Re: [Paddlewise] Skeg Jammers etc

Peter Treby (>> <<) [mailto:ptreby_at_ozemail.com.au] responded to me (>):

> Appeals to authority don't much impress me.
>>Disregard for knowledge and experience is the opposite evil.<<

Not opposite at all. I've got nothing against authorities, as long as their
reasoning is clear. Rather, I object to: "The authority said it so it must
be right". Likely he is, but he could also be wrong. I want to hear the
argument rather than the name before I decide what I think.

> ... lots of kayak designers can't figure it ... One kayak
> company, who's new models are rudderless, trims them very stern down.
>>Which brand of kayaks are these? Good to see someone is bringing out
rudderless models.<<

I got this trim information through my drag test work for Sea Kayaker so I'm
not sure I'm at liberty to reveal the company. The reason is pure
speculation on my part, but I would bet on it. The drag calculations I do
for Sea Kayaker don't take the (likely) increased drag of non-level trims
into account. I'd like them to but I don't have any hard data that measures
the effect reliably enough. Anybody know if that data is available for ships
or racing kayaks?

>>Don't the centre of buoyancy and the centre of mass line up once the boat
is
on the water?  If the CoM is not aligned vertically with the CoB, doesn't
the kayak hull shift in the water until it is? I understand the CoB to be
the point through which the forces counteracting sinking appear to act. If
the CoM is measured along the kayak when it is out of the water, the
difference when the boat is on the water is the mass of the kayaker sitting
in it which may alter both centres. If the kayaker's centre of mass is
determined to be somewhere in the gut behind the navel, then that point
should be aligned vertically over the kayak's centre of mass to have the
boat float in trim.
Picking the kayak up and balancing it determines the position of the centre
of mass of the kayak along its length. Then plonk the boat in the water, and
the centre of buoyancy is at that point. Tell me if I'm wrong.<<

You're wrong. Let's take it to extremes (things are often clearer that
way--a trick my dad taught me). Put a huge amount of weight in the bow
(only) and find the kayaks center of mass by lifting it at different places
until it balances. Put the kayak in the water and now sit on that balance
point (for the sake of example, say the bow hatch). What's going to happen?
Well the center of gravity will still end up over the center of buoyancy but
the kayak might be nearly vertical in the water when it does so. In order to
keep the kayak LEVEL the paddlers weight would have to be further back (far
enough back so that the CofB and the CofM lined up when the kayak is LEVEL).

> 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.
>>Now I'm lost. If the centre of mass of the paddler and the kayak are
aligned
vertically, and the boat is at rest on the water, then the centre of
buoyancy will align vertically through both those centres, won't it,
whatever the plan shape of the hull?<<

Yes, always but the kayak may or not be level at that time. I may not have
been very clear here. Pick the kayak up empty, put a paddler in it on the
water.

>>Do you mean that because Swede form
kayaks likely have more storage room in the stern, and conversely Fish form
kayaks more storage room in the bow, that there is a probable difference in
the distribution of the load placed in each type? <<

No, I was talking about an empty kayak and how its mass is distributed. On
its own (no paddler in it) it would likely sit in the water with a bow down
trim. In order to get a LEVEL trim the paddler will have to sit a little
further back in it than in a fish form kayak.

> However, the easiest way I know of to determine the center of
> buoyancy of most existing kayaks is with a level.
>>Since the centre of buoyancy is at the centre of mass of the displaced
water, and both are aligned vertically when the kayak is at rest, why isn't
the centre of mass of the kayak on land a good approximation to the centre
of mass of the water is displaces when floating. Could you explain this use
of a level a bit further?<<

The level is to find out where the seat has to be in the kayak for the kayak
to float LEVEL on the water. You seem to be assuming that when the centers
line up the kayak will be LEVEL. But that can't be because the centers
always line up when a stable balance is reached but the kayak is only level
when that balance is reached and the kayak is still at LEVEL trim. The level
helps you easily (no math involved) find when that balance is reached and
the kayak is LEVEL.

> I get excellent repeatability on retesting the same hull.
>>You are way ahead of me. Every time I turn the boat radically, it spins
through a different arc. The turning effect depends on a lot of things,
including how close to having a cold wet ear I want to be.

True, so I need to try to be consistent based on some reasonable and
practical standard. Therefore, I have chosen to lean each kayak as far as I
reasonably can without taking on a lot of water (as if I don't have a
spraydeck that fits the kayak--sometimes none I have access to fit the
cockpit). A few splashes are okay but I don't want the water pouring in. In
other words I try to use a consistent criteria for the lean. Sure, if I can
lean more the turn will be tighter. Some kayaks have such poor knee bracing
or are so wide and stable that I can't lean them even that much without undo
risk of a capsize. So in those kayaks I lean as far as I can (or as far as I
dare to). Since other paddlers will be in roughly the same boat I think this
is a logical standard.

> Very little difference since my slight modification referred only to a
rare
> class of kayaks, those already balanced in a side wind...
>>That's something else to think about. What side wind? A kayak balanced in
a
10 knot wind will not be a kayak balanced in a 20 knot wind, for the same
forward speed of the kayak. I suppose a kayak designer should try for
balance in the winds a boat is likely to be paddled in. That will mean that
higher winds will weathercock a boat designed to balance in lower winds. And
a boat designed to balance at 3.5 knots paddling speed in 15 knots beam
wind, will drop off downwind when stationary in the same wind. What
approximate beam winds should boats be designed to balance in?

You are correct that a kayak balanced when moving forward will blow at some
angle downwind (where the forces balance) if it is not moving forward. First
you are not talking of a huge difference due to different wind speeds (or
over normal paddling speed ranges) and secondly you have things backwards at
higher speeds. Wind puts more pressure on the end of a long symmetrical
object angled into the wind than one angled away. Strong winds (force
increases at the square of the wind speed) therefore tend to reduce both
weather helm and lee helm. At zero wind speed no kayaks weather helm or lee
helm. The weather helm at first gets worse with increasing wind speed but
then gets less again in high winds. The worst weather helm is (and this is a
subjective guess) in side winds of about 10 knots and the range where it is
most of an issue are probably in the 5 to 15 knot range. Since this is also
a very common speed range for winds to blow, I try to get the kayak to
balance in this wind speed range. The speed of the kayak across the wind
doesn't matter as much to this balance as long as one is moving at a
reasonable kayaking pace.
If it still tend a little one way or the other then adjust the skeg or slide
the seat or use the rudder or some lean of the kayak to make it turn to
compensate. The closer to neutral you can get the basic kayak hull to be the
less energy will be required to make needed corrections. Our goal is to make
a kayak neutral when it is moving forward at a good cruising speed. Neutral
to the wind and neutral to the waves [in all axis of rotations--yawing
(weather helm and broaching), heave/dip, and side to side (dynamic stability
in waves)]. A neutral kayak is a kayak that is easy to control because you
are not fighting "tendencies" or "forces" all the time.

Matt Broze
www.marinerkayaks.com
***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************