Modifying Bobstay for new EXCEL anchor

[Pelagic]

Presently installing a new Excel Anchor #13 as my main anchor to replace the original CQR

Not easy when both my bow anchors are deployed for stern docking during our windy NE monsoon season...

I will need to modify where my Bobstay Chain attaches to bow spirit, as the longer Excel catches on the original fix point. (Photo)

I see 2 Options which I have sketched below and would like some opinions from those with some structural experience on this:

Option 1: Move the fix point behind the Fwd. Hawsepipe.
Pros:
No chance of fouling the Excel on the Bobstay Chain as it does not pivot like the CQR
Better Angle from the Dolphin Striker as Option 2 is a straighter lead
Neg:
Does not attach directly under the Forestay, so counting more on the deck box structure at bow spirit for rigidity

Option 2: Modify Original by cutting away interference and extending fix point lower to give Excel better clearance.
Pros: keeps the fix point under the Forestay and perhaps aesthetically better
Neg: It almost becomes a straight line from the Dolphin Striker, so does that already reduce structural support?

I am leaning more towards Option 1 as it is a cleaner arrangement for primary anchor....
Opinions ... Option 1 or 2 ???

[BillKny]

Both of the proposed modifications WILL increase the loads on the bowsprit by a LOT, although in different ways.

Doing this kind of change by “feel” or by popular vote on a forum is a disaster in the making.

If you can’t calculate the changes in loads imposed by these changes and then evaluate the consequences thereof, you have no business making these changes on your own. And anybody who has an recommendation here without having detailed dimensions and loading is blowing smoke.

Get a real rigger to help you decide what to do.

[Paul L]

In option 2 you could increase the length of the dolphin striker a bit.

[Pandor]

Neither of your options will result in a satisfactory outcome. You need a load equal and opposite the forestay at the attachment point so that the bowsprit is not loaded in bending.

You should consult a naval architect. If you must make a modification without a professional, it would be much safer to modify the anchor hawse position to move the anchor aft.

[tkeithlu]

As set, the bowsprit is in compression only, with a whopping force in the forestay and the chain. Don't mess with that - the consequence could be demasting, which I suspect would spoil your day.

Find a new position for the anchor, even if that is on the bow beside the bowsprit.

Bilkny has a good point in cautioning against armchair recommendations that involve changing the rig, but recommendations that do not involve changing the rig may be worth your consideration.

[thinwater]

IF you want people to even do a rough take on the math, post a picture in EXACT profile from a good distance so that they can do the trig. The posted images hardly help. Also straight up and straigh down.



In fact, the tension on the bobstay will probably not change very much. There be a slight increase in compression with option 2 and a slight bending moment, depending on how it is done. Option 1 obviously increases the upwards bending moment.


Moving the roller opening is not a simple matter, depending on the structure.


Another possibility, if the others can't be made to work, is splitting the bobstay into two parts at the dolophin striker. I've see it done.

[FabioC]

In absence of precise measurements, Option 1 does not look promising. Option 2 may have a chance to work with some adjustments.
As others have said, before proceeding, a reasonably accurate load computation is required. It is not an overly difficult calculation, if you have some basic knowledge of vectors. A rigger should be able to do it and provide advice.

[tkeithlu]

See the post in the middle of the chain? The vectors are such that that rather small post, with a rather small compression force, can create a whopping tension in the chain. In a common application of that principle, a person can push sideways on a tight chain between a car and a tree, and pull the car out of the mud.

[CDR Smith]

Might a better option be to investigate a different new anchor than the Excel? Maybe a Mantus II (no roll bar) might work. Perhaps if you sent good photos to a manufacturer (like Mantus, for instance) you might find a modern design that would work without having to involve a naval architect/engineer.

[David B]

Originally Posted by Pelagic

Presently installing a new Excel Anchor #13 as my main anchor to replace the original CQR

Not easy when both my bow anchors are deployed for stern docking during our windy NE monsoon season...

