Canoe Sprints

Paddlers exert large forces on the footrests which are forward of centre.

I'd be worried about the strength of a hull that can be deformed by the
water pressure a couple of centimetres below the surface.

At high speed you can see their skins ripple. Apparently this reduces drag in
turbulent flow they experience. I'm not sure if it is automatic or a result of
feedback: I suspect the former.

 

When I was interested in canoes and kayaks in the early 80's I remember reading the racing boats were made as light as possible for very limited use, they maybe lasted only a few races. Obstacle course kayaks especially. Puncture resistance was low and there were stories of emergency repairs in between heats, Some glass and resin slapped on and the hulls rolled around in the campfire to set it all off in time for the next race.

I wouldn't think durability would be a priority issue in those sprint boats, but hull shape/form would be #1. Reasonable durability is easily achieved. The loads on them are very specific, and so the layup could/should be very specific.

I think it would be easy to create a layup that would be light and stiff where it needed to be, say 80% of the boat a stiff epoxy/graphite/foam composite that would blend into areas of a flexible, straight epoxy/kevlar layup of various sized kevlar layers to give varying flexibility.

I think the rippling on dolphins is automatic also, as in layers of fat rippling instead of a conscious muscular action. I wonder if it helps or not, as in, dolphins are fast, but if their skin didn't ripple, would they be faster yet?

Is it a moving series of ripples, or is it a static indentation that occurs at speed? A static indentation at speed is what I envision in the canoe idea. Something that "goes with the flow", "follows the path of least resistance", "rolls with the punch", etc.

Imagine two flexible strips that were attached on the ends hinge like, and at rest were in the shape of say the outline of a football. If that was towed through the water by one end, would the resultant shape be the most efficient shape for moving through the water at X amount of speed, or is that shape just reaction to forces and nothing to do with efficiency?

 

In fact, as a sprint canoer who has paddled these boats, the newest ones made by Plastex, called the Fighter, are made with flat parts running the length of the boat effectively making 3 Keels instead of a U shape. Lift is actually a semi planing, but since the technique of the sport promotes boat bounce, this keeps the boat running smoother when up to speed.

Sabs

 

there is a rule that does not allow any device that would aid a paddler, and this would probably fit under that. Generally, wel like our boats to be a stiff as possible because any deformation can be felt through the boat and can mess you up while paddling.

boats are made in a vacuum mold using carbon and PVC foam around 5mm thick at the beam and double foam above that. Generally they are two part construction, but now they claim to be one piece, which actually looks like they just sanded off the seam.

as for lightness, boats must be certain weights for each event. C1s are 16kg, K1 are 14kg

 

This never applied to beam at waterline. They were diamond shaped so the paddler could draw his blade closer to the centreline of the boat and therefor be more efficient when paddling. This is why they had those radical shapes in the late 90's and early 2000s. They had to retain the beam, while trying to make the boats narrower and had to maintain having the front of the first cockpit, or front wave guard in canoes, be the tallest point of the boat.

Sabs

 

Sabs

I'm not sure what it is you're trying to say?..other than the boats are made out of facets.

It doesn't alter the basic hydrodynamics of the boat, they are not planing.

 

I'm saying that the facets keep the boats from pitching as much as they did before and make them run further out of the water than they did before. I know they're not actually planing in the technical term, but they are not traveling though the water as deep as before.

 

I don't follow?

The principal factors affecting pitching is the GM. If the boat is ostensibly the same size and displacement, other than going from U shape to facets, you are not altering the the Kxx (mass + added mass) nor the GML.

Thus the only aspect that "may" be different, but highly questionable to what degree, is the damping. Unless you're suggesting the facets "act like" bilge keels???? If so, what evidence is there of this and to what degree does it effect the damping coefficients?

 

First let me apologize for not knowing all the physics and math behind what I am trying to explain.

But what I'm saying is that isn't a sailboat. The boat is 17 ft long and 1 ft wide and I weigh 180lbs, so when I paddle the boat at 60 strokes a minute, the boat will begin to bounce up and down in the water because I am kneeling in a straddled position transfering my weight from my kneeling knee to my paddle and front leg. The facets on the hull (in my experience of paddling the boat) keep the hull from diving down the way it usually does. In the paddling world, we mistakenly call this a feeling of planing because usually the boat will sit much deeper in the water.

Here's a link to a youtube video and you can see the boats bouncing.

http://www.youtube.com/watch?v=ZjM6mE7FyGI&feature=relmfu

Notice how the ones labeled plastex (generally with a flatter wave guard) pop up out of the water faster and how the nelo stay deeper longer.

Unfortunately these faceted plastex boats are way less stable than the nelos.

 

Sabs

That’s ok no need to apologise, now I can see from the clip what you’re referring to.

Pitch and in this cause the effects, trim, dynamically, are related to several factors:

1) Inertia Moment
2) Damping Moment
3) Restoring Moment
4) Forcing moment.

I wont go into the maths or physics too much. But term #1, relates to the mass and added mass (the amount of water the moves with the boat, in a simple sense), #2 damping from waves, air, friction pressure etc, #3 restoring this is simple the righting lever (or how much ability to prevent capsizing) and #4 forcing, an external force not related to the boats characteristics, but is then added to the boat, and #1-#3 shall react in “some manner” to disturbance in #4.

