R/C trimaran hull shape and foil suggestions

Discussion in 'Multihulls' started by Mitch M, Nov 29, 2007.

  1. Mitch M
    Joined: May 2006
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Texas

    Mitch M Junior Member

    HI Everybody,

    I am going to start a R/C trimaran built to the F-48 class standard which restricts the hull to a 48" box. The problem is most (if not all) of the current F-48 designs suffer from pitch poling when overpowered. See the following video:

    http://youtube.com/watch?v=Prax02AbpZw

    I would like to try to make some small changes to improve sailing characteristics. I will attach a pdf of the hull and float shapes of the hulls I will be building. Is there anything I should change? I was thinking of adding some freeboard up forward, but keeping the floats narrow. Also, considering moving the entire rig aft as far as it will go. The rudder has a horizontal foil, but I am concerned as soon as the rudder lifts the foil will act a brake and encourage pitch poling.

    Lastly, I was considering moving the foils from the main hull, to the floats/amas. Would this be of any benefit, especially when flying the main hull?

    This topic was discussed in the following link, and it is in German but the pics show floats with flair, increased volume and freeboard.

    http://www.rc-network.de/forum/showthread.php?t=48237

    I would appreciate any input on design changes to improve the hull and foil shapes.

    Best Regards and Thanks,
    Mitch
     

    Attached Files:

  2. Doug Lord

    Doug Lord Guest

    F 48

    Hey Mitch hope it goes well for you! Pitchpoling is the biggest problem with small multies and the best fix -in my opinion- is to use lifting hydrofoils. Foil lift scales down well and if you were to use a system similar to that used on ORMA 60's with curved forward foils that can be retracted in lite air in combination with a rudder t-foil you'd have a pretty fast set up that was very resistant to pitchpole. Thats the second best solution. A boat similar to the F3 (www.microsail.com not for sale!) is the best solution to pitchpole I've ever found in small multies because it virtually eliminates pitchpole and capsize. But it's downside is that the foils can't be retracted in light air and it is complex to build. However, it foils in just over 4 knots of wind but many competitions seem to be held in light,fluky air(for some reason). Either solution is fairly difficult to implement and you should be an expert modeler before attempting it.
    Another solution is to use movable ballast which is legal in the F 48 class. That is much more expensive but workable nonetheless-and easier to do. A mini40 in Europe won a championship using movable ballast a few years ago so I've heard.
    Good design practice in an rc tri is to have the Center of Buoyancy of the ama- when the main hull is flying- forward of the CB of the main hull at static. This allows the weight of the boat to couteract -to some degree- the pitch down tendency of the rig.
    Using any of these ideas requires lots of research.Using an existing design with movable ballast is not likely to work well because of the extra weight.
    Don't be afraid to experiment- thats half the fun of these tremendously exciting boats!
     
  3. Mitch M
    Joined: May 2006
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Texas

    Mitch M Junior Member

    Thanks Doug for the reply. I was thinking this topic might interest you.

    If foils is my best bet, but the lifting part very difficult to do, what about triple rudders (one on each hull) and use T foils on each permantly set at some angle of negative incidence. The forward volume would be sufficent to keep the bow from submerging if I can just keep the stern in the water. Light air days I can just remove the extra rudders.

    I can easlily add some bouyancy to the fwd areas of the amas with just some added depth of the hull.

    Is there merit to moving the rig aft?

    Also, would a biplane type of rig lower the COE enough to eliminate pitch poling?

    Any other ideas out there?

    Mitch
     
  4. Guest625101138

    Guest625101138 Previous Member

    That model gets along nicely.

    It is not easy to confirm it on the video but it seems the jib is backing as the boat approaches the pitchpole. Maybe this is control action aiming to avoid the dive!

    By contrast I am amazed by the controlability of the full size tris in big seas. They seem perfectly happy on one outrigger with little tendency to dive. Although I know they will pitchpole. They seem to be getting some lift from the jib rather than it forcing the nose down.

    So I am wondering if you can improve your situation by getting the drive lower down in the jib. Consider sail cut and shape as well as the shape of the amas.

    Rick W.
     
  5. Guest625101138

    Guest625101138 Previous Member

    This is the one that defies gravity:
    http://youtube.com/watch?v=QZDS3miKQCc&feature=RecentlyWatched&page=1&t=t&f=b
    The main is reefed and it has a small jib but it nevers looks like pitchpoling even when driving over waves. If anything it looks like it will get airborne and flip backwards.

