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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > A formula for seaworthiness?
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Thread: A formula for seaworthiness? Reply to Thread
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Topic Review (Newest First)
09-29-2007 07:23 AM
Tartan34C
Quote:
Originally Posted by catamount View Post
Robert,

Doesn't the factor of safety calculation depend on what stress you assume to be the ultimate stress for which you are designing?

Bob Perry's calculation of FS=4 could result in the exact same specification as RichH's calculation of FS=6 if they are considering different ultimate stress criteria...?
I think this quote from Eric W. Sponberg sums it up better then I could. Just to put this person into perspective, Mr. Sponberg is a life member of the Society of Naval Architects and Marine Engineers as well as a licensed Professional Engineer in the state of Connecticut. Mr. Sponberg is also a corporate member and licensed Chartered Engineer of the Royal Institution of Naval Architects in the United Kingdom. He has a naval architectural degree from the University of Michigan. He is a contributor to both technical and popular magazines and a former technical editor for Cruising World. His web site is www.sponbergyachtdesign.com. The snippet by the way, is from a thread discussing mast design originally appearing 7-20-06 on www.boatdesign.net.

START
“There is not really any "rule of thumb" because generally the loads are pretty easy to identify, and they all come from the righting moment of the boat. Typically what I and many designers do is assume that the wind loading on the sails is equal and opposite to the righting moment of the boat. This moment is assumed to come from a distributed load along the mast multiplied by the center of its area above the waterline or above the CLR, depending on how you like to design. "Skene's Elements of Yacht Design" by Francis Kinney (therefore what used to be standard practice by Sparkman and Stephens) assumed that this distributed load along the mast was uniform, i.e. a constant value up the mast. Other designers such as myself assume that the load is larger at the top of the mast, smaller at the bottom. In my case, the top half of the mast is uniformly loaded, and the bottom half of the mast has an increasing distribution from zero at deck level to the same value at mid-height as the upper half of the mast. To summarize, then, the lower half of the mast is a triangular load distribution, and the upper half is a uniform distribution. The total moment of these two distributions (center of its distribution for each area to the waterline) is equal and opposite to the righting moment of the boat.

I hope that's clear. This is a live load. Therefore, you need some factor of safety on top of that somewhere in the analysis, either an allowable stress under live load, or multiply your live load by some FoS and calculate back to the yield stress, ultimate stress, or buckling stress, whichever is appropriate. Typical factors of safety may be anywhere from 1.1 to 4.0. My personal belief is that if you have to use a FoS over 5.0, you don't know enough about the problem to engineer it properly. FoS over 5 and you are overbuilding.

So to answer your question, there is a fairly well defined procedure for identifying the loads, therefore we don't need to rely on a "rule of thumb".”
END
All the best,
Robert Gainer
09-25-2007 01:32 PM
catamount Robert,

Doesn't the factor of safety calculation depend on what stress you assume to be the ultimate stress for which you are designing?

Bob Perry's calculation of FS=4 could result in the exact same specification as RichH's calculation of FS=6 if they are considering different ultimate stress criteria...?
09-23-2007 07:04 AM
Tartan34C While all the talk about a suitable boat is fine donít forget the luck factor in the equation.

Good luck bad boat bad skipper
Good luck bad boat good skipper
Good luck good boat bad skipper
Good luck good boat good skipper

Bad luck bad boat bad skipper
Bad luck bad boat good skipper
Bad luck good boat bad skipper
Bad luck good boat good skipper

I have known all of the permutations containing the bad luck factor to not make it some of the time. But I have also known some to survive the bad bad bad combination just because the amount of any one category bad was not sufficient to sink the trip. And bad luck with good boat and good skipper will usually survive because the right decisions with good equipment will save the day most of the time.

On the other hand I don't know of any trips that failed while containing the good luck factor even with both of the other components of the equation being bad. I think in the end good luck is more important then skill, experience and proper choice of boat.

But even if you can't control luck you can control the good boat part and good skipper part of the equation and then you have a better chance of succeeding instead of leaving everything to random chance and make no effort at selecting a boat and training yourself or doing any preparations at all. But it is unfortunate that we canít control luck.
Good luck and all the best,
Robert Gainer

Early in the day waiting for the crew so we can start the engines and get to work. We are escorting a kayak race today.
09-22-2007 06:15 PM
Valiente
Quote:
Originally Posted by bestfriend View Post
I don't have enough information to know the answer to this, but aren't full keelers built with considerably more displacement than fin or even cutaways? If that is the routine, then it would also be a function of weight, and where it is put, not just the length of the keel. I am of the opinion that it is more than just one thing. Shape of the hull, ballast, displacement, and keel, all contribute together. I would believe that you can have a very seakindly boat with a cutaway or even fin keel if the rest is designed right. That is what I see with a lot of modern, well respected, "open ocean" boats.
Yes, it's more than just one thing. Full keelers present more lateral resistance in some respects, hence the reputation for good tracking under windvane, but can't get the CG super low (like a "shark" on a stick... ). Internal tankage becomes critical then, as does the total wetted surface. I definitely sacrificed total potential speed for comfort, tankage and a certain amount of "bombproofing" that a steel hull, a box keel (think like a narrow but extremely elongated wing keel, a keel-stepped rudder and a slight improvement in draft. This represented a good compromise, because I didn't have the money for a robust fin keeler with a skeg-hung rudder and a waterline 90% of the total length. There are few boats out there like that, because there are few people out there willing to pay a premium for a performance cruiser that falls on the cruiser side, rather than the racing side.

Shearwater 45, anyone? Put a skeg hung rudder on it and it's near perfect.
09-22-2007 12:42 PM
bestfriend I don't have enough information to know the answer to this, but aren't full keelers built with considerably more displacement than fin or even cutaways? If that is the routine, then it would also be a function of weight, and where it is put, not just the length of the keel. I am of the opinion that it is more than just one thing. Shape of the hull, ballast, displacement, and keel, all contribute together. I would believe that you can have a very seakindly boat with a cutaway or even fin keel if the rest is designed right. That is what I see with a lot of modern, well respected, "open ocean" boats.
09-22-2007 01:10 AM
Valiente I haven't seen more than 40 knots or so, but now I've seen that speed in a narrow fin keeler and in a larger full keeler and the movement is considerably different. The full keeler's movement is definitely more ponderous, and considerably drier (although this could easily be a function of freeboard).

The fin keeler is far more responsive, but it is easier to get "whacked" by a wave mid-tack and it is sometimes hard to get back on course, whereas the full keeler just more or less plows onwards.

Basically, none of this is news to experienced sailors, but it confirms not only what I've read but what I think is going to be more "seakindly" in the long run for us: the full keeler. Yes, we will spend a few extra days on passage, but I can see actually sleeping in 12 foot waves on the new boat, whereas the old fin keeler, while fast, is far too "snappy" to consider sleeping in such oceanic conditions. I mean, I suppose I could, but only with a multi-person crew to "spot" me. The racer-cruiser simply demands a lot of active sailing that the full keeler doesn't.
09-21-2007 09:16 PM
Tartan34C Sailaway21,
Are you advocating a tender boat to reduce the ultimate stress in the rig? It would do that but isnít a tender boat more likely to be rolled over and that puts a greater strain on everything then just the wind load no matter what the wind speed is. Very few boats roll and come up with their rigs intact. So, I think preventing a rollover is high on the list of things to do when in extreme conditions, at least in my play book

If you look at the amount of area under the curve of righting moments for a given design you will see that it takes more energy to roll over a stiff boat then a tender one. I suspect that this means a stiff boat needs a larger, steeper wave to be rolled over compared to a tender one. But a boat with less draft is more likely to slide instead of trip in this situation so a stiff shoal draft boat would be more seaworthy then a deep draft narrow boat such as was popular during the CCA days. I think this is contrary to popular belief and people imagine the perfect offshore boat to be full keel deep draft heavy and narrow. I am beginning to think the perfect boat is wide, (not as wide as todayís boats) stiff and has less draft then in the popular notion.

Now add to this the published report that a wide stiff boat will stay with the profile of the wave and keep her deck edge above water. A tender narrow boat will roll easily and put her deck under which drops the righting moment suddenly and I think a roll is now just a heartbeat away.

Is a stiff boat more uncomfortable? When you get into the type of weather where the boats survival is in question I wouldnít worry too much about comfort. Besides, in that type of wind speed the boat is held down by the wind so stability and comfort arenít tied together. Comfort is now related to wave shape and size. If you have more then 80 knots of wind and seas larger then 60 feet any boat is going to be uncomfortable and the key question is now survivability. This is not to say that when bashing to windward in 30 knots comfort shouldnít be considered a factor. But now its longitudinal stiffness and not stability that determines comfort. Sailing on your ear isnít uncomfortable but pitching is. Now the best boat is exactly the opposite thing. A deep narrow boat pitches less then a wide flattish bottom boat does. I donít plan on spending much time going to weather so my Tartan 34C is the best compromise for what I do right now.

In the end itís six of one and a half dozen of another. All combinations of beam, draft and displacement have been at the same time successful and unsuccessful. The two factors that have had the greatest influence on survivability, every time, time and time again have been luck and skill not boat shape or size.
All the best,
Robert Gainer
09-20-2007 02:02 AM
Classic30 Sailormann, I'd assumed by his talking about "the stresses such a boat endures as well as her crew. Tender may produce the more sea-kindly motion alluded to earlier" he was talking about hull stresses in general.

Thanks for trying to clear things up for me..

--Cameron
09-20-2007 01:23 AM
Sailormann Cameron,

There are a few different uses of the term "tender" amongst sailorfolk. As well as the usage you have employed, it can refer to a smaller boat for ferrying back and forth between a slip and a larger boat, or it can be used to refer to a boat that tends to heel easily. Conversely, a boat that has substantial initial stability is referred to as "stiff", as it takes more force to make it lean over.

I think that the second connotation of tender is what Sailaway meant in his post. The stresses that he goes on to mention, are the forces placed on the windward side of the hull by the shrouds holding the mast in position. His point is that because a boat experiences less pressure on its sails as it heels, the shrouds would be placing less stress on the hull in a tender boat than they would in a stiff one, where the sails remained closer to the vertical.

Hope that I am not making myself sound like a know-it-all, just trying to help you become a bit more fluent in the (ever-confusing) lingo.
09-19-2007 11:48 PM
Classic30
Quote:
Originally Posted by sailaway21 View Post
One area I've not heard discussed, because we really should be talking about boats, not who and how they're sailed, is the debate over stiff versus tender. In my opinion, we reflexively opt for stiff and overlook the stresses such a boat endures as well as her crew. Tender may produce the more sea-kindly motion alluded to earlier. I do not have a boat in this race, but am interested in Mr.Ganier's and other's thoughts on the matter.
A few years back I spent some months on the big white thing you'll see in the "Anchoring Problems" thread. This ship was built in 1916 and is the world's oldest active passenger vessel - meaning it was designed and built as rivetted construction.

One thing I never realised was that, unlike welded steel, rivetted hulls flex. After so many decades of welded patches for hull repairs she now has a really strange tender/stiff/tender jerkiness in any decent sort of sea.

Methinks tender is good - but unfortunately pretty rare these days because it's more expensive to build.

--Cameron
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