Rounding up under pressure

You know something ....

I wish I'd kept my mouth shut now !!!

Blimey - we've had see-saws, centre of effort, move sheeting position ... it's amazing ....

The power of the forums ......

Don't get me wrong ... I admire it and applaud it ... keep it going ... but I do find it difficult to read another one about see-saws etc. !!!!
 
YUK .... I keep meaning to clean her up !!

Everytime I see this photo - I say to myself ... T-Cut me ol'fruit and a tin of wax ... to finish.

Actually it's the straps from the lift that do it ...
 
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1. Get flatter newer sails.
2. Bring the genoa traveller inboard slightly
3. be able to haul the main sail in I could only get it to 10 degrees off the centre line, partly because the boom was too small and and too much curve in it.
4. I could never get the boom 100% flat.
5. Get a more heavy duty boom vang, again too much bend in the boom
6. A larger rudder, but I think the sail shape and position was more important


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I'd add that your boom looks very skinny it's round section with little structural strength. What will happen is as your main powers up, the boom will bend and allow too much shape into the main. Suggest you get a new boom as well.
 
Re: YUK .... I keep meaning to clean her up !!

with a boat that has bluff bows and carries her beam well forward the bow shape when heeled will cause the boat to luff up. Bowsprits etc will help a bit but at the end of the day the hull shape is what is causing the rounding up.

with all boats being a compromise this is the trade off for a high volume hull.

I would prefer to sail my boat but much prefer to live aboard yours with vastly more living space on a similar length.
 
Snowleopard, you have explained the point perfectly. There is more tension because the same load , the boat, is offering more resistance to turning by the same applied force, through a lever of reduced mechanical advantage . In your example where has this extra tension come from? Since the applied force of the wind is the same, and the overall weight of the boat is the same, something has been added and that is resistance to turning due to decreased mechanical advantage. of the force applied. The boat,having mass, by moving forward gains MOMENTUM Theoreticaly this momentum is a force travelling in a straight line forwards along the centre line ( leaving out for a moment the fact that a monohull does not travel along a straight line but askew of it due to the sum of various forces working on it) It is this momentum( a huge force, a multiple of the boats weight which increases as the speed increases) which supplies the resistance to turning. In my previous example of the door that resistance is friction, supplied by gravity and resultant extra effort required to move that body from rest..Elementary mechanics connect to the real world through the rules of physics.
I can only conclude by mentioning my own experiments with this, which are the basis for my opinions offered on this forum today.:
My mainsheet traveller is mounted about 18 inches aft of the companionway. One day, in brisk conditions, the mainsheet track car tackle gave out, the car shot across to leeward just as I was in the way, and pinned me painfully to the side of the cockpit.
Not wanting a repeat of this I determined to move the mainsheet attachment point to a strop fixed between the stern cleats, a sort of flexible rope horse over the tiller head with a shortened tackle, looking not unlike Nigel`s arrangement, and went to sea. Understand I had not worked this out or made any kind of calculations. To say I was gobsmacked would be an understatement.What a revelation and a graphic display of the rules of applied physics. The attachment point had been moved aft by about 5 feet. Sheets hauled in , the boat would hardly move forward to windward at all without immediately rounding up, in not much wind at all.I moved the attachment point forward to a pair of unused turning blocks,same effect,but less so.I concluded that the original position achieved the best handling, but I`m still musing about shifting it forwards onto the coachroof. As has been said,high boom loads, high sheet loads.
What a splendid discussion on a wild,rainy Sunday. This forum is unmatched, anywhere.
Cheers,
Pete
 
Early International Canoes had long booms, around 10 ft, sheeted from the end to the afermost tip of the hull. Later, they were all converted to centre sheeting; the boom was cut down to 6 ft and the sheet attached half way along. The change was made for ease of movement and made no difference to the handling. In your case the difference in handling was certainly down to changes in sail shape due to the different sheeting angle. Either the temporary arrangement allowed more twist or hooked the leech. If you used the original sheet tackle in the new position it would easily over-tension the leach. Either could produce weather helm.

As to the question of where the increased force comes from with a centre mainsheet, try this: get a long heavy piece of wood, put a pair of bathroom scales under one end and read the weight. Now move the scales to the centre. The weight has doubled - where has the extra weight come from?
 
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The boat,having mass, by moving forward gains MOMENTUM .

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No it doesn't, it gains momentum only when it accelerates. A boat travelling at a constant speed has constant momentum.

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Theoreticaly this momentum is a force travelling in a straight line forwards along the centre line... a huge force... Elementary mechanics connect to the real world through the rules of physics.

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As a matter of very basic physics/mechanics, this is completely and totally wrong. Momentum is NOT a force, and does not travel either in a straight line or in a curvy line or any sort of a line. And there is no net force being applied in a fore and aft direction to a boat travelling at a constant speed - whatever forces are being applied are equal and opposite. Otherwise the yacht would accelerate or decelerate (depending on the direction of the superior force).
 
Re: You know something ....

having owned several boats and getting used to all there quirks,

I had a timber boom with a huge ammount of bend in it.

to reduce the bend someone along the way had connected the main sheet on a block and tackle beside the companion way at the front of the cockpit, the main sheet went up to the boom and then travelled down to the stern of the boat where I had the traveller at the back of the boat. This system worked really well, because the extra tackle lightened the effort required to haul the main sail in, in a gust.

However this had no effect on the balance of the boat. Snow leapord, thankyou, I was rattling around trying to put a simple explanation as to what you said into words that could then be understood by others.
 
"Where has the extra weight come from?
With the scales holding up one end the ground would be holding up the other end,offering an upward force equal to the downward force of the weight of that end of the plank, effectively halving the weight on the scales.
With the scales in the middle, both ends would be off the ground, and only the scales would be offering an upward force equal to the weight of the plank. The feet of the scales would be carrying the full weight of the plank,plus the weight of the scales.Since the spring balance in the scales would compress under the weight of the plank, what is displayed is the difference between the weight of the scales and their total weight.
You are of course quite right about the effects on the mainsail, but in this case although present could not account for the immediate onset of diabolical difference in handling.
Cheers
Pete
 
Studiously ignoring the raging debate....

How does the boat handle before rounding - what sort of rudder angle do you need to keep her straight? If this is excessive (say over 10 degrees, or so??), have you got any room to experiment with shortening the forestay to rake the mast forwards? Just an inch, or so may make a noticeable difference. Has stepping the mast further forward effectively been negated by making the mast rake further aft?

As mentioned elsewhere, its about keeping the centre of effort from the sails balanced against the CLR of the hull. Unfortunately both of these move about depending on the angle of heel, boat speed, wind strengh, sail plan and the set of the sails, etc. so it is only ever going to be approximately right. Good boat designers get it right over a wider range or circumstances.

And, of course, if you were my grandmother, and were unsure how to suck eggs, I could suggest that you make sure the main isn't over sheeted!
 
Momentum is not a force.It does not travel in a straight line.
Alright:
A car accelerates to 100mph and cruises at this speed along a straight slippery road. At the end of this road there is a right angle bend which the car attempts to negotiate without slowing down. What happens-it goes straight on.Why? Because the MOMENTUM of its acquired mass makes it go straight ahead in a straight line. This momentum`s resistance to turning cannot easily be overcome by any applied force through the friction of its tyres on the road
Momentum is a force in that it requires an applied force to deflect the direction of travel of the mass that contains it.
Pete
 
Having established where the extra force comes with the plank, now apply the same logic to the boom. With an aft sheet, the loads on the boom are counteracted by forces at the sheet and at the gooseneck. With a centre main, most of the load is carried by the sheet.
 
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Momentum is a force in that it requires an applied force to deflect the direction of travel of the mass that contains it.

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The car carries on in a straight line because of the ABSENCE of any sideways force (in the absence of friction in the road to let the turned wheels do their stuff) to make the car turn.

Momentum is not a force, it is defined as mass x velocity (mv). It is, if you like, an expression of how much energy the car will expend if it hits a brick wall. A car or a yacht travelling at a constant speed in a constant direction (ie. a constant velocity - don't forget that velocity is both an expression of speed AND ALSO direction) by definition has a constant momentum (assuming that it does not acquire any extra mass). If it is also travelling in a straight line at that constant speed, then by definition there are either NO forces at all working on it, or all the forces that are acting on it byare balanced ie. equal and opposite.

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Because the MOMENTUM of its acquired mass makes it go straight ahead in a straight line.

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This is putting it the wrong way round. Momentum does not MAKE a car go anywhere. It is just a fact that an object travelling at a certain velocity will carry on at that velocity until a force exerted on it makes it change that velocity.
 
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have you got any room to experiment with shortening the forestay to rake the mast forwards?

[/ QUOTE ] This depends on your existing main. If it's cut for a backward bend in the mast then altering the rake forwards will have all sorts of effects on the centre of pressure and set of the sail. You're going to end up having to replace the lower inner shrouds and totally altering the designed sail plan if you're not careful.

And isn't momentum a vector? Direction must be a component.

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And isn't momentum a vector? Direction must be a component.

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As mentioned, it's mass times velocity. Velocity is an expression of both speed and direction, so that, yes, an expression of momentum relates to a particular direction.
 
The rudder angle varies with wind strength and point of sailing but never less than about 15 degrees. I have tried making adjustments to the rake i.e. reducing to close to zero to further move CE for'ard. Oversheeting the main is somethng I am very concious of and avoid.

I am rapidly coming to the conclusion that I will have to incrrease the rudder area but it will have to remain very low aspect. There is not much I can do about the underwater shape of the hull or CLR (unless I extend the bilge keels aft).
 
