Rounding up under pressure

aitchw

aitchw

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[ QUOTE ]
The boat I have now rounds up if you press her too hard over ... so washing windows is difficult.


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Thought this needed a new thread rather than causing thread drift.

Nigel, picking up on the your mention in the 'heeling' thread is this tendency hull form induced or due to small rudder blade area (or something else)?

My little boat suffers from over much weather helm and a tiny LAR rudder blade so when hard on the wind a gust will round her up pretty smartish as the already under pressure blade stalls. I have reduced the weather helm somewhat by having mast stepped as far for'ard as I can and increasing jib area with a slightly reduced main. Haven't figured a way yet to increase rudder area without messing up her ability to take the ground.
 
The only way you'll cure the problem is to convert to twin rudders angled outwards a la open 60. Not a weekend project.

Any boat with a wide shallow hull and beam carried well aft will have this problem. On most AWB's form stability keeps the rudder immersed and when it starts to lift out you should be reefed.
 
OK, I understand the logic of that with the beam carried well aft though I hadn't thought about that before but mine is an old boat with the beam pretty much under the mast foot and a narrow transom. The keel form is shallow draft long keel with bilge keels.
 
I have a theory.

With bilge keels as you heel the leeward keel becomes the aerofoil and the windward one just tickles the water surface, losing the flow. The rudder is displaced sideways and upwards from the water flow from the working leeward keel and you need more (weather) helm to bite into the current created by the that keel. Thus you're much closer to stalling the rudder in order to achieve some control than if it was in-line with the keel.
A gust and consequent further heel then removes the rudder from the water flow and it stalls leaving the boat to round up.

If your long keel is only a stub it won't be helping a great deal.

It's a theory of course and I haven't done (couldn't do) the maths.
 
I can see from the pic that you don't have the hull form problem but with a shallow rudder it's still going to lose grip as you heel. Twin rudders angled out so the lee one goes deeper and comes vertical as you heel will solve the problem.

With transom-hung rudders you can have a deep retracting blade or a pivoted stock to kick up if it touches bottom. That's quite common on multihulls with dagger boards.

The cheapest solution is to reef early!
 
Yep - you got it ....

S-annegeneral.jpg


Here you can see I have a small rudder blade and not much I can do about it .............

Mast is raked forward, stepped as far forward as possible, main is reefed before genny etc. etc.

Still I fight weather helm, she rounds up if really pressed ... on occasions broached with that bluff stern and wind up the chuff ....
 
Pity we are talking about older designs and NOT AWB\'s ...

The hull form here is as per photo ....
 
Twin rudders ....

On boats such as being discussed here ... apart from the AWB error earlier by one post !! ... would be lethal on them .... why ?

The safety factor is that the rudder stalls or lifts sufficiently from the water to lose bite and influence ... the boat then rounds up to spill ...

Put twin rudders on a boat such as mine ... no I don't think so - well in fact I would never sail on it ... without a) grab bag handy and lifejacket / eprib etc. .... there would always be a rudder biting to steer .... boat would not safely round up itself ... it would keep going till you do it .. or not as the case may be !!!

The twin rudder concept is near ok on a hull form designed for it ... but not on a heavy older traditional form such amine and AITCHW boat ... sorry ...
 
You seem to have gone through the most logical steps,rig forwards etc.
On very many sailboats the only thing offering any lateral resistance at the stern at all , is the rudder. As you say ,when this stalls the boat rounds up,sharpish..
Where on the boat is your mainsheet attached? If quite far aft to a rail or ring bolt, then try bringing this forward by attaching it to a rope strop fixed athwartships and experimenting, moving it a foot forward at a time and then sailing to try it out. Don`t overdo this It is important to retain the safety aspect of automatic rounding up to spill wind.
the theory behind this that the mainsail, through the leverage of the boom pulling on the mainsheet, is hauling the stern down to leeward, pivoting the bow up to windward, hence `weather helm`.
This is why on many sailboats( monohulls), mainly those with bilge keels or dagger keels, the mainsheet traveller is to be found near the companionway or thereabouts. Fixed any further aft and the boat becomes uncontrollable, shooting up into the wind at the slightest provocation.
These problems became more common as the GRP boat industry got into gear in the 70`s and 80`s, with many boats being designed round their interiors (mast position etc) with fine tuning or balancing carried out in the water using the preproduction vessel. There are notable exceptions,like the Centaur and the Konsort, and no doubt many others, which are extremely well engineered,and despite having fairly short (chord) bilge keels and an aft mainsheet horse, sail very well, with a long-legged easy motion more associated with more traditional designs
If you end up bringing the attachment point forward more than a foot or so, you may have to use a claw on the boom to keep your mainsheet verticle.
Hope this diatribe helps,
Pete
 
Same goes for you, Nigel.Try bringing that (extremely) far aft mainsheet attachment forward. Its easy to do. To clear your tiller, try a bridle and a much shorter tackle.
Pete
 
I know the idea you put ...

