Moment effect of canoe paddle stroke

[jesdreamer]

Videos of racing canoes/kayaks show some submersion or greater draft of aft area during each paddle stroke. If we disregard entry and exit and assume power is applied straight back in direction parallel to water surface at a point perhaps 2ft below water surface please explain how and why this force causes stern area of boat to sink or show increased submersion during the time paddle force is applied. I can explain why stern should sink but I can alternatively explain why bow should sink (stern should rise) -- one has to be wrong but which and why?? (I am assuming boat and paddler to be a single rigid or fixed structure)

 

[Ketch22]

Videos of racing canoes/kayaks show some submersion or greater draft of aft area during each paddle stroke. If we disregard entry and exit and assume power is applied straight back in direction parallel to water surface at a point perhaps 2ft below water surface please explain how and why this force causes stern area of boat to sink or show increased submersion during the time paddle force is applied. I can explain why stern should sink but I can alternatively explain why bow should sink (stern should rise) -- one has to be wrong but which and why?? (I am assuming boat and paddler to be a single rigid or fixed structure)

Are you questioning the paddle stroke as the reason for the boat to squat? The wave train that is produced by a boat is a dual wave compound, one always starts with a crest just aft of the bow, the other starts as a trough just forward of the stern. As the speed of the boat approaches the speed of the wave train the waves line up and the bow starts to float up on it's wave while the stern falls into its trough. High power vessels can climb this "hill" however low power vessels tend to stay very close to it's " hull speed." This is generally; Hull speed in knots= 1.34 x the square root of waterline length in feet.
Each paddle stroke at close to the speed will Accelerate the boat to this limit then it will decelerate between stokes. Giving the impression that the stroke is causing the squat.

 

[jesdreamer]

My question specifically referenced videos of racing canoes and racing kayaks. Racing canoe aft sinkage is less evident because the relatively large freeboard above water surface tends to hide the effect (deep hulls to help avoid swamping the open cockpit). Racing kayak videos clearly show severe aft sinkage during each stroke.These boats are operating at speeds far in excess of Hull Speed and videos in question show little or no bow wave as well as little if any stern wave -- they show no evidence of the hull sunk into the hollow between waves. Water surface is relatively flat and boat length behind cockpit actually submerges enough that if full decking was not present, boat would swamp. This seems totally non-related to hull speed effects.

 

[Ketch22]

My question specifically referenced videos of racing canoes and racing kayaks. Racing canoe aft sinkage is less evident because the relatively large freeboard above water surface tends to hide the effect (deep hulls to help avoid swamping the open cockpit). Racing kayak videos clearly show severe aft sinkage during each stroke.These boats are operating at speeds far in excess of Hull Speed and videos in question show little or no bow wave as well as little if any stern wave -- they show no evidence of the hull sunk into the hollow between waves. Water surface is relatively flat and boat length behind cockpit actually submerges enough that if full decking was not present, boat would swamp. This seems totally non-related to hull speed effects.

It is unlikely that one can somehow avoid the effects of the fluid. I do not know of any canoes or kayaks that exceed hull speed without additional input. In flat water the highest end canoe racers achieve close to 7 mph. Relatively human paddlers will often be closer to 5 mph. Let's just assume a 20 foot boat this would make the hull speed 1.34 x √20 = 5.99 knots or 6.89 mph. This speed is relative to the water, thus a down stream race where the water is moving at 3 mph will give the boat a possible speed of 10 mph. There is always outside inputs as well. If on a river or active wave system in open water the paddler starts to surf they can easily overcome the hull speed. The energy input to do so is from the wave not from the paddler though.

One must also consider that the wave train is a composite. The Bow wave is small and creates several small waves along the hull. The Stern wave is separate it is when the two trains coincide that the bow has more buoyancy and the stern less. Kayaks especially open water boats tend to sink more than canoes. With the small waterplane area of the finely tapered hulls the Pounds of buoyancy per inch of Immersion is dramatically less. Given that the weight of the boat is a constant the kayak has to drop farther to maintain buoyancy. With their fuller centerbody they will tend to rotate more in the center where they are fully supported.

