Study for Sail Area / Displacement Ratio of some cats

[Therapy]

Originally Posted by mikereed100

Dave,


I'm sure you've sailed mono's as well as cats and have noticed that while multi's accelerate more quickly they also lose speed faster than a ballasted boat.

Mike

Now you have gone and done it.


Now it will be another mono vs cat thread. Good for 6 or more pages...........

[mikereed100]

I take it back!!!

[2Hulls]

Originally Posted by mikereed100

I too can envision instances in which a heavier boat will be faster in light air. As you point out it will take more force to accelerate a heavier boat but once up to speed I submit (at the risk of getting all Newtonian) that it will take more resistance to slow one down.

Mike - All other things being equal - a heavier boat can never be faster than a lighter boat in any air - assuming the lighter boat can attain the same propulsive force from its sails.

Never, never, never.

Very true a heavier boat (more mass) will maintain speed longer in a lull - it requires more force to slow it down >> F=ma works both ways.

If considering two otherwise identical heavy and lighter boats experiencing the same lull, the heavier boat will "coast" through the lull better for the reasons you state and be ahead. Once the wind returns the lighter boat will accelerate faster and attain a higher top speed sooner and, I submit, regain the lead due to being faster. Lighter means less friction and less wave making. This has to be the case. Physics demands it.

All of this is about differing mass. Moving ballast - shifting mass - is an entirely different story involving other variables.

Dave

[Robertcateran]

I can invisage situations where the momentum can carry a boat through a dead spot. when it is that windless, I'd start to scull in a light boat. A ballasted boat can lean to ww, and put some shape in their sails with gravity
Robert

[catty]

Originally Posted by 2Hulls

Mike - All other things being equal - a heavier boat can never be faster than a lighter boat in any air - assuming the lighter boat can attain the same propulsive force from its sails.

Never, never, never.

Below is a quote from bill gibbs, owner of very fast cat "afterburner" commenting after being beaten by monos full of "heavy" lead in Ensenada-Ventura race.......


Excellent wind and point of sail for the first 100 miles had us in the lead
at sunset. We reach faster than Magnitude 80, and Loe Real had sailed ahead
into weaker wind. Only 20 miles to go.
Then the wind shut off, 11 hours of light air drifting, and 6 boats passed
us. 5 monos and Loe Real. Those monos are crazy fast in 'no wind'.
I'm not counting the cruzing class boats that passed us motoring :-)
Much better than drifting for the whole race, but the end was brutal!

Bill

[GordMay]

catty:
Where did your Bill Gibbs quote come from?

Bill Gibbs’ “Afterburner” (54 Ft Cat’) finished 7TH in this year’s (61ST) Newport to Ensenada International Yacht Race, behind 1 Catamaran (“Loreal”) & 5 Monohulls (80 Ft “Magnitude”, 75 Ft “Akela”, 64 Ft “Medicine Man”, & 70 Ft “Oex”)

[fastcat435]

Originally Posted by mikereed100

Dave,

I too can envision instances in which a heavier boat will be faster in light air. As you point out it will take more force to accelerate a heavier boat but once up to speed I submit (at the risk of getting all Newtonian) that it will take more resistance to slow one down. This means that the boat will be able to coast through lulls and will be less affected by chop and windage. In a perfect world lighter is faster but in the real world of lulls and lumpy water a heavier boat may have the advantage. I'm sure you've sailed mono's as well as cats and have noticed that while multi's accelerate more quickly they also lose speed faster than a ballasted boat.

Mike

Contrary to what you think it is not the case with a light weight cat , we do not go into the waves but go over them more like a float and hardly slow down . light weight always has the advantage
We have tested this with heavy and lightly loaded FastCats
10 % less weight translates to a 6 % increase in speed upwind and 9% increase downwind
This is tried and proven
I rest my case

Greetings

Gideon

[2Hulls]

Wow, simply amazing. P.T. Barnum was right.

To anyone who really believes heavier can be faster - all other things being equal, I recommend you weigh down your boat right away.

Dave

[Hud3]

Two Hulls,

Intuitively, I'd tend to agree that lighter would be faster, but how do you account for Joli's contention?

