Centreboard Length

[Island Pointer]

Hi Sailors and Builders
I have a question regarding the shortening of a centreboard.
This is a general question and not relating to any particular boat design.
Say that I have a narrow 250mm wide centreboard projecting vertically 900mm.
If I removed 250mm from the bottom of the board, what would be the likely effect on the boat's performance and or stability?

[The Q]

Stability not much change as that's mostly dependant on you Putting your weight in the right place.
Losing 28% of your centreboard will just make you drift side ways more for the same pointing angle, how much is impossible to say as the hull makes up some of the hull resistance effort, especially if it's hard chine and with runners.

[WI-Tom]

I predict you'd be able to sail in water about 250mm shallower than before you cut the tip off.

Tom

[stromborg]

The last post (#6) in this brief thread will give you some guidelines on sizing your boards:
http://forum.woodenboat.com/showthre...ea-calculation

[john welsford]

Hi Sailors and Builders
I have a question regarding the shortening of a centreboard.
This is a general question and not relating to any particular boat design.
Say that I have a narrow 250mm wide centreboard projecting vertically 900mm.
If I removed 250mm from the bottom of the board, what would be the likely effect on the boat's performance and or stability?

Always bear in mind that the smaller the boat, the slower its potential hull speed will be, and as a consequence of that it will need a proportionately larger lateral plane ( centreboard) than a bigger boat that moves more quickly through the water.
So if you reduce the size of the centreboard you'll find that the boat makes more leeway going upwind, and may not tack reliably.

John Welsford

[Hugh MacD]

Always bear in mind that the smaller the boat, the slower its potential hull speed will be, and as a consequence of that it will need a proportionately larger lateral plane ( centreboard) than a bigger boat that moves more quickly through the water.
So if you reduce the size of the centreboard you'll find that the boat makes more leeway going upwind, and may not tack reliably.

John Welsford

Good point from the master. If you do find tacking difficult, you may have to resort to some gymnastics, shifting forward during the tack and back to optimum after to facilitate getting through the wind. Much depends on the configuration of the boat and the rig.

[Canoeyawl]

In terms of "stability", if the centerboard is working correctly it will increase the heeling moment...

[Island Pointer]

Ha Ha, thanks Tom, that is exactly what I was hoping for.

[Island Pointer]

Thanks everyone for the comments.
If the width of the centreboard was increased proportionately to achieve the original board area, would this assist with the drift to leeward?

[Island Pointer]

Thanks for the link Steve.
After reading the content there really was no need to ask the forum the second question.
Ian

[Edward Pearson]

Hi Sailors and Builders
I have a question regarding the shortening of a centreboard.
This is a general question and not relating to any particular boat design.
Say that I have a narrow 250mm wide centreboard projecting vertically 900mm.
If I removed 250mm from the bottom of the board, what would be the likely effect on the boat's performance and or stability?

You're going to cut drag but also cut lift. Without knowing the boat and sail area etc, the result is likely to worse off where it matters.

Downwind, less foil will mean less drag, and you'll be faster if you're not lifting your centerboard up. Most people do, but high performance skiffs sometimes don't for steerage and roll stability.

Upwind, you will have 1/4-1/3 less length and proportionately less lift. Assuming lift is generated equally along the foil (and it isn't) and assuming your boat's centerboard/ sail area etc are currently in balance the net affect will be worse pointing, worse VMG in low to medium winds. In high winds, if you wind up your boat speed first, you have more energy available flowing over your shortened centerboard an actually you might point not only point as high as before but with less drag be going faster. Faster means better VMG and even higher pointing angle - a virtuous circle. The round the cans dinghy racers lift the foil a bit in high winds to windward once they're going well to windward to go a bit faster. Same reason a fast jet needs smaller wings than a slow one of the same weight. They don't normally lift it by that much though...so you're likely going to be worse off.

Roll speeds will be faster but ultimate stability is still dependent on your position and bilge profile.

If you usually half lift your centerboard on a reach then the net geometric center of effort of the remaining foil will be now a bit more forward. That might get rid of weather helm and actually leave you a bit faster or might introduce lee helm, likely not much would change but that depends on the shape of your boat a bit and its current balance. Tacking speed will likely be reduced, but that depends on quite a few factors, but it certainly won't improve things.

You'll exert less leverage on the end of a shorter foil if upturned, but that's probably not too significant. You'll have a bit of exposed slot drag too when up downwind unless you have a gasket.

Typically the end of the foil allthough it's in deeper less disturbed water (good) is also contending with tip vortices that net drag flow obliquely towards the tip. So the water isn't following a perpendicular course accross the foil as a well shaped Greek God of NACA foil intended, but taking a longer progressively more oblique path closer to the tip.

