|
|
18-03-2021, 08:51
|
#16
|
Registered User
Join Date: Jan 2018
Boat: 50ft Custom Fast Catamaran
Posts: 11,832
|
Re: Can someone explain how the AC75 sails so fast?
Quote:
Originally Posted by taxwizz
|
OK, everybody can stop posting. You just won the Internet for today. LOL
OK, everybody can stop posting. You just won the Internet for today. LOL
This was hilarious. I’m imagining a prop down on one of the foils. 200 hp diesel. Everything is just for show. Ha ha
|
|
|
18-03-2021, 09:17
|
#17
|
Registered User
Join Date: Jul 2012
Location: North East USA
Boat: 1975 Tartan 41'
Posts: 1,044
|
Re: Can someone explain how the AC75 sails so fast?
Quote:
Originally Posted by Woodland Hills
.
My feeling is that the farther the AC departs from traditional sailing craft, the less the public is interested. This is proven by the complete ignoring of the series that just ended by the media. Not too long ago the Cup was headline or at least above the fold news, now it’s buried on page 10.
|
I think it was ignored in the USA b/c NBC bought the copywrite and expected $175 to purchase the America's Cup channel. The vast majority of the world 150+ countries were able to stream it live for free. Greedy capitalism ruins the advancement of wind technology for America.
I personally love the AC75 class. Crazy exciting racing! Just a matter of time before foiling is mainstream... there's already a lot of small foiling race boats available for similar costs to conventional racing dinghies (ex. UFO < $8k). Beneteau makes a foiling 32ft production boat at ~$200k (Figoro), And there are large performance cruising yachts getting up on foils, albeit at high cost. It won't be long before foiling is mainstream... Why wouldn't you want the smooth and fast ride over the top of the waves, vice plowing through them with a day-sail range of 100+miles compared to 60mi?
|
|
|
18-03-2021, 11:42
|
#18
|
Registered User
Join Date: Sep 2014
Location: Annapolis, MD
Boat: Irwin Citation 34
Posts: 192
|
Re: Can someone explain how the AC75 sails so fast?
The speed of the wind bears no direct relationship to the ultimate speed of the boat. A sail simply captures a portion of the energy in the wind to maximize the forward force vector. It is simple physics that a sailboat will accelerate until the water resistance, i.e. rearward force vector, equals the forward force vector. The speed of the wind is basically irrelevant. The only relevance is the energy that can be captured from that wind; and as the wind speed goes up, so does the amount of captured energy, but the ultimate speed of the boat has little to do with the speed of the wind, but more so with the resistance of the water.
This is why ice boats can so easily exceed the wind velocity--the resistance of the ice on their skates is so small.
|
|
|
18-03-2021, 12:12
|
#19
|
Registered User
Join Date: Sep 2010
Location: Boat in Greece
Boat: Lagoon 400
Posts: 1,432
|
Re: Can someone explain how the AC75 sails so fast?
Quote:
Originally Posted by Muaddib1116
I can understand how they can sail faster than windspeed going up wind, but I don't really understand how they were doing that downwind. I assume it has something to do with the fact they they were basically tacking up wind and downwind symmetrically, but I can't figure it out.
|
True wind is behind the beam, but at high speed the apparent wind is moving so far forward that the sails feel an upwind direction of the AW.
|
|
|
18-03-2021, 12:16
|
#20
|
Registered User
Join Date: Jun 2019
Location: Bay of Islands New Zealand
Boat: Morgan 44 CC
Posts: 1,136
|
Re: Can someone explain how the AC75 sails so fast?
True wind is everywhere, so is apparent wind. Only if the boat is stationary is true wind and apparent wind the same.
|
|
|
18-03-2021, 13:53
|
#21
|
Marine Service Provider
Join Date: Jan 2019
Boat: Beneteau 432, C&C Landfall 42, Roberts Offshore 38
Posts: 6,364
|
Re: Can someone explain how the AC75 sails so fast?
that is apparently a very true statement...
|
|
|
18-03-2021, 14:13
|
#22
|
Registered User
Join Date: May 2015
Location: Lamb Island, Queensland
Boat: Northshore 33 ft sloop
Posts: 105
|
Re: Can someone explain how the AC75 sails so fast?
You will get the picture if you analyse the velocities in VECTOR Mechanics.
|
|
|
18-03-2021, 14:26
|
#23
|
Registered User
Join Date: Apr 2013
Location: Penobscot Bay, Maine
Boat: Tayana 47
Posts: 2,123
|
Re: Can someone explain how the AC75 sails so fast?
The difference in drag became pretty apparent when they both fell off their foils during race 8.
My Raymarine instruments allow me to toggle between true and apparent wind and I like to try to guess what the true wind will be before I switch from apparent wind at various points of sail. Of course my top speed is only about 8.5 knots but it’s still pretty interesting to see the difference when I’m on a beam reach.
|
|
|
18-03-2021, 14:58
|
#24
|
Registered User
Join Date: Apr 2016
Location: Charleston, SC, USA
Boat: Island Packet 380
Posts: 13
|
Re: Can someone explain how the AC75 sails so fast?
