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Sail Trim and Aeronautic Theories.

Oct 22, 2014
15,713
CAL 35 Cruiser moored EVERETT WA
Boy did I get my thinking scrambled a couple of months ago.

I was chatting with @DrJudyB about the planning for our sadly canceled 2020 Circumnavigation of Vancouver Island, when the topic drifted to the new sails I have on my boat. That is when she quietly dropped the "Aeronautic" bomb in my lap. "What do you know about slot trim?"

Before I thought about it, out popped my years of sailing training from the late 60's and 70's... "Have you read the work of Arvel Gentry..."

No. I had heard of him having followed the Americas Cup racing, but most of my youthful experience came from being on the water racing dinghies and small sloops. Learning to read the wind and adjusting the sails to get the most speed out of the boat and beat my competitors. Not a lot of aeronautic theory study. It would have gotten in the ways of surfing and chasing girls.

So I got Mr Gentry's work off the web and started to explore the mathematics and aeronautic associations. Bam, my head took another blow. All that stuff about "Lift" I learned to get my pilots license was disrupted by the vortices of the math.

I became a believer that I can learn new things. I have always believed you need an open mind and when presented with new data you need to evaluate it and compare it to the data you have. If the data changes your thinking you need to embrace it, not discard it.

Sorry about the long preamble. I am writing this to share some ideas on trim. Not my ideas, but Gentry's and followers of his theory. I want to encourage you to read Gentry's work. (https://gentrysailing.com/theory.html)

I find it to be the reason for the sail designs now being made. Why a new design sail will give your boat a boost in sail speed.

As with all theory, we need to take this and apply it to the sails on our boats. We get out on the water and see another boat slipping by us. We ask, what are they doing we are not. The crew says, hey they have a tweaker on their jib sheet. So you send someone to get a tweaker. Why?

Gentry's work will help you understand the theories involved... Then I came across a link to North Sails website "The Science behind your Sail Trim". Ok I like science. Click... (the-science-behind-your-sail-trim-induced-drag-part-1). Again Gentry pops up, and like all good click bait ideas there is a helpful idea on Main sail trimming and the effect of managing your leach. Ok, there is also a shameless link to a new book . We are all adults we can choose to buy or not buy.

What I liked was the info about the leach and the "top telltale". How this "telltale" can inform you about the "induced drag" being imparted to the mainsail based on your trim.

Take a look. I would like to hear your ideas on this subject.

No drugs were used to induce this mind blowing confession.
John
 
Jun 25, 2004
1,108
Corsair F24 Mk1 003 San Francisco Bay, CA
I love this stuff... but I don’t recommend buying the book “Art and Science of Sails”, 2nd edition. I did, and it’s a waste of money, IMO. It has numerous egregious editorial or factual mistakes that will confuse you if you’re new to this topic. It’s a pretty, overpriced, cocktail table book in sore need of a good editor.

I bought the first edition (1990), and the second edition when it came out in 2016. I liked the first edition. But I hated the second edition(2016) because of the numerous errors. Maybe the “revised” 2nd printing of the 2nd edition is better.... but I’m not spending a penny for it.

I have a copy of Ariel Gentry’s collected published articles and scientific papers, plus a lot some of his less well-known, privately written stuff. I had the honor to get to know him a bit 20 years ago via phone and email.
 
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May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
i love this stuff too. for me, as you know, i'm a huge frank bethwaite fan. i love the trimming , i love the understanding of the wind, i love the detail.
"down in a puff, up in a lull" while accurate, i need to know why, which only left to more interest
 
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Jun 25, 2004
1,108
Corsair F24 Mk1 003 San Francisco Bay, CA
Yup, I have read all of Bethwaite’s books too. Including his latest.

They’re superb with regard to the interactions between wind, coastal lands, and water. A must read for sailors.
 
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May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
i tried the site john gives and i get a security warning not to go there.
so, with these new theories, is it really for the sail designer or is it the way i trim my sails.
on the alden i have a non movable telephone pole for a mast so my rig tune is for 'one size fits all'. on the Ascow the mast is very flexible and can be shaped to a huge degree on the fly by the crew.
are there new tricks to trimming i should learn?
 
