Synthetic rigging

[Guest]

Ya all have me lookin into stability now and I came across this.....Ted Brewer from Good Old Boat magazine "However, a very stable vessel may be uncomfortable in a seaway, as it can develop a snap roll. Back in the "good old days," when there were still coasting schooners carrying lumber from Maine to Boston and New York, it was not unusual for the skipper to hoist heavy weights to the mastheads on windless days in order to raise the center of gravity and slow the roll. This was particularly necessary if there was a leftover sea or swell from a storm offshore, as the snap roll of the heavily laden schooner could damage the rig. "

I tried mine today in light conditions with 20kg at the top of the staysail halyard but it wasn't a good test. Yacht stability is not a simple subject at all.

[rigger 47]

Ya all have me lookin into stability now and I came across this.....Ted Brewer from Good Old Boat magazine "However, a very stable vessel may be uncomfortable in a seaway, as it can develop a snap roll. Back in the "good old days," when there were still coasting schooners carrying lumber from Maine to Boston and New York, it was not unusual for the skipper to hoist heavy weights to the mastheads on windless days in order to raise the center of gravity and slow the roll. This was particularly necessary if there was a leftover sea or swell from a storm offshore, as the snap roll of the heavily laden schooner could damage the rig. "

I tried mine today in light conditions with 20kg at the top of the staysail halyard but it wasn't a good test. Yacht stability is not a simple subject at all.

 

I did mention before about making a vessel more tender - basically raising the centre of gravity.

 

A challenge with sailing ships now with the tougher stability requirements is making the rigs strong enough and safe to work in - not much can be done to make the crew strong enough to hold on. I have been thrown off a yard due to a very violent roll - harness caught me - some minor injuries were the result.

On another more tender vessel that had a wide beam to length ratio I have been out on the course yard and had my left boot get filled with water, yet I had never felt like my grip would fail me.

 

bbaya try looking up 'roll testing' - that might be of some interest.

[Guest]

Had an interesting debatewith TL on our recent voyage, I contended that the catamaran was extremely stable making it "flat", but in a seaway the "snap roll"was very tiring. A slower period of roll would be more comfortable.

[Guest]

Maybe these ships need a mast to haul some weight aloft Rigger. This is really interesting stuff. I can see that if an anchorage is rough then just hauling aloft some weight will calm the situation by a large amount. BBay has this problem and finding an easy solution is neat. She has great initial stability but this probably is true when she is upside down but she is very well ballasted so should come upright with help from waves. Cats are a lost cause , can't imagine any thing helping them in this regard but imagine they don't get the whip effect as much as a single hull. I have not much experience with cats but it would be interesting to haul some kg's aloft and see what affect it has.

Still it all points to problems if the weight is reduced aloft by a significant amount by using synthetic stays. No doubt great for a racing yacht but not good on a cruiser unless she has too much weight aloft.

[Battleship 100]

Instead of Hauling weight up the rig wouldn't more sail area be better?

You move faster and have the steadying effect of the sails.

[ex TL systems 63]

mono or multihull I reckon the same boat with no weight aloft ie no mast or rigging will be much more stable. Of course it will have also a lower center of gravity as well, in the same way weight at the ends of a boat induces pitching so it should be better to keep weight central and low, cats would be better to have water tanks under hull floors than on the bridgedeck. My cat has gone from a short temporary 15 kg mast to briefly no mast to now a tall mast at over 100 kg, there is noticeably more rock and roll movement at sea now and it would be nice to take 20 kgs away from the masthead. Think of a mono rocking sideways at anchor , if all the weight is in the keel then that wants to point straight down and stay there, so slowing the motion, but if the aloft weight /force is similiar [less weight but more leverage] then doesn.t it want to keep rocking like a balanced pendulum, with the only thing slowing it being the resistance of the keel in the water. I would want less weight aloft not more.

[Guest]

Yes sail area solves the problem but sometimes we don't want sail area like when reefed down early say like when setting up for night. You may have less sail than ideal but feel better because there's less to do in the dark and on mine that would give her a violent motion. Also at anchor as mentioned.

I don't think the amount of weight needed for this affect is enough to seriously affect the stability of the yacht in regards to righting after a knockdown. In this position the keel has maximum leverage and the mast is likely floating so I doubt any negative impact is likely.

Its a delicate balancing act for sure but its interesting to keep in mind for the control we can have over the boats motion. Sure some would be negatively affected by extra weight aloft, initially tender vessels would not like it but really stiff vessels benefit if they have not been well balanced in this regard. exTL, yours would likely benefit from synthetic rigging.

What about a hose inside the mast that gets pumped full of water when at anchor?

