Moving the clr aft

Discussion in 'Fiberglass and Composite Boat Building' started by frank silver, Feb 3, 2012.

  1. frank silver
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Kill Devil Hills, NC

    frank silver New Member

    Good morning all! I have a 1976 Tratan TOCK. I have owned the boat for three years and have fought a battle, unsuccesfully, with weather helm from the start. I understand the relationship of the CE to the CLR and have properly tuned my rig. I have invested in a new mainsail and normally sail with a variety of headsails between 110% and 140%. It seems that no matter what I do the weatherhelm takes over at abour 12 knt of breeze. Rarely do I ever reef the headsail, and rarely am I able to sail with full main and mizzen. I sail with several other ketches, a morgan OI 41, a Shannon 38, a Person 365 and a Tiburon 36. These boats all carry full sail in much more wind than I am able to and having sailed them they do not have near as much weather helm. Being a racing sailor, I think my understanding of proper sail trim is substaintial.

    I am considering altering the keel design to try and aleviate the WH. One of the two thoughts are to add a lead keel shoe (1,500 lbs) to the bottom of the keel. This shoe would follow the entry angle of the keel and extend appro 8" when finished and glassed. It would extend the bottom of the keel about 10 inches aft. I would fair the shoe into the original. The idea being that I would be increaseing the righting force, and increaseing the weight low and aft.

    The other poosibility is to fill the areea between the keel and the skeg partially. The idea being to increase the surface area and thus moving the CLR aft.

    My concerns with idea #1 is multi facetted. By adding that much weight, approx 16% of the total ballast I would significantly alter the SA/D and slow the boat way down. The other concern is the practivcle aspect of forming and attaching the weight.

    My concern with thought two is I don't know if CLR is effected only by weight or by surface area as well.

    Obviously I would like to have no more draft than I must have, nor would I like to carry around a bunch of extra weight.

    I am not a designer, but a pretty through thinker and hands on guy. Any and all knowledgeable thoughts would be greatly appreciated.
     
  2. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    I would look at what could be done with the rig before rearranging the bottom profile.
    What has been the experience of sistership owners?
    How much could the mast be angled forward?
    Is there some system in place (traveller, etc.) to flatten the main?
     
  3. frank silver
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Kill Devil Hills, NC

    frank silver New Member

    Alan, there is a traveler in use..... The head stay is a Hyde roller furler that can not be adjusted. I am unable to shorten the forstay because of this furler/headstay.... There are fore and aft lowers with the forward lowers tensioned to induce mast bend. The mast is chocked at the deck in a nuetural position. Back stay is pretty snug to induce bend as well...she carries weather helm under main and job alone but obviously witht he mizzen it is amplified.

    Visually there is no rake to the mast.
     
  4. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 144, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    unless your hull has a reputation for taking more weather helm as it heels, adding more ballast to the keel will not affect weather helm much. That would be very costly and not have a much affect on weather helm I suspect. The profile area below the water line (and it distribution fore and aft) are what affect weather helm.

    How about a fixed fin in front of the rudder? better yet, a larger rudder with more area ahead of the rudder pivot. That should also reduce weather helm. I am not familiar with the rudder configuration on your hull, so I am not sure if this is feasible. It is far easier to alter the rudder only rather than modify the hull.

    Also, would these alterations disqualify your hull from the normal rating handicap? If so it would mean there is not much club racing you can do that would count. It would be far easier I suspect to argue that an altered rudder does not give you a performance advantage vs. major changes to the keel and ballast.

    Play with the rudder first, easy to experiment with and make changes if you do not like the results.
     
  5. frank silver
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Kill Devil Hills, NC

    frank silver New Member

    Petros, it is my understanding that the TOCKsdo tend to have a lot of weatherhelm. I understand that was a comon trait with other boats similiar in design such as the Whitby 42. The rudder is skeg hung and I have thought of altering that. It was suggested that one soluton might be to try to balance the rudder some by extending it deeper and building it forward. I have thought about this and think I can accomplish that. I do understand the difference between rudder pressure and weather helm. I thought about increasing rudder size as well but I didn't see how that would move the CLR aft. We do not run out of rudder effectivness generally. We may have to sail with 12 or more degrees to keep the boat straight but generally we are able to keep it straight....slow and straight.

