Hey everyone,
I will be building a shearwater sport soon and customizing it as a sailing kayak. I want to add a retractable skeg to act as a centerboard near the bow of my boat. Should I install it on the centerline or just off center to avoid compromising the strength of the hull?
TIA
The skeg box & etc. should add more than enough strength to offset any weakness induced by cutting the skeg slot, thus a centerline position should be fine (and recommended - otherwise you'll have to create some fancy shimming as a part of the install to keep the skeg vertical). BUT...
Near the bow?! That will tend to create a lot of weather helm. Note that I'm not aware of any sailboat of any design that uses an underwater foil "near the bow" - and that ought to lead you to question your idea on positioning.
Are you also going to rig amas/outriggers or a centerboard? Seems any underwater foil intended to develop a center of lateral resistance will need to be nearer to the fore-aft midpoint of the boat (under the forward part of the cockpit). If you're taking anything like a normal sized skeg, I also believe it will be too small to be very effective. Will you also be installing a rudder?
If you haven't looked at a basic diagram of the forces involved in making a sailboat go, I recommend doing that. Something like this
https://en.wikipedia.org/wiki/Forces_on_sails
-Pay attention to how the fore-aft distance between the CE of the sails and the CR of the centerboard needs to be relatively small, and offset by the force of a rudder in order to steer a straight course. A larger distance between CE and CR means a lot of lee or weather helm, and a lot of rudder angle (and thus drag) to offset it. At some point the rudder becomes insufficient to offset large amounts of lee helm (dangerous) or weather helm (the boat rounds up into the wind and goes nowhere).
If you haven't looked at the CLC kayak sail rig kit on this website, I'd recommend doing that.
I might be missing something, but with what I'm seeing in my head based on your question about the retractable skeg, even with a quality sail rig, without a properly placed, properly sized underwater foil you are probably never going to achieve anything other than a beam reach.
I give a hearty "haul-away" to anyone willing to do some experimental designing, but making the experiment a built in part of the boat (like a skeg near the bow) might not be advisable. When experimenting, the first-draft installations are probably best done with duct tape and bailing wire until you've got something that works.
P.s. I sail my Chess 17 (with rudder) and a Windpaddle sail. I can only sail about 45 degrees either side of dead downwind. Otherwise I paddle. A kayak with a sail rig can easily sail upwind (about 45 degrees either side of the direction from which the wind is coming), but to do so takes a proper centerboard, or flat sided amas (like a Hobie Cat), and a real sail rig.
You know of course if it is a center board it is not a "skeg", but similar....sort of. If a center board it would be larger/longers than a typical skeg. Location would likely be where your feet are and go back towards your seat. For me that area is already crowded.
Small boat sailers often move weight to the windward rail. I'm thinking you can't do that in the shearwater sport. So the size of sail and CB would be small and limit upwind performance.
Really, really, really good advise from Grumpy and Bubblehead.
There are some examples of successful forward centerboard sailboats. Phil Bolger's Cartopper comes to mind, a brilliant design I have long admired. However, Bubblehead is right--such a thing would need to be worked into the design of the rest of the boat from inception. In Cartopper's case, the rudder is oversized to assume a significant part of the lateral plane area.
I've also seen examples of designs with "tandem" centerboards--one forward, one aft--the idea being to get sufficient lateral plane with less depth, to provide a means of adjusting helm balance, or to work around some interior arrangement better. Maybe something to try with your Shearwater Sport? Again, you're going out on a limb here when you start adding stuff to a design after the design is already done.
.....Michael
   Thanks all for the advice, I greatly appreciate it! I am going to do some more research, but essentially yes the goal was to use a retractable skeg foundation but make it bigger to account for the lateral force. I’m thinking it will need to be close to the center of forces from the sail and rudder. I have an 1.2m2 upwind sail and rudder set up on my rec kayak that I have now and I’ve been able to achieve a close haul but its not as efficient due to only having the small rudder.
Perhaps the move is to go for a larger rudder and consider adding a centerboard or skeg later on.
Think about some sort of leeboard also