Bill,

So it is safe to assume that the I-17 rudders do not belong on the boat either,.

...since they are identicle to I-20 rudders.

..from a designers point of view,....what should the I-17 rudders and d-boards look like?


regards,

Bruce
St Croix

There is one advantage of canting the hulls that was not mentioned here: if you cant the hulls and keep the same crossbeams, the total beam will increase and so the max. righting moment.

Or, if you cant the hulls and keep the same overall beam, the crossbeams can be made lighter.

Cheers,

Yup, and another I do not think has been mentioned is that the lee side of the lee hull is more vertical so producing less of a wave / vortex - whick looks great and is very usefull for filling up slower boats as you pass, but must be slow chucking all that h20 out !

Bruce,
This is a question that only the I17 designer can answer quickly. If I tried to answer it, I would have to say, " well if you assume this, bla...bla...bla..., but if you assume that, then bla...bla...bla. and I don't know the this or that because I did not design the boat. The proper size boards and rudders for the boat depends on how the designer loaded them up. For a given sailplan board area plus rudder area equals a constant. If you load the boards more then the rudders can be smaller. If you increase the load on the rudders, then they should go up in size and the and the board area can come down. The best answer is check with the boat's designer.
Bill

Is it any indication that the I20 rudders (old style) do have a stall tendency if you don't quickly (and perhaps 'properly') counteract a gust?

Bill,
Ok,..thanks.....I get the pic

regards,

Bruce

Jake,
The technical committee of the I20 class has decided that larger rudders on the I20 make it a better boat. On an airplane or a sailboat when a foil stalls, it is being asked to generate more lift than it is capable of. The lift per square foot of lifting area exceeds the capability of the foil. This is called wing loading or foil loading and is measured in pounds per square foot, for example. There are two ways to solve this problem. Either reduce the load or increase the foil area. The I20s that I saw at the Steeplechase this past year had the mast leaned way back. I can see the rudders carrying a bigger load than they were originally designed for. Bigger rudders are required to carry the increased load.
Bill

Hi Bruce,

I am attaching a picture of the NACRA A2 going upwind. From this picture, you will know as much about how it sails as I do. As you can see, the leeward bow does not submerge in flat water.

I agree that sailing with only 150 sq. ft. of sail area (mast and sail combined) will be a challenge!

I understand that Morelli and Melvin have just finished building the last platform that will be supplied prior to some minor tooling changes that are being incorporated for the NACRA A2. When those changes are completed and resulting NACRA A2 production begins is unknown to me. I don't think the first boat from NACRA will be delivered until sometime in July, but that is a guess. I am not first on the list. In fact, I have been having some recent vision issues that affect my ability to sail competitively. I believed that these could be resolved fairly quickly, however, my Opthomologist told me yesterday that he would not change anything for the next two years because he thought my eyes probably would not stabilize in less time than that and any changes could be detrimental in the long term. The long and the short of this is that I have cancelled my order (major dissapointment) until I know that I will be able race again. I am planning on being back for the 2006 sailing season.

Who knows, maybe I will come back on a new I-17R instead of the NACRA A2. It sure is a fun boat to sail and race! I think the A2 will also be fun, just no spinnaker.

I am glad that my prior posts have been of value to you.

Hello Bruce,
You could do this as a starting point. Assume that the foil loading between the CB and rudder are in the same proportions on the I17 and I20. If this true, then a starting point on calculating correct CB and rudder size for the I17 is to take the I20 CB and rudder areas and scale them down by the ratio of the I17 sail area divided by the I20 sail area. By varying mast rake, you can shift load from CB to rudder or from rudder to CB.
Bill

Bill, Bruce;

I recently raked my mast forward about a foot(using the trapeze rake measuring method)and the boat seems to act a whole lot better. I have to be really over pointing to get rudder cavitation now. This and a lot less rotation has really sped my boat up.

Tom Turlington
I17R #124

Bill and all,
Ok,.here are some number.
Took them off the Performance site

TOTAL Sail Area ( s.f.)

I17normal ( not R) 160
I18 227
I20 247

As a percentage increase sail area, using the I17 as a basis

I 17 100%
I 18 141% ( main and jib)
I 20 153% ( main and jib)

Correct me if I am wrong, but wasn't the I-18 the first in this family of design?
And all the I series use the same rudders and d-boards.
Therefore, the I-18 has the rudders and boards that came off the designers computer as THE correct configuration.

So, if we now switch the basis to the I-18, for this is the correct design boards/rudders;

I 18 100%
I 17 ( 160/227) x 100 = 70.46 of the size needed, so current I17 rudders/boards are 30% ish too big.

I 18 100%
I 20 ( 246/227) x 100 = 108.37 of the size needed, so the I 20 current rudders/boards are 9%ish too small.


Someone check these ratios and see if these numbers are good.

I will be honest, and as Tom stated also,. the I 17 rudders do not fail.

I also know from a local I20 racer, that the stock I20 rudders do not do the job.....which is probaly why the retro rudder is now available.

....ok, enough of this stuff, lets just go sailing..

Bill, congrats on the RC30 cleaning up in the Caribbean. Tell the owner to come to St. Croix in Feb 2005.

regards,

Bruce

eehh Brobu

Didn't we go through all this a while back, including the math etc. Then we came out at 30 % as well.

It may be more efficient to search the thread of back then and read it all over again to see what you have forgotten since then. The answer you are looking for are there already.

Wouter

Wouter,

ok,....I will quit beating this dead horse..

..I'm off to go sailing.

Bruce

I've been sailing today ! Good F5-6 plenty of spray, plenty of Kite flying......and my plates were a long way up most of the time......