How to calculate load on bow eye (to size new one)?

[Sunbeam]

I've got a question for the more engineering-oriented. I am going to replace my bow eye, which is the original, of unknown provenance, and attached somewhat suspiciously on the inside of the boat (stainless encased in something and gelcoated over - not optimal).

I will, of course, use the new bow eye to help secure the boat to the trailer; but I would also like to be able to use it to secure the anchor rode (snubber) without any undue worry. I'll attach and back it properly. What I'm not sure about is ... how do I calculate the potential loads in order to decide which "strength" to buy? From past anchoring, my feeling is that the most stress is probably put on when waves come into an anchorage and the boat surges against them, but I don't know how to put a number on it for the bow eye (granted, it will be snubbed, but still, the snubber gets stretched tight eventually). I realize that the chain I have (and especially the shackle) have set working/breaking load ratings, but I don't know how the strain put on those links relates to that put on the bow eye.

Of course I can just make a set-of-the-pants judgement, and choose one (which, if I have to do, will probably lean toward overkill), but I thought I would ask the question here, since we have some pretty sharp minds in the group :thup

This is the type of bow eye I plan to put on the boat. They list working and breaking loads for each size, so I can know what I'm buying.

http://www.wichard.com/rubrique-White-0 ... 00-IM.html

Thanks!

 

[thataway]

The 5/16 U Bolt has a SWL of 5291 lbs. That should be more than enough to hold anything, even under the most severe conditions. I believe that William Van Dorn's book on "Oceanography and Seamanship has the loads for various wind, and sea conditions. Earl Hinz may also have some in his book on Anchoring. If I get time, I'll look those up.

You can read about these forces at:

http://www.boatdesign.net/forums/boat-d ... 45116.html

Don Dodds wrote a good article:

http://mantusanchors.com/wp-content/upl ... vealed.pdf

ABYC has their tables: and a storm anchor for a C Dory 22, would pull somewhere just over 5,000 lbs.

As for the trailer--the biggest load would be a panic stop--and that would be a forward and upward load--and I am quite sure less than the 5291 on the bow eye.


I would lay in a few more layers of 1708 with epoxy before putting in the new bow eye. On the other hand, I think what is on my C Dory 22 is adequate, and will just leave it be. You are backing up any snubber with the main cleat, which should have fender washers or a plate under it.

In my boat, I have become intimate with the anchor locker, as I have done a drain hole surrounded by solid glass, and then faired the floor so the locker will drain any water out. The glass is well overlapped and at least half an inch thick--there is also a loop threaded onto the current U bolt to tie the bitter end of the anchor. I measured my U bolt, and the SS shank is 11 mm thick.

This may be a situation which can be over thought! When will you be on the boat with a load on the anchor line more than 5,000 lbs? I think very unlikely!

 

[Sunbeam]

Thanks for the input. I, too, have spent a fair bit of time with my head in the anchor locker. I have the drain and locker bottom/tabbing squared away, but have not done anything with the bow eye. The reasons I don't like it are as follows:

1) General distrust because a few fittings I have removed were not adequate or adequately installed.

2) My guess is that it is stainless, probably of unknown provenance.

3) The after end is encapsulated in something that feels like caulk, and then gelcoated over. Likely not backed up, certainly deprived of oxygen, and the fasteners cannot be accessed.

Since I am going to use it as part of my ground tackle system, I would like to trust it, and I don't trust what is there now (mostly because it's an unknown). I'm less worried about the strength/installation as a trailer bow eye. Although I suppose since the boat/trailer can weigh 5,000#, then there can be at least that much force on the bow eye. Can there be more (?).

Will there be over 5,000# of force on it when the boat surges repeatedly at anchor? That's what I don't know. For all I know 3,000# could be overkill, or 10,000# could be inadequate. I just don't know how to calculate that sort of load, take into a account an unfair pull, etc. (I know that on shackles, for instance, a side pull reduces its rating.) I have a great chart I have used to calculate anchor and rode size over the years, (which interestingly has everything to do with the windage of a boat, and little to do with its weight -- took me a while to get my head around that). Hmm, maybe I can use that to shed some light on the forces involved, come to think of it. I guess I worry more when I'm trying to sleep at anchor

I don't feel I probably need a naval architect, but I figured that someone with a better understanding of physics and how to calculate forces than me might be able to shed some light on it. Is that over-thinking it? Maybe, and if so, guilty as charged :wink: Maybe it's like how I use a torque wrench, when folks with much more experience than me can just "feel" a specific torque by using their hand and a wrench and know it's about right. I guess when I'm not sure about something by gut or experience then I feel better with some calculated numbers to go by. This can avoid overkill as well as "underkill."

