Design flaws: adapting canoe sail rig to 14' Starcraft???

Discussion in 'Boat Design' started by Tikajack, Jun 28, 2019.

  1. Tikajack
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    Tikajack New Member

    I have a 14' Starcraft "tinny" to which I'd like to adapt a Grumman Canoe Sailing rig. The sail area is 42 square feet. I plan to use leeboards instead of centre daggerboard. Steering would be with a 'steering oar' arrangement in order to assist in coming about. I read on another post that flat-bottom boats will not come about easily. As a newbie, I would like to hear comments on whether the design could work. Could this arrangement work without a jib? I would NOT expect speed, simply quiet propolsion. A mock-up picture pdf. of the boat and proposed sail is attached.
     

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  2. rwatson
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    rwatson Senior Member

    Anything will WORK. Heck, you will get blown about in a tinny without a mast in a decent wind.

    A rudder will be more useful though. If you have an outboard in the way, build two rudders, one for each side of the transom.

    Make the Leeboards adjustable at first, so you can experiment with their position.

    Finally, when sailing, keep your stern as high out of the water as possible, even if it means a bit of water ballast in the bow.
     
  3. messabout
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    messabout Senior Member

    Tikajack; You can actually sail that boat but it is not going to sail like a purpose designed sail boat. The problem is that the Starcraft is meant for planing with an outboard motor for propulsion. Viewed from the side, the bottom is in a straight horizontal line while moving toward the aft end, or transom end. That means that the transom will be dragging when the boat is in a level condition.

    Rwatson has advised to keep the transom above the water surface. He is telling you that transom drag is not good. At slow speeds the transom drag will not be too serious. "Low speed" might be 2 kilometers +/- per hour which is dreadfully slow. A sailing boat has a curved bottom profile that has the aft end rise up above the water level. Not to worry you can have good fun with this boat but do not expect much speed. Many another person has rigged a powerboat for sailing. It will actually sail if you get the components to work in harmony with one another. That might take some experimentation which can be part of the fun. Go for it.
     
  4. sharpii2
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    sharpii2 Senior Member

    You'll probably have to move the mast back by several feet, or you will have a Hell of a time getting it to sail up wind. The draggy transom will tend to make the bow point downwind. Also, the leeboards can be no further forward than the widest part of the hull. You can probably get away with one rudder, even if it is offset all the way to one side of the transom. The steering sweep will wear out your arm.
     
  5. Tikajack
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    Tikajack New Member

    Many thanks to "messabout" and "rwatson" for encouragement and for suggestions regarding 'transom drag'. One question that I am still pondering: "without a jib, would it be advisable to locate the mast as far forward as possible, something like a catboat rig?" Thanks for any suggestions on that.
     
  6. Tikajack
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    Tikajack New Member

    Thanks "sharpii2" - you've answered my question about how to position the mast. Are you saying that the centre of effort on the sail should be aft of the centre of resistance of the boat in the water (including the 'draggy transom'?
     
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  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I wouldn't get too distracted trying to build twin rudders that work together. Could have twin rudders and only use one, and have the other either locked straight as additional lee-area, or kickup out of the water. Or just move outboard to one side and rudder to the other. I've seen guys with similar boat with about 15hp off to one side and he said it works fine and is much more ergometric and thus greater real world safety.

    But what I think this guy needs is some basic formulas of where to put leeboards AND mast, and the basic feedback symptoms to look for so he can fine tune the rig. Example: If it keeps wanting to turn into the wind when he uses his weight to keep boat level what does that mean? If it does the same but only when he keeps weight centered and lets wind tip boat what does that mean? (I'd like to know that stuff, too)
     
  8. messabout
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    messabout Senior Member

    Use a scratch pad to draw a picture of the boat from a birds eye view. No need to draw the boat. Just the top view of the sail and the top view of whatever you use for lateral resistance. The sketch need not be anything more than two lines. Let an arrow push on the sail, as if it were the wind, at some location that you think is the center of effort.....maybe a third of the way back from the mast. Lets say the boat is on starboard tack and the wind is coming over the right side of the boat. It is trying to push the sail to leeward. The centerboard or dagger board is trying to keep the boat from going sideways to leeward (the left side of the boat) Place an arrow on the opposite side of the board from the one you placed on the sail. The arrows represent forces. One going right and one going left. If the center of sideways resistance of the board is aligned at the center of effort of the sail then the boat will have no tendency to turn into the wind or away from the wind. If the center of lateral resistance of the board (lets call it the CLR) is behind the center of effort of the sail (CE) the boat will tend to turn left or downwind. If the CLR is ahead of the CE the boat will turn to weather (to the right side.)

