Model Yacht Balance

I have built three Mistral US1M class model yachts. One planked and two from the same mold.(the mold wasn't taken from the planked boat) The plans call for the mast to be about even with the front of the keel. On all of them I have to have the mast at least two inches forward of this. This works out to having the CE 5 or 6 inches forward of the COLR, this with the keel, bulb and rudder in place. I am using the US1M "A" rig. I have tried 3 or 4 different sets of sails, one set was professionaly made by Gordon Stout. The Mistral is the same hull used as the ODOM with a different set of sails. I've talked to the ODOM guys and they place their mast according to the plans. The ODOM sail plan has the CE at or about the mast, same as the rig I'm using. The boat sails fine with just a touch of weather helm in most winds. I don't get lee helm unless I'm just ghosting along. I've tried more or less every adjustment I can think of. I'm suspecting it's something to do with the hull but this morning I checked the rocker against the plans to see if I screwed up building the plug. It seems to match fine. I suppose I could just accept it but it just doesn't make sense and it keeps nagging at me. I have to build so much adjustment into the boat because it's so far from the plans. I want to build yet another boat with more or less permanent attachment points but until I figure out what I'm doing wrong I hesitate to start.
Any ideas would be appreciated.
Thanks
Don

 

Don, having the CE 5 or 6" ahead of the CLR sounds way too much(5" =12.7% LWL) and the boat is well balanced this way?
Could you post some pictures -side view-(out of the water) and sailing?
Is the CLR (or CLP center of Lateral Pressure)
position shown on the plans? Does it include all or part of the rudder area?

 

Here's a link to the plans.
http://www.modelyacht.org/us1m/pdf/us1mmistral.pdf
It doesn't say with or without attachments

I'll look for some pictures
Thanks
Don

 

Don; The US1 class is well established and the Mistral has been around since the mid 80s. It is almost a sure thing that you have some issues with the sails. It is real easy to have the max draft too far aft which creates weather helm and forces you to move the mast forward. Typical US1 sails have a main with approx 10" foot and 60" luff, while the jib is about 11" on the foot and 54" on the luff. If you stray very far from those general dimensions you can get into problems.

Another thing is sail material. If you are using something like spinnaker cloth you have shot yourself in the foot. The only way to fly is to have your sails built by a pro who will probably use 3/4 ounce Trispi. That stuff will hold its' shape and a good sailmaker will get it right the first time. Hint: contact Rod Carr who will build a great pair of sails for $120 plus $9 shipping. ( I just recieved a new suit this week) See Carrsails.com He's in Redmond Wash. so you are not far away. Forgive the shameless plug. I am not a salesperson but I can give you first hand, unbiased, advice. The bonus for using proven sails is that the boat will go faster and point higher. There are other sources for high quality sails such as Black or Bantock. Bantock UK will cost a fortune but they are superb. There are also some very good Aussie model sail makers.

 

I have tried a set of Gordon Stout sails with the same result. I use Trispi for my sails. I'm starting to wonder if I'm just overpowering the sails. Unfortunately I don't have a wind speed gauge.
Thanks
Don

 

Mistral and boats of similar design change their personality with different angles of heel. When laid over severely weather helm is a common result. Sometimes it is so severe as to make the boat uncontrollable. So if you are sailing in brisk air the problem will likely persist. "A" rigs are manageable within a limited wind speed. You have not mentioned the weight and depth of your ballast. If you have followed the plans then you'll have somewhere in the region of 5 pounds. Too little ballast and excessive heeling is a probability. I hope you can work out the problem

 

There's a long thread on the topic of hull balance (resistance to yawing when the boat heels) on the RCBoats forum:

http://www.rcsailing.net/forum1/showthread.php?t=3436

I also presented a paper at the last CSYS on the topic. PM me here or there with your email if you'd like to have a look.

Cheers,

Earl

 

Thanks Earl, I've already read it. Couple of times in fact. Good stuff.
Thanks
Don

 

Can anyone provide a link to a site that has all these hydrostatic terms explained? And which are synonymous with which. I just started using Delftship and they seem to use slightly different terms.
Confused(as usual)
Don

 

Reconciling the terms would be a good idea; alas, I am now submerged in the project of writing the M Class history for the next AMYA Quarterly.

It would be interesting to see how the Mistral fares when analyzed using the metacentric moment hypothesis.

Cheers,

Earl