Thanks for the links - I'll take a look at them :thup I'll also see if I can dig up my anchoring chart and report back if anything from it looks useful for this.

PS: Come to think of it, maybe since my shackle is the "weak link" in my ground tackle (since there is no high test shackle that will fit into 1/4" high test chain, darnit!), then there is no point in having a bow eye that is any stronger than that? (Except for trailering of course.) Sure wish I had taken more physics and engineering in college...

 

[thataway]

Going back over Van Dorn and Hinz, along with ABYC tables. The most that a C Dory 25 would be subject to is 4,000 lbs, including a 2x factor for surge loading in a n elastic mooring system. Your 22 will be less than that.

Both of these gentleman's and my own experience is that surge loads become a factor in an non elastic--such as all chain--anchoring system, and the load is from the waves, rather from the wind. In the past I have given the experience, when I was anchored along side a friend in a boat half the displacement, but using all chain, without a snubber, or elastic system. We both had 3/8 High Test Chain--I had both nylon rode, and snubbers. His chain was connected to a chain pawl and no shock absorbing. We had constant 50 knot wind--and a series of big swells came into the anchorage. His chain broke. Mine held, even though I had a much larger displacement,and windage vessel.

It is not going to happen at anchor. On the trailer, you are dealing with weak links--chain down to the trailer--type--usually PC, and week fittings at the shackle and eye bolt. 2" wide strap, SWL 4000 to 5000 lbs, if in prime condition--but sitting in the sun, weakens both the strap, and the thread which forms the structure of the loop at the shackle--which is a very much unknown. I have not only put the stern ratchet strap over the boat, but on long trips, put a bow ratchet strap, thru the bow cleat athwart ships.

There are a whole bunch of formulae in Van Dorn's book--but his conclusion is that the loads from wind alone is much less than ABYC tables. Waves are the big issue. Interesting, some of the formulae have been annotated by the author in his own handwriting--so he must have hand corrected at least some of the publications.

 

[Sunbeam]

Thanks for the additional info - much appreciated.

I agree about the waves. The couple of times I felt worried on Powell were not due to wind, per se. We were securely anchored for a blow. But eventually waves came into the anchorage (was hard to find a spot where the wind wouldn't get in at some stage of the blow as it went around - 3 a.m. naturally). The surge forces caused by the waves were what caused us some concern. I didn't have the ground tackle set up optimally yet, the weak point being my lead over the bow. So I aim to improve that. I'll start with the bow eye (attaching the snubber there) and see how I like that system (since I want a trustworthy bow eye in any case); then perhaps go to chocks/strakes on the bow if I want to lead the rode that way. I have a windlass waiting to go on, so that will probably change the leads some.

Thanks again for checking your sources and giving me the benefit of your experience.

 

[T.R. Bauer]

The last thing I would spend time worrying about is the bow eye you have pictured. Your rode, tackly hardware, the anchor set itself, and just about anything else will give before that sucker if done right. Hell, you could swing the whole boat on it from the bottom of a crane for days on end......

 

[Arend]

When we are anchored for Halibut 250ft deep out on the Westcoast, I use 2.5mm diameter monofilament line with a 850 lbs rating.
Even when subjected to large swells, wind and waves it has never broken the line.
The 700-800 feet of line does stretch quite a bit when the boat gets pulled "uphill" on the swells.
We do not trust to stay on this line for overnight anchoring in those rough conditions.
Hope this info is somewhat helpful for you.
I cannot see how any 22 would ever exceed 2000 lbs when using some shock absorber, let alone 5000 lbs.

 

[Will-C]

I wonder how much an asteroid could weigh? Better figure for the worst case scenario.
D.D.

 

[Sunbeam]

This reminds me of something I am genuinely curious about:

I find that in boating, RV-ing, cabinet-making or any number of other endeavors, there is a range of attitudes and comfort levels. A functional cabinet might be a cardboard box duct-taped together, or might be a work of art crafted to a high standard and engineered to be super light. Or anywhere in between.

