Dagger boards versus shared lift

General Sailing Discussion
J Drew
Professional
Posts: 104
Joined: September 9th, 2013, 12:39 am
Boat Make/Model: SC 20
Location: n. florida

Re: Dagger boards versus shared lift

Post by J Drew »

Bill,
I'm glad you explained that equation, and now I can say that I understand parasitic and induced drag.
For the record, I wasn't trying to introduce the thought of changing your design, I know it works.
I simply got off on a tangent about lift when looking at the symmetric board.
Two different types of lift. I was thinking of lift in the sense of pointing, not to be confused with lift involved with induced drag that you have explained.

And as for me taking an aerodynamic class, there's a lot of things I wish I had done differently, I can only fix them one at a time.
If a frog had wings, he wouldn't bump when he hopped
Bill Roberts
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Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Dagger boards versus shared lift

Post by Bill Roberts »

Drew, I don't mind if you change the design, just change it for the better.
J Drew
Professional
Posts: 104
Joined: September 9th, 2013, 12:39 am
Boat Make/Model: SC 20
Location: n. florida

Re: Dagger boards versus shared lift

Post by J Drew »

So let's get back to shared lift.
and for comparison we will use
ARC21 which uses boards forward of the beam
Hobie 16 with banana shaped hulls
SC17 fairly straight elliptic hulls w/o boards

If you look at a side view of each hull, you can see the forward point of shared lift on the 21 is the boards.
On the h16 it is questionable where this point is because of the rocker in the hulls, but my assumption is that raking the mast back puts the CE in that variable rocker
On the SC 17 the elliptical shape with unique forward buoyancy puts the forward point of shared lift closer to the bows in comparison to the h16.

My question is, if the distance between the two points of shared lift is greater (in ratio) does that increase hull speed?
Based on the slightly incorrect thought that water line itself increases hull speed.

So how far off am I? And what equation can I study next
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Dagger boards versus shared lift

Post by Bill Roberts »

Drew,
The top speed limit on beach cats is pitchpole. Here's the situation, The forward sail thrust acts at the sail center of effort about 10 to 15 ft above the water depending on the size of the rig. The hull drag occurs about 6 ins below the surface of the water. The forward sail thrust and the hull drag are in opposite directions. This creates a a torque, a force times a lever arm, tending to push the bows underwater. Let's call that torque the pitching torque and it is equal to the forward sail thrust times the lever arm, 10 ft for example.. The hull tends to provide a resisting torque. The hull resisting torque is equal to the distance from the hull's center of buoyancy to the total boat plus people center of gravity times the total weight of the boat plus people. When the pitching torque becomes greater than the restoring torque, the boat "pitchpoles". Max boat speed was achieved just prior to pitchpole.
Question: What can be done to reduce pitching torque? Ans: Lower the effective center of effort in the sail plan.
Question: What can be done to increase the hull's restoring moment? Make the hull longer to increase the distance from the hull center of buoyancy, bow down attitude, to the center of gravity of the boat and people. Let's consider a given length boat here. When the bow goes down, the center of buoyancy moves forward. We all know that when the bow goes down further, the people on the boat move aft. Both of these actions allow the sail thrust to be increased and the boat goes faster. When the bow is as low as it can go and the people are as far aft as they can go, the restoring moment is at its max value and the sailors and boat are at max speed.
Question: Within a given boat length, what can be done to maximize boat restoring torque? This comes down to what can be done to maximize the distance between hull center of buoyancy and boat center of gravity with sailors at max aft position. As the bow or bows goes down, the center of buoyancy migrates forward in the hull increasing the distance between CB and CG. Well what can we do to increase it more? Ans: Make the bows wider, fatter. That makes the hulls draggier so I don't like that answer. What else can I do? Make the hulls taller, more volume with no increase in bow wedge angle! Oh, well how about that. My hulls are already have taller than any other beach cat bows of the same hull length. Now what can I do? Well the taller bows will let you push the boat harder and get a little more speed out of it. But you are sailing the boat right next to disaster. If the boat has flat decks and you get solid water on top of those flat decks, you are on your way to pitchpole as the boat dives deep and stops in an instant. On the other hand, if your boat has decks that are shaped much like the bottom of the boat, arched or highly curved decks, when these decks become the bow, they will split the water just like the bottom of the boat does. There will be an increase in drag and the boat will begin to slow down and the sailor will sense this and they will sheet out and slow down a little more and the bow will pop up again and you are still on your happy way.
Back to the flat deck boat: The boat pitchpoles and luckily no one was hurt. Now the boat is turned over and this guy and his girl friend try to right the boat. Well, they aren't heavy enough and they have to wait for help. In the mean time they are drifting north rapidly and hoping someone sees them turned over and are going for help. If this boat had a shroud extension system and captive mast step. they could right the boat themselves and sail home. In this case they did not and the current in the ocean carried them to Ft Pierce, FL from West Palm Beach overnight, 40 miles north and they were rescued the next day off Ft Pierce by the Coast Guard. Friends and relatives spent a very anxious night wondering how their sailing friends made out in the ocean overnight turned over or broken down.
Question: Are you glad you own a SC/ARC catamaran sailboat? Did you know all of these characteristics were designed into your boat in 1976.
More questions later?
Bill
Last edited by Bill Roberts on September 3rd, 2016, 10:29 am, edited 1 time in total.
J Drew
Professional
Posts: 104
Joined: September 9th, 2013, 12:39 am
Boat Make/Model: SC 20
Location: n. florida

Re: Dagger boards versus shared lift

Post by J Drew »

Bill,
That clears up my misconception about water line and boat speed. I assume the location of the two points of shared lift is simply to balance the center of effort?

I was checking out an article on PlayStation. What you have just explained, seems to be evident in its rework. They added 20' to the nose and sloped the tip of the bow up, giving it more forward buoyancy. From what I saw in the pics, it has fairly round decks. The bow shape isn't plum either ... Hmm, wonder where they got the idea?

I think I'll double check the functionality of my shroud extenders

Thanks Bill for a good mental exercise
Bill Roberts
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Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Dagger boards versus shared lift

Post by Bill Roberts »

Glad to help, Drew.
You are right, shared lift is just another way to balance the CE and the CLR.
The equation of max hull speed equals 1.4 times the square root of the LWL applies to keel boats and power boats.
It does not apply to high fineness ratio hulls like beach cats. Power boats overcome this limit by climbing over the bow wave and moving into the planing mode.

Playstation: Playstation first went to sea trials at 100 ft long. Top speed was limited to 25 knots max speed due to near pitchpole tendency. The bow was running low and had everyone on the boat with white knuckles. It was a scary ride. The boat/design was not ready to race across the ocean. Back to the boatyard and the boat was extended to 120 ft length. Also the bow and deck immediately behind the bow were raised 8 ft. From the side view, the boat's shear line looked like a snow ski jump. Back to sea trials with 20 more ft of forward hull length and a much taller bow. Speeds in the 30 to 35 knot speed range were easily and safely attained. Playstation was the first big cat to blow the doors off the trans Atlantic crossing record.

More equations: We could say that pitchpole resistance is proportional to hull length squared. Squared because as you add hull length you are also adding hull volume so it is a double benefit to add hull length. So we could say that the increase in pitchpole resistance for Playstation was something like 120 ft squared divided by 100 ft squared. The increase in pitchpole resistance for Playstation was like a 44% increase. With the much taller bow, the improvement was at least plus 50%. That converts to 50% more sail thrust and major top speed improvement.

Keep those shroud levers and Avibank pins working real good.
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