Spinnaker Theory

Technical discussion of ARC products
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Kevin Keller
Professional
Posts: 338
Joined: September 24th, 2006, 2:56 am
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Spinnaker Theory

Post by Kevin Keller »

I haven't flown any of my spinnakers yet however it has come up in discussion down at my club. People can't wait to see it up on my boat.

I have never flown an asymmetric spinnaker. One thing I am being advised about is when a gust hits to turn down wind. This goes against my natural tendency to point higher. I am not sure if I understand why you would do that. Are you trying to stall the spinnaker to keep you from blowing over?

Kevin
Bill Roberts
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Location: Stuart, Florida

Re: Spinnaker Theory

Post by Bill Roberts »

Kevin, no you are not trying to stall the spinnaker. When the hull flies TOO HIGH with spin, bare off a little. Why? When you bare off,the resultant force from the rig is aimed more forward. The boat has much more forward stability, 20ft long, than it has sideways stability, 12ft wide. So, bare off and things settle down.
Go sailing first in light winds to get use to the feel of the boat under spinnaker. Gradually work your way up in windspeed and finally you will be sailing downwind with spinnaker and flying and flying the hull with total confidence. The tell tales will be flying from abeam and the white caps will be chasing you from the rear. The spinnaker and the main will be trimmed in tight. To depower the mainsail, travel the main out. Do not ease the mainsheet. This might let the spin bend the top of the mast forward. Remember the mainsheet and leech of the mainsail are your backstay to the mast.
Kevin Keller
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Boat Make/Model: SC-20, ARC22
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Re: Spinnaker Theory

Post by Kevin Keller »

Thanks Bill. Can't wait to try the spinnaker but it's been way too windy. Waiting for them to die down when I am home.
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Spinnaker Theory

Post by Bill Roberts »

Kevin,
The spinnaker on your boat when trimmed and sailed properly, it is acting like a big Genoa sail. The airflow will be across the sail. You need luff taletells to see when the flow is attached and flowing across the sail just like on your jib. The spinnaker and main are trimmed in tight. Do not think of the spinnaker as acting like a "big catcher's mit". You don't sail this sail like it is a barn door pulling you downwind. This is super slow. There is another world of downwind sailing waiting for you to discover it and learn how to sail it. You can sail downwind faster than the speed of the true windspeed and be flying a hull. You can look behind you and the white caps are following you and you are flying a hull with the relative wind just forward of abeam. At first you wonder, "how am I doing this"? You have just been introduced to Americas Cup level of sailing downwind. Your boat is one of the few boats that will do this.
Kevin Keller
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Posts: 338
Joined: September 24th, 2006, 2:56 am
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Re: Spinnaker Theory

Post by Kevin Keller »

I finally got the opportunity to use the spinnaker. It was in about 11-13 kt winds. We used the supplied bag for launching and retrieving it. It was fun and I look forward to trying it again when i can. I have to say it was a mess of lines on my deck.

I plan on purchasing a top down furler for it from Tom. Do you have any experience with the top down furler? The winds here in Hawaii have been blowing pretty strong all summer in the upper teens. Do you think the furler works well in the higher winds? You guys are the experts. Others are skeptical of it working in the higher winds out here. I'm all for simplifying as much as possible.

The others all use a snuffer.

Thanks for the explanations above.

Kevin.
Bill Roberts
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Re: Spinnaker Theory

Post by Bill Roberts »

Hi Kevin,
I have no experience with top down furlers.
I sail mostly distance races so spin up and down many times during a race is
not a problem.
Graig, with an ARC22 on White Bear Lake where Tom sails has a top down furler
and he loves it. He thinks it is great. It makes the crews job with spin up and down much easier.
I'm sure you will like your top down furler.
Mac M
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Joined: June 13th, 2012, 5:05 am
Boat Make/Model: SC17
Location: Lugoff, SC

Re: Spinnaker Theory

Post by Mac M »

Kevin,

I put the TDF on my 20 and am happy with it. I loaned the boat to a friend to race at regatta recently and it excelled rounding A mark. I followed them around and they gained a boat length or 2 with the momentum they were able to carry with the fast deployment. It didn't look to be quite as big of an advantage at C mark though. I'll attach a video I made when I first installed the system. Keep in mind the wind was super light, th sail unfurls quicker with some breeze.


https://youtu.be/6SY-hGtA9dI
Kevin Keller
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Posts: 338
Joined: September 24th, 2006, 2:56 am
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Re: Spinnaker Theory

Post by Kevin Keller »

Thanks Mac. Too bad you are selling it. I am asking $10k for mine out here in Oahu after putting $19k into it. Hard to find a place to keep it though. Our club is full at the moment.

