RE: [Paddlewise] Skeg Jammers etc

Peter Treby [mailto:ptreby_at_ozemail.com.au] wrote:>>> <<<
<Snip>
> Paul Caffyn ... is reported to have told Chris that if he didn't use a
Nordkapp
> >with a deep draft rudder he had no chance of succeeding. ... I remember
thinking what a silly comment by Paul,
>>>You don't suppose Paul might have been a bit tongue in cheek, perhaps
just
slightly less than rock solid adamant with that comment do you?<<<

You'd have to ask Paul. As I recall, Chris reported it as though Paul was
very serious. I don't suspect Paul was anything but serious when giving
advice to a paddler undertaking what could be a life threatening journey and
seeking out his advice and knowledge. I'll bet he believed it and was
serious about it (just not correct). I've read Paul's opinions on rudders on
more than one occasion and do not share them (except maybe for certain
kayaks). They may well work for him in the kayaks he paddles but how many
different kayaks has he compared (and what has he had the most practice at).
Appeals to authority don't much impress me.

> If I had my rudder fitted (fell off years ago and I never put it back)
Of course it fell off, they all do at some time don't they?

 >I think it is useful to check the balance of the boat by lifting it.
 >Straddle the boat, grasp it under the coaming at the CoB point, and lift
 >slightly. If both ends come off the ground, OK.This makes sense, but the
trick is knowing where the COB is located.  Any
> thoughts?
>>>Matt states the Centre of Buoyancy is near where your navel is. Drop a
perpendicular from your belly button and mark the seat with a waterproof
marker pen!<<<

Not necessarily, lots of kayak designers can't figure it and some who can
may purposefully put the seat in another position for a reason. One kayak
company, who's new models are rudderless, trims them very stern down and I
suspect this is to make them handle better and have less need of a rudder in
most conditions. One reason we don't do this (except as a position of the
sliding seat) is that there is a drag penalty, at least at faster speeds,
with a stern heavy trim. I learned this from doing sprint tests with the
sliding seat in various positions. Our original Mariner had an 18" slide
range with trim in the middle. With my 180 pounds 9" back from level trim I
was 4% slower than at level trim (which was the fastest). Nine inches
forward of trim only lost me about 1/2%. Marking the water level at the
stern in both extremes of seat position in this 16' or so long waterline
showed a 4" vertical difference. That created quite a trim change. Olympic
flatwater kayaking experiments have also shown trim to be the fastest,
although at one time many believed a bow heavy trim kept the bow from rising
as much at hull speed (and that would raise the hull speed limit) the test
didn't work out that way. yes the bow didn't come up as high, unfortunately
it plowed deeper into the bow wave apparently at greater cost in efficiency
than if it could rise higher.

 >>>In practice I am guessing that the location of the CoB along the
length of the kayak is around the same point as the Centre of Mass, which
may be wrong, but for most kayaks is probably OK for practical purposes.
Perhaps you could measure your kayak and put the data into a hull design
program, and have the CoB calculated.<<<

If I recall correctly, the center of mass and buoyancy are at the same point
(longitudinally and laterally anyway) when the boat is floating. If you move
the center of gravity the center of buoyancy changes accordingly because the
boat is free to move. In more simple terms the CofB is always directly below
the CofG. This can lead to some confusion though because the center of
gravity of a kayak you pick up is not the same as it is when it is on the
water with a person in it. A Swede-form kayak is likely bow heavy when
picked up at the longitudinal center of buoyancy (with a paddler in it) and
the opposite for fish-form. it would be impractical to pick up the kayak
with the paddler in it (maybe he could climb in when it was balanced over
the log though).

While it can be done mathematically with great effort (due mostly to all the
measurements that must be made). The math itself, Simpson's Rule, I believe
is quite easy. However, the easiest way I know of to determine the center of
buoyancy of most existing kayaks is with a level. Put the level in front of
you in the cockpit (given a reasonably straight or consistently curved keel
line in that area--that allows measurements with the level) and then move
around in the cockpit until the level is, well, level. Mark the point under
your crotch.
If using a hull design program it most likely will be able to calculate the
center of buoyancy for you (such as Paddlewise's Robert
Livingston's--available to download free on our website) .

> However, perception and reality can often be two different things. Until
we
> get some objective measured results with many different kayaks using both
a
> rudder and not using a rudder (by paddlers equally competent both ways) we
> don't have good data, only opinions.
>>>Very true. Even if you have the same paddler try paddling in two
different
boats in similar conditions, there are so many variables which are hard to
control that the trial will never be anything like a mathematical thought
experiment. Makes me wonder about those turning tests you conduct to compare
different boats. Perhaps one way to make trials better is to conduct many of
them. One thousand one mile downwind runs over the same measured course in
each boat tested, say. Since that sort of thing is so laborious it won't be
done, we only have subjective impressions of kayak performance to go by. I
wonder if an analysis of sea kayak race results would be useful. Perhaps
one-design sea kayak races, the same hull, variously fitted with rudder,
skeg, or none. <<<

I get excellent repeatability on retesting the same hull. This consistency
gives me a certain level of confidence in my timed results. I don't get
nearly as good consistency by counting strokes (although I still do as well
on the spin tests). The hardest part about your down wind experiment is not
that you would have to do very many trials to get reliable results, you
wouldn't if they were consistent. It would be the difficulty in keeping the
wind and wave conditions even reasonably consistent between even a few runs.
Time, wind speed and fetch control wave size. Fetch is the only variable
here that you could control.

> With kayaks that don't weather helm you probably ought to store the first
10
> to 20 pounds in  front of ones feet (if there is a possibility of extreme
> winds blowing off shore anyway) but after that the next 40 pounds should
go
> into the rear so you won't induce weather helm unnecessarily with a bow
> heavy trim.
>>>Hey Matt, this is a little different to your recommendation to simply put
twice the load in the rear as the front.
I think the loading and balancing has to depend upon the particular boat. I
still don't like loading differently for different conditions. You may load
up for a downwind run, say, and then want to return upwind. Or, the wind may
swing.<<<

Very little difference since my slight modification referred only to a rare
class of kayaks, those already balanced in a side wind, and we are only
talking about the first few pounds of the load before switching into the 2
to 1 stern to bow GOAL if loading heavier. It should be noted that this goal
is rarely achieved with a heavy load so it is very hard to overdo it and not
harmful to control if you do. The only penalty for overdoing it is just a
little more drag at fast speeds (but much less than a rudder adds). There is
no need to load a heavily loaded kayak any different for different
conditions. The only time a bow heavy condition would likely be an advantage
would be with a light load going into quartering head winds or head seas or
in winds so extreme one could barely get the kayak to turn into them (but
again with a heavy load this is not likely to be a problem because the ends
can't blow around quickly).

> Lifting the kayak to test the balance only works with light loads and/or
strong backs.
>>>I suppose there is some back damage risk. If I can do this, I am sure
many
kayakers can. You only need lift the boat slightly to check the balance.
Although I haven't tried this, two paddlers should be able to pass a 2''
sling under the boat and lift to check the balance point. It may be possible
to balance the boat on a log if there is one. A check is better than finding
the boat misbehaving out on the water.<<<

I've heard the teeter it over a log one before too. With a heavy load this
is asking for damage with many kayaks. It is also unnecessary. How many
times are you going to have to repack everything to get it right. Even if
you get the balance perfect the heavily loaded kayak is likely to handle the
more difficult conditions better if it is out of trim some towards the stern
(even though in reality this is very hard to achieve given the relative
storage volume available in each end if you make it your simple goal the
first time you pack you will end up with a better handling kayak the closer
you come to achieving that goal. What could be simpler. Load all heavy dense
items (and bags) in the stern and the light ones in the bow. It is an easy
simple to follow rule and you only have to load the kayak once, no lifting,
balancing or logs necessary. One more simple rule. Float the loaded kayak on
the water to see that it floats level side to side before getting in. If it
doesn't, shift a few liters of water from one side to the other until it
floats level. I like to take some of my water in clear 2 liter soft drink
bottles because they make this sort of thing easy (and they can be stomped
flat for more space when not in use and reconstituted like a balloon if you
want to refill them again).

Matt Broze
www.marinerkayaks.com
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