A proper explanation does require vector mechanics, but I believe none of the above posts has the complete explanation. The key concept is the wind (via the sails) pushing the foils (keel) against the water. A good analogy is you can shoot a watermelon seed out of your fingers if you squeeze it, with a speed much faster than your squeezing. Water is basically incompressible, so most of the wind energy "squeezes" the boat ahead, as long as the hull is not DDW. You might also think of a kite, which requires a kite string to fly -- if the string breaks, the kite falls to the ground. (Another, more difficult analogy, is the cutting point of a scissors, which moves much faster than the blades of the scissors when you cut something.)
|
|
|
18-03-2021, 16:45
|
#25
|
Registered User
Join Date: May 2018
Location: New Hampshire
Boat: Pacific Seacraft 37
Posts: 61
|
Re: Can someone explain how the AC75 sails so fast?
How do they sail so fast you ask? Simple. They are made out of “Youcantafordium”.
|
|
|
18-03-2021, 17:12
|
#26
|
Registered User
Join Date: Jan 2017
Location: Kingston, Ontario
Boat: Hans Christian 43
Posts: 382
|
Re: Can someone explain how the AC75 sails so fast?
Check out the blackbiord landyacht!! 50MPH dead downwind in 20mph wind???
|
|
|
19-03-2021, 04:48
|
#27
|
Registered User
Join Date: Jun 2016
Location: Brisbane Australia
Boat: Schionning Waterline 1480
Posts: 1,987
|
Re: Can someone explain how the AC75 sails so fast?
Quote:
Originally Posted by Rowglide
Check out the blackbiord landyacht!! 50MPH dead downwind in 20mph wind???
|
Re: Blackbird Land Yacht.
This is very intriguing. My head is screaming BS. But I've spent a while checking this out and smarter people seem to think it works although thier arguments haven't done it for me yet. I'll spend some more time on it but it seems like you are doing more work than the energy you put in.
I ended up going down the rabbit hole that is surfing the internet and found interesting subjects such as wind shadow gliders which also seem to defy physics and can fly an oval course continuously at up to over 800kph (current record) in less than 80kph winds.
__________________
Regards
Dave
|
|
|
19-03-2021, 14:10
|
#28
|
Registered User
Join Date: Jun 2016
Location: Brisbane Australia
Boat: Schionning Waterline 1480
Posts: 1,987
|
Re: Can someone explain how the AC75 sails so fast?
Quote:
Originally Posted by Dave_S
Re: Blackbird Land Yacht.
This is very intriguing. My head is screaming BS. But I've spent a while checking this out and smarter people seem to think it works although thier arguments haven't done it for me yet. I'll spend some more time on it but it seems like you are doing more work than the energy you put in.
I ended up going down the rabbit hole that is surfing the internet and found interesting subjects such as wind shadow gliders which also seem to defy physics and can fly an oval course continuously at up to over 800kph (current record) in less than 80kph winds.
|
Ok, I think I have it.
My stumbling block was when the propellers pitch velocity equalled the air speed I hadn't considered there would be any motive force generated by the propeller which is not right. The angle of attack needed so the cart can continue to acceleate is in the propeller blades. They are still rotating at 90° to the air flow (relative to the ground) and as such even though the pitch of the blades matches the air flow, the blades are still forcing the air to go around the assymetrical foil shape of the blade so still creating lift. Providing the air speed at which the foil shape is neutralized and the angle of attack is neutralized occur at different air speeds it will work.
Because it can always maintain an angle of attack the carts velocity is only limited by drag. No different to the AC75's or any other sailboat except the boats don't have a differential to allow an angle of attack when sailing at 0° or 180° to true.
Voila.... What a good boy am I.
Give me a little while to enjoy this before you destroy it.
__________________
Regards
Dave
|
|
|
19-03-2021, 14:18
|
#29
|
Registered User
Join Date: Feb 2012
Location: Toronto area when not travelling
Boat: Nonsuch 30
Posts: 1,669
|
Re: Can someone explain how the AC75 sails so fast?
OK, I have a different take. The boats from this challenge are now obsolete for the next one. If I buy one of them, what sort of PHRF rating will I get for the Wednesday night beer can races?
__________________
Have taken on the restoration of the first Nonsuch, which was launched in 1978. Needs some deck work, hull compounding, and a bit of new gear.
|
|
|
20-03-2021, 01:38
|
#30
|
Registered User
Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 6,451
|
Re: Can someone explain how the AC75 sails so fast?
Newton done it, F=ma they are not displacing any water which almost entirely gets rid of the form drag and the low surface area of the foils generate low skin drag.
__________________
Satiriker ist verboten, la conformité est obligatoire
|
|
|
|
|
Thread Tools |
Search this Thread |
|
|
Display Modes |
Rate This Thread |
Linear Mode
|
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
|
Advertise Here
Recent Discussions |
|
|
|
|
|
|
|
|
|
|
|
|
Vendor Spotlight |
|
|
|
|
|