Design to avoid Broaching

[farjoe]

Hi,

First of all by broaching i mean the tendency of a normally balanced boat to turn abruptly into the wind when a gust hits going upwind.

Recently I have been sailing on a modern design monohull which seems to suffer a lot more than other boats in this regard.

One reason for this behaviour seems to be the tenderness of the design which causes it to heel considerably in the gusts and then turn savagely into the wind.

Are there other design reasons which affect a boat's behaviour in this regard?

[masalai]

Broach (sailing)
From Wikipedia, the free encyclopedia
A sailboat broaches when its heading suddenly changes towards the wind due to wind/sail interactions for which hull hydrodynamic reactions cannot compensate. This happens when the aerodynamic force on the rig greatly exceeds the hydrodynamic force on the hull, usually due to a sudden increase in wind strength. In small boats and dinghies, broaching often leads to capsize (see Death roll). In larger boats broaching can lay the mast horizontal, putting both rig and crew at risk. It can be particularly dangerous when racing other boats at close quarters.

Sailing in variable, gusty winds is a common cause of broaching when sailing on a close reach and in strong gusty winds on a broad reach. When sailing on a broad reach, the helmsman must be careful to make sure that the wind is not allowed to cross the stern. If this happens, then the sails may fill from the leeward side, resulting in a sudden reversal of the direction of thrust as the sail moves from one side of the boat to the other. This is called a gybe, and it can be a dangerous maneuver even when performed under controlled circumstances. When it happens unexpectedly, the boom may live up to its name, as it crosses the boat under full load and may sweep unwary crew members overboard, unless a preventer is rigged on the boom.

One of the less obvious causes of broaching is an oscillation developed when running downwind. When running dead downwind, the sails are set with an angle of attack of nearly 90 degrees to the wind. The high angle of attack causes turbulent bubbles to form on the leeward side of the sail, and when there is insufficient attached flow to keep these bubbles attached to the sail, they separate. The sudden separation of a turbulent bubble drops the lift of the sail to nearly nothing, which causes the boat to rock slightly. At angles of attack near 90 degrees, lift can generate on the leech, or trailing edge of the sail. As the boat rocks due to separation of turbulent bubbles, a constructive interference is formed, as bubbles begin to shed from alternating sides of the sail. Once started, the oscillation builds quickly, especially in modern ultra-light displacement centerboard boats, which lack the damping effects of a large keel. Once this oscillation reaches a certain point, the extreme change in heel angle will cause the boat to broach. Spinnakers generate lift from the top, rather than the side, and thus they are less prone to this form of broaching than other modern sail types. Square rigged vessels also generate lift from the tops of their sails, which in fact, combined with the large keels they traditionally use, makes them immune to this type of broaching.

Another cause of broaching is encountered in heavy seas and high winds. If the bow of the yacht is not kept pointed into the waves, then the waves will push the bow aside, turning the boat side-on to the waves. Once side-on to the waves, the waves will roll the yacht side to side violently, causing severe discomfort to the crew, the yacht may broach and may even capsize. Since a sailing ship cannot steer unless the sails are providing forward motion, and since heavy seas are most often from the same direction as the wind, a sea anchor may be used at the bow in extreme weather to keep the bow pointed into the wind

Read the above from wikibedia, just by typing "broaching" into google......

[masalai]

Probably your boat had too much sail up and an inexperienced skipper, a couple of capsizes and other mishaps and you will all learn.... - - not the boat, but the skipper and crew pushing too hard... It is very difficult to design out "idiots and fools" in a sailboat - or any boat.... Good seamanship and learning from someone experienced and knowledgeable is a start... - in dinghy or other small boats....

[Finlander]

Originally Posted by farjoe

First of all by broaching i mean the tendency of a normally balanced boat to turn abruptly into the wind when a gust hits going upwind.

I think you are describing weatherhelm, not broaching. We had a similar problem with an early model Nauticat 33. The solution was to add a bowsprit and second headsail, which thereby moved the sail area forward. It was perfectly balanced after that.

[Earl Boebert]

This subject was extensively studied by model yachtsmen in the free-sailing days. PM me with your email and I'll send you a paper I wrote on the topic.

Cheers,

Earl

[Guest-3-12-09-9-21]

When I was sailing on a 37' heritage one-ton we had a bunch of broaches during the Swiftsure race out of Victoria, BC - after looking at the rule book we came up with a double headsail scheme - with one hanked on as a headsail and another one used to 'replace' our spinnaker, attached only at the head, foot, and on the spinnaker pole on the clew of the sail.