Its the point 1/3-1/4 the way back where the force is generated, the nose is curving water around to that point and the bit behind is returning without hopefully getting detached (draggy) flow. In the end it's planform/ aspect ratio which is the biggest determinant of performance down there (assuming you've got enough basic area). Things like NACA section, tip profile, nose profile are all secondary and relatively minor marginal gains, so reducing your aspect ratio will reduce drag but reduce lift leaving you worse off, at least around a typical upwind downwind race course although due to the the tacking upwind and the longer length and thus time, that biases upwind performance...

Whats made you think of sawing some off?

[skuthorp]

Most of my sailing is in either shallow water, or water with complex sand banks. I have a normal and a shorter centreboard, and ghosting over sand banks requires you to either lift and drop as you go, or cant the boat over enough to clear the sand.
BTW, we sail at times in a tidal flow strong enough to overcome the available wind when coming about. So you throw the rudder over and lift the board enough that the bow goes through the wind and then drop it again so the tide helps you turn.
I even have 2 different profile lee boards for my canoe for much the same reasons.

[Edward Pearson]

If the question were...I have 5" of water under my keel, what's the best foil arrangement to sail best I can?

A centreboard half up will perform very poorly because the water is now running mostly along it, rather than accross it. There's a change in the lateral center of effort to think about and you're getting lift (approximately) proportional to immersed depth under the keel, but you've now that wetted area adding drag behind the effective leading edge.

On the other hand a daggerboard even a partial immersion presents it's NACA chord profile to the water, at all times. Even if there is insufficient immersion, at least what is there is working as efficient as it can be.

So 5" of immersed daggerboard under the keel will work to generate more lift and have less drag than 5" of immersed semi raised centreboard.

Twin daggerboards canted out, becomes vertical with heel, and greater immersed depth (though the boat might rise on it's bilges). You could have asymmetric twin daggerboards canted for the most efficiency. This is how 'modern' bilge keelers work, and they work pretty well with less draft than a single deeper keel version. I've seen a paper that showed two half size bilge keels gives equivalent roll speed reduction to a single double length keel.

The other option is the leeboard. This can offer one more advantage. Between the water surface and the bottom of the keel (lets say 4 inches) it's immersed doing work, before the centreboard or daggerboard even pokes it's nose out. In turbulent flow, in the path of weed, a bit out of reach from the windward rail, and there is no 'end plate' affect from the hull's bottom...but it now means you've got a 9" board immersion rather than 5". When all others are 'board up', the leeboard is still working that 4".

[Island Pointer]

Edward Pearson. Thank you for the comprehensive explanation.
The reason for the saw off? I intend to sail in shallow water and I was looking at the pros and cons associated with a shorter/wider board and tilting the centreboard and the resulting lee helm.

[Island Pointer]

Edward Pearson. Thanks again. I think your statement "On the other hand a daggerboard even a partial immersion presents it's NACA chord profile to the water, at all times. Even if there is insufficient immersion, at least what is there is working as efficient as it can be." puts my issue into perspective. I had been considering producing a second shorter/wider centreboard that could be employed at destinations where shallow water is likely to be encountered. A similar concept as described by "skuthorp" but both boards would not be aboard at the same time.

[sp_clark]

I had been considering producing a second shorter/wider centreboard that could be employed at destinations where shallow water is likely to be encountered. A similar concept as described by "skuthorp" but both boards would not be aboard at the same time.

Wider? Tough with a daggerboard if its trunk's not able to accommodate the extra width. Centerboard, if pivoted from forward, ought to need no reduction as it can be raised when in shallow water enough to clear while that does add to increasing lateral resistance and subsequent change in helm.

Going out in deeper water with the original, you find your course takes you into shallow water, simply raise the board to adjust draft.

Whacking 1/3 off your original board seems an uncompromising act unless you stick to depths where the shortened board is 'enough'.

[The Q]

You've not actually mentioned what type of boat you have (unless I missed it) a photo might give us ideas.
With only 5 inches of water to play with I'd be tempted to add twin 5 inch bilge keels / over large bilge runners. They'd protect your hull from grounding as well.

[Canoeyawl]

The best boat I ever had for sailing in shallow water was a low sided flat bottom skiff. I could sail it standing up and by moving my weight a little could just touch the chine into the mud. When I could keep that going on (by watching the trailing mud track) she made no leeway at all!
The rest of the time a lee board on a pivot worked very well

[Lemsteraak]

Centerboard shape is a really important factor. If your centerboard isn't shaped, you can lower drag and increase lift by shaping. You can find some great articles on shaping on the Internet, it is something of an art and science. We sail racing dinghies and will spend a small fortune to have a good board.

We got an absolutely beautiful racing boat that the owner allowed us to use for guest skippers. We found the boat was slow upwind, just didn't point or tack well, so we spent a day swapping sails and trying all sorts of things to the rig. Last resort we flipped the boat and swapped out the centerboard. The boat was then up there with the leaders, dramatic change. The old board was the one that came from the builder, flat sides and blunt edges.