Kayak Sailboat

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It has been my desire to have an extremely small and portable sailboat. A kayak hull may be modified with the correct hardware. This article will be updated as a means of recording this project as a whole as well as various updates.


Previous Experience

My background comes from experience with small sail craft. I have sailed windsurfing boards as well as single person sailboats such as Sunfish, Escape Captiva, and even a Hobie 16. Each has beneficial attributes to it's design which can be used in the design of a small kayak sailboat. A minor discussion based off of experience is provided since the final resulting kayak sailboat design will be a hybrid of both a small sail craft and a windsurfing board.

Design brainstorming and layout comes from the design of easy-to-use single person sailboats. One such boat is the Escape Captiva. This boat is very stable and features an unstayed mast. Being unstayed, it is easy to set up and there is no risk of the stays interfering with the boom. This is beneficial in the sense when the mainsheet is let out, the sail merely acts as a flag and can even rotate 180 degrees unhindered. This design does experience higher moment loads at the base of the mast so requires more structural integrity there. The mast is also built into a furling so that the sail can be reefed easily while under sail. The centerboard can swivel back which is useful in shallow waters and the rudder can kick back as well. This boat is 11.5ft long and has a maximum sail area of 67ft^2. The hull is made out of polyethylene (PE) similar to most sit-on-top kayaks in the market.

Even smaller boats, such as a windsurfing board, give even more insight into design of a kayak sailboat. Being much smaller, they are very susceptible to changes in center of wind thrust/drag as well as the moment caused by the wind on the sail. Here the rider uses his or her own weight to counteract the moment on the sail. This requires the rider to be very high up, essentially acting as a stay. Steering is done through subtle differences in sail drag against the oncoming wind. The rider merely has to tilt the sail forward or backward in order to change this center of drag which in turn turns the board. This is a very notable characteristic of small sail craft as a change in this drag location may require the rider to compensate with the rudder.

Kayak Sailboat Prototype 1

Previous work and experimentation involves the modification of a windsurfing board. This was most easy as the board already included a movable keel and was quite old, old enough not to fear its destruction. The hull was that of an old windsurfing board approximately 11 feet long and made from HDPE with foam interior. The rudder was mounted using through bolts and a brass hinge. The rudder control surface itself was first made from wood but that was found too buoyant so later a plastic one was constructed. Considerable changes were made to the control stick with the inclusion of second linkage so that the boat could be easily steered without forcing the rudder out of reach of the pilot. The mast was made from the end of a wind surfing mast and secured using a steel threaded tube and a threaded base bolted to the board body. A pulley with a spring loaded cleat was affixed at the middle of the board to act as the mainsheet. The sail and the rudder assembly were the additions made to make this into a full fledged sailing vessel.

But alas, this boat was not without its faults. The floating rudder had to be changed out for a solid plastic one in order to keep the control surface in the water. The base of the mast experienced some high moments when the wind became gusty causing the fasteners to tug at the body. When the mainsheet was let out under sail there was a noticeable change in the center of thrust causing the pilot to adjust the rudder quickly. It was also difficult to keep the vessel level in the water due to the force of the wind. Seating was limited as the boom could easily hit the pilot on the head and the center board kickback seen in red positioned at one’s crotch. There must be better ways to build a mini sailboat.

Current Available Designs and Commercial Products

Before starting this project it is best to benchmark commercially available products. For this I have focused on products that act as mods to any kayak versus kayaks that are designed to be sailed with included hardware. So far I have found two prominent designs from [Kayak Sailor]
and [Falcon Sails]
. Both designs have very good documentation and videos online for installation.

The Falcon Sails system features a stayed cat-style sail and mast setup for any kayak. This setup requires stays which mount to the kayak's frame via fasteners. There are a total of five stays, one to the bow and two on either side. The line for the mainsheet appears to go to another fasten point on the hull. Sail material is made from dacron fabric and comes in a variety of colors. There are two sail sizes available, 1 meter and 1.3 meter squared surface area. The entire setup costs approximately $550 and includes fasteners. This system does not however supply a centerboard or a rudder which are critical to a stable sailing stance. Instead the user is required to use their paddle as well as a kayak rudder if the kayak is so lucky to have one.

The Kayak Sailor is the most comprehensive kit I have yet found. It boasts a stayed cat-style sail but this time with three stays which anchor much higher on the mast. Sails are available in 1.4 and 1.6 meter squared surface areas. The mast, boom, and sail assembly can also fold up in a moment's notice which is useful when switching from sail mode to kayak mode. The mast is mounted on a cross-shaped frame which is fastened to the kayak's body. The two arms of the cross include centerboards which can pivot back in shallow waters. The kit does not provide a rudder.