It made the center of effort much lower and totally stabilized the wild oscillations we were getting using the spinnaker. It made people look twice, and throw up protest flags, but they ruled in our favor since we, in fact, only had one headsail and one 'spinnaker' according to the rule book.

The biggest problem we had was that there was so much power involved that it actually started driving the boat under water at one point.

[farjoe]

Originally Posted by Finlander

I think you are describing weatherhelm, not broaching. We had a similar problem with an early model Nauticat 33. The solution was to add a bowsprit and second headsail, which thereby moved the sail area forward. It was perfectly balanced after that.

I suppose you can call it weatherhelm but only during gusts, at which point the boat gets overwhelmed. Otherwise the boat is quite balanced with just a little weatherhelm as it should be.

Maybe the rudder is too small?

To answer Masalai.

I agree with you that a more experienced helmsman would be able to handle the situation better.

However my observations were made after noting how the boats handled under the control of 3 different helmsmen

[Earl Boebert]

Sounds like a classic case of an unbalanced hull (vs. rig imbalance). I'd bet a beer (I'm cheap :-)) that the hull has a wide transom and a narrow entry. Roll -> pitch down -> yaw.

Cheers,

Earl

[farjoe]

Originally Posted by Earl Boebert

Sounds like a classic case of an unbalanced hull (vs. rig imbalance). I'd bet a beer (I'm cheap :-)) that the hull has a wide transom and a narrow entry. Roll -> pitch down -> yaw.

Cheers,

Earl

Earl,

You may be spot on because indeed the boat with the problem does have a wider transom than the others. I am eager to read your paper on the subject.

Most modern design have wide transoms. Does it mean that this sort of thing is becoming normal?

[TollyWally]

Think of how a funnel would roll across the floor compared to a pipe.

[Earl Boebert]

Originally Posted by farjoe

Earl,

You may be spot on because indeed the boat with the problem does have a wider transom than the others. I am eager to read your paper on the subject.

Most modern design have wide transoms. Does it mean that this sort of thing is becoming normal?

As I understand it, the very wide transom designs are used on ocean racers to provide planing ability on mile after mile of broad reaches and runs (a real NA may correct me on this) and the "look" of these boats becomes more of a fad than a justified feature for inshore craft. Broad transoms are OK provided they are balanced by a suitable degree of buoyancy in the forebody. Somewhere else on this forum somebody noted that (if memory serves me) newer TP52s have gone to less sharp entries to reduce crew fatigue from fighting the wheel on beats.

Cheers,

Earl

[Paul B]

Originally Posted by Earl Boebert

Somewhere else on this forum somebody noted that (if memory serves me) newer TP52s have gone to less sharp entries to reduce crew fatigue from fighting the wheel on beats.

Cheers,

Earl

Whoever wrote that has no idea what they are talking about.

[kenJ]

Originally Posted by farjoe

Hi,

One reason for this behaviour seems to be the tenderness of the design which causes it to heel considerably in the gusts and then turn savagely into the wind.

Are there other design reasons which affect a boat's behaviour in this regard?

I think the term rounding up applies here. Sounds like the rudder needs to be larger or longer to maintain control in gusts. Pro-active sail trim can also help the situation.

[tspeer]

Originally Posted by farjoe

...One reason for this behaviour seems to be the tenderness of the design which causes it to heel considerably in the gusts and then turn savagely into the wind.

Are there other design reasons which affect a boat's behaviour in this regard?

I'm surprised nobody's mentioned the fact that the tenderness itself is a big factor, here.

When a yacht heels, the rig is no longer driving from over the deck, the center of effort has been moved to leeward. So now it has a lever arm that wants to turn the boat to windward. Think of a biplane rig with just the leeward sail sheeted in. The greater the heel angle, the greater the lever arm from the rig.

Obviously, keeping the yacht on her feet will help to avoid this source of weather helm.

[farjoe]

Originally Posted by tspeer

Obviously, keeping the yacht on her feet will help to avoid this source of weather helm.

That is certainly very true but it is not easy to always find sufficient crew to do so.

So what are the alternatives.....

Should one aim for lee helm in the upright position so that when the boat does heel, the weather helm is not so heavy?

When reefing, should one reduce the main first rather than the genoa to push the forces ahead of the keel again to try and counteract this source?

These seem to make logical sense to me but what is sure nobody does this whilst racing.