Weight Stories...

A discussion of weight and displacement in another thread got me thinking about weight stories. Can I tell a couple of weight stories? Pleazze..

Okay, okay... now sit quietly and listen.

Many years ago Bob Walstrom gave me some advice, " to be successful at yacht design, watch your weights". Another thing he said was, " If you are designing big boats, really watch your weights". This was some of the best advice I've ever received.

A number of years ago the office I worked in was approached to submit a proposal for a 90'-100' cutter. The boat was to be built at a very prominent European custom yacht yard. I was handed the project, I made some drawings and put a displacement figure, which I've forgotten, on the drawing. The drawings went to the builder..., "gee, it's awfully heavy" they said. I was being honest about what I thought the boat would weigh. No good, we lost that project to another very prominent designer who claimed the boat would be 20,000 pounds lighter. He did pay me the compliment of using my profile and arrangement for the boat. When she was launched she floated 5.5" deep, about 27,500 pounds overweight. About 11% overweight, no small thing. That's an example of displacement being set by market pressure.

Last story, I promise.
Recently I was working on a large (70') motoryacht project. It was a new, longer version of the builders current model. Eventually the discussion got round to what engines they should order. "Well, what is it going to weigh?" I asked the plant engineer. "85,000 pounds" I was told. (This is the number in their brochure) "Really, are you sure?" I asked. Oh yes, the last one floated right on her lines when she was launched. Okay, can I go measure this existing boat? Which I did, took the measurements back to the office, and sank the hull to match. She was fully loaded with all owner's items aboard, full tanks, dinghy with outboards, etc. She was floating at 116,000 pounds! I don't think the builder wanted to hear this, and perhaps he did not believe me. I'm no longer involved...

All the best, Tad

 

Yes I've seen that too. A lot of women don't like to admit their real weight either.

A few years ago I was involved in an offshore race boat project. When it came time for the final weighing all involved from the directors to the apprentices chipped in a few bucks each and made bets on the weight. So I made a rough mental count of the stuff I knew about and the stuff I didn't and came up with a figure and put it on the list. The weighing was done and my edu-guess was within a couple of kilo's of the true weight.

Seems quite a few people didn't like the naval architect with his spreadsheet winning so about half "forgot" to pay up. Like I told them exactly how much resin to use. Still got enough for a bottle of black label Jim Beam though.

From memory the boat was something like 5500 kg (50 ft class 1 diesel)

Brett

 

LOL! ....give us more! Give us more!!!!

 

I have a couple questions for you guys who design or build boats for a living:

How often do you find new boats in a light ship condition coming in over design weight as opposed to under design weight? Any sort of ratio there you'd hazard a guess at?

And to really open a can of worms, what do you think the major reasons for ending up with a heavier than designed vs a lighter than designed vessel are?

 

Let me take a punt at this one.
Number one would be owners adding stuff to the boat that wasn't in the original spec - an extra watermaker here, a washing machine there, or perhaps a few more built in lockers up on the bridge....
Number two would be builders changing stuff - the design says 4x2, but I always used 4x3 before....and builders always know better than designers...

So, how close was I?

 

It is almost impossible to go under weight. You need to use materials at least the size the design shows. And you need to fill in all of the gaps.

I estimate wood at 30% above the expected density. That covers any adhesives, fillers, and epoxies used.

I estimate epoxy at 20% above the optimal. This gives me a good feeling rather than any weight savings.

 

How often under

Unless a very agressive weight control program is in place, the boat never comes in either at or under weight. Use of margins is an important element of such a weight control program, and any design needs well thought out margins.

Note also the Society of Allied Weight Engineers http://www.sawe.org has a number of very useful references for free download regarding weight control, margins and so on.

 

When I do a weight guestimate, I always include "a little" for paint (often forgotten), some for soakage, and a 5% margin for structural weight, always situated 6" above the calc'd VCG.
I have been a few pounds out in my day <blush> but usually am within 1/4" on the lines when launched.
The builder and how well you know them plays a large part in this art. If your builder is eric Goetz, then you can assume a tight weight budget and exact resin/fabric ratios. If it is Joe Blow-gun then you have to add bigger margins.
Steve

 

When I do my estimates from scratch, I take the time and care to be right. Fitout is the biggest headache so I am always generous with each individual weight and then add 10% to the lot at the end for margin.

If I am extra carefull and use methods of confirmation such as weighing parts, previous boats etc then the end margin can be reduced to say 9%. lol

Boats always get heavier - never the other way.

 

I'm not at all surprised to hear it's easy to come in overweight and tough to come in underweight, and that it can be the fault of either a poor weight estimate with insufficient margins or builders not following the design.

May I now ask the designers if, for new designs, do you routinely do detailed, line item weight tables; or do you compare to a built design with known weights and scantlings and apply scaling factors? And if you've done it both ways, do you find a great discrepancy between the two methods (from Brett's last post it seems as if this may typically be less than 1%)?

 

Chris, I think you misintepreted me hence the . I was hinting at even if you think that you have a good/accurrate grasp of the weights going on you still need a decent margin tacked on. If you manage to predict weight within 1% every time then you are doing fantastic.

 

Someone in Ron Hollands office got the job of doing the estimates for Mirabella V.... not a bad effort either!

 

Probably still 3 tonnes of safety gear to go on...

 

LoL!!!

This is a great thread. I,m sure there are more stories out there. PLEASE!

Gary

 

As I understand it Mirabella V was within 1% of her final weight estimate, not the contract weight estimate and she was at least 40 tonnes over that due to MCA fire insulation requirements!

We have found FRP boat to be a particular problem with weight build up in the structure and have found we need to work with a 5% overlap margin, 7% estimating margin and 10% build margin, so not to have too many problems.