I will need to modify where my Bobstay Chain attaches to bow spirit, as the longer Excel catches on the original fix point. (Photo)

I see 2 Options which I have sketched below and would like some opinions from those with some structural experience on this:

Option 1: Move the fix point behind the Fwd. Hawsepipe.
Pros:
No chance of fouling the Excel on the Bobstay Chain as it does not pivot like the CQR
Better Angle from the Dolphin Striker as Option 2 is a straighter lead
Neg:
Does not attach directly under the Forestay, so counting more on the deck box structure at bow spirit for rigidity

Option 2: Modify Original by cutting away interference and extending fix point lower to give Excel better clearance.
Pros: keeps the fix point under the Forestay and perhaps aesthetically better
Neg: It almost becomes a straight line from the Dolphin Striker, so does that already reduce structural support?

I am leaning more towards Option 1 as it is a cleaner arrangement for primary anchor....
Opinions ... Option 1 or 2 ???

No way option 1 - no way!
That is totally not structurally sound. The bobstay is directly under the forestay to transfer the load from the forestay. Back from there - even by a little bit, is introducing stresses that area was never designed to handle.

Think of the stresses involved with the boat pitching in a decent sea. With the bobstay moved that far aft, it would be an interesting exercise to see how long it took for disaster to occur.

Hopefully you can do something around the option 2, but that area will need to be beefed up to allow for what you intend to remove.

Get an engineer - or at least a rigger involved, and good luck with the project.

[Dsanduril]

For all of the reasons posted above I don’t think Option 1 is good, and Option 2 is questionable. How about Option 3, extending the dolphin striker? If pushing the chain “down” would serve your purposes this might be feasible with a relatively modest engineering effort.

[Pelagic]

Thanks to all for your comments and cautions.

No worries, I am only beginning my research, so not in any hurry to execute.

Luckily, as Owner’s Rep for design and build of new Superyachts, I’ve been fortunate to consult with some of the world’s best naval architects and structural engineers on rigging issues. So I have a good understanding of the forces involved, but not dumb enough to just guess!

I simply use CF to help organize my thoughts, make a list of data needed and brain storm with experienced technical sailors. I do this both to help myself and share to help others.

The final solution will be verified by 2 different engineers, once I can give them the structural Data (Scantlings) of the integral bow spirit, Sail areas, vectors and detail dimension drawings of “as Built” rigging situation.

Firstly, I am committed to keeping the staggered 2 anchor layout on the bow spirit as it is a wonderful and elegant solution for many reasons.

Option 1 is based on the Scantlings of the Integral Bow spirit.

Being Dutch built of high tensile Corten Steel, with a large centerline longitudinal beam and doubler in way of Forestay, everything on this boat is overbuilt like a tank, which is the only reason I would be considering it.

Option 2 is more of a “fine tune”, but I first need the clearance to bed the new anchor home and measure vector changes and deployment issues.

The great thing with steel, is that you can cut and reweld properly without loss of strength, so today I will do that and measure scantlings and vectors.

My first cut will remove the obstruction and second cut will be to fine tune the bedding of anchor
Stay tuned!

[Pelagic]

Oops, downloaded the wrong photo for first cut.
I will just nibble away at where it obstructs to see how much modification is needed for deployment

[Alan Mighty]

Would it not be possible to change your anchor roller (a larger diameter roller, elevated in a larger fitting) so you increase the angle that the anchor shank makes to your bowsprit, thereby changing the angle and orientation of the fluke of the anchor - just so the fluke then clears the lug to which your bobstay attaches?

[Uncle Bob]

Originally Posted by Alan Mighty

Would it not be possible to change your anchor roller (a larger diameter roller, elevated in a larger fitting) so you increase the angle that the anchor shank makes to your bowsprit, thereby changing the angle and orientation of the fluke of the anchor - just so the fluke then clears the lug to which your bobstay attaches?

This was my thought as well.