 

[jesdreamer]

It appears to me that you are doing everything possible to escape my question --
First off, I believe most racing kayaks are around 16-18 ft, well below the 20ft range so speeds you quote would be well above hull speed.
Second, YouTube videos clearly show aft sinkage during each paddle stroke with racing kayak on apparently quite flat water --
Force of paddle blade some 2ft below water surface would appear to create a moment tending to rotate hull/paddler combo about a pivot point located somewhere --
1) -- Does paddle blade force create a moment tending affecting the boat/water relationship??
2) -- If so, does this moment tend to affect boat trim??
3) -- If so, does it tend to depress aft end of boat or bow end??
4) -- If so, approximately where is pivot?? (somewhere within paddler body or thereabouts??, up toward bow??, back toward stern??
5) -- At speeds in question is there a Bernouli effect behind widest area of hull with each surge in paddle related speed?? Enough to increase draft out toward stern?? (within the area where cross section volume reduces from max to zero)
6) -- If both effects might be present, is there any way to estimate relative importance as far as aft area draft is concerned??
7) -- If aft draft increases with each stroke as illustrated in YouTube videos, Might the pusulating or periodic increased trim angle retard speed??

 

[Vedward]

I didn't read your last post, too complicated. The power of the stroke comes from the shoulders & arms which are well above the centerline of the kayak. This means a torque is being applied to the kayak. I can vision this as a rotation that causes the aft to drop, but since there are some many steering components to the stroke, the front could drop also. My main point is that the canoe or kayak is not a simple line in the water, but a 3-D object with other moments and forces under consideration.

 

[Ketch22]

Jesdreamer You are asking some good questions that I will restate to your points after a bit of exploration.
The moments that are produced on a kayak are complicated to say the least. They are very small with a recreational paddler involved and often go over looked. As a person gains skills and/or takes classes the technique affects all of your questions. Higher end kayaking requires proper positioning, this would have the feet engaged at the foot braces and pressing down slightly at the heels and the thigh braces to engaged with the top deck at a similar pressure to the feet. To maintain control the paddler needs to fully engage the rectus abdominis muscles and the spinus erectus this will isolate the core from fore and aft movement. The obliques however are somewhat loose and utilized to assist in the rotation of the shoulders for full paddle stroke.
The function here is to develop a moment in the legs that opposes the moment developed in the upper torso. While paddling, the hand that is closest to the water is pulling back mostly and the opposite hand is bracing and actually pushing forward a bit. In a video one can often see the "uphill" hand relaxed with open fingers which is common to assist in circulation. This opposition however creates a relatively solid couple between paddle and shoulder.
It is also in this phase that a paddler varies the length and twist of the paddle to make course corrections and turns.
In the forward stroke phase the leg moment is balanced against the shoulder moment. The net force is against the fulcrum of these two forces. That point is usually 2-4 vertebrae above the pelvis depending on paddler physiology. This point is held rigid by the engaged core muscles and so the net moment is the force developed by the paddle engagement at a distance of (let's assume) 3 vertebrae above the pelvis to the actual waterline of the boat. This being forward in direction and above the waterline it will tend to increase the draft forward.
As the legs are oriented fore and aft and the paddle stroke variations are across the beam Yaw moments are quickly developed and allow easy control.

So along with your numbers:
1) Yes, the moments developed by the paddle will always affect the hull/water relationship. It will tend to depress the bow in forward paddle but easily overpowers the hull in other axis.
2) Only in theory, a typical high efficiency Kayak will have about 5lbs of buoyancy per inch submersion forward of the midpoint. If a paddler were to develop 6lbs of force against what is normally a 2-3 inch lever the affective change would be approximately 1/2 inch of draft change down by the bow. This assuming that the paddler could actually make a perfect stroke without compensating for the change they would feel.
3) Already been covered in this response.
4) Already been covered in this response.
5) Bingo, two points for a perfect interpretation. It is the Bernoulli effect that creates the wave trains that the boat cannot escape from. As the hull narrows the water must fill in the void that is developing. This is part of the trough that leads the stern as it is impossible for the water to react until after the imbalance is apparent. (please stay to the end for more on this)
6) There is but it is quite complicated. Once again please stay until the end
7) Bingo, You are so close to the answer, The angle (at a very minute level) has a huge effect on the speed and the power requirements of the boat.