However to simplify things. If the boat was level trim….just simply floating, if you added a weight up fwd, by a piece of string hovering above the boat and left this weight on the boat, the boat would alter its trim. The centre of gravity would move fwd, owing to the weight. This means the centre of buoyancy, to remain in equilibrium, must also move fwd. The result is a simple trimming moment, so the boat trims. In this case by the head, the boat has less draft aft and greater draft fwd.

If you now removed the weight, by lifting it, with the string, the boat shall return to its original level trim. This is now no outside ‘force’ disturbing the boat.

The factors that effect the trim, are dictated by the BML, which is simple the Waterplane Inertia/volume. This value is added to the distance of the KB (centre of buoyancy). You then take away the KG (centre of gravity) to arrive at the GML, what is called the longitudinal GM, or metacentre. The greater this value the greater the “stiffness” or resistance to trim if you will.

When that weight is added it causes a trimming moment. To calculate how much trim the boat shall experience, we have what is called the MCT or moment to change trim, by 1cm. The principal factor effecting this, is the GML.

So the greater the GML the less trim, for a given weight that is added.

If you now added this weight again to the fictions boat but once placed on it, immediately removed it, the boat would pitch, oscillate, or in your words, bounce. It is no different to pushing down the front corner of your car then letting go..what happens..it bounces up and down …the better the shock absorbers the less bounce. On a good car the front of the car would bounce once and stop. On a knackered shoch, or a typical American car that is designed only for straight line driving, the car would bounce up and down a lot before coming to rest.

So the shock absorber, or “damper” has an effect on the motion of the car as you press it down. You can feel this going over bumps too.

This is basically #2, in the list. The factors that change or alter the amount of damping is complex, but suffice to say, you wont see any appreciable difference between a U shape and a facet shape of boat like those shown in the video. However, the rate of oscillation (or bouncing) has a “fixed” vale. This is unique to each boat. This is called the period of pitch.

The period of pitch has one of the principal factors that effect it as, yup, you’ve guessed it, the GML. Change the GML considerably and you change the natural period of pitch.

So, what you are experiencing is the combination of a simple trimming moment by the “additional” weight being added (the reaction force of pushing the paddle and moving with your knee fwd), the natural period of pitching of the boat both being combined into one, which is called dynamics.

The facets could be U shape or any other shape. All this part is doing is providing buoyancy. If the displacement of a U shape and facet shaped boat is the same, then the buoyancy shall be the same. The only difference shall be the longitudinal distribution of the buoyancy. The actual effect this has is pretty much negligible, on such boats. The restoring moment (#3), the GML will only be altered by changing the inertia of the waterplane area.

If the buoyancy is the same, and the only difference is the U to facet shape, the change in inertia of the waterplane area, again, shall be negligible.

If the boats you used were much shorter and fatter, then of course things would be very different. But on the long slender boats you use, it makes very very little, if any, difference.

However, as a proviso. If the facet shape has more buoyancy available, above the static waterline than say the U shape, then this does, to a small degree, change the motions. But on a boat of this type/size, not a great deal.

PS...if a very tall heavy person (say 2.20m and say 150kgs) ran a race with a very small light person (say 1.5m and say 40kgs) , the "bouncing" would be different..as this would effect the GML

 

Is there any difference in pitching between a boat with zero average velocity and one with a non-zero velocity if the what ever is forcing the pitching is the same?

Is the unsteady/time dependpent part of the pressure distribution opn the surfadew of the hull the same when the pitching at zero average velocity compared to pitching at non-zero average velocity?

 

Yes, because the period of encounter is different. How much is another matter.

Little unsure what you’re saying owing to the spelling/grammar.

However, to an extent yes. Since whatever is causing the disturbance, occurs at different longitudinal hydrostatic pressure distribution from that which is stationary to that which is not stationary; which may or may not, also alter the waterplane inertia too.

Also, the damping coeff’s of heave and pitch are affected by the encounter frequency of the oscillation.

In other words, there is a reduction in damping with an increase in frequency of oscillation.

But as before, how much is another matter.

 

(I've learned to be very careful with my choice of words and sentence structure with posts in certain threads here.)

Consider the situation with a boat moving in calm water with a weight oscillating internally causing significant pitching (such as an occupant moving fore and aft), and the frequency of the oscillations being independent of the boat's velocity through the water. Will the magnitude of the oscillations be independent of the velocity of the boat through the water?

 

If the weight is “swinging” back and forth then the angular motion describing its period of oscillation shall be affected by the fwd movement to some extent owing to it changing the radius of gyration about a given axis. Since its location from the centre of rotation is constantly changin.

It also alters the restoring moment since the weight may be on the ‘active’ side when needed. In the same way active water anti-roll tanks do.

But how much depends upon other factors. Since a 1tonne weight moving on a 10,000 tonne vessel shall remain unaffected. Whereas a 1 tonne weight on a 10kg boat shall be greatly affected.

 

All true. But I asked whether there will be any difference in the amplitude of the pitching as the average velocity of the boat through the water changes if the boat, the moving weight and the amplitude of the movement does not change? And the weight and amplitude of it's oscillation would be large enough to cause significant pitching.