How good is the Rocna?

[Kenomac]

Originally Posted by Cottontop

I remember some testing (Panope's?) in which the Ultra reset very well, better than most, and wondered if the SS was the reason. If shedding the bottom contributed to resetting.

Just a hunch, but I think the weighted tip and the missing roll bar have more to do with any resetting advantage.

[downunder]

Originally Posted by Kenomac

Just a hunch, but I think the weighted tip and the missing roll bar have more to do with any resetting advantage.

[Spindrift NH]

This one held me through a Cat. 2 hurricane.
I guess that's good enough.

[Spade Anchor]

Originally Posted by Kenomac

Yes we have. We've owned and used a 55lb galvanized Rocna for two years on our Hunter and now a 45kg stainless Ultra for four years on our Oyster, but you can go on believing whatever makes sense to you.

The Ultra anchor also has a weighted tip, but unlike the Spade, the Ultra also has an air filled bouyant shaft in order to insure landing on the bottom correctly every time.

How do we know it digs in quicker? I can watch it every time following the drop. I let out 1:2 scope the Pam reverses slowly as I watch the very shiny flukes dig in prior to letting out more scope from the bow. Easily seen through clear or eaven slightly murky water.

Air Filled buoyant shaft? Ultra really does a good job marketing.
They do not have an air tight shaft. Even if it did have an air tight shaft, the buoyancy it would create would be a non factor (like a balloon covered in stainless steel). The lead in the tip is what creates the ballast that gives it the self righting action.

[Dockhead]

Originally Posted by Spade Anchor

Air Filled buoyant shaft? Ultra really does a good job marketing.
They do not have an air tight shaft. Even if it did have an air tight shaft, the buoyancy it would create would be a non factor (like a balloon covered in stainless steel). The lead in the tip is what creates the ballast that gives it the self righting action.

Umm, I would be willing to bet, if you run the numbers, a hollow shank would provide significant buoyancy and make a big difference in balance. A ship in fact is nothing but a steel covered balloon, right?

Is the Spade shank sealed?

[Spade Anchor]

Originally Posted by Dockhead

Umm, I would be willing to bet, if you run the numbers, a hollow shank would provide significant buoyancy and make a big difference in balance. A ship in fact is nothing but a steel covered balloon, right?

Is the Spade shank sealed?

A ship has a whole lot more "trapped" air. To have any ballast effect the shanks inside hollow area would need to be at least 4x the size of the metal. So if the shank weighed approximately 20 lbs than it would need a hollow area of about 3ft by 3ft and at least 3ft long. That would be for a minimal effect.

The Spade anchor is a 3 piece design that is hollow and not sealed.

[Dockhead]

Originally Posted by Spade Anchor

A ship has a whole lot more "trapped" air. To have any ballast effect the shanks inside hollow area would need to be at least 4x the size of the metal. So if the shank weighed approximately 20 lbs than it would need a hollow area of about 3ft by 3ft and at least 3ft long. That would be for a minimal effect.

The Spade anchor is a 3 piece design that is hollow and not sealed.

Your numbers are way off -- about two orders of magnitude -- the volume you mention is nearly a cubic meter and will have roughly a TON of buoyancy.

To completely offset the weight of 20 pounds of steel in a shank, so 9kg -- that is, to make it weightless in the water -- you would need a volume of air of only about 0.009 cubic meters or 9 liters, so about 21cm on a side, or 8" on a side.

But you don't need even that much buoyancy to make a significant difference in the center of gravity of the anchor.

This idea sounds like something worth following up on.

[Spade Anchor]

I threw this together quickly so apologizes for the poor quality.

[a64pilot]

I believe if you have a sealed anything and send it off be re galvanized, you may have a problem.
Fresh water weighs 8.34 lbs per gallon, so if had a full quart of airspace, then you have of course 2 lbs of buoyancy.
I believe my Rocna shaft is solid? Roll bar is not.


Sent from my iPad Pro using Cruisers Sailing Forum

[Dockhead]

Originally Posted by Spade Anchor

I threw this together quickly so apologizes for the poor quality.

Thanks for that, but it doesn't have to float, in order to have a useful alteration of the center of gravity!

[Spade Anchor]

Originally Posted by Dockhead

Thanks for that, but it doesn't have to float, in order to have a useful alteration of the center of gravity!

It has no affect. It does not sail to the bottom or anything. It just sinks straight to the bottom.
Its a clever marketing strategy that is about all it is good for.

[Dockhead]

Originally Posted by a64pilot

I believe if you have a sealed anything and send it off be re galvanized, you may have a problem.
Fresh water weighs 8.34 lbs per gallon, so if had a full quart of airspace, then you have of course 2 lbs of buoyancy.
I believe my Rocna shaft is solid? Roll bar is not.


Sent from my iPad Pro using Cruisers Sailing Forum

Yes, but it wouldn't be a problem on a Spade, because you've got the lead to deal with anyway. You could just put a plug in it with instructions to take it out. But I bet hardly any Spades get regalvanized anyway because of the complexity of the job.


Concerning the buoyancy issue:

The main difference between Spade (and the Ultra copy) and other new generation anchors is BALANCE. Spade has (a) hollow shank; and (b) lead ballasted tip. Both of these are more expensive to make (hence the price difference). This makes the top light (and strong), and the bottom heavy, and that's why no roll bar is needed to orient the anchor to go into the bottom.

The hollow shank is like a space frame construction -- much stronger for a given mass of steel. But it is much more complicated to fabricate, so more expensive. It's already ahead of the game without any buoyancy, but buoyancy could make a big difference in the balance and perhaps make it even better. A fairly small force applied at the top of the anchor can make a BIG difference in how the anchor goes down.


By the way - Peter Smith's design for the Rocna was based on a simple piece of water-jet cut steel plate as a shank, to save cost, but in order to keep it from grossly unbalancing the anchor, he designed it to be thin, which required it to be made of high strength steel. All the unpleasantness a few years ago came from a very stupid attempt to save costs further and cut corners on the steel used.


Another advantage of the Spade is the finely made very sharp tip. This also adds costs, but really improves performance in hard bottoms.

[Dockhead]

Originally Posted by Spade Anchor

It has no affect. It does not sail to the bottom or anything. It just sinks straight to the bottom.
Its a clever marketing strategy that is about all it is good for.

You didn't understand what I am saying.

IF the shanks were sealed on Spade or Ultra, the trapped air would add buoyancy and RIGHTING FORCE, to keep the anchor oriented towards the bottom. You'd have to do some fairly complicated calculations to know how much extra force you would get, but I guess it would be significant, at least on the Spade, which has a rather more voluminous shank than the Ultra.

This is a question of righting moment just like boat stability, and it directly goes to balance, which is very important to how anchors perform.

[a64pilot]

It really wouldn't take any complicated equations, drill a hole and measure the amount of fluid it holds.
The hole would be tapped for a pipe plug, or if your the manufacturer I guess you sacrifice one for testing.


Sent from my iPad Pro using Cruisers Sailing Forum

[Dockhead]

Originally Posted by a64pilot

It really wouldn't take any complicated equations, drill a hole and measure the amount of fluid it holds.
The hole would be tapped for a pipe plug, or if your the manufacturer I guess you sacrifice one for testing.


Sent from my iPad Pro using Cruisers Sailing Forum

The complicated part is figuring out the center of gravity and righting moment, not the amount of buoyancy.


Note that trapped air has a negative effect as well -- it reduces the underwater weight of the anchor. But I think this doesn't matter at all -- weight in the shank is not doing anything for holding; it is just unbalancing the anchor.