Weight Stories...

Weight procedure

Basically, the classic comm'l/military weight process is:

1) Use parametric estimates for initial studies to home in on a basic guess and to optimize. This is usually done with design synthesis programs like "ASSET" or whatever a firm has in house, now.

2) Do a two digit estimate, again usually parametically.

3) Meanwhile, determine weight limits ("envelope") for stability, speed, draft, etc.

4) Look at available margins between 2 & 3 and assign allowables to systems so each system has a weight budget.

5) Design systems, doing a detailed weight estimate for each.

6) Begin construction, and track actuals by weighing, and see how you are eating into your margins.

7) Panic

8) A miracle happens.

9) Boat exceeds estimated weight, but there is still margin left over so you are OK.

 

BrettM says:
"If you manage to predict weight within 1% every time then you are doing fantastic."

LOL!! I think "If you manage to predict weight within 1% every time then you are cheating!" may be more realistic.

Steve

 

What I really like is our guest's "Miracle" part, that really helps!

I'm sorry but I would say that the 1% folk must be working in some parallel universe. In my experience, as soon as construction starts, the project weight is out of the designer's hands. Of course if the boat floats wrong, it's the designers fault.

Obviously it was Mr. Holland's fault that he didn't foresee that extra 40 tonnes of junk that MCA demanded. No, that's just the way large projects work, the larger they are, the more complex, and more difficult to keep up with.

Going through my black book I find projects that are 10% over, 13% over, 21% over, and one that floated 3/8" high on launch? A recent really big project started out at 490,000ish pounds, and ended up, (admittedly a slightly larger boat), at 588,000. That's the way it goes.

Part of it that the owner often doesn't really focus until he or she is standing in the main salon, then, very often, they will say, "oh, by the way, I just ordered this, and this, and do we have room for such & such". The poor builder can only say, "no problem". Usually the designer is informed of this the next day. So you become prescient and all seeing, based on past experience your designs become the heaviest boats known to man. But,... no contracts for you, your designs are too old-fashioned, ha!

But, it keeps us entertained and off the street.

Take care, Tad

 

I find that on even much smaller projects than those above, the weight seems to creep up. This happens to me near the finishing end of the project. I've not a long list of projects to build a guestimate on, but my creeping occurs with all the finishing materials and little bits and fudge factors that I did not add into the original computations. Of course, I now know that all of these must be accounted for to begin with. My projects have been small enough that I could make weight measurements (with moment calculations and a bathroom scale) and it always looks like the weight is going well, or even under predictions, until right up near the end when the boat starts eating between meals and balloons by about the 20 percent Tad mentions.

Good judgement is the result of experience and experience is the result of bad judgement.

Anyway, I'm glad that Jeff has gotten me off the "guest" list. In my case, I guess it was more like the "taxi squad" or "injured reserve" list.

 

Very early in my career I landed a contract to be the on-site weight engineer for a project to build a steel catamaran research vessel for a government agency, who were VERY specific about what the maximum draft and trim of the vessel was to be. I inherited the job from a predecessor who had been on the task for the first few months of a year-long build schedule. The predecessor had begun the task using a ledger format of hand calculations.

After the first week I had reviewed her work and entered the results in my new-fangled computer thingy (an Apple IIc, for those here who are elderly enought to remember the little critters), did some projections, and came up with a number to bring to my first production meeting. I bravely informed the assembled suits that projections showed that the vessel might be overweight, but I had barely gotten my feet wet and the figures might need to be adjusted. No panic yet, please, just preliminary information.

A month later I presented a report to management that stated quite clearly that the vessel would be 10% over design weight and well beyond the 3% margin called for by the client. I included in my report where the weight overruns were coming from (mostly wiring and interior outfit) and my reccommendations (I was a cocky little bugger in those days, wasn't I?) of what areas could be looked at to reduce weight. I was thanked for my report and told that my reccommendations would be considered.

I did a second comprehensive report some months later in which I confirmed that the overweight trend was unchanged, and that the areas that I had predicted to be the major culpprits were, indeed, the areas of most weight overrun. I then suggested that there were few places left where the yard could reduce weight aboard, but with immediate and drastic actions the damage could be reduced to explainable levels. Again, I was thanked for my report and told that my reccommendations would be considered.

The week before launch day I presented my penultimate report, in which I gave the directors of the yard the bad news that the boat would go in the water ten tons overweight, 4" too deep, and out of trim 13" by the stern. I also suggested that they begin to formulate some form of coherant explanation of this to the client, because if I were to be asked I would have no option but to be honest and show them the warnings I had issued during the previous months. This would not support the yard very well. My report was met with stony silence.

The next morning I was called to the office of the Building Superintendant. The atmosphere was not pleasant. After some preliminary roaring and blustering, he succinctly told me that, "we have been building ships at __________ Shipyard for dozens of years, and they all floated! We don't need some pissant college kid telling us what to do. If you were on staff I'd fire you right now!" Properly chastized for doing my job, I returned to my desk to do launch calculations.

A week later the boat went in the water and was prepared for trials. Subsequent measurements taken afloat confirmed that my predictions of draft & trim were accurate to within 1.25%, and well outside of the acceptable parameters of launch as laid out in the build contract. In the epilogue to this tale, large sums of money was lost by the yard, senior management (including the "we have been building" guy) was decimated by firings and re-assignments, the boat was not capable of doing the job it was intended to do and had to be refitted and assigned to other tasks, and in the end the yard was sold to a consortium because the directors could not cope with the large debt left by the botched build job on the cat.