I don't have the ability (desire/etc.) to get to that utmost latter standard, but I do like to know things are "right" (by my definition), and that I am happy with them. In the case of the bow eye, for example, I have seen many (many) stainless hardware items (everything from tangs to fasteners to shafts) that had crevice corrosion to the point of near breakage (or actual breakage) before they were discovered, failed, or replaced. The combination of "unknown origin" stainless and a lack of oxygen is not a good one. My bow eye has both (score!). So I want to replace it. Me being me, I could probably do a decent job by "guessing" combined with a bit of overkill. I know I won't go wrong buying a good product like the Wichard. But it could also be calculated (we used to hire pros for that on big jobs, but I no longer have those resources at hand). This would probably mean I could go with something smaller/lighter vs. going with overkill just to be sure. I was simply curious if anyone here knew how, or maybe had done, the calculations and who would be willing to share their info. Forces can be tricky (to laypeople like me, anyway).

But it seems to bother some folks (in this and similar threads). At least I get the impression that my "worrying" about it stresses them (although it's just part of a mostly pleasant process for me). I'm not sure why though. And it's not like I'm "wasting" boating time. No, I'm camped in a nice spot in the woods, working on some other projects (not boating related). But of course I'm thinking about my C-Dory, which will be getting my attention soon

Anyway, I am sorry if I cause anyone bother. I know that each person has their own comfort level when it comes to structure, aesthetics, expense, time, etc.. I don't expect others to live, think, or plan my way (or vice-versa) -- but I do still like to share in community and I appreciate the vast knowledge base here, so I post.

On the bow eye: It's looking like I can now choose one of the Wichards with a pretty good idea it will serve the purposes well, and (equally important on a small, light boat) without resorting to serious overkill. Next, I just need to see how long I want the "legs," so that they will go through the bow/stem and whatever backing/reinforcement I decide to use. I'll sleep better knowing just what I have in there, and how it's installed, so I'll have a happy ship

Thanks again for the input - most helpful!

 

[Sunbeam]

In case anyone else is looking at one of these U-bolts (Wichard "Watertight" series), here is an annotated diagram that's also in my album.



The base is a screen shot from the Wichard 2014 .pdf catalog, with a few notes/mods. Reason is, that first I looked them up on the Wichard website, and they have a photo and list the dimensions, but don't label what is "A, B, C," etc., so less helpful. I finally found a labeled diagram in the downloaded catalog.

Then, I noticed that the "regular" U- bolt had the leg and overall length lines shown how you would expect, ending at the bottom of the fastener. But on the Watertight one, they showed the bottom measurement line up a ways, as if they might be allowing for the fastener/washers. Or... maybe just a mistake. So I called them. The first call ended in some frustration and "It's just how the French do it, they probably allowed for the fastener." I called again and spoke to Martina, who was very helpful and saw what I was getting at. She took a moment to measure one in real life, and we deduced that the lines were just put in wrong for the Watertight one, and so it is almost certainly actually measured as you would expect, and the lines are just wrong. I modified the "C" line on the photo to show what I mean. The "L" line should be similar, but I left it as-was to show the original line.

I have used Wichard products in the past and found them to be very good. The U-bolts are 316 stainless. I had a shackle with a molded "saddle" made of the same material that the "Watertight" molding is and it seemed very good. I plan to order one of these for my 22.

Sunbeam

 

[rogerbum]

This reminds me of something I am genuinely curious about:

I find that in boating, RV-ing, cabinet-making or any number of other endeavors, there is a range of attitudes and comfort levels. A functional cabinet might be a cardboard box duct-taped together, or might be a work of art crafted to a high standard and engineered to be super light. Or anywhere in between.

I don't have the ability (desire/etc.) to get to that utmost latter standard, but I do like to know things are "right" (by my definition), and that I am happy with them. In the case of the bow eye, for example, I have seen many (many) stainless hardware items (everything from tangs to fasteners to shafts) that had crevice corrosion to the point of near breakage (or actual breakage) before they were discovered, failed, or replaced. The combination of "unknown origin" stainless and a lack of oxygen is not a good one. My bow eye has both (score!). So I want to replace it. Me being me, I could probably do a decent job by "guessing" combined with a bit of overkill. I know I won't go wrong buying a good product like the Wichard. But it could also be calculated (we used to hire pros for that on big jobs, but I no longer have those resources at hand). This would probably mean I could go with something smaller/lighter vs. going with overkill just to be sure. I was simply curious if anyone here knew how, or maybe had done, the calculations and who would be willing to share their info. Forces can be tricky (to laypeople like me, anyway).

