Weathecocking a 20 foot kayak? Why?

-- Last Updated: May-11-07 6:06 PM EST --


Question: I realize that shorter, rockered and flatter bottomed sea kayaks have a tendency to weathercock. For instance, although I enjoy Prijon boats, basically all of them including the popular Seayak, largely because of the hard chine and flat bottomed structure, will weathercock--sometimes fiendishly. Of the Prijon boats, the least weathercocking is the Barracuda, with it's shallow v-bottom and minimally rockered 21 inch beam.

Why then do I typically read about very narrow surf and racing kayaks weathercocking. An example would be the Sea Kayaker review this month of the Valley Rapier, at nearly 20 feet and 20+ inch beam, the reviewers mention weathercocking significantly, and one even suggests that it's best rudder down most all times.

I would think weathercocking -- which I understand is related to boat trim, paddler position etc.--with all things being equal, would be minimal in a longer, thinner, less rockered boat.

Thoughts?

PS the Valley Rapier comes out in plastic this year.

why do you think that
weathercocking, with all things being equal, would be minimal in a longer, thinner, less rockered boat?

Weatherhelm Has To Do With Trim
not length.



sing

A 50 ft sailboat
will weathercock if (if properly designed) slightly, its really a safety thing, more so with sailboats, but with a kayak you would want it to turn up into the wind if you stopped paddling as the direction of the wind is generally the direction that the waves come from. And in a narrow boat, if you were hurt, having the bow sitting into the sea would be a good thing.



Also the current trend with race boats seems to be towards slightly more rockered to give the boat a little more manuvurability making the boats more rudder rudder dependent, but still fast. At least thats what I see.

If I was Mr. Wind, and someone…

– Last Updated: May-11-07 8:27 AM EST –

...asked me to push a boat around, spinning, I'd choose to push the shorter, fatter, flatter bottomed and highly rockered boat... a longer boat would, intuitively, be more stationary in the water to spin/cocking, with a digging v-hull and longer waterline.

Weathercocking results mainly by the pressure difference between the bow and the stern as the boat moves through the water... low pressure at stern compared to bow (see, I watched my Nigel Foster videos). Perhaps the pressure gradient would be higher in a faster boat at a faster speed... even though at similar speeds, the less waterlined, more rockered, and wider beamed boat would be the more weathercocking of the boats.

I don;t know, but for those that paddle both types of boats, they might know.

Rapier
It’s not really a sea kayak but more of a racing boat. Sea Kayaker should get racing skippers to review it.

Wind strength and water conditions also play a part in weathercocking and some weathercocking is very desirable.

It seems to me
that a majority of sea kayak weathercocking is from how much kayak sits above the water and if the kayak has a pinched stern keeline and how far below the waterline it sits.

Sing’s got it right.
Trying to remember all the stuff from my sailing days…

Any boat, regardless of length, when viewed from abeam (from the side), has a natural “pivot point” called the center of lateral resistance, located somewhere along its length.

If this point were exactly in the center of the boat, then the boat would be termed “neutral helm”. Ie; wind from either side would be just as likely to cause the bow (front) to turn downwind as the stern (rear).

In a kayak, where the is no keel, the easiest way to either cause or correct the tendency of the boat to point either end into the wind is by adjusting the fore-to-aft positioning of the weight in the boat. This weight could be either the occupant, if you can move your seat, or weight you place in either the front or rear hatches.

Starting with the example of the “neutral helm” boat, if you WANTED it to have “weather helm” (the front of the boat pointing into the wind), move the weight toward the front of the boat.

This causes the front of the boat to sit lower in the water, and exposes (relatively) more of the rear of the boat to the wind, allowing it to be blown downwind, thus pointing the front of the boat into the wind.

Conversely, if the boat already had TOO much tendency to point into the wind, shift some weight toward the rear.

You can get a lot more technical than that, but this should get you where you want to go.

Bob

It’s all about pressure
A stationary boat will not weathercock. In fact it will eventually turn so that it is beam on to the wind.



When a boat is moving forward, the bow wave generated by the boat is an area of high pressure. Likewise, the wake leaving the stern creates an area of lower pressure.



As a result, when the wind hits the boat from the side, the bow is anchored in place by the high pressure of the bow wave, but the stern is free to blow downwind. That causes the boat to point into the wind.



If the stern was higher pressure than the bow, then the bow would blow down wind while the stern stayed anchored and you would have a lee cocking boat.



When you are not moving and there are no waves and wakes being generated, both the bow and the stern will be effected equally by the wind and they will both try to blow downwind (assuming the boat is trimmed equally) and it will come to rest 90 degrees to the wind.



You can affect the way wind acts on the boat by adjusting trim to make the stern “stick” more in the water, or you can use a skeg or a rudder to make it more difficult for the stern to blow down wind.



Many boats such such as my Nigel Foster Shadow, are designed to be “loose” in the stern. This makes it easy for the stern to “skid out” and make a quick turn.

Boyancy?
A bigger volume boat would float higher in the water and be more affected by the wind.

Trim and geometry would determine if it turned to weather, beam or to lee.



That would be my first guess.



Tommy

ANY boat
that is not underpower will slowly turn sideways to the wind.



Next time your on a lake and see someone drift fishing check the position of the boat to the direction of the wind. It will be sideways.



Because kayaks are so light it happen quicker and you have to fight against this tendency.

Balance WC with Rudder size
The most efficient way to balance a boat that weathercocks is to add an efficient foil shaped rudder. If the boat still weather cocks with the rudder down then it simply means that a larger blade is needed. Most surfski manufacturers offer rudders of different sizes to suit diffent paddler weights and conditions.



All serious race kayaks and high performance sea kayaks should be designed to be balanced with a rudder deployed full time. Still there may be some tweaking needed to rudder area to suit one’s weight, speed, and conditions.



I’ve used a rudder that was too large and the boat lee-cocked. I simply cut meat off the bottom of the blade until I achieved the balance I needed.



Sea kayaks can easily be adapted in a number of ways to suit the user’s needs. Whether changing seats, backrests, rudder blades, day hatches, bungee, etc…there are a number of reasons not to base your buying decision on a quick try at a demo day. Or like in the case mentioned above, a magazine review by someone who may not be familiar with the type of boat they are reviewing.



My guess is most folks choose a kayak at a demo day mostly based on seat comfort. This is why we see so many cheap little plastic kayaks with big goofy padded chairs.



Also be very leary of the “experts” advice you read on Paddling.net, including mine. If you read my profile you will see I am a forward stroke performance-based paddler and my opinions reflect this.

SK Review
They may not have been the best people to assess the race-ability of the craft but I think they provided some good info for the sea-kayaking sort of person interested in perhaps getting one of those.



I also liked the calculated drag data, best they’ve gotten at 6kts, but still incremental over something like an Epic 18 at 5kts. Still difficult for the average weekend paddler to sustain 5kts over flat water, I’d guess. It would certainly pull away with a more serious paddler and no doubt even more so running downhill on ocean swells.



And, it looks pretty neat, especially in profile.



Mike

Waterdock…listen to him!

I mean listen to waterdock…
That may have sounded wrong. Waterdock is absolutely correct regarding pressure differences. Long directionally stable kayaks can be harder to correct in wind etc. Under stern rudders help a lot with this. Another excellent reason why smaller, less powerful paddlers should be wary of long boats.

if you had gone with me
to flatpick’s presentation on boat design at Canoecopia like I told you you should, you wouldn’t be asking this question right now!

Weathercocking
has less to do with wind acting on the boat, & more to do with wind acting on the water - or more correctly, it’s how the bow wake effects the stern of the boat after experiencing the winds influence.

A boat will only exhibit weathercocking tendencies while moving forward & creating a bow wake in a wind enviroment (without wind - there is no weathercocking, without forward movement - there is no weathercocking).

As stated earlier, a boat that is simply adrift in wind will turn beam-to the wind.

While it is correct that water pressure differences are responsible, it is (IMHO) the “wind to wave/bow wake” inter-action which plays the significant role.

It is the wind driving the bow wake against the stern of the boat on it’s windward side which generates more ‘pressure’ on that side then on the leeward side, forcing the stern downwind & the bow upwind.

Dropping a skeg locks the stern into deeper water/more stabile conditions.

Trimming the boat by adding weight to the stern submerges the stern into less turbulent conditions also & raises the bow which generates less wake-line. By that I mean that the boat will provide less waterline distance bow-to-stern for wind driven wake to have an effect upon.

Did not see the review yet …
Sea Kayaker mag ‘experts’ stating a rudder designed race boat weathercocks without rudder ??? … sigh.



I feel bad for Peter.

Yeah…
hard to make sense of this… Until you realize the orginal post has been edited.



Racing boats are not your ordinary tour boats and are designed to work with a rudder.





sing

salty…listen to him!