Would the spi work like a kite? Kites lift into the air becasue the string prevents horizontal motion and the angle of the kite is such that the air is deflected downward forcing the kite up.

I have seen on larger boats (mono-hulls) people dangling off the bottom of the spi for fun. Is that spi can lift a person up, why would it not lift up on the end of the spi pole which would lift the bows up too. It would seem that a spi would direct the airflow to the leeward side of the main and jib as well as downward which would lift the bows due to Newton's 3rd law.

Am I understanding ths correctly or am I way off base?

I'm sorry to say but your are making one of engineering rookie mistakes. Much like the example of where the sailors blows in his own sail to propel him along.

The spinnaker does definately pull upwards at the end of the pole. However it very much pulls downward at the mast hound fitting. As the mast adn spi pole are fixed relatively to eachother this means that these forces will largely level eachother out.

This is called systems thinking in engineering. You must learn to regard a fix composition of elements as one whole system where internal forces have no meaning when looking at relations (newtons laws) that are taking over the whole (system)

Wouter

Wouter,

I am not to familiar with a spi as I have never actually sailed one before (budget constraints). I think I will need to get a closer look at one in action to fully understand this.

Which way does the air flow off the of spi? Where is the center of lift on a spi? Which direction, approximate, is the force vector of a spi?

Upon further thought I think I know what is going on with the spi.

My assumptions are:

1) All the forces on the boat act about the center of buoyancy.
2) Forces created by the air acting on the sails can be reduced down to a center of effort and a vector eminating from the center of effort

Now, depending on where the center of effort is on a spi and at what direction (vector) the force is acting from that center of effort will determine what torsion is acting about the center of buoyancy (CB). If that center of effort (CE) is below the major horizontal chord of a circle whose center is at the CB and a radius at the CE, then any horizontal component of that force (in 2 dimension looking at the port side of the boat) will produce a clock wise torque, therefore lifting the bows. Also, any vertical component of the force will produce a much larger clockwise torque.

If the CE is above the major chord of the circle described above, then any horizontal component of force will produce a CCW torque about the CB while any vertical component of the force will still produce a CW torque.

In my above explaination the vertical and horizontal components I am referring to are of the force that is perpendicular to the cirlce described above. This force is a component of the spi's overall force vector. I hope this makes sense to you all. In my head it is perfectly clear.

Since I don't know where the CE is on a spi I can't be sure if this is why everyone thinks it creates lift.

Now, as to why a spi "settles" the boat down on a reach. Take the same perspective looking at the port side of the boat. I knwo for a fact that the CE of the spi is out in front of the CB of the boat. This means that the spi is dragging the boat forward versus pushing it. As you know it is easier to pull something in a straight line than it is to push it.

I guess I should have taken that technical writing course as an elective instead of wine tasting. This is much easier in person with a pencil and paper.

Let me know if I need to clarify anything or if I am making absolutly no sence at all.

Wout, pull on the end of the pole is vertical, or a tiny bit aft of vertical. Pull on the hound fitting is far more forward than down, thus the net effect is that the spin generates forward "pull". Tell me if I'm wrong, but the lift of a sail flying free is perpendicular to its luff. This image of an 18 skiff, admittedly with an exaggerated pole length and luff angle compared to most cats, shows the general orientation of the kite to be maybe 35 or 40 degrees aft of vertical. This will clearly degrade the amount of forward thrust available to pull the boat through the water. However, the total thrust or pull generated by the kite is unchanged. If the vertical component is greater than the "tripping" component of drag caused by the greater speed, then the kite will without doubt lift the bows.

Andrew,

We are talking about common spinnakers on beach catamarans here, not extreme examples that really have no application on beach catamarans. Sure enough you and others could device a contraption where the spinnaker will lift the bows, although this is much more difficult than you think, but that still doesn't say or proof that spinnaker setups as commonly used on Cats actually lift the bows or not.

Also one of the documents sciencific analyses was done on the Cherub skiff boats. A monohull with a long spinnaker pole and an angle back spinnaker. Still the numbers showed a setup that didn't lift the bows despite the fact that these boats ride with their bows high in the air as many other skiffs.

Also from my own experience; I could get my bow to fly on a 49-er just be placing our weight way back on the wing. Under spinnaker it flew less high when the weight was similary placed back.

But lets stick to the catamarans setups.

Wouter

I haven't studied your post in all details but the theme enclosed in them is correct. In the end, as I think you say, the spi creates one overall force that can be devided up into 3 components. In summary these are :

-1- a horizontal force along the centreline (drive)
-2- a horizontal force perpendicular to the centreline (pushes the boat to lee)
-3- a vertical force

These three forces create in turn 3 moments around the CE of the hull and daggerboards becuase the CE of the spi sail isn't at the same place as the CE of the hulls and daggerboard.

Force 1 and 3 result respectively in a counterclock wise moment (when looking at the boat from port) and a clockwise moment. Wether the bows are lifted or pressed down in absolute sense dependent on which one of these two moments is larger than the other. On beach catamarans the moment linked to force -1- is bigger than the moment linked to force -3- and so spi doesn't lift the bows in an absolute sense although the vertical force does lift the whole boat out of the water a little bit. To makes things simpy right !

If this is what you were getting at then yes I think you are correct.

However as we discussed elsewhere this is only part of the story that explains what sort of effect adding a spi has to a catamaran

Wouter

I installed the Hobie Cat snuffer system on my H16. It's incredible! The bows get lifted and you just take off. We've used it on deep reaches and down wind legs and have just flown by other H16's. My wife, normally afraid to pitch poll (that's another story, I should say stories) is pleased with the spi's affect. I don't recommend using it in wind lighter than about 8mph when racing under portsmouth. You'll take a hit of about 3mins to the hour.

cincy kid

Well,

I'm glad Wouter is done, now we can go on exchanging our experiences with the spinaker on the Hobie 16.
MY experience is that the "diving capabilities" of a H16 diminish when using a spinaker downwind.
I suggest giving it a go with the spinaker. It is fun.

Although for a lot of H16-diehards adding a apinaker is a bit of defiling (correct word ?) the H16-class.......