Originally Posted by Joli

When you consider meter boats, one trick in light air was to sink the boat (ie. open the bailers and flood the hull) This allowed the boats to carry through the lulls once momentum was built.

[2Hulls]

Hud - Joli is correct. A heavier boat will coast through a lull better than an otherwise equal lighter boat. Physics demands it. Same goes for completing tacks faster.

It's the payback that hurts - and building the momentum in the first place.

The laws of motion also demand that when the wind re-fills, or a boat recovers from a tack, the lighter boat will accelerate faster and attain higher speed (this assumes the lighter boat can actually complete a tack). This is not rocket surgery. Adding weight to a boat cannot possibly be beneficial from an overall performance standpoint - assuming the propulsive force of the sails remains constant for the two comparisons. When more weight affects sail power via heeling or other factors, changing in F in F=ma, "otherwise equal" no longers applies.

Dave

[Joli]

Originally Posted by Hud3

Two Hulls,

Intuitively, I'd tend to agree that lighter would be faster, but how do you account for Joli's contention?

Hud, Dave is not taking into account that stabalizing the rig gives a higher propulsive force (ie. F is not constant). If F were a constant the "trick" would not work and Dave would be correct.

That is also the reason you switch down to a wind seeker when drifting conditions exist. Since the sail is smaller it cannot provide as much drive as a larger sail but does provide more drive then the larger sail in light air, flow stays attached. Total F is larger...........

[Hud3]

F = ma
F = m(dv/dt)

If two boats, one heavier and one lighter, have the same sail area and the same underbody, the lighter boat will speed up faster in a puff and slow down faster in a lull, due to frictional drag on the hull. It would seem to be a theoretical draw to me.

In Joli's example, two identical meter boats, I can see the strategy of adding mass, once speed is achieved, working to keep the boat moving through the lulls, leaving the lighter boat behind.

[Hud3]

Originally Posted by Joli

Hud, Dave is not taking into account that stabalizing the rig gives a higher propulsive force (ie. F is not constant). If F were a constant the "trick" would not work and Dave would be correct.

That is also the reason you switch down to a wind seeker when drifting conditions exist. Since the sail is smaller it cannot provide as much drive as a larger sail but does provide more drive then the larger sail in light air, flow stays attached. Total F is larger...........

What you're saying is, do anything that works to keep an aerodynamic sail shape in the light air conditions? And a heavier boat reacts less to anything that tries to "shake the air out of it's sails"?

[2Hulls]

Originally Posted by Hud3

It would seem to be a theoretical draw to me.

In Joli's example, two identical meter boats, I can see the strategy of adding mass, once speed is achieved, working to keep the boat moving through the lulls, leaving the lighter boat behind.

Then the logical conclusion has to be that adding even more weight is even better. "Go ahead, load 'er up to the gunwales, boys!!"

The theoretical draw gets shattered when considering the adverse effects of extra friction on the heavier boat by virtue of sitting lower in the water - more surface friction and more wave making.

If the case is not having extra mass starting out at the dock, but rather adding mass underway - once speed is achieved - this is deceiving for another reason. Where does that mass come from? From the non-moving water below the boat? How is that water (extra mass)accelerated up to the speed of the boat? This isn't free. It takes F that otherwise could be used to keep the original mass moving. Thus, taking on more mass underway has to result in slowing down the boat. There can be no other outcome.

Again, you can't get sumpin for nuttin.

Rremember, all of this is based on an assumption that the propulsive force of the sails is not affected by taking on more mass. In light conditions, this is very probably the case. In heavier conditions on a mono when heeling comes into play, this may not be the case.

Dave

[Joli]

Originally Posted by Hud3

What you're saying is, do anything that works to keep an aerodynamic sail shape in the light air conditions? And a heavier boat reacts less to anything that tries to "shake the air out of it's sails"?

Sure. When you add weight the rig is stabilized, momentum built can carry the boat on through chop. If you go to a windseeker you have altered SA/D just as if you had added weight to the boat. Is there a limit to how much weight you want to add, off course, just as there is a limit to how small a windseeker you want to go down too.