But it seems to bother some folks (in this and similar threads). At least I get the impression that my "worrying" about it stresses them (although it's just part of a mostly pleasant process for me). I'm not sure why though. And it's not like I'm "wasting" boating time. No, I'm camped in a nice spot in the woods, working on some other projects (not boating related). But of course I'm thinking about my C-Dory, which will be getting my attention soon

Anyway, I am sorry if I cause anyone bother. I know that each person has their own comfort level when it comes to structure, aesthetics, expense, time, etc.. I don't expect others to live, think, or plan my way (or vice-versa) -- but I do still like to share in community and I appreciate the vast knowledge base here, so I post.

On the bow eye: It's looking like I can now choose one of the Wichards with a pretty good idea it will serve the purposes well, and (equally important on a small, light boat) without resorting to serious overkill. Next, I just need to see how long I want the "legs," so that they will go through the bow/stem and whatever backing/reinforcement I decide to use. I'll sleep better knowing just what I have in there, and how it's installed, so I'll have a happy ship

Thanks again for the input - most helpful!

I don't think you've caused any bother and I don't think your worrying stresses me out. That said, I do worry a little about your worrying. To me there's a fine line between spending time to make the boat near perfect, and getting the most use/value of the boat. We all draw those lines in different places and I have no doubt that by the time you're done with your boat - well we're never really done ;-) - it will be a far better boat than the one that left the factory. But for me, I'd rather be fishing than spending all the time I'd need to spend to make the boat near perfect (but it gets a little better each year). You on the other hand, seem to love spending that time improving that boat and of course, for you, that's great! It really is "whatever floats your boat".

 

[Will-C]

Sunbeam,
Don't worry about stressing me out. I was just poking a little fun not being nasty. Pick out the best bow eye so you can rest in peace, well you know what I mean.
D.D.

 

[Sunbeam]

Roger,

Thanks for the feedback. I do hear what you are saying. There are many people with "less perfect" boats out using them more than I do mine (so far), and that is a good thing! I tend toward the opposite, which is both a blessing and a curse, but I am having fun, I do love my boat, and what I do gives me satisfaction, so it mostly works for me. That said, I don't "love" working on boats, and if I could buy one already how I wanted it, I would. (It's not impossible, but rarely happens just at the time I'm looking to buy something.) And too, there is the fact that after working on bigger/older boats, wherein the projects were more complicated/heavier/expensive/long/daunting, it's kind of fun to be doing such manageable ones on my C-Dory, which started out in great shape anyway.

Sunbeam,
Don't worry about stressing me out. I was just poking a little fun not being nasty. Pick out the best bow eye so you can rest in peace, well you know what I mean.
D.D.

Ha ha :lol: Thanks, I do know what you mean. Sometimes I just bristle a bit because why should my standards bother someone else, and as you can guess, it's not the first time I've heard similar (although, make no mistake, there are *plenty* of things on my boat that are not up to snuff, but still I know it comes across like I'm a perfectionist). Part of it is that I just really enjoy learning about things (such as how stresses are calculated), so there is that facet too. A long time ago I worked in parts, and as you can imagine, I had a ball. Nothing but details, learning about stuff, looking for "unfindable" things, and solving mysteries. Pure fun!

I'll have to wait for the right time to tackle the bow eye, because I need to remove the existing one in order to see how long the legs should be (also including whatever reinforcements I add). It will be nice if the new one has the same distance between the legs - I'll report back once I do something with it.

 

[thataway]

I would remove the current bow eye, measure it with your micrometer, and then find the Wichard, which has the closest diameter and spacing. It undoubtedly be stronger than the one which came from the factory. That way you will not have to do much modification to your boat, and have something which makes you happy.

I have given you the information from the two acknowledged experts in anchoring loads. If you want the complicated formulae, I will be happy to supply them to you in a PM.

I don't follow the logic of a "Waterproof" U Bolt, since you will/should be sealing it as you install it with 5200.

Now if you were going to tow the C Dory 22 at 25 knots (and people do tow boats bigger than this hundreds of miles at high speeds), then you would be looking at a total redesign of the tow attachment point--which would mean beefing up the bow glass on both sides, plates inside and outside the hull welded in the place of the bow eye, and even cables going to the transom. That type of load is not going to happen.

The trailer loads are so dependent on the setup on the trailer, bunk carpet, vs slicks, rollers vs bunks, @ of bunk area, other straps, stern strap attachment etc. Too many variables unless you were specific about all of these--and even then???

