Getting Chain Out of the Bow

[thinwater]

A systematic approach to snubber sizing--and on anchor threads I never expect anything to be systematic other than homage to the god of big chain--would have the units of energy, not length, since it is energy we are absorbing. The calculations would be for relatively shallow water, since that is generally the more severe test, and where catenary and the motion of the chain through the water do not help. I've anchored to all chain and to mixed rodes in very strong conditions in shallows, so this I have a feel for. I've seen rollers and bows torn apart by the forces of chain snatching tight, and many anchors pulled from the bottom by the yank.

The energy might be measured as the product of displacement and wave height, since that is the source of peak force in most cases.

The snubber sizing would be in the same terms; at peak stress (15% BS perhaps, to give sufficient fatigue life?) how much energy is absorbed? Not hard to calculate, if the elasticity of the rope is known. Nylon rope is not far from an ideal spring at relatively low elongation and rate, which this is.

Since wave height is relatively fixed (a standard storm is the same for all boats), and elasticity and %BS are relatively fixed, it is small wonder that a % of LOD would be a reasonable fit for line that is of normal size. Makes sense, a good relationship.

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For deep water it would be interesting to see a calculation of the energy required to move the chain up and down through the water. This would require statics, fluid dynamics, and a good model of the direction of motion (up/down is very different than fore/aft, and I'm sure there are both). But since I don't anchor in deep water, I'm not really interested in doing it. But it is a VERY different case from shallow water. Ironic, that the place chain might help most (deep water) is the very place where the penalty is the greatest (weight).

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How do folks decide how much weight is worth to them? In my case, I have in mind how much more I would be willing to pay for the same boat built 1000 pounds lighter. Thus, I know what I will pay to save each pound. How do other people decide, bearing in mind that not all "cost" can be measured in $$ (safety)? For me, weight is worth about $20/pound, which makes high strength chain an absolute bargain, even if it meant carrying a bit more and replacing it a bit sooner.

(That $20 figure is an average--more at the mast head or ends, less in the middle.)

[carstenb]

Originally Posted by Dockhead

By the way, Carsten, it's dAvIt, not dIvAt -- I think you're having a dyslexic moment! Davit - Wikipedia, the free encyclopedia

I don't think the main issue is trim. I think the main issue is the polar moment of inertia around the horizontal athwartships CG. The mass in the very bow inhibits the bow coming up when it meets a wave. My boat's bow is fairly fine, maybe not as fine as a Sundeer's, but not that far off.

Your point about getting rid of the davits is well taken. But removing them will also have the effect of reducing the polar moment of inertia, so will both hurt and help bow rising to meet the seas, at the same time.

As to trim -- indeed, I checked it carefully when I put all that chain on board, and was pleased to see that trim was not detectably changed. But as it turns out, trim is not the whole story . . . . . .

Thank you fro being so kind as to label it dsylexia - my better half has labelled it old age (sigh).

Obviously, moving your dinghy on board will have the effect of moving your weight closer to the boats meta center and thereby increasing the boat tendency to be trimmed on its "toe". This is counter productive to trying to keep water off your decks.

So first you need to move an equal amount of weight an equal distance from your bows. I don't know what you dinghy/engine weighs but lets say 150kgs just for the sake of argument.

If I remember correctly, I measured the with of your dinghy to be just under 2 meters, plus some centimeters that hangs out from the stern to allow access to to your bathing platform. let's say 250cm in all

Moving that on deck required you to move your stern scepters out 80 cm to accomodate the dinghy. 250 minus 80 leaves 170cm

so you are moving 150kgs 170cm.

Now you have to move some combination of this from your bows towards the stern (or remove the equivalent weight from the boat). Easiest method is to simply remove chain. or move 75kgs 340cm towards the middle of the boat or move:

wait for this:

If your boat is 16.5 meters long and you are trying to counteract the movement of weight - any combination of equivalent weight/distance will serve. If we assume that the center of floation of your boat is in the exact middle (it isn't) then the forward half of this distance from center of floatation to bows will be 8.25 meters. Now the calculation is simple 8.25/1.7 = 4.85. 150 kgs/4.85 = 31 kilos.

Therefore moving (or totally removing) 31 kilos of weight from your bows to the center of floation will outweigh the effects of moving the dinghy.

Moving more than that will make your boat sit on its "heel" which you may not want, as this will have other effects (perhaps on your speed or pointing ability)

[Terra Nova]

Weight in the very ends of a boat is BAD.

Don't compromise your all chain rode for very little weight savings.

On TN I have installed a (free) used Nilson vertical capstan windlass near the base of the mast. (photo shows it between the dorade box and the mast) This came off a 50' boat and so was sized for 3/8" chain. This location allows it to be used for other purposes, with turning blocks at the base of the mast and along the full length toe rail-mounted tracks. (see photo)

In the head, next to the mast, I built a tall (skinny) vertical chain locker whose base rests virtually atop the keel's 10,000 lbs of internal lead ballast. This locker holds 270' of 3/8 G4, plenty strong for a 24,000 lb boat. A large, heavy, stainless pipe (~3' long) leads the chain from the windlass's chain pipe to a point directly over the center of the chain locker, which drains to the shower sump/pump. The chain locker takes up space in a rather small head. But its height I made such that it makes a great place to brace yourself while shaving or brushing your teeth.

[Cheechako]

Originally Posted by skipmac

I am assuming the second locker will not work automatically with the windlass. I see it mainly as storage and plan on having to hand feed the chain down the pipe from the main locker.

So would not be using when anchoring frequently. When moving around a lot, doing ICW or island hopping I would keep the appropriate quantity of chain in the main locker. Potentially all of it. Just move the chain into the lower locker when necessary, mainly on a long passage. Of course never stow the chain until well at sea since one never knows when an anchor could be needed in very short order.

Yeah, some sailing couples manage to make this work, one below and one above. The person below gets to pull chain thru the pipe so it doesnt stack up and jam. So maybe a good compromise if you can get at it well, and it's only for the less used part of your rode. Like maybe a 300 ft rode and the last 100 below? It amazed me when I tried it how even a pipe that was at least 45 degrees jammed up.

[Cheechako]

Originally Posted by Terra Nova

Weight in the very ends of a boat is BAD.

Don't compromise your all chain rode for very little weight savings.

On TN I have installed a (free) used Nilson vertical capstan windlass near the base of the mast. (photo shows it between the dorade box and the mast) This came off a 50' boat and so was sized for 3/8" chain. This location allows it to be used for other purposes, with turning blocks at the base of the mast and along the full length toe rail-mounted tracks. (see photo)

In the head, next to the mast, I built a tall (skinny) vertical chain locker whose base rests virtually atop the keel's 10,000 lbs of internal lead ballast. This locker holds 270' of 3/8 G4, plenty strong for a 24,000 lb boat. A large, heavy, stainless pipe (~3' long) leads the chain from the windlass's chain pipe to a point directly over the center of the chain locker, which drains to the shower sump/pump. The chain locker takes up space in a rather small head. But its height I made such that it makes a great place to brace yourself while shaving or brushing your teeth.

Those Freya's are sweet looking machines! I love all the deck space...

[Terra Nova]

Thanks. I like the looks, too--1960's.

[DoubleWhisky]

Originally Posted by Dockhead

You assume (like I used to) that G70 chain is more brittle than G40. But is it really so? Are we sure about that? And if it's not, what does that do to our other conclusions?

There is some of misunderstanding here (not first time, so I blame my insufficient command of English).
I'm rather sure the G70 chain is not more or much more brittle than G40.
It would make it unusable in too many circumstances.
On the other hand I'm quite sure G70 can become brittle after suffering prolonged shock loads. Tension or stress (I'm not sure about proper English word here) resulting from such a loads tend to accumulate in the structure of high tensile steel and after a time it becomes more and more brittle. It was quite understandably explained by Andrew Troup in the Bigger is Better, Part 2 thread, if I remember correctly.

Originally Posted by Dockhead

Coincidentally, my favorite snubber is 10 meters and 18mm -- coincidentally, 60% of my LOD :lol:. Pure coincidence!

May be not so pure coincidence
I read a lot about snubbing on this forum, as myself I'm not subbing expert anyway. I noticed that a lot of people advocate the use of the snubber not shorter than 60 % of LOH (or LOD, if You prefere), and more in rather demanding conditions. For me it creates "rule of thumb" situation.
But if one will think a little more about it...
You are - of course - adjusting the size of the snubbing rope accordingly to Your boat weight. So, for longer (and heavier) boat the "60 % of LOH/LOD" snubber is not only longer, but also considerably thicker and stronger. Really, nylon in action is close to the spring, so with bigger boat You have not only longer, but also heavier, harder spring...
May be - just may be - there is some statistical correlation, making the "60 % of LOH/LOD" snubber (made of appropriately sized line) suitable for average medium displacement boat in average conditions?
I do not know, but I'm not quick to disregard any "rule of thumb" resulting from accumulated experience

Cheers,

Tomasz

[Cheechako]

Well... if I remember my metallurgy correctly, the more heat treated it is, the closer the Yield strength is to the Ultimate strength. ie: about the time it starts to permanently deform, it breaks. Where a mildly heat treated chain may stretch more and still survive. But in the end, the higher heat treated version will be higher in strength before breaking... so not sure it's a down side... other than more corrosive...

[DoubleWhisky]

Dockhead, may be I found an 8 mm anchor chain for You (and for me also)...

Have a look here:

Duplex steel anchor chain

I think it is well worth to investigate further

Cheers,

Tomasz

[savoir]

Originally Posted by DoubleWhisky

Dockhead, may be I found an 8 mm anchor chain for You (and for me also)...

Have a look here:

Duplex steel anchor chain

I think it is well worth to investigate further

Cheers,

Tomasz

Looks expen$ive

[Dockhead]

Originally Posted by savoir

Looks expen$ive

My thought exactly! Do we know roughly the price?

[Terra Nova]

Stainless steel work hardens, and would be my very last choice for anchor chain, except for display purposes

[Dockhead]

Originally Posted by Terra Nova

Stainless steel work hardens, and would be my very last choice for anchor chain, except for display purposes

Does it work harden more than any other type of steel? I never heard this, but if it's true, I will be glad to be enlightened.

[Wotname]

Originally Posted by Dockhead

Does it work harden more than any other type of steel? I never heard this, but if it's true, I will be glad to be enlightened.

Yes, SS work hardens at an alarming rate. Just one bend and that bend is much harder than the surrounding metal.

[DoubleWhisky]

Originally Posted by Dockhead

My thought exactly! Do we know roughly the price?

Somewhere between 20 and 30 Euros.

About hardening - it is Duplex steel 1.4462, not regular SS, and I'm not sure if the problem is common to both kinds of steel.