I got a very nice letter from the Coast Guard (who would be administering the vessel in its new assignments) thanking me for the very thorough and accurate weight records that I had prepared. They found them to be very helpful in the major refit that they did on the boat within two years of her launch.

Weight & centres, eh? Don't need to pay much attention to 'em.

 

Airplanes are nice. Do they have the same weight creep that boats have?

I am looking at building a small boat under 25' and weights are 1500-3000#. Displacements allow perhaps 1000-1500# for crew and supplies.

A boat 20% overweight (300-600#) cuts deeply into the supplies.

 

Weight engineering

It is worth noting that the aircraft industry has an engineering discipline called "weight/strength analysis" wherein structure is analyzed based on formulations of load resistance per weight. This allows rapid optimization of structure.

 

Hi, it might be a little unfair to compare with areo-buisness as these boats are more "one-off's". But my question is: how is your good figuers on weight-budget compared to budgets on labour-, cost- and time-budgets. Are these figuers more "equal" to projects in other branches where a one-digit-over-% seems to "never happen".

I don't want to turn this interesting thread into new subjects but the low figuers you manage to keep seems to be very good.....

Olle

 

Oh, yeah, and they have a distinct tendency to come out tail-heavy, too, which makes them less stable. And being over-weight with an airplane can make the difference between flying and not. For example, the Boeing X-32 could only be flown in the vertical take-off mode with the intake cowling and fuselage panel doors removed to save weight. At sea-level. Before the summer got too hot at Patuxent River.

From a Lockheed preliminary design review press release "The remaining significant issue involves higher-than-expected preliminary weight estimates of F-35 structural elements. The issue - mostly attributable to structural arrangement, load paths and design immaturity (owing to the early stage of the program) - is under intense study." If even Lockheed says they're overweight and under "intense study" I doubt the structures staff is spending much quality time with the family these days!

You can bet an aircraft manufacturer has a whole staff of weight engineers that absolutely obsess over every weight gain. An aeronautical engineer will sell his grandmother for a 10 lb weight savings. Ok maybe not for 10 lb, but for sure for 15. But they do an amazing job once they have things dialed in. Ive heard of Boeing airliner that came in within a couple of digits of its predicted weight.

I think what makes the difference with aircraft is you have to live within the design gross weight. So if the plane is overweight, that has to come out of payload. Airplanes are individually weighed and the center of gravity determined, and reweighed after major overhauls, etc. And before every flight one figures the weight and center of gravity to ensure they're within limits. There are systems that can be installed in the landing gear to measure the weight and balance in real time for display in the cockpit.

Sometimes the limiting factor on weight can be surprising. For example, the Israeli Air Force found out the limiting factor on the gross weight of their F-4's was side loads on the gear when taxiing. So they taxied to the end of the runway, then filled up the aft tank to more than the normal max gross weight because the airplane only had to go straight for takeoff! Other airplanes will do a mid-air refueling after takeoff because they can fly at a higher weight than the gear will stand. So in general, I think aerospace practice is to keep very close tabs on the weight and cut the margins as close as possible.

 

This is a really interesting thread, particularly as I have the weight problem right now.

We just finished building a one-off 48' power boat and it came out 30% overweight, according to the designers original calculations. I know how hard it is to control the build end, weight saving costs serious money. Looking at the end result, the weight increase looks like a combination of adding some extra equipment, extra structure required for certification purposes, the inevitable consequences of building a 'superyacht' interior and and a weight estimate at the design stage intended to coincide neatly with the known displacement. (existing hull extended)

I've just done a rough analysis of other boats around the same size (LOA and Displacement) and configuration, and the weight of our boat falls just under the mean. This analysis is not that accurate, but I remember doing this excercise at college many moons ago.

Can anyone tell me if designers would normally do this kind of analysis? if so, is there a good model to use?

I know that builders have a tendency to overbuild, but the team here are used to building race boats so they understand the problem.

marcus

 

What kind of margins were included in the orginal weight estimate and did you weigh the hull/deck at any stage before completion? It's also a good idea to weigh items as the come into the shop and go on to the boat, for example engines can be well over the weight stated by the supplier. This can't stop weight build-up but it can where and when the weight went on, and correct it before it's a major problem

 

Although designers (some, at least) are justifiably due a bit of chastisement for doing somewhat optimistic weight calcs at the front end of a custom project, the client must bear some culpability too because he rarely is interested in paying for a thorough weight analysis. This forces us designers to estimate weights by ratios, averages, past knowledge, and best guesses. I am sorry to be a bit blunt, but you often get the accuracy that you pay for.

 

we weighed it several times with load cells i.e. hull and structure, deck on, near completion, weighing the kit due to go on. It seemed to check out and not be a big problem, but it went seriously over at the end.

 

An area we have had problems with is the electrics, particularly when working with unfamiliar regulatory requirements. It’s quite easy to think that it’s just a couple of wires, but in a fairy simple boat we can be talking tonnes.

I agree you get what you pay for when it comes to weight estimates and the trouble you can pay big time at the end of the build when the think does not work!…

 

Parametric

Reyling and Diadola did a paper in about 1991 with parametric weight estimating techniques - in SNAME's Marine Technology. Calkins has some methods in his 1983 SNAME Transactions paper.