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

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

  1. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Fair enough Rwatson. Please know that I mean no harm if and when I disagree. I'd be more than pleased, honored in fact, to buy you a beer so that we could continue our friendly arguments.
    rwatson likes this.

  2. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Perhaps a little bit of clarification is needed here.

    1.) This plan can work because it has been done before. Rwatson is right about getting the hull to trim down by the bow to reduce the area of transom under water. The less of it there is underwater, the less drag there is going to be.

    2.) If there is a significant amount of transom still underwater, the balance of the boat will be pulled aft. The Siren 17 I once owned would not self-steer downwind. I once forgot to pull up the outboard and, to my amazement, I discovered the boat would then stay on a downwind course without my hand on the tiller. This was not good for either the engine or its locking mechanism, but it did show how I could get the boat to be stable on a downwind course. All I would have to do is fashion some sort of small drogue. Yes, it would make the boat somewhat slower and less efficient, but it would beat the pants off trying to fashion some sort of expensive vane steering device or spring for an auto-pilot and a power source for it. The immersed transom would act just like a small drogue. It would tend to pull the transom upwind. It is for this reason that the Center of Effort (CE) of the sails has to be moved back. This is to prevent having a lee helm in which rudder correction is needed to get the boat point upwind. On a typical sailing dinghy The CE is ahead of the Center of Lateral Resistance (CLR). This is because, as the dinghy heels, the effective CLR tends to move forward. When this happens, a modest weather helm (rudder correction needed to get the boat to point downwind) can become an arm numbing -monster weather helm. It can become so bad that the rudder acts more as a brake than a turning force. Then, just as Rwatson warns, the boat will refuse to come about. On your proposed boat, the CE may well have to be behind the CLR because of the draggy transom. But one must be careful not to over do it. There is not much experience in rigging this type of hull with a sail, so it may be a good idea to put the leeboards where they need to go and set up a temporary tri-stay system for the mast, so its best placement can be found through trial and error. These three stays can attach to "C" clamps on the gunwale. Once the best placement is found, a proper mast step and/or proper stay attachment points can then be installed.

    3.) Although modest fore and aft weight shifts on a typical sailing dinghy can change its balance significantly, such may well not be as effective on a boat such as yours. This is because a typical sailing dinghy has a bottom which curves up in the stern, so the transom ends up above the Water Line (WL). If one shifts the boats Center of Gravity (CG) aft, some of the transom ends up being immersed but very little. What also happens is the CE of the sails moves aft. The taller the sail plan is, the more true this is. This aft movement of the sail's CE overwhelms the little bit of drag of the slightly immersed transom. The boat will tend to want to round up much more than normal. The reverse is true if you move the CG forward. Strangely, on some conventional sailing dinghies, the opposite is true. This is because the deeper immersed fore-foot moves the CLR further forward than the CE moves forward, when the boat trims down by the bow, so this time the boat rounds up. When moving the CG aft, it will tend to round down (point downwind) because the rising fore-foot will have less grip on the water and the CLR of the hull will move aft further than the CE of the sails. Your hull has a straight run aft and a bit of a fore-foot. It will trim far less fore or aft with a given fore and aft CG shift than a boat with a more curved run aft will. It's kind of like replacing the curved rockers of a rocking chair with straight ones but not quite. My main point is that once the fore and aft trim of your boat is settled on, it will be much harder to change its balance by shifting its CG slightly forward or aft. I suggest you rig a long tiller or even a drag link and a push-pull stick so you can sit one thwart further forward than you do when driving the boat under engine power. This will trim the stern up and the bow down and minimize the amount of draggy transom still in the water.

    4.) Although your boat will not sail as fast as a conventional sailing dinghy of the same size and sail area, it will sail if it is properly set up. It will be significantly slower, but may reach 3.5 to 4 kts in a good wind. A more conventional non-planing sailing dinghy can be expected to be about a knot faster.
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