Latest News: Forums Technical Compressive Loads

  • This topic is empty.
Viewing 11 posts - 1 through 11 (of 11 total)
  • AUTHOR
    POSTS
  • #3884
    matoi
    Member

    I apologize for my ignorance, but was the original 1957 Proctor Wayfarer designed with a ‘spreader-less’ mast?

    If so, why not try living without them?

    I like to spy on other boat and dinghy classes for possible setup ideas, and in many areas it looks as the development runs in circle… For example the very new and relatively high-tech class called Seascape18 comes with a carbon mast but – without spreaders. Take a look:

    http://www.seascape18.com

    It is a sealed tube, engineered in such a way that it bends the way racers would want, and all halyards running externally…

    Regarding the jib tension – it seems that recent trend is towards halyard systems that lock on the mast and then the tension is brought by a purchase under the furling drum…

    http://www.karver-systems.com/site/us_produits_hook.php
    http://www.karver-systems.com/site/fr_produits_emmagasineurs.php

    They say there is some benefit to reducing the compressive loads in the mast.

    Speaking for myself, I’m not crazy about this lock/purchase gadget as it seems quite complicated. But if someone stole my mast, or if I broke it, I’d put some effort in investigating the possibility of having a new one without the spreaders. Of course, I don’t care about racing, and like the idea of having as few holes in the mast as possible (especially in the area where it is most likely to break – half distance from shroud hounds and jib halyard sheave to deck).

    Best wishes,

    Mato

    #7916
    Swiebertje
    Participant

    @matoi wrote:

    http://www.karver-systems.com/site/us_produits_hook.php
    http://www.karver-systems.com/site/fr_produits_emmagasineurs.php

    They say there is some benefit to reducing the compressive loads in the mast.

    Bolocs!
    The forces on mast and stay (or luff) are exactly the same regardless where the tensioning device is located. Tension on the stay (or luff) will compress the mast, that is simple (secondary school) mechanics. I for one, want my Genoa touching deck to close the “air leak”. That is far more important then the location of the tensioning device. (Mine is flat on the top of the CB case and the mast it still compressed 🙂 ).

    #7917
    Anonymous
    Inactive

    @Swiebertje wrote:

    @matoi wrote:

    http://www.karver-systems.com/site/us_produits_hook.php
    http://www.karver-systems.com/site/fr_produits_emmagasineurs.php

    They say there is some benefit to reducing the compressive loads in the mast.

    Bolocs!
    The forces on mast and stay (or luff) are exactly the same regardless where the tensioning device is located. Tension on the stay (or luff) will compress the mast, that is simple (secondary school) mechanics. I for one, want my Genoa touching deck to close the “air leak”. That is far more important then the location of the tensioning device. (Mine is flat on the top of the CB case and the mast it still compressed 🙂 ).

    Actually you are wrong.

    Try thinking about the luff of the sail as a means of pulling down the mast.

    If you anchor the top of the sail onto the mast, then pull (say) 400lbs down on the foot of the sail, then you are applying (approximately) 400lbs force acting downwards at the top of the mast.

    If, instead, you route the halyard over a block at the top of the mast and bring it back down to deck level (as per a conventional halyard) then in effect the masthead sheave introduces a 2:1 purchase. Whatever force you apply on the tail of the halyard is approximately doubled in terms of the downward pull on the top of the mast.

    The critical difference is that the system in the link locks once the sail is up, so there is no longer any load on the halyard and it works as per my first example – ie the compressive load is (approximately) whatever tension you apply to the luff, not double that amount.

    It was fashionable at one point to use main halyard locks where you hoisted the main and it latched at the top of the mast so that the halyard was no longer under tension. This was exactly the same principle, and solely for the reason of reducing compressive load.

    #7918
    Swiebertje
    Participant

    @John1642 wrote:

    If you anchor the top of the sail onto the mast, then pull (say) 400lbs down on the foot of the sail, then you are applying (approximately) 400lbs force acting downwards at the top of the mast.

    If, instead, you route the halyard over a block at the top of the mast and bring it back down to deck level (as per a conventional halyard) then in effect the masthead sheave introduces a 2:1 purchase. Whatever force you apply on the tail of the halyard is approximately doubled in terms of the downward pull on the top of the mast.

    No, at the end of the day you still want your 400 lbs tension on your luff to get the same trim. It is correct that there is a 2:1 purchase but it only means you have to pull the halyard half as hard to get that same rig tension.

    #7919
    bigal
    Member

    Sorry folks , I know I passed matric physics as long ago as 1950 but surely the sheave at the top of the mast acts only as a guide – it provides no gearing !

    Only if you pass the halyard thru ‘ a block at the top of the mainsail and back to a fixture at the top of the mast do you get a 2:1gearing .In other words you have to pull twice as far to get the mainsail up but it is half as easy !

    I appreciate that you younger people have got us to the moon but some basics don’t alter .

    #7920
    matoi
    Member

    Swiebertje – you are wrong in both your posts here,
    bigal – I think you misread something, it’s not about mainsail halyard.
    John, I would agree with almost everything you said.

    It is true that in both systems the same halyard/luff wire tension yields the same tension in the shrouds. But if the system I mentioned was applied to a W rig, there would be about 20% less compression in the mast than with our standard system.

    The influence is less than 50% because a big part of compression comes from shrouds. If the mast was indefinitely stiff, we could do away with shrouds, and then all the compression would come through the foresail luff wire and the difference would be closer to 50%. But, since we don’t have such masts yet, we still need tensioned shrouds. These push the mast downwards too – and equally in both systems. An even better benefit, in case of such yet nonexistent masts, would be that all the worries about breakage could be easily forgotten in the first place.

    Wether this 20% difference would make any significant impact on safety on such a small boat as a Wayfarer, I do not know. There are things which make much more difference on big boats than on very small ones. But, perhaps all this would be interesting to discuss with someone involved with mast and rigging manufacturing like Seldons or similar. Unfortunately I don’t have any such contacts, so it’s up to someone else.

    Best wishes,

    Mato

    #7924
    Swiebertje
    Participant

    Mato,

    The theorie behind the device you referred to is that we currently have the luff AND the halyard inside the mast pushing the mast down. It is true that force on the top halyard sheave has to cope with the luff tension AND the tension of the part of the halyard inside the mast. Ignoring the fact here that the luff is not fixed at the mast foot but some distance away at the bow, This is the 2:1 purchase John mentioned, BUT the halyard inside the mast does not contribute to mast compression. It runs over another sheave at the mast foot, and then back up to the halyard rack. The tension on the sheave at the foot is equal to, but in opposite direction, to the force at the top sheave. These two forces balance one another and hence the tension on the halyard part inside the mast does not contribute to mast compression.

    Think of it this way: If you were to tie a piece of bungee cord from the mast top to the goose neck. Now, if you would tighten that bungee real hard, how much pressure would it put on the mast foot? Answer: None at all, because the force art the top of the bungee and the force at the goose neck cancel out one another.

    Only the forces that try to bring the hounds closer to the mast step count if you are talking mast bend. And those remain the same, regardless where the tensioning device is located.

    #7927
    matoi
    Member

    ‘compressive loads in the mast’ in question here don’t have anything to do with mast bend or pressure on mast step. It’s about compression inside the mast itself, within the material of which it is made.

    If we make a paper tube as a simplified model of a mast, and then stretch a piece of elastic gum from one end of it to the other – the elastic will create compression within the paper and the tube will crush. If we had a block on one end of the paper tube and lead the elastic around – it would have been even easier to crush the tube…

    Now, what I’m really interested in is this: imagine the elastic over our paper tube tensioned just to the limit before the tube collapses. If we hold the tube at one end, all is well as long as it is still. But try shaking it and the inertia of the tube will add up some pressure to that exerted by the elastic – resulting in a collapse.

    So if we transpose this to a situation at sea where the boat is (over)loaded with cruising gear, the weather deteriorates and some short and steep waves begin to repeatedly accelerate and decelerate the boat… How do our masts feel? Are they so strong and overbuilt that it doesn’t bother them at all, or do they quietly protest: “Ease that jib halyard dam***, put on some sort of a running backstay for chri*** sake !!!”….

    Majority of experienced Wayfarers probably developed an attitude towards these things intuitively. As I’m packed with only limited experience, I would appreciate anyones thoughts on this.

    Cheers!

    Mato

    #7928
    Anonymous
    Inactive

    Hopefully this explains why a fixed anchor point at the top of the mast is different to a halyard running around a sheave and back down to deck level.

    Consider the following (very rough!) sketches…

    In 1A we use a turning block to double the force we apply when dragging a weight along the floor. We can clearly see that removing the block and fixing the rope directly to the weight as in 1B halves the mechanical advantage.

    In 2A we have transposed the exact same system vertically. Instead of a weight, the block is now pulling downwards on the ceiling. Again it has the effect of (approximately) doubling the force applied to the running end of the rope. Again we see in 2B that removing the block halves the mechanical advantage.

    In 3A the block is no longer anchored to the ceiling, but instead to a sheave at the top of a mast. Everything else remains the same, and whatever force we apply to the rope produces (approximately) double that force applied downwards to the top of the mast. 3B demonstrates again that removing the block and anchoring the rope at the masthead halves the downward force on the mast.

    Note that it doesn’t matter whether the tail of the halyard is cleated halfway down the mast or led via a block at the bottom and into the boat – whatever tension is in that rope between it’s effective bearing points (either sheve to sheave or sheave to cleat) will be transferred to the mast as a compressive load. The system discussed earlier in the thread removes this load as there is no longer any running load on the halyard where it runs up the mast.

    Here is a more detailed analysis of the forces acting on a typical rig:

    Assume that angle “a” is the angle between the shrouds and the mast – lets say 15 degrees, and “b” is the angle between the forestay and the mast – say 30 degrees. We now apply 400lbs tension onto a conventional job halyard.

    This gives a direct compressive load of 400lbs on the mast between the mast foot sheave and masthead sheave. There is a also an indirect compressive load due to the tension in the forestay, as shown in the right hand vector diagram. Rj is the forestay jib tension (400lbs), which can be resolved into a horizontal component Xj of 200lbs, and a vertical component Yj of 346lbs.

    For the mast to stay where it is, the horizontal component must be resisted by the shrouds. Therefore they must exert an equal backward force of 200lbs, shown as Xs in the left hand vector diagram. This in turn produces a vertical component Ys of 746lbs.

    So the total compressive force on the mast is halyard tension + Yj + Ys = 400 + 346 + 746 = 1492lbs.

    But if we anchor the halyard at the top of the mast so that only the luff wire provides the tension, then we can remove the 400lb halyard compression and the total compressive force acting on the mast drops to 1092 lbs, a reduction of 27%.

    (sorry for any drafting errors – typed in haste!)

    #7929
    matoi
    Member

    Yes!

    John, I have been using only slightly different numbers (taken from drawings based roughly on class rules data):

    a = cca 22 deg (angle between jib luff wire and mast)
    b = cca 12 deg (angle between shroud and mast)

    Rj = 70 kg (tension in luff wire)
    Rs = 2 x 150 kg (tension in shrouds)

    I got Rj by measuring it some time ago, and also by calculating from the equation 2 x Xs = Xj (oddly enough, the measurement was quite close with the calculated result: 64 vs 70 kg 😮 )

    By the way Xs is a little tricky to calculate since the shrouds are not in plane with the mast and jib halyard, so the angle of spreaders to centerline must be also taken into account – I used
    c = cca 66 deg

    Anyway, weather our numbers are perfect doesn’t matter – it’s all probably a very simplified model since in reality the mast bends, twists and loads spread through the material in very complicated ways. But these sketches and figures do give an idea of relationships and show a rough order of magnitude of these loads.

    #7943
    Bob Harland
    Participant

    NB This topic is now locked
    Thanks for your contributions.

Viewing 11 posts - 1 through 11 (of 11 total)
  • The topic ‘Compressive Loads’ is closed to new replies.