), the result is you can't furl the main, or put the spinnaker down.
Now with a jib, another problem arises, most jibs for most other boats are designed around a rigid mast, the mast on the TI is everything but rigid. For this reason most of my jibs have masts, and roto-furlers. I have separate halyards (2 halyards) for my spinnaker and my jibs, which operate independently so I can raise or lower either (same with furlers).
Actually running a jib from Hobies mast topper system would actually work pretty well in my opinion. However I would prefer to easily run either the spin or the jib and switch back and forth depending on conditions without having to unclip sails or re-run lines. If I get the Hobie reacher (pretty likely), I will likely add a second pulley on hobies mast topper (right next to the main), and run a second halyard line for a rotofurling jib (the jib will probably be on an aluminum mast). I currently have a similar setup, and have been using it for a while, and am able to furl/unfurl either the jib or the spinnaker separately. Running the spinnaker sail from a snuffer should work as long as the jib is furled when the spin is deployed (really never any need to run both at the same time anyway).
Whatever jib you design, it pretty much needs to be able to contend with the floppy carbon mast which waves around up there 2-3 feet (next time out just watch the top of the mast in different conditions). Some guys tried to use jibs from other boats, and ran into troubles with the jibs getting too loose (potato chip), this is why I always slip a mast in there (the mast also makes roto-furling way easier).

Hobies spinnaker kit shouldn't effect the performance of the mainsail (meaning it's a pretty good design)

One thing I found out early on is if you add a front stay and a rear stay, and side stays, it kind of kills the main sails ability to work (I think the main sail was designed from the ground up around that carbon unstayed mast (actually I think it's a standard windsurfer mast). Hobies mainsail is a cool design, but makes it really hard to add jibs.

I don't have any good advise for anyone, everyone kind of needs to figure this stuff out for themselves. I'm extremely happy with the setup I've been using for 5-6 yrs now with few problems, and haven's made any changes to anything in a long time.

FE

Fusioneng:

Thank you for the very clear, time consuming explanation. I don't have my mast top swivel yet, but got some rope and ran it through the top of the mast with the mainsail rolled up. So, I didn't have the normal pull power, but when I pushed on the mast, I noticed that the line loosened. What I found myself doing was gradually tightening the stay/halyard line as I gave the mast a push to the back (representing pulling on the mainsail control line). That worked OK and when I get on the water, and the sail isn't pulled back, I could: #1. adjust the one line from time to time while sailing, loosening (to straighten the main) and tightening (to keep the luff of the jib tight) as needed. 2. Have two lines, a forestay attached to the jib and a backstay attached to opposite ends (not sliding) so the top of the mast has little movement fore and aft. Either system will protect the powerful pull of a spinnaker and possibly 45sqft (thought to a much lesser degree) jib.

I think the choice of solution #1 or #2 may be based on the efficiency of the mainsail when it isn't permitted to bend back as designed. Does it loose a lot of efficiency?

I got my furling system today. I chose Barton due to pricing and seeming to be more than adequate. In the living room my sail furled up better than I thought it would.
...mark...

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...mark...

Enjoying a 2012 TI
w/ hakas, a stainless steel
mast topper & furling jib

fusioneng wrote

Actually Bob, looking at Hobie's instructions, I believe that the only knots in the halyard/backstay are figure eights at either end (spinnaker head and top grommet for retrieving).

I had initially thought there might be a knot above the spinnaker head so that the masthead could be pulled back against this stopper knot to tension the "forestay"(eg the spinnaker) while locking the position of the masthead by tensioning the "backstay".

The instructions disproved my theory, and the absence of such a knot means that the forces on the masthead are basically downward (apart from sideways flex from the sails inherent with the design.....)

Incidentally, I have discussed the Hobie configuratoin with a local sailmaker, who confirmed that Hobie's use of a single line as halyard, backstay, foot tensioner and retrieval line is typical for off the beach sailboats.

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker