A question for the mathmaticians

Pythagoras theorem of right angle triangles ... BUT not strictly true as the start is not 90 deg, and second the stem fitting is usually well below mast foot - so final is not a 90 deg either.

As to cutting - the olkd fashioned way is sharp cold chisel and hammer on iron block. It has to be a good sound hard smack otherwise you end up with splayed end.
Another way is to clamp in vice, use lashing out along the wire a cm from vice and then a metal cutter disc in grinder.

But best is the riggers guillotine.
 
[ QUOTE ]
Pythagoras theorem of right angle triangles ... BUT not strictly true as the start is not 90 deg, and second the stem fitting is usually well below mast foot - so final is not a 90 deg either

[/ QUOTE ] Of course we are working with right angled triangles.
One has a base of 13 feet the, other 14 feet. Taking the height of both as 40ft + an unknown amount, x, for the height of the mast step above the stem head is the only approximation. Call it (39.99 + x) feet if you like when the mast is raked by a foot.
scan0006.jpg
 
Oi
I didnt say I couldn't do decimals of feet, just that it's much easier to do in metric.

I was brought up to do duodecimals and £, s. d. as mental calculations because "ready reckoners make you lazy" and calculators were only for Directors use.
I assumed that you have some sort of adjustment on your forestay, but if you really need to calculate it to the nearest 1/32" I think I would emply a rigger. /forums/images/graemlins/smile.gif
 
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Move some weight from the stern to the bow

[/ QUOTE ] Looks as though she trims about right to me.
Also looks as though the mast foot is not very much above the stem head so let x=0 in my calculations above.

fweatheronmooring.jpg
 
Yes thanks for that...but the proof is in the pud, and basically that is a starting point.

The old Halberder is a super boat in just about every possible way except my boat has excessive weather helm. Other owners, however acknowledge it is present but not to the level that I appear to suffer.

A before anyone mentions it, my sails, although not expensive are pretty new by cruising standards (1 and 3 years). So it has to be some attention to the mast rake next, and after that I will be adding some ballast to the 'well' ... I have about 350kgs of lead ingots that I can experiment with.

Interestingly I had a chat with Alan Hill the designer, a couple of years back and he said that some boats appeared to go dead straight and some didn't !

Thanks Nick
 
Straight mast and raked mast are two totally different things - you can have a raked mast that is totally straight or a vertical mast (where the top is directly above the foot) but has a pre-bend in the middle (usually forwards) ...

To correct weather helm then you have to move the center of effort forwards or the pivot point back ... but it may just not be possible to correct and you have to reduce sail instead ...

We have a reasonable amount of weather helm when healed, and letting the mainsail right off makes no difference as the power is in the masthead rigged genoa ... put a couple of furls in that and sheet the main back in and it's much better ...
 
I've come into this a bit late so I'll be succinct. Calculate angle of rake of mast - 1/40 radians or 1.43 degrees. Use Pythagorus to calculate length of forestay when mast is vertical and use cosine law to calculate it when "mast angle" is at 91.43 degrees rather than 90 degrees. This gives forestay lengths of 42.06 and 42.37 feet ie. shorten by 0.31 feet or 3.7 inches. A smallish difference in height between mast foot and stem fitting will be cancelled out by the subtraction.
 
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How do you know it rakes 12"?

[/ QUOTE ]

That's what it said in the original post. I assume that it was measured by the time-honoured halyard with weight on it method.
 
My 2 ha'porth

If adjusting the mast rake proves inadequate to reduce weather helm,
you might like to consider buying some more anchor chain for the foc's'le, moving a water tank further forward ( ie under the vee berth),or emptying an aft tank, adding more cruising weight to the boat generally but low,low down, and maybe having a look at the propellor aperture and prop (does it fold?) as you <might> be generating bad turbulanece across the rudder. I say <might>.

The more a boat heels and the more it squats aft, the worse is the distortion to waterflow past the hull, irrespective of where the sailplan lies in relation to the fore and aft underbody hull profile..but you knew that.

So perhaps this is the underlying reason behind the designers comments that 'some go straight and some don't go straight'?

Lastly, and expensively(!), an independent windvane such as a Hydrovane will steer the boat extremely well whilst you lock off the wheel at such an angle as to balance out any remaining weatherhelm using the rudder as a trimtab to the keel, on whatever tack you are sailing on.
 
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This one is going to run and run!

[/ QUOTE ] Rubbish /forums/images/graemlins/grin.gif
Simple question.
A simple application of Pythagoras's theorem.
Out pops the answer with no scope for further debate! /forums/images/graemlins/wink.gif
 
Yes Vic its a simple question its just that we dont seem to know if we have right angle or not, and even the guy that built it isnt sure about the rig so while the question is simple sorting out the answer with all the unknowns and unknowables is going to be quite entertaining, has anyone suggested a balanced rudder yet? or perhaps a cutter rig? both would reduce the weather helm.

/forums/images/graemlins/smile.gif
 
You know I've heard someone else say that as well. Now you have said it and I starting to wonder !

In my book.. to reduce weather helm you raise the bow and lower the stern thus moving the center of lateral resistance aft. But you say move it forward....Can't be correct can it...

I did speak to one gentleman who said this to me and thought it was because by doing so it tilted the mast forward ! And he was an evidently experienced yachtsman and boat builder.

In this case I would be very happy to be proved wrong as I will just buy a 100m of 10mm chain and bung it in my chain locker !

But I don't think so........

All best and thanks for all the interest ... Nick
 
It depends if the weatherhelm is being caused by the centre of effort of the sails or the centre of gravity being behind the centre of resistance of the hull.

Trimming the bow up and down changes the centre of resistance only very slightly but loads of water , engines, batteries, gas bottles etc in the stern moves the C of G back a lot. When the c of G is behind the C of R the boat rounds up when the boat rolls off vertical due to wind or waves.
 
Well weather helm is a combination of of many things, most of which we can't do much about.

However the C of E we can alter by raking the mast fwd or aft and the C of Lat resistance we can alter by trimming the hull. Its not really C of G as in aircraft trimming but of course the loading is the only way we have to control the C of Lat res.

So if you trim the down the stern the C of Lat res moves aft and the bows will tend to fall OFF the wind......Add to that raking the mast forward ( or reducing aft rake) and you move the C of E fwd adding to the same effect.

So its a combination of the 2 but even so one change in isolation will work on its own.

The effect of weather helm due to roll I don't pretend to understand apart from the fact that generally tubby hulls are less good that long slim ones......but as the boat heels the effect of the mast and sails exerting their effort well off to the side of the hull provides a turning moment that produces Weather helm.

I suppose what you get in a really well balanced hull is an underwater form that as it heels combats the 'lever' effect produced by the mast and sails..

All best Nick
 
Trimming the hull up at the bow and down at the stern cannot really be achieved without adding weight. Its this weight that makes the stern heavy so as it heels the stern will go down pivoting about the C of R turning the boat into the wind.

If you can't correct weather helm by reducing sail aft then the problem is weight distribution.
 
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A before anyone mentions it, my sails, although not expensive are pretty new by cruising standards (1 and 3 years). So it has to be some attention to the mast rake next, and after that I will be adding some ballast to the 'well' ... I have about 350kgs of lead ingots that I can experiment with.



[/ QUOTE ]

Isn't that what I said..?

Regds Nick
 
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