 

[Sunbeam]

I would remove the current bow eye, measure it with your micrometer, and then find the Wichard, which has the closest diameter and spacing. It undoubtedly be stronger than the one which came from the factory. That way you will not have to do much modification to your boat, and have something which makes you happy.

That's what I'm thinking too. And looking at the leg lengths, I'll probably end up with one that's a "notch" up from the minimum strength anyway, because I want extra length to accommodate backing/reinforcing.

II have given you the information from the two acknowledged experts in anchoring loads. If you want the complicated formulae, I will be happy to supply them to you in a PM.

I think what you already gave me is fine, and thank you for it. Your mentions also reminded me that I had the ground-tackle load calculation article and tables from an older Pardey book, so that gave me something to go on as well. I had forgotten about that.

II don't follow the logic of a "Waterproof" U Bolt, since you will/should be sealing it as you install it with 5200.

I hear you, and I'm not necessarily going to be relying on their fitting to be "waterproof" -- I'll have to look it over and see what I think once I have it in hand. I don't tend to use 5200, but will use a good bedding/sealing compound if it seems called for (depends on what the white/black collar seems like). The reason I think I will like those fittings is that that white/black "collar" on the U-bolt is likely a nicer shape to put up against something (such as my gelcoat/stem) than just a flat stainless plate. I don't *know* that for sure though (although I have had a shackle of theirs that had a molded line "saddle" that I believe was the same material and it was pretty nice). Actually, I may have had one of these U-bolts in the past too, but I can't remember. I'll probably order one of these and one of the "usual" ones with the two stainless plates just to see which I like better. I'll report back when I do.

I understand about the variability of trailer loads. And really I think there are many other weak links in trailer geometry such that I will never get them all eliminated, so really, I wouldn't be changing the bow eye just for trailering. It's anchor snubbing that's driving this (although the improvement won't hurt for trailering, of course). Once when a friend was driving my car/sailboat/trailer and went into a skid/series of fishtails at highway speed on an icy patch, it was not the bow eye that gave way; but rather the whole bow stand "moved aside," and thus allowed the boat to move forward past it (as it scraped the port-side opsides :cry ). Luckily my over-the-cockpit strap held, as did the bow attachment down to the trailer frame, so the boat stayed on the trailer and was relatively easily repairable (although there were several damaged places). I might have aged a bit in the passenger seat though!

 

[T.R. Bauer]

The weakest link in your system is its maximum rating no matter which one of the suckers you put in.

 

[Sunbeam]

The weakest link in your system is its maximum rating no matter which one of the suckers you put in.

I'm not sure I understand what you are saying. I do get that ground tackle is a system, and that a system is generally no stronger than its weakest link. But I'm not sure that's what you mean (?)

If that is what you mean though, then the reason I posed the question is that I wasn't sure that the forces on a bow eye would be "equivalent" to, say, the shackle, which is otherwise my weakest link by quite a bit (because I have 1/4" high test chain, but it is not possible to get high test shackles to fit it). So in other words, say the shackle is rated at 1500# working load and 2000# breaking load (I'm making those figures up for the sake of discussion, although I have the real ones elsewhere for reference). Then does that mean the bow eye need only be rated that high to be "equivalent"? OR are the forces on the bow eye different so that in order to be "not weaker" it needs a higher WLL/BLL? In my lay-person's mind, it just seems like forces on the fixed bow eye might be greater than forces on the "fluidly placed" shackle (that is between the chain and the rope, and generallly gets a relatively straight-line pull). And what about trailering? These questions are why I posted.

 

[BrentB]

Having a better part is ok but proper installation is important too.. It seems to me that one limitation is the hull thickness in area. Is it re inforced with fiberglass mat or just buildup of extra epoxy ? With a strong pull, the eye could pull out like a tooth.

 

[thataway]

This is the type of rig which is used for towing 25 foot boats at up to 25 knots across open water and the Gulf Stream. In these boats, the hull interior is re-enforced with layup of several layers epoxy/cloth, such as 1708 /then mating SS plates on the inside of the hull. The strength is probably limited by the shear load of the 5/16" Bolts--The shear strength of each of these bolts is at least 3,500 lbs. (total over 40,000 lbs) The weldment would be more than a 1/2" SS U bolt. The load is spread out over a substantially greater area.

So if the U bolt, and its specs is not enough--you can always go to this more substantial fitting.

 

[BrentB]

cool
thanks

Do we need twins? :wink: