# stability with speed

Discussion in 'Stability' started by griff10, Aug 30, 2009.

1. Joined: Aug 2009
Posts: 6
Likes: 0, Points: 0, Legacy Rep: 10
Location: rockford, IL

### griff10Junior Member

This is one I don't think I've ever seen posted. Most know that boats tend to be more stable when they are moving compared to at rest. Been working on designs that are extemely narrow and now this is getting important to understand (for me). Can anyone explain why this is?

Thanks,

Bill H.

2. Joined: Oct 2008
Posts: 6,227
Likes: 417, Points: 83, Legacy Rep: 2488
Location: Japan

"...Most know that boats tend to be more stable when they are moving compared to at rest..

can you explain what you mean by this?....and how this relates to being 'extremely narrow', in your definitions/understanding

3. Joined: Aug 2009
Posts: 6
Likes: 0, Points: 0, Legacy Rep: 10
Location: rockford, IL

### griff10Junior Member

stability

Well the boats I've been using are less than 20" wide and nearly 20' long. They are totally unstable at rest and are fairly stable once moving. Some forces obviously are at play here that I don't understand.

Look at Olympic kayaks and Surfskis for examples.

Bill H.

4. Joined: Sep 2007
Posts: 1,481
Likes: 61, Points: 48, Legacy Rep: 304
Location: SF bay

### Squidly-DiddlySenior Member

When water is flowing past the hull, and the hull trys to roll

the force of the water running past it will want to continue in the same path it was traveling a split second before.

That is a force that doesn't exist on a stationary boat.

The fact that the hull has already altered the flow of the water somewhat doesn't matter, what matters is the force and momentum of the flowing water at that instant the hull tries to roll. This will be both positive and negative pressures.

Also, certain aspects of certain hulls may be set so that at speed and 'at level' equalized pressures are set up, and putting the hull out of level will increase pressure on the downward side of the hull.

This is sort of like a 'stabilizing sail' on a boat to keep it from rocking. The sail might cause the boat to heel to one side, up if it heels too much the wind will be spilled, and if it is upright the pressure will increase. Better to be heeled to one side and have less rocking and rolling.

All this is just my own 'feeling' on this issue. I may be part or completely wrong.

5. Joined: Oct 2002
Posts: 3,486
Likes: 95, Points: 58, Legacy Rep: 1148
Location: netherlands

### yipsterdesigner

think you mean dynamic stability?
a boat moved by a force experiences different motion than at rest
on choppy waters you be way better off in a moving boat

1 person likes this.
6. Joined: Aug 2009
Posts: 6
Likes: 0, Points: 0, Legacy Rep: 10
Location: rockford, IL

### griff10Junior Member

stability

Actually in tippy kayaks on flat water it's more stable while moving than sitting at rest. I'm sure this is magnified because the boats are so narrow and the degree of stability is so low to start with. Expert kayaks that actually have any positive stability at all (some have negative stability) have that in the range of 5 or less ft/lbs., but they all gain stability when moving.

Bill H.

7. Joined: Dec 2007
Posts: 2,565
Likes: 113, Points: 73, Legacy Rep: 1650
Location: Finland/Norway

### TeddyDiverGollywobbler

Can you get yourself to reed C A Marchaj, Seaworthines The Forgotten Factor? There's a lot to read about stability issues (too much to quote here).
Static vs dynamic stability pages 112 to 121.. Some also relevant with canoes.
Teddy

8. Joined: Oct 2008
Posts: 6,227
Likes: 417, Points: 83, Legacy Rep: 2488
Location: Japan

yipster is correct, you're talking about dynamic stability, stability when underway.

Many factors are at play when considering at rest and moving. Since at rest, there is no paddle in the water. This paddle provides directional stability as well as it own restoring force on the hull when heeled. The paddle is not always providing horizontal thrust, there is a vertical and transverse component during the swing too.

The hull shape too...and hence the relationship between the KB and KG, or centre of buoyancy and centre of gravity. Then how these are affected with heel angles (and in some cases list too, since the paddler/rower is moving transversely too). For a long slender monohull, the dynamic stability, that is the work done in heeling the boat through an angle, is the product of the angle and displacement, or immersed volume. Since this is also affected by the hull shape above the waterline, this plays a major factor too. Some long slender canoes are different, some have minimal above water hull shape some have a lot, this affects the restoring moment too. Also how the person is sitting in the hull affects the KG which affects the aforementioned. The same is true longitudinally too...this plays a part in yaw etc

It is not so straightforward many aspects of the hull, shape, KG and KB are all at play, not to mention the addition of an external force and its resluting effects, so to speak, of the paddle, which are not present at rest.

9. ### Guest625101138Previous Member

Bill
There are two things worthwhile trying to get first hand experience with dynamic stability.