With our sheets on the horse as is .. the boom is effectively pulled in at the stern end and undercuts the sail ... causing a rounding moment ... bring the sheet more forward then sheets the boom straighter ...

My main would make it a serious structural problem to rig sheets futher forward ... worth a look ... but attachment would need serious underpinning ....
 
Re: Twin rudders ....

I disagree. While luffing is the normal practice in a dinghy, a safer option for a cruiser if overpressed is to bear away onto a broad reach. It brings the boat upright and restores control plus it reduces the apparent wind.

Rounding up runs the risk of losing steerage way and control.
 
Snowleopard ... if I was in UK ...

I would invite you on board to demo that ..... I think you would change your mind - for my boat - after you pick yourself up out of the water !!

I understand what you are saying .... but in fact the rounding up of a boat is an extension really of the weather helm aspect ... an in-built safety aspect of a boat ...

Put a less experienced person on the helm and press the boat ... I would feel much happier with the boat taking over and rounding up than they trying to bear away ...

We all know that as a boat rounds up to the wind that the hull digs in on its lee-side ... if you bear away lee-side will lift ... but unless you are careful to adjust all sheets as you do it the wind now is attacking the sails at more direct angle and PUSHING you over rather than sailing you over ....

I think I'll stick with rounding up when pushed to extreme ....

As to the downwind bit and broaching ......... thats where I like the downwind tacking idea to get around that ....
 
Pete, that's something (again) I have never considered and I can see the point. Problem is in small boat there is precious little cockpit space and to bring my mainsheet attachment forward would create an unwanted obstacle in that already restricted space. The mainsheet is attached to a traveller right on the aft edge of the cockpit at present.

As to the 'diatribe' I will happily read everything you want to post. It's always helpful to widen the scope of the way I think.

I don't want to paint a picture of a bad mannered little tub, she most certainly isn't. Idiosyncratic, yes. What I'm trying to reach is a point where she is as well balanced as I can make her without losing the essential safety aspects of her behaviour. She is a small boat with all the limitations that imposes but I want to get her to the point where she will handle F3-4 without a battle. She doesn't carry a lot of cloth, current jib+main is only about 100sq.ft so I don't reckon she should be overpowered in a low F4.
 
I thought at first you were joking about moving the mainsheet in, but now I'm not so sure. But just in case some poor soul reads and decides to start moving his mainsheet - moving the mainsheet fore and aft makes no difference to the turning moment on the stern. If you move the mainsheet from the end of the boom to closer to the mast then you are just increasing the strain on the main sheet, a greater force closer to the mast having the same turning moment as a lesser force further out.

What matters is the centre of effort of the sails (in relation to the centre of lateral resistance of the hull when heeled).
 
Re: Snowleopard ... if I was in UK ...

The other point about bearing away under these circs is, at least momentarily, you need more bite from the rudder at just the point where it is likely to give up anyway. Could take the matter out of your hands. If you did have more to go at you would definitely need to ease the main as you fall of the wind. Not to do so would be pretty risky.
 
Re: Pity we are talking about older designs and NOT AWB\'s ...

I see you have probably about the equivalent blade area I do relative to LWL though close to 2:1 aspect and not much more bilge keel area, so I can also see the possible similarities in handling. My rudder plate is slightly less than 1:1 (don't laugh!).
 
Simon cr.- You misunderstand - its where the mainsheet is attached to the boat hull thats important. Whilst you are absolutely correct in pointing out that its the centre of effort of the sails that is crucial, this point has already been covered by referring to mast position and the theory of moving the mast forward and with it the centre of effort of the sails, relative to the centre of lateral resistance of the hull.
Also, the sails have NO centre of effort until they are filled with wind AND hauled in and attached to the hull. Its where this attachment is, relative again to the centre of lateral resistance of the hull, and the actual physical lateral resistance relative to the point of sail,and the speed through the water of the hull relative to the wind pressure, which is the nub of the thing.Hence a boat moving slowly hit by a gust will round up violently,yet the same boat, moving at hull speed, will react less violently to a proportionaly larger gust. This becomes much more complicated now, involving a host of variable factors like hull form ,weight,wetted area, hull shape when heeled etc,etc.
The only thing I can say to you doubters, is try it. Those of you with mainsheet travellers across the cockpit who may wish to `reverse prove` this theory by moving your attachment point BACKWARDS by making up a strop across your aft cleats or whatever and experimenting, don`t do it in strong winds, and make sure those on board are hanging on tight.Even moving it aft a couple of feet can result in violent rounding up in quite moderate conditions.
Your point about increasing loads is taken,but they are taken by the boom more than the mainsheet. A foot or so forward of the end won`t make much difference.
Another diatribe.
Cheers,
Pete
 
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