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The rudder angle varies with wind strength and point of sailing but never less than about 15 degrees.

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I'm not an expert, but this sounds like quite a lot - Would a *real* expert care to comment? /forums/images/graemlins/smile.gif

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I have tried making adjustments to the rake i.e. reducing to close to zero to further move CE for'ard.

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Has this made any difference (better or worse)? What sort of change have you tried? (I'm an (ex?) windsurfer, so I *LIKE* mast rake as a variable /forums/images/graemlins/grin.gif It has worked wonders with our 22' er, too. (Incidentally, Lakesailor's response about sail shape, I think refers to mast bend, rather than rake. Changing the rake can be done without introducing the complications (s)he mentions.)

Increasing rudder size seems to correct the symptom, rather than the cause, but at some point, this will be the only viable option. I suppose the other thing to look at is the trim of the boat (i.e. fore-aft weight distribution). If there's too much weight in the bow, it will tend to be buried as the wind pressure builds, moving the CLR forward, and causing the boat to round up. Maybe another avenue to explore if you haven't got there already.

At some point, you will hit the limitations of your boat design. Is there a class association where you could find out how well she sails against the class norm?
 
Wow what a load of argument even a fair bit of bovine you know what.
Moving the sheeting point on the boom will not change round up tendency. Mine is midway with the traveller at the main entrance hatch. The advantage is less rope required the disadvantage is more force required and a tendency to bend the boom. The boom takes up a fixed position relating to the hull and it doesn't matter where the attachment is the turning moment boom to hull is the same.
Now re rounding up. I reckon the weather helm theory of centre of forces becomes irrelevant as the boat leans over and the asymetric hull in the water and sail pressure asymetry have a far greater roundup force. In fact it is better to reduce jib area to minimise the effect simply because it is easy at the same time to dump the mainsail pressure by easing sheet. It is also desirable to reduce mainsail area. Just don't hang on to a large jib cos the theory says get more pressure forward.
A deeper rudder is another excellent way to aleviate the problem. I was able to increase the depth of my transom mounted dagger rudder by about 2 inches very easily. I carved some urethane foam to rough size and shape and glued on the tip then layed up layers of fibreglass over the foam and up onto the sides of the rudder. Sanding and filling made it all smooth. A thickening of the rudder is also good to aleviate stalling certainly you should be looking for a classic airfoil shape with a max thickness in the front half and a nice semicircular leading edge. If you have simple steel plate rudder you could improve things enormously with epoxy build up to give it a good shape. The extension of the bottom of the rudder will probably be quite frangible if you do hit anything it will pull off without damaging your original rudder so don't worry too much about accidental grounding.
Lastly new less baggy sails especially jib will also aleviate round up tendency.
So when sailing a round up tendency will require a pull of the tiller to keep it on course. This lifts the stern to windward which in fact tends to lif the whole boat to windward offsetting some of the leeway. This is good until the point where the load on the tiller translates to drag on the rudder to slow the boat down and of course as soon as the rudder starts to stall it is bad news. It is quite fascinating to watch over the stern of a transom mounted rudder as it stalls and you can definitely hear the turbulence. So keep the tiller angle down by increasing rudder area. olewill along time campaigner of 21ft very tender racer.
 
Snowleopard.
I wondered where you were going with the plank.We are not concerned here with the transfer of energy from the sails to the hull to move it along, but with the turning moment applied to the hull by the main sail. Since the gooseneck is very close to the theoretical pivot point its turning moment is near zero. the total load applied to the mainsheet is the same no matter where it is mounted on the boom, since it is the only point where resistance to its pull is applied( ignoring kickers for the moment).The tension in a tackle mounted to the centre of the boom and to an attachment on the boat vertically beneath it is much higher because of the boats increased resistance to being pulled aside from this point because it is closer to the pivot point.More force, more wind, must be applied to this centre point to achieve the same pulling effect as the lesser force applied to the attachment point at the end of the boom and the end of the boat in the same way that pulling open a door with the handle mounted in the middle requires more applied force (pulling power) than pulling open the same door with the handle mounted near the edge.The door simply offers more resistance to being pulled from this point. Your centre mounting on the boom is an extreme example and shows a convergence of the forces involved.Whilst the boat offers more resistance to being pulled round, the boom ( the applied force) offers more resistance to being pulled in for exactly the same reasons so any advantage to be gained must be with the boom attachment at or near the outer end and the hull attachment vertically below it ,ie within limits.
All this concerns attempts to `tune out` a tendency to weather helm existing in a boat - as found-. the weather helm is there caused by any one of a multitude of factors including hull form,wide beam, bluff bows (as has been mentioned earlier),mast position, rig design,sail cut and the tension applied to halyard,foot and not least, leech line and in my humble opinion,mainsheet position. Are there any yacht or rig designers out there who can settle this , by showering us with formulae or leading us through fields of prismatic coefficients and metecentric parameters ?
 
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