    That is too simplistic because the sail is never in the same place for long. That means when you let the sail out a way, the CE moves forward because of the rotation of the sail around the mast. When the CE moves forward, even a little bit the boat again will try to turn downwind. The common remedy for that little detail is to let the board CLR be somewhat forward of the CE. That is commonly called "lead" and the amount of lead is a wild guess in this boats case and is best discovered by experimentation. I rather doubt that the Starcraft hull will have much lateral resistance because the chines are rounded and not very deeply immersed.

    In cases where the boat has hard chines or other elements that cause some lateral resistance then there is some arithmetic to do.

    Whew! I do hope that that clumsy explanation will be of some help.
     
  9. rwatson
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    rwatson Senior Member

    The opposite.
    You need the CE to be in FRONT of the Centre of Resistance IF you want to be able to tack properly. You need the bow to be pushed away from the wind without operator adjustment on rudders.

    If you take my advice to have experimental adjusting Leeboards, make their position further back than you think best, so you can move them forward with experimentation.

    All this CE is pretty academic on a 15ft, flat bottomed boat. When the occupant shifts a feet forward or aft, the whole calculation will be upset. Ask any small dinghy sailor. Then, as messabout said, the actual angle of the boom will have a big effect on overall balance.

    You will end up with the Leeboards set in the most useful position after you have experimented for a while, and it won't be any kind of fixed formula.
     
  10. Blueknarr
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    Blueknarr Senior Member

    Tikajack,
    Back to one of your original questions. I have found that flat bottom boats tack easily and quickly. It is sailing in a straight line that may provide difficult.
     
  11. sharpii2
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    sharpii2 Senior Member

    It should be aft the usual CLA (Center of Lateral Area) because of the draggy transom. It may have to be so far aft that some of the Boom extends past the transom. If you look at MacGregor 26m's, you will see the rig is considerably far aft. And these boats have centerboards which can be placed relatively far forward. Your boat will have leeboards which really need to be at the widest point of the hull. Here's an example of a boat of this type I sketched some time ago. It has two masts so is probably not the best example, but it can give you some idea.

    FisFow4.jpg
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    how about leeboards (or swing keel) that swings back so that CLR moves with CE? I guess you'd lose a certain % of LR, but you'd stay on course with less rudder drag. Maybe use the moving CLR as the rudder, more or less, and use the rudder mostly as a fixed unit. I'd just feel better knowing the boat was 'balanced'. Using the rudder to correct for CLR/CE diffs seems like driving a car with a messed up front end that wants to pull to one side and needs hand on wheel at all times.
     
  13. messabout
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    messabout Senior Member

    I rarely disagree with Rwatson. In this case I do disagree.
     
  14. messabout
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    messabout Senior Member

    RWatson I know that you speak "Strine" so perhaps we are really on the same page but confused because of a different interpretation of the kings english. Please help me to understand your reasoning. I am certainly not above learning things about which I may have been misinformed. I am often enough proven wrong.

    You want the boat to have lee helm? Man that is a dangerous setup. If it has serious lee helm the skipper is helpless to bring the boat into the wind in order to tack or even to avoid those shoal rocks over there close to leeward. . Sure enough if the boat has too much weather helm the boat will be a ***** to tack, especially in a brisk wind. In almost every case a tiny bit of weather helm is traditionally designed into the system.

    The relationship between the CE and the CLR can be thought of as a lever system. Imagine for a moment that the board or whatever constitutes the lateral resistance is fixed and rigid. The sail then pushes sideways with respect to the fixed point and the boat will pivot about the rigid CLR. Which way do you prefer that it pivot? Move the pivot point fore or aft and the rotating moment will make itself evident.
     

  15. rwatson
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    rwatson Senior Member

    Everyone calm down. Of course, the average boat doesn't want over much weather helm or lee helm, I am talking here about a fifteen-foot flat bottom dinghy. If he doesn't get a bit of Lee helm, then the bow won't come around on a tack. Especially if he also has an outboard that probably isn't in the rudder loop.
    The boat will need to be able to have the bow push the boat over to the opposite tack, and not get caught in irons.

    Of course, everyone totally ignored the point I made about the constantly moving CLR, and even the CE as the boom is adjusted, let alone the presumably single bit of moving ballast.
     
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