Kevin
Bruiser
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Joined: July 1st, 2013, 6:37 pm
Boat Make/Model: RC 27

Re: Spinnaker Theory

Post by Bruiser »

We have a top down fuller for our spinnakers/code zeros. It works way better than previous retrieval systems. We also have had a bottom up furler and that did not work so well. This top down system worked well in big wind too. The trick is to not have much pressure in it when furling.
Kevin Keller
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Posts: 338
Joined: September 24th, 2006, 2:56 am
Boat Make/Model: SC-20, ARC22
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Re: Spinnaker Theory

Post by Kevin Keller »

Thanks Bruiser. Good to hear. I'd love to see a picture of a 27 with a spinnaker. Where are you located?
Bill Roberts
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Location: Stuart, Florida

Re: Spinnaker Theory

Post by Bill Roberts »

The vector diagram below explains what is going on in this picture. The true wind is off the rear quarter yet from the trim of the sails it can be seen that the boat is sailing into the wind, the relative wind. Notice the trim of the main. The main is trimmed in tight on the sheet and the car is in the center of the rear beam and the tell tales are flying perfect on both sides of the mainsail. The main thinks it is sailing to windward. The flat spin is sailing just like a big jib. The skipper is sailing to the tell tales on the luff of the spin just like it was a jib. The relative wind is only 37 degrees off the bow. This is AC boat type sailing downwind. Miss a shift or sail on the wrong tack for 10 seconds and you have lost 100 yards or more relative to a boat that sailed the favored tack.

How do you sail the boat/sails in the situation described by the vector diagram below? Keep in mind the 14 mph relative wind vector. The boat is balanced here, good speed and flying a hull. What happens when you encounter a puff? The boat accelerates and the hull is flying higher and on the move. All velocity vectors are increasing. The one you are most interested in is Vrel. Everything was fine when Vrel was 14 mph. How do we get it back down there? We have two choices. We can head up or bare off. If we head up, the included angle, 45 degrees, starts increasing and Vrel increases even more. Vrel is already too great and the hull is lifting faster. The time now is 2 or 3 seconds after we encountered the puff. Another option is blow the chute. The luffing flogging chute will turn the boat over anyway. Last option bare off. The 45 degree plus angle between the true wind and boat speed vectors begins to decrease and Vrel begins to decrease. Get it back to 14 mph and everyone is happy and you are going faster and deeper than you were before the puff hit. Sooo, now what do you do when you hit a lull and the hull comes down? Vrel is below 14 mph. Well let's head up a little and see what happens. Well son of a gun. We increase the below 45 degree angle of the vector diagram and even though Vtrue is smaller, we can make Vrel go back up to 14 mph. Now we are sailing slightly higher than we were before. So in the puffs we bare off and in the lulls we head up. As the wind naturally puffs and lulls and we are baring off and heading up to hold Vrel constant we are sailing a slight S shaped path through the water. So to sail downwind fast with the chute you have to turn your hat around backwards and think backwards and be smooth and quick.
Attachments
2011.jpg
Downwind Velocity Triangle RC30.jpg
Last edited by Bill Roberts on November 2nd, 2016, 6:39 am, edited 1 time in total.
gahamby
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Boat Make/Model: SuperCat 15#315
Location: Falls Church VA 22042

Re: Spinnaker Theory

Post by gahamby »

Bill,
The leech on the main in that picture is almost parallel to the mast all the way up! Talk about more cowbell, what's the ratio foot/head on that mainsail?
ps: Is the main brailed up on the bottom?
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Spinnaker Theory

Post by Bill Roberts »

gahamby,
The picture is 5 years old. That sail is 8 years old . The sail is reefed along the foot. It is a sail for a 45 ft mast on a 41 ft mast. In the Mug Race I have to use an RC27 mast/rig/sails because of a bridge at mid race, the Green Cove Springs Bridge. The taper ratio on that old sail is a little less than 0.4.
Look at the picture and you will see a very large amount of overlapp between the leech of the spin and the mainsail. Note that spin is trimmed to the rear beam. This spinnaker overlapp turns the airflow on the leeward side of the mainsail so as to override or take out much of the normal velocity direction gradient that would be in free stream airflow in the max overlapp region. The spinnaker bends the airflow on the leeward side of the mainsail so that the angle of the airflow entering and exiting the the leeward side of the mainsail is almost constant rather than having the normal variable twisting direction gradient in it. Therefore the reduced mainsail leech twist.