[Guest]

Thanks Rigger,

got onto this.......Michael Kasten and I quote "On boats, it has been well proven that distributing weights away from the roll axis, say into the structure and the rig, is extremely favorable to a vessel's dynamic stability. By direct observation, we have learned that boats that have been dismasted are much more likely to be rolled over. " and

"We find that comfort and seakindliness are enhanced by keeping beam to the least amount necessary for initial stability and / or for sail carrying ability.

Conversely we observe that adding ballast will be counter-productive in terms of comfort."

So by reducing weight aloft will have a detrimental affect on dynamic stability!

"A dismasted sailboat is more likely to capsize due to the greatly reduced "Roll Moment of Inertia" and the consequent relative ease with which a heavy roll can be suddenly induced. This cannot be demonstrated by any kind of static analysis as one would normally expect. Said differently, while the dismasted boat obviously has 'more' static stability without its mast, in the ocean where dynamic forces are at work, the effect is the opposite...!"

Incredible! And....."but the mast is some 25 times more effective at resisting being put into motion...!! " and "Many years ago during the Fastnet Race, rigorous analyses done after the loss of many vessels revealed that the boats which had concentrated ballast, light structure, and very light rigging suffered excessively due to their harsh rolling motions which caused many dismastings, consequent capsizes, and widespread seasickness. "

here's the link.....http://www.kastenmarine.com/beam_vs_ballast.htm

So we need to be very careful with weight aloft! 25 times 20 kg is 500 kg! So my lump of lead aloft is going to have a considerable affect on the motion.

I noticed when I went from 12mm halyards to 8mm an affect on the boats motion but everyone I told thought I was nuts but this was real!

[Guest]

And...."By comparison, boats with heavier structure, lesser "ballast ratios" and heavier rigs resisted being "thrown" into severe rolling, had a much more seakindly roll motion, were easier on their rigs, did not lose their rigs, did not capsize, and did not experience nearly the degree of sea sickness among their crews. This is counter-intuitive because undoubtedly the heavier vessels had a higher center of gravity and therefore less "static" stability. However due to their distributed masses they had much greater "dynamic" stability, which enhanced both seakindliness and seakeeping ability."

Racing Crews beware!!!!!!!!!!!!!!!!!

[Island Time 1,235]

Be careful BB, that you do not over-simplify things. As you said, yacht stability is a complex subject.

 

20 KG at the top of a rig could cause a capsize, and will certainly reduce the stability angle. Remember that 20 KG on the end of a 15m lever gives a moment of 300 KGF.M . - or, perhaps you might compare that to 300KG on the deck, if that is 1m above the Vertical GC. To counter either of these weights you's need 150KG additional weight 2m below the GC, in the keel. That is quite a bit...

 

Also, don't discount the other CG's, the transverse CG and the longitudinal GC. Longitudinal CG mostly effects pitch, but transverse definitely effects roll, and hence roll moment. Water tank placement further from the roll center also adds inertia. And lower, and has no weight when immersed...

 

Even though it is absolutely true that reducing the moment of inertia will make a vessel less dynamically stable, the degree of change is what is important.

 

Is it worth risking your ultimate stability to increase the roll period, when a small course change (or more sail) will often change the movement as well.

[Guest]

Yes. Mine needs it IT. I should explain. She's 32 feet by 11 feet by 5 feet, displaces ten tones. Her mast is aluminium and about ten metres tall. I can carry a large genoa and the full main in 30 knots without going past 20 degrees of heel, hard on the wind. She will be doing 7 to 7.8 knots with a clean bottom. This shows her stiffness and sail carrying power which is really great in flat water. She really is a "racing ferro' as KM jokes. But in a cross sea or chop her motion is uncomfortable, even dangerous. I struck 2 gales from the north and northwest last april going about Ranfurly, east cape, resulting in a really steep sea,somewhat overhead and the motion was radical. So she needs slowing down , calming down in her lateral motion. A lot of this liveliness could be due to not having much cruising stuff aboard. I also have the watertanks under the floors and over the keel and the rather large batteries(120kg) smack in the center behind the mast and under the table. So the weight is very central and below the waterline.

I had heard that weight aloft was x7 weight on the rail but 25 x is another ballgame.

IT this is a nice way to go about the problem. Its easy to get that weight off in a hurry if it becomes a liability and its certainly an interesting experiment.

[Island Time 1,235]

BB every boat is different. You cannot say 7x or 25x or whatever until you know the distances of the center of the mass from the center of gravity...

 

To take someone's word (mine or anyone who is not a trained naval architect) for this is a risk. I'm just saying that the same result could likely be achieved by moving the boats tanks, batteries etc.