There is a decent amount of documentation, photographs, and videos available online. Customers appear to reach decent speeds in excess of 5 knots without much tipping of their kayaks. These serve as good examples for a design since they fit on most any kayak. Mounting is of most concern since thermoplastic hulls, primarily PE, are prone to fatigue. Under sail, the mast must resist some high bending moments which are supported by the stays or in the case of an un-stayed sailboat, the mast mounting frame.


Kayak Hull with Module Callouts

The design starts off with the body of the kayak. Design work will pivot around this constant to provide the best sailing characteristics without damaging the hull. For this I have acquired a Lifetime Hydros kayak which cost roughly $200 on sale. This kayak very small and easy to transport. It can support a single passenger up to 230lbs and is 8.5ft long. It features a hollow molded polyethylene.

Module Breakdown

Mast Module

The most loaded module on this design is the mast module. This includes hard mounts to the kayak hull which have to be able to take the brunt of the wind loading. There are two ways to attach a hard mount to the hull; mechanically and adhesively. Loctite provides an adhesive made specially for polyethylene plastic called [Loctite Plastic Bonder]
. The datasheet only reports shear strength of this bond but not peel strength. Hard mounts attached using this could potentially be loaded in both mode I and II peel. To investigate this, 1 inch strips of HDPE were cut and bonded to a aluminum surface. Both surfaces were sanded with 280 grit and cleaned thoroughly with alcohol. Five minute epoxy had an incredibly weak bond while the Loctite produce was much better. The failure in the photograph shows adhesive stuck to both surfaces. Still the strength of these adhesives "feels" too weak for being the only means of attaching a hard point.

Peel Test Surfaces

Any means of mechanically fastening the hard point require the use of a blind fastener. There is no easy way to get a washer and nut to the backside of the hull shell. The best option chosen for this are well nuts. These employ nut fixed in rubber which when threaded through with a screw can widen and load the hard point from the back side. Each hard point will feature two well nuts. Online there are reported strengths of well nuts in the following link. http://www.hansonrivet.com/rubber-nut-threaded-inserts.htm I assume this was done using aluminum plate and not plastic so the numbers here may not be correct. I have done calculations for a wind gust at 30mph and it yielded a factor of safety of about three. Calculations for this I may include at a later time.

Hard Mounts

Hard points are tailor made to fit into the current hard points which were made by the kayak manufacturer except but much stronger. They will have two well nuts as well as the plastic bonding system. They will also have a central threaded hole to which the mast frame will be secured.

These hard points will then be used to mount the mast plate which also houses the centerboards. The mass plate is made from S-glass/Carbon composite epoxy material. I kept the glass side up since glass behaves better in compression. This plate is bolted into the hard mounts in four locations with 1/4-20 stainless steel bolts. In the center is the mast mount which is made from cast aluminum and has two threaded holes on the sides for fasteners to secure the cylindrical mast base into the hole. The mast base is a six inch galvanized steel pipe with one hole drilled in it to secure into the mast mount. The upper half will be epoxied into the hollow mast tube using a Magnolia brand epoxy. Surface prep including sanding and alcohol wiping will be done before bonding.

Mast Plate and Mast Mount

Rudder Module

Rudder Assembly

Centerboard Module

Centerboard and Mast Step



Sea Trials

2nd Sea Trial

The first sea trial featured the fully assembled ship. It worked to much my surprise able to reach a max of what I estimated 2 knots! There was considerable lean during a reach but that could be counteracted by the pilot. The centerboards did well to keep the boat true and with enough momentum, the boat was able to come about and tack into the wind. Jibes were more successful.

During the first sea trial, the rudder came free and the boat lost all control authority. The rudder pin was held in by a short length of rope. The mainsheet was then removed from the boom and the sail let luff while the boat was paddled to shore. The stainless steel pin acting as the fulcrum of the rudder was modified to remedy this. A small hold was drilled at the tip and a stainless steel pin used to secure it from ever coming out again.

The second sea trial was aborted mid test. The wind became very gusty and difficult to predict. Control of the boat was kept although another problem came from the rudder. The construction of the rudder did not keep the aluminum conduit used to turn it out straight. It would sag and bump against the back of the boat and the pilot would have to lift it to make sure it did not rub too much or did not keep the rudder out of the water. This must be changed in a later mod. The mast and sail setup could be improved. Taking inspiration form the Escape Captiva and the Hobie Bravo, a furling could be installed. Here the pilot could easily open and withdraw the sail depending on the wind conditions.