You know you have the right twist in the mainsail when the tell tales are all flying especially on the leeward side top to bottom. Sheet the main in slightly more and the square top tell tale falls. Ease the mainsheet until square top tell tale flies again. Then the sail is trimmed correctly. Remember tall masts are exposed to more velocity gradient. Shorter masts are exposed to less velocity gradient.

I went back and looked at the picture and I see leech twist especially in the top 1/3rd of the height of the sail.

The picture was taken at about 45 degrees off the rear corner of the boat. The foot of the sail is along boat center line. This means the apparent width of the lower part of the sail, foot etc, is about 70% of what it actually is, 12 ft. The top of the sail is twisted out 10 or 15 degrees so it appears closer to its actual width, 56 ins. Sail taper ratio is 0.40 .
Bruiser
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Joined: July 1st, 2013, 6:37 pm
Boat Make/Model: RC 27

Re: Spinnaker Theory

Post by Bruiser »

Hi Kevin,

We are located in Jacksonville, Florida. I wish I had a picture of the boat with the spinnaker up. I will work on trying to locate one. It might be helpful to add, that we changed from a conventional spinnaker (like the one Bill has in his picture above) to what the sail makers refer to as a Code Zero. The main difference I think is the material. Our spinnaker is more like a big jib now, with the sail cloth like that of a jib rather than the soft material of a spinnaker. It is top down roller furling and works very well. This is like the set up the America's Cup cats have.

We just had a race in the Gulf off of Ft Walton, Florida and our big competition was an ARC22. This boat was I think Bill's most recent design. This is basically a Super Cat 20 (tall rig)with newer designed and longer hulls. In light air this boat has a better power to weight ratio than our RC27. Previously when we raced against this boat with (what I call a conventional spinnaker, regular spinnaker cloth) we could not stay with them in the lighter air conditions. (It is also quite possible that we did not sail the boat as well as say Bill could have, jury is still out). Also, I must add, the last time we raced against an ARC22 was 7 or 8 years ago. We just had a race against them using our new Code Zero sails, we had a down wind leg from Ft Walton to Pensacola in the Gulf, about 35-40 miles. Here is my take of what transpired, as we went neck and neck for those miles. Previously, the ARC22 would walk away from us in what was lighter air. Now, we had a very good speed advantage when we built up our apparent wind. The challenge was, the ARC22 could go a little deeper than we could with their apparent wind and a conventional spinnaker. On the other hand we could go higher with our code zero. At one point, the ACR22 got a puff from behind and tried to roll us to weather. We could go higher and fend off their attempts to the point that they could no longer keep the chute flying. So their next move was to try and go lower than us.

As we sailed down the beach, we would be going a little higher but a little faster as well. The result when we jibed back was we crossed right behind them. We had plenty of speed potential so we just heated it up and sailed over the top of them. They tried to stay with us rather than going as low as possible so we ended up putting time on them. The point to all of this, is I think we have a faster set up than we had with the conventional spinnaker. I am talking just about speed, as for recovering the spinnaker onto the boat versus rolling it up, it is night and day. You know, maybe a week for us to get the spinnaker into the bag. (something the ARC22 had to contend with) However being able to go higher to build the speed we needed, did not seem as easy as with the previous spinnaker.

Also, a G-Cat 6.0 had a conventional spinnaker with top down furling and they had no issues with it, so the as far as the furler goes, top down is good.

Regarding Code Zero versus Conventional spinnaker, I think Bill is of the opinion that the conventional is still a better choice for all conditions. Time will tell (and a few more races). In the short term, I am happy with our Code Zero results so far. Just the ease of managing it alone made the boat so much more fun.

Cheers,

Rich Brew
Kevin Keller
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Posts: 338
Joined: September 24th, 2006, 2:56 am
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Re: Spinnaker Theory

Post by Kevin Keller »

Rich,

Thanks for the information. Sounds like it was an exciting race.

I've ordered a TDF from Tom and look forward to getting it. One of these days I would love to get a ride on a 27 or 30. I wish one would fit in at my club.

Thanks,

Kevin
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