 

There is no question that in the ultimate storm a heavy, steel, full keeled, narrow, low free-board vessel is more likely to survive than a modern performance yacht.

 

However, the modern performance yacht may be fast enough to avoid the storm altogether...

 

Every boat is a compromise.

 

My original training (NZCE) was in mechanical engineering. A long time ago... and I do not consider myself an expert in yacht stability in any way.

 

Before making changes to your vessel that will effect stability, my advice would be to consult a trained naval architect who uses your vessels measurements etc ..... Otherwise it's just guesses.

 

At anchor, by all means have a play, at sea it's more risky.

 

It is good to see you are making an effort to understand this though

[rigger 47]

Bbay if you can find the original plans with the specs on materials used in the rig (ie type of wood) then you should be able to work out the difference in weight, then work out what you would need to add at the mast head or part way up.

but was the original wooden mast hollow? I know a lovely yacht with a hollow wooden mast - incredibly light and very strong.

 

My old yacht had a 34ft long deck stepped wooden mast - 2 of us could carry it - so had to be less than 80kg.

Anyone have a figure for a 34ft / 10.3m alloy mast for a keeler?

[Guest]

The 25 x is Michael Kasten's remark from the above link. I went on the Atkin's Website and measured the "Thistle" (which mine is) and then measured the mast and the mast is around 25% longer than the vessel. On mine the mast is the same length as her deck. So I'm in the right room at least.

 

But really, this is about synthetic rigging, which is going to be a lot lighter than stainless so we do need to be very careful about what we use.

 

A timber mast is usually around say 120mm at the top so plus 2.5 metres will be an extra 18 kg in spruce. Probably 20 kg in Douglas Fir. Plus a few kg for the extra rigging wire. I'm thinking mine was setup for Wellington weather.

[Chewing Gum 17]

Cav 32 mast was very short at only 34 ft and she had a 50% ballast ratio. there was no way that boat was uncomfortable in a seaway so I think your problem lies elsewhere. I don't like the idea of adding weight up the rig at all.

[Guest]

I don't have a Cav 32 Chewing Gum. Rigger a solid spruce mast at ten metres would go to about 150kg spruce being around 500kg per cubic metre. An hollow spruce mast would be half that so your figures are pretty spot on. I guess a timber mast has some admiral qualities in regards to stability, extra weight aloft plus it would float if it hits the water. The difference between the hollow wood and solid makes me think that traditionally yachts have not been too sensitive to weight aloft because it would have been the builders discretion as to which way it went. Look at the weight the Junk Rigged yachts have aloft and its pretty obvious that we can safely add weight to the mast.

[rigger 47]

Look at the weight the Junk Rigged yachts have aloft and its pretty obvious that we can safely add weight to the mast.

 

Go careful.

Many vessels have been lost by just adding a little more weight. The thing is everything can be good until you get a little more wind, roll a little further then you reach the point where it all goes bad.

I know of one sailing vessel that has a calculated stability curve that is scary - all good to a point then she will not come back. The thing is she felt great and all of a sudden there is a strange feeling like she is hesitating and deciding what to do, possibly at the angle of loll and only wave action brought her back?

Just go careful.

[BNG 44]

Has anyone considered form stability as a contributor to rolling motion in this so-called thread about Synthetic Rigging?

 

I'd be leaving the theory with the Naval Architects well ahead of putting weight aloft. The old narrow gutted displacement launches used to swing a bucket out on their booms to slow/stop roll motion, have you tried poleing your weights out BBay? Might be safer than something swinging around aloft.

[Guest]

Thanks Rigger and everyone concerned about safety. I hear you. By simply hauling some weight aloft I think it is the safest way to test this. Theories are fine to a point but this way I get to see what happens in real time and if I don't like it I can just drop the weight in a second. I've just lashed it to the inner forestay ( with 3 separate fastenings ) plus the staysail halyard. If the halyard came apart for some reason it is still attached to the inner forestay. Look its only 20 kg, its not a huge amount of weight on a ten ton yacht.

BNG its form stability I'm trying to overcome. I have too much of it! As mentioned the yacht is designed for a mast roughly 2.5 metres taller, boom that overshoots the end of the yacht and all in timber. Someone had better ideas than Bill Atkin and put a shorter mast and boom, less sail area and probably because they lived in Wellington. I've been trying to bring this back to the original thread but it does relate in a pretty serious way. I think synthetic rigging will affect the stability negatively by reducing weight aloft. Thats counter intuitive and thats the danger.

[Guest]

Well went out today in winds gusting to 25 knots and the weight has definitely slowed the motion. Much more comfortable and smoother in her motion now, remarkable difference. It heels exactly as before ( not very much) but it returns much slower. You couldn't get a more effective way to slow the yachts motion.