1. Try riding a bike with rear wheel steering. Or a bike with reversed geared steering.
2. Try riding a surf ski with a bow mounted rudder.

There is a very narrow pedal boat called Wavebike that uses dynamic stability once under way. The hulls are only 12" wide and 20ft long. It would operate in surf with a strong rider. It has a huge rudder for its size so very powerful righting forces. Having the high seating position helps with dynamic stability because it increase the roll moment of inertia so the response time is less demanding.

There are small outriggers on the Wavebike with spring release so the deploy once released.

Narrow hulls are very similar to bikes once under way although the point of application of the forces is different.

Rick W

#### Attached Files:

• ###### DragRace_2.jpg
File size:
76 KB
Views:
701
10. Joined: Mar 2005
Posts: 3,644
Likes: 185, Points: 63, Legacy Rep: 2247
Location: Pontevedra, Spain

### GuillermoIngeniero Naval

I didn't know kayakers call 'dynamic stability' to the leaning of the body to right the boat. (The essential sea kayaker: the complete guide for the open-water paddler)

But talking a kayak's increased transversal stability under way due to lift, or hydrodynamic stability (the term 'dynamic stability' has a different meaning in naval architecture, related to righting energy derived from the static stability curve), it depends on dynamic pressure and drag acting on the hull, in a, let's say, similar way the gyroscopical force acts in a bike. It will greatly depend on type of hull section (and, of course, speed), presenting chined hulls the stronger effect and rounded ones the lesser, but it is always small.

It seems Waterbike's stability rather depends on a canard fin under the hull directly beneath the WaveBike's handlebars.

A couple of good links to kayaks and the like stability.
http://www.guillemot-kayaks.com/guillemot/information/kayak_design/kayak_stability

Cheers.

#### Attached Files:

• ###### WaveBike.jpg
File size:
19.8 KB
Views:
2,247
11. Joined: Oct 2002
Posts: 3,486
Likes: 95, Points: 58, Legacy Rep: 1148
Location: netherlands

### yipsterdesigner

good links Guillermo. apart from kayaks let me trow in the new giro stabilizers
i was under the impression they are for at rest comfort at sea
as at speed dy or hydrodynamic forges to an extent allready neutralise heave pitch yaw etc
dont really know if those giro stabilisers are fully used underway..
a quik google say's yes http://www.seakeeper.com/applications_luxuryyachts_photogallery.php
its sort of relevant to the thread but me, no i am not convinced i want one

12. Joined: Mar 2005
Posts: 3,644
Likes: 185, Points: 63, Legacy Rep: 2247
Location: Pontevedra, Spain

### GuillermoIngeniero Naval

13. Joined: Oct 2002
Posts: 3,486
Likes: 95, Points: 58, Legacy Rep: 1148
Location: netherlands

### yipsterdesigner

almost every year, send me a pm please

14. Joined: Aug 2006
Posts: 3,497
Likes: 146, Points: 0, Legacy Rep: 2291

### ancient kayakeraka Terry Haines

It is not just transverse stability that is affected by kayak speed. As a kayak picks up speed there is a noticeable reduction in the yawing caused by the double bladed paddle. I estimate the improvement can be as much as 2:1. It is more noticeable in short, beamy kayaks with hard chines and keels or other longitudinal edges than in long, skinny rounded ones.

15. Joined: Oct 2007
Posts: 2,936
Likes: 139, Points: 63, Legacy Rep: 1593
Location: Arlington, WA-USA

### PetrosSenior Member

I have spent a lot of time in narrow sea kayaks and have notices this issue. The yaw stablity increases because when the hull yaws it generates a large vorex off the stern, creating drag that want to pull you back in-line. This effect is greatly noticeable if the hull is equipped with a skag or even a small strake or similar device that generates a vortex off the rear when yawed.

I suspect, especially in a hard chinned hull, that you get similar vortexes along the skin increase in strength with speed, that also help the roll stablity. and this is what you are feeling as you increase speed. Evan a round bottom kayak will shed vortexes off the skin, just not as strong as the hard chine hull. It would work something like this, if you roll the kayak to one side, the deeper side creates a stronger vortex, which take engery and has the effect of slowing or stiffening the roll. The side that lifts out of the water gets a weaker vortex, softening the resistance against the hull. This would also have the effect of yawing the hull, and as the hull want to correct the yaw, it will also add some correcting rolling moment to the hull. These forces are not large, but they are noticeable.

While the paddling motion helps stability, even if you hold the paddle out of the water while moving, or when sitting still, you are more stable when underway. The effect I believe is essentially caused by the water moving over the curved surfaces of the hull in a non-symetcial way.

I do not know of any way to model this behavior, you would have to take measurements of righting moments on the hull both statically and with water flowing over it (in a flow tank perhaps). And like anything else, different hull shapes will have different amounts of correcting moments.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.