hi all,
i visited a catamaran regatta this last weekend and couldn't help but notice that almost all the cats had a lot of backward rake. the isotope cat i have is 16 foot, supposedly configured for f-16 racing. has daggerboards, jib, etc. the owner info i have mentions mild backward rake if needed...just wondered what this might do to help or hinder.

The large rake on a catamaran is a function of the relationship betweeen the speed of the catamaran, the drag on the hulls, and maintaining a workable center of effort on the mainsail.

In short, the cat moves at such high speeds, the hydrodynamic drag on the forward moving hulls greatly encourages the center of effort forces on the mainsail to move well forward of the front beam, (where the mast foot is located; as the main is moving through air, it has significantly less drag than the water-steeped hulls when moving forward). Crew and skipper will counter this effect to a degree, by moving back on the boat, but can only balance the forward force/effort only to a defined point (the back of the boat). As the center of effort moves forward of the mast foot (front beam) the boat hulls begins to go downward (trip over itself) into the water. The "stuffing downward" of the hulls can result in the "pitchpole", wherein the hulls submarine into the water, (and cause a nice boat cartwheel-google "catamaran pichpole". It is the most spectacular of sailing capsizing!).

To counter this effect and to generate greater boat speed, catamaran masts are raked backwards, allowing the hulls to pop up at speed, and on some, plane across the water. Thus, mast rake results in both greater boat speed and better control over the boat at high speeds. The effect of a raked mast is best visualized, not by looking at a catamaran, but at a windsurfer at planing (high) speeds. Those folks lean back, pulling the sail back with them and scooting the mast foot forward relative to the main. They begin to plane their boards, and go much faster.

As might be expected, mast rake is a dynamic control, and is commonly adjusted by the the teams to compensate for different wind conditions.

very nice description! would it be correct to infer that too much rake can also slow down your boat in lighter breezes? at the regatta the first day had breezes in the steady 10-15 knot range. next day not so much. still it didn't seem that anyone really changed any configurations much from the two days. racing ofcourse became much slower the second day. it could be that i wasn't as observant as i could have been to notice the changes that might have been made. still i did notice considerable backward rake on the second day. wish i could have been out on the water observing instead of stuck on the shore a mile from the course.

one other thing i wonder about. the isotope i have has the original sails. although they are in excellent condition i wonder how they might be compared with 'high-performance' sails?

In the yore of catsailing the theory for daggerboard boats was to put the center of power (the sails) over the center of lateral resistance (board) and use the rudders for steering.

Historically, along came the Hobie cats with no boards, just banana hulls. They discovered that moving the mast aft they put the load over the rudders, which then acted as daggerboards, and the boat performed much better.

The next phase of evolution was to rake the mast farther aft on boats with boards as well, moving the power aft so it would allow both the boards and the rudders to have underwater lift.

This has also fit well with the modern rig that sports a headsail of some kind (i.e., spinnaker, Hooter, Screacher) the puts more power out forward.

There are also boats with skegs near the aft end of the hull, i.e., Dart, Wave, etc., that move the center of power way aft to make both the skegs and the rudders provide underwater lift. This has also proven very effective.

Bit of history,
Rick

Thanks Rick, are you back in the Keys yet or up in Mich?

I adjust my mast rake so the helm feel doesn't change as the windward hull is lifted out of the water. The boat doesn't round up or bear away as the gust hits. Once I have the setting worked out I make side stays the right length and get rid of all adjustment, same rake for light or heavy wind.

Succinct explanation Rick,

Darryn

keep in mind, as always there are many variables that go into the equation. Every boat is different. some boats (like the Hobie16) need alot of rake, others dont.) Hull shape is a big factor in this also things like how "tucked under" the rudders are, the forestay tention... etc.

Probably best to talk with others with your exact boat and discuss their experience and then experiment on your boat.. try to keep all other variables static (same tention on forstay, same wind, same tides, same weight on board, etc)...

Tuning of the rig is, in my experience, often skipper dependent, and boat dependent. Many of the high end F18 sailors will tune according to wind, but I know good sailors that 'set (rake) it and forget it'. A significantly raked mast can, in theory, waste some wind energy, but this can be easily compensated for by repositioning the crew weight more forward on the boat. Obviously, as the boat floats, as this has the effect of pitching the rig forward, thereby counteracting the rake of the main. Just FYI, the same sailor may or may not tune the "bend" in the mast by tesioning the diamond wires on the main and rig tension prior to a race as this has another effect on the positioning and shape of the main. But that is another story!

I am not familiar enough with this boat to accurately comment. Ask people that own it, and know it well...they will be the best source of info.

In my experience, old sails are often powerful in moderate breezes, but are often hard (or even impossible) to trim. This is due to the chord (depth) of the foil shape being 'blown outward' with time-they do lose shape over time. Such sails can be fast, sometimes overpowering, in a fresh breeze, but painfully ineffiecient and slow and horribly ineffective in light 'drifter' breezes. As for general technological improvements, I think sailcloth and shape, and constancy has generally improved over the years.

We are at Put in Bay, OH, beautiful island is the western shelf islands of Lake Erie. Heading south the first of Oct, with a stop at the Outer Banks for the Wave North Americans at Duck, NC.
Rick

Isotope tuning is different than many other catamarans (especially the H16). It has a balanced sailplan with the mast almost vertical and the centerboards (not daggerboards) fully down (going upwind). Some isotope sailors use no rake (mast vertical). I prefer a little aft rake to give me a slight weather helm. Not much, just enough to provide tactile feedback on the tiller and for the boat to head up if I fall off. I find that a bit of weather helm also helps me point better. Once the rig is set, I can make fine adjustments by moving my weight forward or back on the boat.

Going downwind, you can shift to neutral or lee helm by adjusting the centerboards (how much is dependent on wind conditions) and the sail set.

Regards,
Eric