    I take it from your responce that by creating verticle surface area the pressure would help to reduce the WH. Is that correct?

    This boat is not used to race at all. With a rating of 162 we can not even begin to sail to those numbers. I wish we could. Our windward performance is mediocre at best and when we reach the WH kicks our butt.
     
  6. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    I have owned a boat that had to be corrected for balance because of weather helm. The solution for me was a conversion to a cutter with a flying jib set out on a bowsprit. Using a sail rather than changing the underwater shape allows the use of the boat with or without the "correcting factor".
    The boat is probably a good sailer when not heeled enough to move the CE aft but the hull probably gets extremely assymetrical when heeled due to unbalanced ends, the stern being wide at the waterline.
    Whatever you do, whether altering sailplan or underwater profile, you probably won't improve the boat on every point of sail, but rather you will likely improve reaching ability a lot (with a sprit and jib) and windward ability will possibly suffer a bit.
    It may be a "choose your poison" situation. It may also be that adjusting for weather helm to where you get better overall performance will create lee helm at low speeds, which is no problem if you don't mind, but be aware the basic hull shape is likely causing the problem and that's something you can't do much about.
    Boats with balanced ends tend to have less helm (than your boat) at higher angles of heel.
    Also, if you have a tiller, measure your tiller angle when the weather helm is at its worst. Pressure and angle are two different things. A larger boat feels much more hard-mouthed than a small one because the helm pressure is developed from a much larger rudder. The angle is really the significant factor. Do you really have to crank the helm way off to windward?
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 478, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You can't add balance to a skeg hung rudder, it's like adding a brake and really screws up the flow to the blade.

    I addressed this issue on a similar design years ago and made a new, bigger rudder, along with rig tuning. The rudder area increased by about 40% and it did eliminate most of the heavy helm feel.

    The first thing you should do is check to see how much rudder deflection you need, in a 10 knot and again in a 15 knot breeze. At 10 knots you should be under 5 and at 15 knots, you should be close if not on 5 degrees. If you are experiencing more rudder deflection at 15 knots of wind then 5 degrees, you have a weather helm issue. If on the other hand, you are in the 5 degree range in 15 knots of wind, you have another issue, not weather helm. It's an important distinction and needs to be verified.

    Assuming you are cranking in 10 degrees of rudder deflection, just to keep her on a close hauled course in 15 knots of blow, enlarging the blade can help quite a bit. Rather then the forward sloped trailing edge, make the new blade with a straight up and down trailing edge. This will add about 30% - 35% more area to the blade. Before cutting and building, tack some plywood on, to simulate the increased area and take it for a sail. This will verify you're on the right path, before committing to materials and cutting.

    My notes on this design suggest this was a known issue and changes were made, to address the issue. Contact S&S and see if they can offer suggestions as well. The updated plans are likely available too, which would show the modified skeg/rudder assembly.

    [​IMG]
     
    1 person likes this.
  8. frank silver
    Joined: Feb 2012
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Kill Devil Hills, NC

    frank silver New Member

    Par, I contacted S&S and they gave a project number for a keel shoe and a bow sprit addition....should the rudder addition continue verticle to the hull or stop at the height of the original rudder? There is no doubt about the amount of rudder deflection required.
     

  9. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 478, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Assuming way more then 5 degrees, the longer sprit and shoe may be the ticket. If you want to go further, the new trailing edge height will not gain anything being above the fairbody at the top of the current rudder. I suppose you could put a little "jog" in the rudder blade top and stay right behind the fairbody termination, maybe a stylish little whoop-de-doo on top, just to finish it off. This area is in pretty turbulent water to no real gains, but it will look nice if done well. If it was me, I'd just come straight back off the top of the rudder, maybe a 4" diameter radius on the top and bottom corners, bracketing the new trailing edge.

    I have no reports about weather helm on the modified TOCK, but I'll look around and see if the sprit and shoe made enough of a difference.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.