So with all that has gone before one must see that I have consciously and often referred to the kayak as a vessel or boat. This is on purpose as they are just little boats that operate with a limited power source and inside a carefully prescribed envelope. There are many variables that affect each other and composite power requirements. The aforementioned hull speed formula is actually just an approximation, for each hull type and shape the real world variable must be experimentally determined in a test basin. Most important is the wetted surface as it is the largest predictor of drag. In actuality an increase in waterline length almost never creates the theoretical increase in speed as the wetted surface drag develops faster than the speed gain increases.
Interesting enough look at a few videos of large freight vessels. The Bulbous bow (a large knob on the front of the ship) if properly designed can achieve a 10-15% increase in fuel efficiency (please read as reduced Hp requirement) just by the action of reducing the bow wave.
You are spot on in assuming that it is the draft change that is causing the speed "limit." It is coming from the wave train and the vast difference in density between water and air. When Paddling a good paddler can momentarily exceed the hull speed as a small light boat only requires a small amount of power. However this power requirement is non-stop and the paddle stroke is intermittent. As soon as the paddle is reducing power the hull drops below the "limit" as the drag is continuous.
I can demonstrate several GPS tracks where my average speed is close to hull speed but my demonstrated speed, even in smaller portions, is not above. This is precisely where the debate of Fishform (waterplane area biased forward) vs Swedeform (waterplane area biased aft) comes into play. However that is part of another discussion.

 

[Baluncore]

Force of paddle blade some 2ft below water surface

The paddle blade is only just below the water surface. It would never be 2 ft below the surface.

Rotation of the paddle shaft during a stroke gives a varying sideways force that eliminates directional moment. That is why a “single Canadian” = C1, with a single ended paddle, travels in a straight line while being paddled continuously on one side with a J-stroke.

The kayak sits in a long curved hole in the water. The forward thrust of each stroke immediately pushes the nose up onto the water ahead while the stern drops into the hole previously occupied by the centre of the hull. The water then moves to reposition the hole under the boat.

Slalom kayaks exceed the hull speed when cutting into or out of eddies. Because the stern immediately buries itself in the water, the effective length of the boat is reduced which makes it possible to spin the boat to face quickly in a new direction. The stern is deliberately low buoyancy and the paddler leans backwards, so the stern will sink in that situation. The rear deck is flattened or even concave so as it surfaces, leaning the hips and boat left or right will fine tune the exit direction. Keeping it level will accelerate it forward as the stern pops up from underwater accelerating it in the new direction.

 

[CWatters]

The boat accelerates during the power stroke. The centre of thrust (the blade) is lower than the centre of mass and/or centre of drag so that would cause a torque that rotates the boat bow up/stern down.

 

[Baluncore]

The boat accelerates during the power stroke. The centre of thrust (the blade) is lower than the centre of mass and/or centre of drag so that would cause a torque that rotates the boat bow up/stern down.

The mass of the paddler is being continuously repositioned relative to the centre of buoyancy of the hull. The paddler weight is maybe 90kg and the boat less than 15kg. The 3D paddle forces and the changes in those forces during each stroke of an experienced paddler is very complex. Boat pitch rotation due to paddle moment is not really important when considered along with things like lifting the paddle from the water at the end of a stroke. A beginner is fearful of capsize and so gains stability by allowing the paddle to rise to the surface by the pressure of water from below as the boat moves forward. That actually slows the boat down. A more experienced paddler forcibly lifts the blade at the end of the stroke so that water flows backwards from the upper surface of the blade. Additional thrust can be obtained compared with a beginner's more relaxed stroke. That vertical paddle force is applied at the end of the stroke, behind the CofM and buoyancy, so will tend to pull the stern downwards as it applies additional forward thrust.

The best paddlers blend their complex strokes smoothly, so they do not look like they are doing anything special. They use their brain and muscle memory to control boat balance and attitude, then allow the water to do the work. Other paddlers fight the water all the way and make it look really exciting, but they exhaust easily in big water and do not win in competitions. Anyone who thinks they can explain changes in boat attitude during a stroke with a single vector paddle force needs to learn to paddle a kayak on a rapid river, or by surfing waves at the beach.

