Elliptical top and the AC72

[havliii]

I am imagining things? or does the bow of the AC72 closely resemble the Supercat design. Aside from the reverse bow it sure looks like a Supercat to me! Oh how I wish we could see the section drawings and overlay the two. Bill, please chime in here, what are the pros and cons of the reverse bow.

Andy

[Bill Roberts]

There is no benefit to the reverse bow, reverse stem slope. There are significant losses. The reverse bow is part of the idea to slope the foredeck downward as it approaches the bow to reduce the sideways projected area of the forward hull length. This is supposed to reduce the sideways aerodynamic drag of this part of the hull. Does it or not, I don't know, I have seen no test results about reduced sideways hull drag but I have seen other sailing results. Today "PITCHPOLE" is much more prevelant than it was 10 years ago. There is nothing slower than "PITCHPOLE".
People have forgotten the PLAYSTATION experience. Playstation first went to sea trials at 100ft long with a conventional sheer line, about level. The top speed the team could get the boat up to was about 25 knots and the boat was very near pitchpole. This test and speed limit was repeated several times. It frightened everyone on the boat.
One day after sea trials the boat was pulled and major hull modifications followed. The hulls were cut off just in front of the main beam and a 20ft extension was added. The foredesks were removed and the shear was raised several feet at the bow. The foredeck looked like a ski jump as it approeched the bow. The aft end of the boat was extended 5ft to retrim the boat's waterline.
Now back to sea trials. This time the boat could go 35 to 40 knots before pitchpole concern was reached. What's the difference?????
FORWARD HULL VOLUME! Forward hull volume is like reserve horsepower! It lets you drive the boat harder before pitchpole. Pitchpole, pitching moment, is caused by the forward component of sail thrust. Restoring moment resists pitchpole and it is equal to the total weight of the boat times the distance from the center of gravity of the sailing system to the center of bouyancy of the boat with the boat in a max bow down attitude, CB migrated forward. The smaller the forward hull volume, the shorter this distance is going to be and the greater the tendency for the boat to pitchpole. If the hull design can only withstand a smaller forward component of sail thrust, the lower the top speed of the boat will be.
ON an RC27: Did anyone ever notice that the main beam and rig on the RC27 are 16ft aft from the bow. I wonder why that is ????
Did anyone ever notice that the PNs of the RC27 and 30 are like the same as the million dollar+ Formula 40, an all out world's best racing machine. You could buy ten RC27s for one Formula 40. What's going on here???
Now do you understand why Oracle pitchpoled so easily? Can you understand why the AC45s pitchpole so easily?

[havliii]

What's up with the Nacra F20c? Seems like they pi**ed away a ton of money on R and D to come up with a boat that is only rated one tenth of one percent faster than a Tornado, 57.9 against 58 for the Tornado. (That's the only number I could find on the F20c.) So what in the world is up with that? The Tornado design is like 40 years old, with all the massive computing power at their disposal you would think that Melvin and Morelli could better the Tornado by more than a mere .1% ??????

They claim to have used a lot of their AC design work in the F20c, so if that is the case, why not just scale the Tornado design up to 72 feet and see if it out performs the AC72's?

Here's a video link to the F20c:
http://www.youtube.com/watch?v=82CDZcpqyNU

Looks and seems like an incredibly fast boat in the video, until I saw the numbers.

[Bruiser]

What's up with the Nacra F20c? Seems like they pi**ed away a ton of money on R and D to come up with a boat that is only rated one tenth of one percent faster than a Tornado, 57.9 against 58 for the Tornado. (That's the only number I could find on the F20c.) So what in the world is up with that? The Tornado design is like 40 years old, with all the massive computing power at their disposal you would think that Melvin and Morelli could better the Tornado by more than a mere .1% ??????

They claim to have used a lot of their AC design work in the F20c, so if that is the case, why not just scale the Tornado design up to 72 feet and see if it out performs the AC72's?

Here's a video link to the F20c:
http://www.youtube.com/watch?v=82CDZcpqyNU

Looks and seems like an incredibly fast boat in the video, until I saw the numbers.

I think his point is..is that the numbers are wrong. What is happening with boats like the M20 and F20 is that they want to sell them. They are very expensive and therefore they MUST win. They way to do that is get a very good rating, meaning a number (slower) than it actually is. Say like a tornados rating. Then you go out and "kill" everyone on corrected time and the boat is viewed as "amazing". When the M20 came to the US the Portsmouth number was 3 to 4 points lower than it is now. Since that time they have done a lot of work on the mast and added curved boards and the rating goes up? They are saying all those changes made it slower?

Since Darline passed away, the person who used to handle Portsmouth ratings at US Sailing, the system has become political. Very similar to PHRF now. So if you have clout with the right folks shazaam you get a favorable rating.... So my guess is the F20 is pointing at the M20 rating and saying we are like them. Basically we have new designs given politically favorable ratings...

Disclaimer: All that said, it is impossible to get any rating system to fairly adjust for the wide variables in boat speed potential of the different designs. As an example the RC27 and 30s rating is probably "fair" in lighter air with the waterline drag etc. when compared to a Hobie 16 etc. However as the wind picks up I see the advantage going heavy to the RC's as they have the ability to sail better than the formulas can account for. The same is true for these new designs. They can out perform the formula approaches to giving them a rating. All the more reason a rating similar to the older tornado is very wrong. Bottomline the rating systems are not useful for the purposes of taking this stuff seriously or getting any sense of a boats speed potential. It is just to give one a general idea of how they are sailing from one boat compared to another.

[Bill Roberts]

The only boat that has gotten any separation from the Tornado PN is the ARC22. The 22's PN has only gone down since it was first issued.
It took three years for Darline, US Sailing , would issue a PN on the RC27 because she said there was not enough data, race results, to base a PN on. Today evidently PNs can be issued when the first boat hits the water and the boat can't sail to that PN. ARC and RC PNs have only gone down since their first issue and the class doesn't have rock star sailors like the Tornado class. Today ARC and RC PNs are based on large data bases, years of race results. Their PNs are real like the Tornados and the H16.
In Europe I am told that the M20 class is going no where, no growth. In the US it is not growing. The performance for the cost is just not there. Again the ARC22 is the only boat of similiar size that has demonstrated performance separation from the Tornado.
Why is this??? The 22 has a higher sail area to weight ratio than the Tornado and it has a higher righting moment to sail area ratio than the Tornado. So, the 22 has more power to weight ratio and more righting moment to power ratio than the Tornado. Relative to a race car the 22 has more horsepower and more traction to put that power to the pavement than the competition. Basics make sailboats go faster. Carbon is not magic!
By The way: There is something I forgot to point out. When I designed the 22 using a SC20 hull as the plug, it was done in such a way that 2ft could be removed from the transom and the result is a 20ft hull. Install wagon tongue steering and this puts the tiller crossbar behind the rear beam, no beam saddles moved. The tramp now is wide open, bigger than ever. The boat would have slightly more fore and aft pitch than the 22 but it still has all the 22 go fasts but in a 20ft version. If some grand race occurred that only 20footers could race in, a 20ft version of the 22 could easily be on the starting line. Why was this done? ANS: The Worrell 1000 when the class of boat entered was open to any 20 footer.

The elliptical hull shape is superior. The elliptical shape comes with inherent very desireable stiffness characteristics due to shape alone. I was over in Tampa the other day and I saw an old World War II submarine. Here's a ship where millions of dollars was spent to determine the optimum hull shape for a vessel that spent most of its live moving ahead at the waters surface at low drag and guess what? The hull cross section is an ellipse. The SC family of beach cats has been elliptical hull shapes since its beginning, 1978. The equation of an ellipse is aX**2 + bY**2 = c**2. This equation generated the hull cross sections for all SC and ARC and RC catamaran hulls.

Novice, don't be so hard on the PN system. There is nothing wrong with the system especially with mature boats, boats with at least a couple of seasons of "race data" behind them and in US Sailing's data base. New boats with an "assigned PN" can have political influence
in them but put a couple of sailing seasons of data on that new boat and the "cows come home", the real PN stands up.
There is no formula to calculate max boat speed for a catamaran or to calculate PN for a given boat design. Catamarans are not limited by a magic "hull speed limit" like monohulls. Catamarans are limited by forward stability which determines max speed at pitchpole. If you want to go faster, the sure way and the inexpensive way is to go longer, BUT DON"T CHANGE ANYTHING ELSE!!! BTW There is nothing less expensive about a catamaran to increase an additional 10% in size than to make the hull 2ft longer. Two more feet of hull length in the right place is cheap and can do wonders, 10 square feet of laminate, resin and foam, 2ft by 5ft around. What a deal!

Also: Starting a new boat off with a high PN is a double edged sword. A high PN says the new design is no faster or very little faster than another older design. This is not a strong selling position. Then why buy the new design, which is unproven, if it is rated only a fraction of a point faster than an old established class/design? Rating a new design with a high PN might get you a few beer mug trophies, but big whup: if the new expensive design can't run off and leave the old boats in the dust like it is advertised to do, then why buy it ? It is the bang for the buck thing. So far I don't see any new design outrunning the ARC22 which is the SC20tallrig, 1980, with 2ft added to the bow for more pitchpole resistance and the same everything else design. The SC20 tall rig PN was 62; the ARC22's is 57 with spin. The boat's PN speaks for itself. Boy, that 2ft of hull did a whooooole lot for the SC20tr's PN. Put some rock star sailors in the class and the PN might go down another knotch or two.

[Bill Roberts]

Hey Guys,
What's up? I write all this stuff up and explain everything and then I don't hear a peep out of anyone?
Do you all agree, disagree or what? Have any questions? I didn't mean to turn everybody off.
Bill

[havliii]

Gone sailin' Bill, Sunday was awesome, 17-20 kts and big waves on the Potomac and Chessy Bay. We had a blast, my regular crew and best mate, sailed with the dolphins and generally ripped up the river.

My weapon of choice is my SC20, tho I cut it down. Now don't go all ballistic on me, I'm a 'trailer sailor' I gotta rig and be sailing in less than an hour to be happy. I love the boat but 12 feet don't go down the road so it's now a 9 foot beam boat. (Okay that's not really street legal but don't tell the gendarmes) Go figure the PN on an SC20 with a 9 foot beam. My wife and I total all of 310 lbs so we really can't hold her down but what the heck it's a rocket. It's leaky as all get out but ya know the extra ballast actually doesn't hurt a bit.

As for the elliptical top, I don't know why every cat in the world hasn't copied that design, actually maybe they have. It simply rocks. Kathy and I got stopped dead when we buried the bow into the waves yesterday but the SC is 'old reliable' never ever once did we feel a pitchpole coming on.

I picked up a great tip from one of your 'ancient history' posts on this forum, the river and bay were white caps from edge to edge yesterday so I furled the jib on the beats to weather and damn if the boat didn't go faster and point higher than I ever thought it would.

Keep posting, Bill, I'd argue or debate with you, but that would be kinda like a one legged man in an a** kicking contest.

All the best,
Andy

[Kevin Keller]

Can you explain why the boat would go faster with the jib furled?

Kevin

[SC15Av8or]

My guess would be less drag and "cleaner air" over the main sail/wing ??? I know team Emirates romped all over Luna Rosa with out a jib sail at all. Luna Rosa had theirs.

Lets get back to the hulls how ever. I understand the great benefits of the elliptical hull as I started as a hobie driver when I got into this sport and experienced the excitement of a pitch pole. Thank you Bill for making that a lesser thing to worry about. How ever why have they turned the hulls upside down from the way a SC is ?? The bow at the water is longer then on top and they have all done this, even on the AC45's.

AC72sx3b[1].jpg

Is this like on large ships that have a bow bulb ?? Putting a pressure wave out in front of the boat in the water. Obviously it does not matter when the boat is on its foils. Is it like a sculling row boat where the length of the boat helps with weight over the water less drag??