May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
predicting the wind and then being ready to maximize my push is a hoot
 
Jun 25, 2004
1,108
Corsair F24 Mk1 003 San Francisco Bay, CA
are there new tricks to trimming i should learn?
Probably, but you gotta “see” it in a different perspective in order to learn new tricks. If you have never read Arvel Gentry’s articles and papers , they will change the way you “see” lift and drag through wind and water.

In my own experience, sailing modern high performance craft has been a humbling experience. Things look different at 15 or 20 kts of boatspeed, especially when you’re going faster than the wind . New experiences beget new questions.
 
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May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
predicting the wind and then being ready to maximize my push is a hoot
 
Oct 22, 2014
15,713
CAL 35 Cruiser moored EVERETT WA
Jon. I got that as well using my Mac. The site provides the theory that Gentry developed.
 
May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
ok you two, i'm hook. i just read the science for what i have observed and really, what i have observed trimming my sails. i can feel when the alden sails are pulling their best. i have on brother that will endlessly sneak the main in , i can always feel the boat slowing.
look, i've read the first essay twice now. the saturation into the brain is slow. i'll keep rereading till i have it down. then on to the next essay of his.

doc. , john, thanks for sharing.
the Ascow is the other end of the scale but the feel is still the feel for grabing all the horse power.

Ascows are a hoot
 
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Joe

.
Jun 1, 2004
7,391
Catalina 27 Mission Bay, San Diego
I love this stuff... but I don’t recommend buying the book “Art and Science of Sails”, 2nd edition. I did, and it’s a waste of money, IMO. It has numerous egregious editorial or factual mistakes that will confuse you if you’re new to this topic. It’s a pretty, overpriced, cocktail table book in sore need of a good editor.
Must agree with Judy. The first edition copy of "The Art and Science of Sails" by Tom Whidden is a bible for anyone who sails a classic design monohull…. which most of us do.. Fractional or Masthead rigs are thoroughly covered. It won't be a lot of help for high tech, foiling craft that have become popular in some circles... because, of course, it was written so much earlier. Whidden was Connor's tactician back in the old AC days and his company, North Sails, became dominant as a result.

I recommended this book to everyone here over the years... but... unfortunately it was discontinued...So... when the 2nd edition came out... I eagerly ordered it.... and.... was thoroughly disappointed. Yes...it has lots of color photos, and some charts... but I felt it was mainly a testament to North Sails.. a promo if you will...
My early years on this forum found me referencing the first book over all others.... even over my #1 sailor's library recommendation, The Annapolis Book of Seamanship" by John Rousemaniere. Because It was/is all about how sails work and how to use them. How they are made, the changes in design over the years, etc. What sails you should have in your inventory for various activities... racing, cruising, daysailing, offshore passagemaking, big boats, small ones..... it explains everything... So if you can find the first edition.... I'd check used bookstores, or nautical bookstores like Seabreeze in Pt Loma, San Diego
 
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Oct 19, 2017
6,855
O'Day 19 Littleton, NH
Wow JS. Really going after the big subjects here. First weather and now sail trim.

I'd like to participate, but this is really going to require more time for serious considered thought and research. In the mean time, I'll just follow along and listen while the big guns talk.

-Will (Dragonfly)
 
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Joe

.
Jun 1, 2004
7,391
Catalina 27 Mission Bay, San Diego
Before I thought about it, out popped my years of sailing training from the late 60's and 70's... "Have you read the work of Arvel Gentry..."
Re: Arvel Gentry: a number of years back...at least ten I'd say... there was a regular poster on this forum who constantly referred to the Arvel Gentry text(Geez, I can't remember his name, but he's the guy with the very beautiful brightwork on his large classic sloop) But, I mean he was ALWAYS citing it... so I took a look and learned something... but imho it lacked giving credibility to the "feel" or "sense" one develops when he understands his boats particular tendencies and personality. I mean... you can stick 50 or 60 tell tales all over your rig.. make your boat look like it's a wind tunnel test model... but...uh, not me. I confess after reading the piece I tried sticking tells all over the mainsail and the jib leech and wherever he recommended... but I got nothing from it that I didn't already know, I think. I remember when my wind vane broke and I thought.. "oh, crap! how am I gonna sail good now?" And then I remembered: I never had a wind vane on my Laser or Lido 14. I do have a flag flying off backstay... but, you know, the odd angle hurt my neck. So rather than crane it up every so often, I just started trusting the hair on the back of my neck, the wavelets on the water surface, THE SAILS... and most of all the three sets of tell tales on the luff of my headsail, and the three singles on the leech of my main. And I rediscovered sailing. You watch your sails and let them tell you what's happening and what to do..
I urge everyone to take a look at the Gentry piece Don't be intimidated by the tech talk, just wade through it as best you can and remember that the more user friendly sail trim tutorials will have incorporated Gentry's concepts already. Which means most of what you need to know, you've already been exposed to... you're just learning where it came from.
 
Jan 1, 2006
5,953
Slickcraft 26 Greenport, NY
If memory serves me, and that's not a certainty, Gentry sailed a Ranger 23 and was very active in the class association. That's how I got familiar with his work. I was looking to buy a Ranger 23. Then I ended up buying a Ranger 29. It may be time to re-visit his writings - and I've got the time now.
 
Jan 1, 2006
5,953
Slickcraft 26 Greenport, NY
I got the unsecure website message when I went to the gentrysailing website. Did anyone ignore that? Or is there a correction in the address. I wouldn't think an obscure sailing website would be a place for a scamer to set up.
 
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May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
chapter 17. sails. in high performance sailing, seems to agree with gentry. same stuff. i think. am i correct? are bethwaite and gentry selling the same concepts.

what about the fokker effect on multi mast vessels? or cutter rigs?
1589727781829.png
 
Oct 22, 2014
15,713
CAL 35 Cruiser moored EVERETT WA
I never had a wind vane on my Laser or Lido 14.
Those were two of the boats I sailed in my youth. We used mom's knitting yarn (I sneaked from her basket... Maybe that is why the sweater she knitted had a small hole in it) tied to the stays and pushed away from the dock. It was all feel and judicious use of the Mark1 eyeball.

I never thought about putting wind telltales on my sails. It would spoil the look. What I did help from Gentry's work, was the description of how the air moving across the sails reacts. How changing the trims modify these movements changing the forces.

I trimmed the sails and trimmed them again, shifted body position, changed the course, changed the heel, all this done in reaction to the feel of a plastic 14 foot boat. No science involved. Trying to understand Gentry's theories helps to explain the results to expect, the changes I learned by feel and experimentation.

When I first got SV Hadley, I was in constant motion. Trimming this, adjusting that, correcting the person at the helm for drifting while I adjusted the sails. (Boy that bugged me. I set the course then trimmed the sails then the dude shifted the course 5 degrees which screwed up the trim) Again this was because the feel was not right. Finally a good friend, who was manning the helm, as I was reaching again to adjust the course asked "Do you ever sit back and just enjoy the ride?"

I thought... No.

Having a context to use rather than adjusting to feel gives me a calmer attitude towards the trim of the boat. I am more purposeful with my actions. I suspect it makes the boat faster.
 
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Oct 22, 2014
15,713
CAL 35 Cruiser moored EVERETT WA
Jon, I am reading for the first time, Bethwaite's Fast Handling Technique. Just started last night. Lot of discussion of early sailing training. Not yet into the current day thinking of sail shape, form and structure which I believe is the essence of Gentry's theories as applied to sailboats.

But I think that Bethwaite is building on Gentry's theories. I think Whidden, from NorthSail, has incorporated Gentry's ideas into his book. I listened to a podcast he did this year. He talked about Gentry theories in connection with his Americas Cup sailing.

Certainly Bethwaite applied some of his flying experience into his sailing concepts in the same way Gentry applied his knowledge of aerodynamics.

As I said, the actions of the telltales on the Mainsail leech, as explained in the NorthSail article helped me to take the theory and produce a trigger for actionable trim adjustments.
 
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