    The high clew on the jib has to reduce the fullness higher up in the sail than a low cleat position.

    For your boat you would lose some efficiency by raising the clew on the jib but it might give some lift as well. There may be an optimum position that does not detract too much from sail effficency in light wind but will keep the drive down low and give some lift in the bow when pressed.

    Rick W.
     
  6. Doug Lord

    Doug Lord Guest

    F48

    Mitch,I would suggest that you shouldn't try adding freeboard/buoyancy forward because that won't stop pitchpoling it will enhance it. Moving the rig too far aft will be detrimental to the tacking ability of the boat. One simple thing that does work and is good back up in any case is to use small foils mounted on the bow of each ama at about a 6° angle of incidence relative to the static waterline. Area on an F48 would be about 2 sq.in. each side of each ama bow(about a 3/1 aspect ratio of each foil minimum).This type of foil works most of the time to prevent pitchpole-but they don't add to speed as do other foil systems. The ama should be canted so that it is vertical when the main hull is just clear of the water. Don't use three fixed t-foils -they'll hurt you in light air. Don't forget to get the CB of the ama- when the main hull is flying- a bit ahead of the static main hull cb.
    You could experiment with a lifting foil(with or without an altitude control system) and a t-foil on each ama if you get rid of any foils in the main hull and design the boat so that the forward lifting foils are retractable and so that the rudder foil on the windward ama is clear of the water when the boat heels. If the boat was to float level neither ama should touch the water.
    Keep in mind that the drag of foils of any type in the water is ,apparently, proportionately greater on a model than on a full size boat. So the foils should be thin-6-7% t/c ratio.
    The advantage of the F3(Rave) foil system is that the hydrofoils develop all the RM for the boat virtually eliminating capsize and pitchpole.They can be 100% automatic with no radio control required.They are very easy to use needing no adjstment after they are first set up. The disadvantage is that they require a lot of study and work and unless you design a retractable system they will hurt you big time in non foiling conditions.They also require multiple rigs like any other high performance model sailboat.
    Sailing one of these boats without lifting foils OR with lifting foils that DO NOT develop RM with the main hull flying is exceptionately difficult and will require lots of practice.
    Just some musings -hope it helps!
     
  7. Guest625101138

    Guest625101138 Previous Member

    I was out pedalling on the lake yesterday and manage to bury the bow of my long slender hull while wave riding behind a barely planing small ininflatable that was carrying a bit to much weight to level out. The wake was 1-2 ft and boat doing maybe 7 or 8 knots. I could have kept diving the bow down if I cranked harder. It made me reflect on your pitchpoling problem.

    I thought of a solution using involving the hull shape. It is shown on the attached slender monohull although hard to depict. The underwater section has a flat lifting portion on the upward part of the bow rocker. The above waterline portion has an upward tapering flare. I believe both would provide lift if pressed and they do not detract from light air performance.

    I can provide a dxf or fbm file if you want to look closer.

    Rick W.
     

    Attached Files:

  8. tnflakbait
    Joined: Nov 2007
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: California

    tnflakbait New Member

    Here are a few ideas:

    -Lifting foils may help
    -Higher freeboard in the bows ie PLAYSTATION
    -Lower aspect rig w/ same sail area
    -put the battery on a track of some sort and use it as moving ballast


    good luck!
     
  9. Mitch M
    Joined: May 2006
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Texas

    Mitch M Junior Member

    Thanks Doug, tnflakbait and Rick for the replys.

    Those are all great suggestions.

    I think I will also experiment with moments of inertia. If I can get the boat to make it through a gust by increasing the moments of inertia that might work as well. I remember a SNAME article that I read back when I was in college that delt with this and it didn't take much weight to double the moment of inertia. Especially if placed at the tip of the mast.

    Mitch
     
  10. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 95, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer

    bringing weight out longitudal may also stabilise motion
    anyway, those boats really seem to fly, great work
     

  11. missing
    Joined: Feb 2008
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: usa

    missing New Member

    Mini 48

    Mitch,

    I have ghost train hulls, beams, 2 sets of foils and materials required to complete. (carbon, kevlar, sail material, masts, etc.)

    It may save you a lot of time... email me.

    cogitopartners@gmail.com
     
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.