There has to be a reason I just do not know. Then why does NACRA make their bows like a milk carton??

[havliii]

Can you explain why the boat would go faster with the jib furled?

Kevin

Kevin,

At certain wind strengths a jib WITHOUT BATTENS creates more drag than lift upwind. Here is a quote from Bill's original post in 2003. Hard as this is to believe, the difference is unreal, I could point like crazy and had better speed, and as a additional bonus the boat was easier to control with less overturning moment. While Bill would like it taken down this is not possible for me so I just furl it, still a ton of drag but way better than using it.

As far as the jib goes: you need to practice taking the jib down out on the water. Roll it from the top down to the foot. Leave the tack and jib sheet attached and snug. Roll the jib up and tie it along the spinnaker pole with a couple of ties. This should be no more than a five minute job and neat. When you see that bay covered with white caps, JIB DOWN! I do not know if you are sailing with jib battens or not but the time when jib battens really pay off is in a stiff breeze. Without jib battens the jib blows up like the throat of a bull frog and you can't point and you have lousey speed. The jib becomes a net drag.

to read the entire post search for this topic: Great Bay Race
the post was dated November 17 2003

Andy

[Kevin Keller]

Thanks Andy, that makes sense. I don't think it would help me since I currently have a very flat jib with battens.

I really like knowing why we do things rather than doing things because it I've been told it should be done.

Kevin

[Bill Roberts]

The first rule of depowering any beach cat with a jib in overpowered conditions is "TAKE THE JIB DOWN" or roll it up neatly and tight
anyway you can get it out of the wind. In strong wind conditions you may have to turn downwind for 30 seconds to get a snug and neat rollup. If you have waited too long to turn downwind for a neat rollup, then roll it up sloppy and ragged and flapping. Do the best you can to stop it from catching wind. In overpowered conditions the sail area that is not being driven, sail area that is luffing or has a large bulge in it to windward, is creating drag which is subtracting from the thrust that a partially used or driven sail is generating. Let's assume we have a 100ft square sail. In windy conditions we can only keep the boat balanced driving 80ft square The 20ft square of undriven sail area is making drag and is subtracting from the 80ft square of driven sail area for a net forward thrust proportional to 60ft square of sail area.
Bill

[SC15Av8or]

Lets get back to the hulls how ever. I understand the great benefits of the elliptical hull as I started as a hobie driver when I got into this sport and experienced the excitement of a pitch pole. Thank you Bill for making that a lesser thing to worry about. How ever why have they turned the hulls upside down from the way a SC is ?? The bow at the water is longer then on top and they have all done this, even on the AC45's.

AC72sx3b[1].jpg

Is this like on large ships that have a bow bulb ?? Putting a pressure wave out in front of the boat in the water. Obviously it does not matter when the boat is on its foils. Is it like a sculling row boat where the length of the boat helps with weight over the water less drag??

There has to be a reason I just do not know. Then why does NACRA make their bows like a milk carton??[/quote]

[Bill Roberts]

See answer above at Aug 01 2013 at 12:57.
There is no benefit. It is a fad that will pass.
Bill

[Bill Roberts]

Professional,

Back to your comment about elliptical hull shape benefits. There are two main benefits to the SC elliptical hull cross section shape.
1. The elliptical deck shape makes the deck as 'clean and as streamlined' as possible in the air and underwater. That is why a near pitchpole situation can be recovered from with a streamlined foredeck shape. There is no abrupt and sudden increase in drag as the foredeck is pushed under water. Drag does increase but not with a 'jerk' that throws everybody forward and pitchpole occurrs in an instant as with flat foredecks.
2. The bows on SCs are 'taller' than other boats of the same length. WHY?????
Taller bows have more volume trapped inside and this volume pushes up with a force of 64 pounds per cubic foot of air when pushed underwater. This is the restoring force that resists PITCHPOLE.
SUMMARY: SCs are much more pitchpole resistant than other beach cats because the elliptical hull shape is much more streamlined.
SCs are much more pitchpole resistant than other beach cats because the bows are taller.
Bill