Underwater foils --- Center / Effort and Percent / Chord?

[Paul B]

Originally Posted by epoxyman

They are reaching they are triradial laminate sails in the later stages of life but still have fairly good shape. The 155 takes a nice shape but has a bit too much stretch such that as the wind fluctuates it needs fairly constant adjustment. When all of this is said and done satisfactorily I plan on investing in a new mainsail and 155, possibly by Quantum or North.

It will probably be easier to get things "said and done" if you get someone from North or Quantum to come down, measure the boat, and help you get the rig sorted. If you are ordering a new suit of sails they will probably do this as part of the deal, maybe not.

Be careful about getting advice over the internet. There is more BS than truth flying around in the ether, including things written in this very thread. You should take everything with more than a grain of salt, and I include what I write in that.

Attached is a quick, back-of-the-envelope sketch regarding the rudder shaft location I mentioned earlier. I used a simple trapezoid planform, sort of from memory of what the old C30 rudder looks like, for clarity and ease of calculations. I suggest you don't build a new rudder to that shape, but for this sketch it serves the purpose.

I noted the shaft should exit the hull perpendicular to the hull slope. Your Capri probably doesn't do this, so you can disregard that suggestion to keep things simple. If you do this it makes it easier to have a close fit of the top of the rudder to the hull (less turbulence), and you can spin the rudder 180 degrees and scull the boat backward into the slip.

The angle of the shaft after it enters the rudder will depend on your shaft angle coming through the hull and the general layout of the rudder. For this example it is 5 degrees. In practice it will be different.

If you are going to make your own new rudder think thin for performance. A 10% chord thickness would be good. I doubt you will achieve that. No more than 12% should be your goal. It is nice to shorten the top chord length to minimize drag at the surface pierce, but shorter chord means thinner rudder for the same percentage. Thinner may not be achieveable based on the size of the shaft, based on the moments required.

When you engineer the shaft what parameters will you use? A speed of 15 knots, full stall, with the force at half span? Using 120K psi for the shaft material?

Remember, a rectangular shaft has better moments than a round shaft. Distance from the centroid provides better gains in moments than wall thickness. Distance from centroid is limited by the thickness of the rudder. Rudder thickness is limited by the percent thickness of the top chord length.

If you do your own composite shaft with a hardwood core be cafeful the wood is well sealed, especially the end grain. Swelling of the core can cause interesting problems with the carbon sheath.

Get your rig tuned with a pro's help, get your new sails, and go rip it up with your existing rudder. You should be fine. A new rudder will be better, but not more than maybe 3 seconds per mile (about one boat length).

When you race in any breeze where the crew is all hiking (8 kots+) you should be looking at 1100 to 1200 pounds of crew weight. We found that to be optimal on a similar MORC boat. More is faster upwind in 12+, but slower downhill. Less is significantly slower upwind without enough gain downhill.

Your mast is the old Kenyon 4060 section. It is a good choice for the two spreader rig on that boat. It is just a bit overkill, but still bends nicely, especially with the newer sail materials these days needing less bend than the old dacron sails. Learn to use the MORC checkstays to fine tune main trim.

[BOATMIK]

This is really good - the advice from Paul B is the critical factor. That the Capris have no problem. So it is more than likely a problem with sailing method or expectation of how it "should be like" rather than "how it is".

The one factor that would cause the problems you are mentioning is improper use of the mainsheet traveller - but could be something else.

So it sounds like you need info on how people are setting up their boats and how they are sailing them. And just to make sure that no past owner has modified things in strange ways.

So...

Lots of people here - membership list of over a thousand is here among other things (1000 boats indicates that there is unlikely to be any major probs with the boat type
http://kb.sailboatowners.com/brand?model=84

There's a forum here - I'd post in the busiest sections as well as the specific one for your boat.
http://www.sailboatowners.com/forums/index.htm?fno=20

Here is some info on their level of performance. Looking at it these boats have no major design problems!
http://www.sailboatowners.com/forums/index.htm?fno=20

Also the class association - a reasonable website - but gives contacts for the people who are taking care of the class.
http://www.catalina30.com/

Hope this helps.

[epoxyman]

Mostly light fluky east coast USA winds which is why I get away with things as they are. But I love sailing most when the winds are honking, and the spinnaker is my favorite sail.

[rayk]

Originally Posted by epoxyman

Mostly light fluky east coast USA winds which is why I get away with things as they are.

Fluky winds are really a curse....
Having to change sails slows you down no end
The fastest guys out there think really carefully first before putting up any sail.
Risk of putting up too little or too much sail at the start makes or breaks them.
Almost no one puts up too little, but many put up too much.
A top guy can miss out on the tin, but be honest enough to say he misread the conditions, and as a consequence was forced to make an unscheduled or extra sail change.
You can beat the other guys or sail faster with the right sails for the conditions.
Many times the boats following the leaders over the finish line can be over canvassed but cant afford to lose any more time to correct it.
Quite often those guys with more sail up are racing against each other for third, fourth, or fifth place.

But I love sailing most when the winds are honking, and the spinnaker is my favorite sail.

Me too. Best fun ever Boat can travel at its max WL speed.
Just gotta keep her stable and under control while you tear along.
When the old girl is getting out of control off the wind, she usually has more power than she can put into forward motion.
No problem as long as you can stay on top of her, but correcting her near broaches might be costing you more than the extra amount of boat speed thats getting squeezed out of her.

The next blowy day, try trading a little power for control, and see how you go.
Trim your sails for efficiency, and the affect on performance will be felt, when the boat is behaving herself.

note: I think you have got a bunch of good guys on your side here, and tackling performance from different sides will give you a holistic approach to the problem.

[epoxyman]

I just wanted to take a moment to thank all of you for the very thoughtful and detailed input; it's all read carefully and you can be certain it will figure into how I approach things.