 

[jesdreamer]

As a retired ME I have developed a hobby of designing and building small paddle powered boats -- not really kayaks but also not really canoes. I have felt the aft sinkage performance during each stroke to be a significant engineering challenge. And I and have found at least one significant observation or analysis in each of the posts to date -- Some really good points have been made. I guess this forum does not notify posters of new posts so I missed the later posts which really do get into the problem and pose some very understandable actions and reactions --
1) -- Ketch22 on 9/4 -- "As the speed of the boat approaches the speed of the wave train the waves line up and the bow starts to float up on it's wave while the stern falls into its trough. High power vessels can climb this "hill" however low power vessels tend to stay very close to it's " hull speed."
2) -- Ketch22 on 9/5 -- "Kayaks especially open water boats tend to sink more than canoes. With the small waterplane area of the finely tapered hulls the Pounds of buoyancy per inch of Immersion is dramatically less. Given that the weight of the boat is a constant the kayak has to drop farther to maintain buoyancy. With their fuller centerbody they will tend to rotate more in the center where they are fully supported."
3) -- Ketch22 on 9/6 -- "The net force is against the fulcrum of these two forces. That point is usually 2-4 vertebrae above the pelvis depending on paddler physiology. This point is held rigid by the engaged core muscles and so the net moment is the force developed by the paddle engagement at a distance of (let's assume) 3 vertebrae above the pelvis to the actual waterline of the boat. This being forward in direction and above the waterline it will tend to increase the draft forward

"Yes, the moments developed by the paddle will always affect the hull/water relationship. It will tend to depress the bow in forward paddle

"a typical high efficiency Kayak will have about 5lbs of buoyancy per inch submersion forward of the midpoint. If a paddler were to develop 6lbs of force against what is normally a 2-3 inch lever the effective change would be approximately 1/2 inch of draft change down by the bow"

"It is the Bernoulli effect that creates the wave trains that the boat cannot escape from. As the hull narrows the water must fill in the void that is developing. This is part of the trough that leads the stern as it is impossible for the water to react until after the imbalance is apparent.

"The angle (at a very minute level) has a huge effect on the speed and the power requirements of the boat. (Ketch22 is referring to trim angle during aft sinkage during each stroke)

"Most important is the wetted surface as it is the largest predictor of drag. In actuality an increase in waterline length almost never creates the theoretical increase in speed as the wetted surface drag develops faster than the speed gain increases.

"it is the draft change that is causing the speed "limit. (Ketch22 is referring to the trim angle resulting from aft sinkage during each stroke)

"This is precisely where the debate of Fishform (waterplane area biased forward) vs Swedeform (waterplane area biased aft) comes into play. However that is part of another discussion. (I would like to get into that discussion as related to laminar vs viscous drag along hull contour)
4) -- Baluncore on 9/7 -- The kayak sits in a long curved hole in the water. The forward thrust of each stroke immediately pushes the nose up onto the water ahead while the stern drops into the hole previously occupied by the center of the hull. The water then moves to reposition the hole under the boat. ( A very sophisticated explanation & exquisite way to look at the situation)

"Slalom kayaks exceed the hull speed when cutting into or out of eddies. Because the stern immediately buries itself in the water, the effective length of the boat is reduced which makes it possible to spin the boat to face quickly in a new direction. The stern is deliberately low buoyancy and the paddler leans backwards, so the stern will sink in that situation. The rear deck is flattened or even concave so as it surfaces, leaning the hips and boat left or right will fine tune the exit direction. Keeping it level will accelerate it forward as the stern pops up from underwater accelerating it in the new direction. (Question was for straight ahead motion -- but the Slalom proceedure might help shed light on design)

So if we consider each of the highlights above it seems as though paddle stroke tends to dive the bow but actual result at or near hull speed is increased aft draft as hull area behind max beam sinks into wave trough while bow is riding up on the front wave -- yielding trim angle which makes the hull climb uphill -- This seems to make sense (at least to me) --

