Gfilui

I have to contend with 100km+ winds with my 17.5ft canoe, so I've tried pretty much every method of attaching a canoe to a roof you can conceive, and the one that works the best is this one:

It's called the keelover. It uses four brackets to hold the gunwales so the craft doesn't slide side to side or front to back, two boat straps to hold the canoe down on the roof rack, and one on each the bow and stern to prevent the canoe from twisting and torquing on the roof rack. If you want to to be extra secure you can use two straps front and back.

I've used the foam blocks, and every method of strapping and tying a canoe down to roofs and racks with ropes and ratcheting straps. The keelover out performs them all.

Position the canoe centered between your cross bars. You can use foam padding (e.g. pool noodle) between the crossbars and gunwales if you want to protect your gunwales. With stout cordage using a truckers hitch knot or Cam Straps, tie the canoe to the cross bars. Tie the bow and stern to the outside corners of the vehicle. This prevents the canoe from rotating from a crosswind.

Check the tension of all the knots after 5 or 10 miles, then every time you stop afterwards. The trucker's hitch knot or cam straps are easy to adjust during these stops.

Here's a pic:

This is difficult, since every boat/ vehicle combination is unique and so there is no one "silver bullet" method for rigging that will work for all. However, there is one principle that I apply to each rig I do, that allows me to adapt to almost any scenario: prevent motion in all six directions- Up, Down, Left, Right, Forward, Backward.

The vehicle itself pretty much takes care of Down. One or more straps tossed over the boat and secured to the roof (or possibly run through the door frames and tied inside) solves Up, and with a little tension, begins to resist Left and Right.

For forward and backward and to finish off left and right, I typically do V-shaped harnesses with the point of the V at the bow and stern, and tying off to where ever I can find on the vehicle. This has included running boards, bumpers, bases of rear-view mirrors (be careful with this one- in fact, I don't actually recommend it) trailer hitch receivers, and even the under carriage.

You just have to ensure that the angles of these harnesses are opposite the direction of movement you want to check, i.e. a rope that runs forward prevents Backward, but not Forward (you can't push on a rope).

Interesting how everyone is carrying upside-down.

We have two 5m fibreglass kayaks (based on Zegal Greenlands) that we carry on the roofrack - right-side up. I have made two strong rectangular plywood boxes for each kayak, and each has a substantial, form-fitting EVA foam insert that the kayak sits in. These sit under the internal bulkheads in the kayak, and I use rated straps to hold them down - very firmly - and a lead rope to the front bull-bar. (The kayaks sit somewhat behind the front of the car - which is a significant 4WD.)
Yes, there must be some lift generated by the hulls, but the proof of the system is 5,000km traveled on a recent holiday - including days on very rough tracks in the north-west of Australia - with no movement!
This system allows me to slide the kayaks onto their holders (which are quite high), and access the storage hatches.

Canoes are easier than kayaks because the gunwales provide four nice contact points on the roof rack bars.

The first step after getting the boat on the rack is to position it in the best location compared to the front and back of the car. This will depend on the relative sizes of the boat and the car. First find the rough center of gravity, where an equal amount of weight is in front of the front bar and behind the rear bar. You do this by pushing up on either end until the force required seems roughly equal.

You probably want to move the boat further back from this point. The main disadvantage to having too much of the boat over the front windshield is that it blocks your view more than necessary. (It's also worse for aerodynamics) But you don't want to move it so far back that it will fall off before you tie it. Also, the more of the boat that is sticking out behind, the more likely you are to hit something with it.

Now attach a strap over the boat on each of the rack bars. You can do this with rope, but webbing straps are faster and also easier to get tight. You can use either a strap with a buckle to form a loop or a ratchet type strap. All you are trying to do is get the strap under the bar on either side, and then tightly over the boat, which holds the boat firmly down on the rack.

Add some twists to the strap where it rests against the boat. At highway speeds this will reduce the noise from the strap vibrating against the boat.

Now go to either end of the boat and try to move it, side to side, up and down. The straps should be tight enough that the whole car moves, not the boat relative to the rack.

If you are just going a few miles in the city, many people who do this frequently will stop there. But I also secure the bow and stern to the car.

Once you have attached the back of the boat, you may not be able to open the hatchback or trunk (boot), so load that first.

You can use ropes or straps at the bow and stern. I have a set of four ratchet straps, so I use the remaining two. If you use rope, tie a trucker's hitch, which helps to get it tight. Find something sturdy at the end of the boat to attach your strap. And you'll have to find an attachment point near the bumper of the car. Ideally your car will have tie-down loops under the car at either end. I match the bow and stern ropes to the number of tie downs. If there is one, I do one strap and if there are two, I make a triangle.

Once you get going, check your straps after a few miles, and occasionally after that. Some kinds of straps can stretch when they get wet, so re-check if it rains.

So the standard solution to this (a good quality rack with foam pads on the crossbars, cam straps, plus a bow and stern line) is a great example of the following basic engineering principles which require no math and can be applied to a great many practical life situations:

Failure Modes/Effects Analysis (FMEA): basically think through what might go wrong and take more precautions for failures that might kill someone vs. failures that would just scratch paint, dent roofs, damage the boat, etc. Specifically, do not let the canoe come loose from the car on a freeway for any reason. If it’s bouncing around destroying your paint that’s your risk to accept, just make sure it can’t come completely free and cause a high-speed accident for the people behind you.

Redundancy / avoiding single-point failure: one way of achieving safety is to have more than one system in place. The classic example is a twin engine aircraft: two completely separate engines which are not connected to each other at all and even have their own separate gas tanks provide a real margin of safety. However there is only true redundancy if the systems are independent. For example two ropes tying the boat to the rack provides redundancy to a rope breaking but not to the rack coming off the car.

Six degrees of freedom: Solid objects in ordinary life move exactly six ways no more no less. They translate three ways (forward and backward, left and right, up and down) and they rotate three ways (roll, pitch and yaw). So for each way it might want to move, something (a rope that pulls on it, a bar that pushes on it, something grippy that keeps it from sliding) needs to stop it.

Leverage: it takes less force to pull on the end of a lever than it does to pull somewhere in the middle. So for example if the canoe is trying to pitch up and down or yaw side to side because of wind, a rope at the end will have less force on it than a rope close to the middle.

“Human error” is often avoidable: use of ropes introduces opportunities to do something wrong with your knot tying. It’s harder to screw up using a cam strap. This point is debatable and will depend on your personal familiarity with knots. Cam straps like ropes must be checked to verify they are in good condition. Closely related: inventing new things is risky, it’s hard to anticipate everything that can go wrong. If you can use something off-the-shelf that someone else already tested a lot, that’s usually much safer.

Making products is hard and you often get what you pay for: built into the price of a good rack from a top brand are things worth happily paying for like materials, workmanship, manufacturing quality control, engineering, testing. It’s so worth it. I’ve loved the Yakima and Thule racks I’ve owned, amortized over years of use their cost per trip was minimal.

So all of these come together in the standard solution:

Buy a good rack, and strap the boat to it using foam pads that not only protect the boat but also provide friction that keep the boat from sliding around when the straps are tight. In addition use bow and stern lines, each in a triangle from one point on the boat to some point on the corners of the car, to keep the boat from yawing or pitching in the wind, and provide independent redundancy that keeps the boat at least attached to the car even if the rack fails somehow. Take advantage of the experience of the boatbuilder instructors who have probably done this a lot to come up with a great solution for bow and stern lines for your particular car (ways of wrapping around the bumper to hard points underneath, or bringing a strap through the gap beside the hood to attach to some hard point under the hood).