Now if you have read this far I have some thoughts we might debate --
1) -- Cut off back end of the hull per Kamm theory and take advantage of the wake hollow where water thinks the hull has the added length for wave making action of the longer hull but with friction drag of the shorter hull
2) -- Widen hull at rear to get more buoyancy and less sinkage during paddle stroke
3) -- Narrow down the bow for less buoyancy, use reverse front edge as with current naval practice and America's Cup along with lots of tumblehome so bow dives & cuts into facing water instead of riding up -- all to get negative trim during paddle stroke so hull drives downhill instead of uphill
4) -- Ketch22 discusses kayaks as being so narrow that they have little on no excess bouyancy and almost operate fully submerged. He also describes friction as the major drag factor -- Off and on I think a wider boat might be better as it might sit more on top of the water but it does take a lot of wetted area to get such an arrrangement -- which approach would net the lesser wetted area??

 

[Baluncore]

The top speed of displacement hulls is fundamentally determined by length.
The rules for kayak competition hulls requires wetted surfaces be convex.
We paddle a compromise when those two are combined.

( Enable email notification. When you do you will get one email. If you do not visit the thread it will not send any more. Reading the email is not sufficient. If you delay your visit and fail to notice another page has appeared, it will assume you have not read the posts, so will not send more emails until you do.)

 

[jesdreamer]

The top speed of displacement hulls is fundamentally determined by length.
The rules for kayak competition hulls requires wetted surfaces be convex.
We paddle a compromise when those two are combined.

Skilled kayak paddlers consistently exceed "Top speed" as determined by hull length. "Top speed" of displacement hulls is defined as a function of length because as speed increases it will reach a point where rear of hull starts dropping into trough (lower water level) between bow and stern waves and this occurs at higher speeds with longer displacement hulls. This creates a trim angle of hull as bow tries to climb up the bow wave so hull has to be propelled "uphill" and this condition demands a major increase in power -- this is referred to as "hull speed". Since this slight trim angle is so critical in causing major increase in power required, I have become interested in the stern submersion of racing kayaks during each stroke since it appears to indicate a self generated similar trim angle -- And I visualize a drag effect similar to that at displacement "hull speed" --

Shorter hulls and those with transoms have a greater chance of getting into Semi-displacement mode where instead of 100% support by water displaced, there is some % of hydrodynamic support due to forward motion -- I realize that transoms carry their own set of drag complications.

The rule requirement for convex surfaces would not prevent significant tumblehome curvature from bottom of hull up toward gunwales -- Instead of the typical cross section of increased beam with elevation from keel, we could have a wide beam at keel and progressively narrower beam as we progress up toward deck -- a side benefit would be improved paddle access into water w/o paddler having to reach outward so far

 

[Baluncore]

Skilled kayak paddlers consistently exceed "Top speed" as determined by hull length.

"Top speed" is not defined without specification of power. Hull speed is dependent on the power available to push the hull up the hill and out of the hollow.

 

[jesdreamer]

"Top speed" is not defined without specification of power. Hull speed is dependent on the power available to push the hull up the hill and out of the hollow.

My statement that an experienced paddler can consistently exceed "top speed" as dictated by hull length was in error -- What I meant to say was that an experienced paddler can consistently exceed "Hull Speed" as defined by hull length. All the references I have read define "Hull Speed" as dependent on hull length ALONE w/o any reference to power which may or may not be available -- It is generally accepted that increased hull speed requires more power and when "Hull Speed" is reached there would be a very large increase in power required in order to go any faster. It is also generally accepted that this large increase in power required to exceed "Hull Speed" is as Baluncore states -- "to push the hull up the hill and out of the hollow"

Since the power required to get the hull to overcome this resultant minor change in trim angle at "Hull Speed" is so great, my original post was in quest of an understanding just why aft area of a kayak shows increased draft (increased trim angle similar to the "Hull Speed" situation) during each paddle stroke -- it seems to me that paddler is creating his own trim angle and related increased drag to overcome during each stroke --

 

[Baluncore]

There are many different kayak designs for different speed and manoeuvrability requirements.
The minimum length and the width of a competition kayak is specified in the rules for the event. In slalom kayaks the paddler adjusts the trim angle (as you call it) by leaning forwards or backwards, the ends of the boat are deliberately low buoyancy so the trim can be more easily adjusted. They have long points, to meet dimension requirements. The ends of the boat also have minimum radius dimension.