boat weight estimates

A boat weighs the sum total of all of its component. But as I can only guess some of them my boat estimates cannot be to accurate. I have tried to use the estimation methods in "The nature of boats" but the results i get are a joke. When I estimate hull weight times 1.3 plus engine fuel crew etc the numbers I get are about 60% of what they should be. Can anyone tell me where I'm going wrong or give me a method which works.
mik

 

For accurate estimate, there no other method as calculating item by item, and ADD MARGIN and SERVICE LIFE ALLOWANCE. Getting realistic estimates requires a lot of experience; some designers are getting very attractive numbers which are not possible to follow by the builder. Actually, I would say weight estimates of often a key for any boat design; it is more important than hull shape

I described method we use at initial design stages, for planing monohull powerboats - see papers of Chesapeake Powerboat Symposium 2012. We use this method for power catamarans as well.

For the first guess, would be good to look at similar boats (and make sure the numbers You are looking at are real figures, not advertised figures).

 

Well said, fully agree.

The weight estimate is easily overlooked or given little attention because it is not "sexy" or have fancy colour plots or words that sound important. Yet it is the most fundamental aspect of the design. Get it wrong and the whole design is wrong, regardless how much pontificating and boasting one has done with hull shape/resistance/structures etc.

It takes years of experience to arrive at a good weight estimate. Since one needs a database of "things" and what they weigh, coupled with the actual design that one is actually doing, beit a monohull catamaran and the type such as passenger ferry or patrol boat etc, and whether a simple code of compliance, such as local regulations/ISO or a full blown HSC Code or SOLAS, all play a part in the final weight estimate.

You should break the weights down into the standard format, no matter whether you see them as important or necessary, everything has a weight!

1 Structures
2 Mechanical
3 Electrical
4 Communications & Control
5 Auxiliary systems
6 Outfitting
7 Armaments
8 Fluids in systems

Each group is broken down into smaller and smaller details. An example of a group 1 breakdown is given below.



You can breakdown each line even further by itemising each structural part such as each frame in another spread sheet which is linked to the summary. Takes a while to set up, but once done, it is done for all vessel as you merely copy it.

Once added up to arrive at a final total lightship, as also noted by Alik, you then add a margin to that weight and depending upon the client and other factors another margin for life-time growth.

A rough rule of thumb, add 10% to the lightship weight if you're unsure of the design weights and the actual design may also be a new one with little database to validate or verify. If you have very few weights to arrive at a total, use 20%.

Good luck.

 

Weight estimates separate the men from the boys or more appropriately the professionals from the amateurs. The whole design revolves around getting this segment as right as practical. The only other practical option is, to not tell a client where to paint the boot stripe, until after launch day.

 

I tell them to paint it 2" (or more) higher than my design waterline and that it does not detract from the looks of the boat to have the bottom paint showing. Actually I do like the boot top high rather than at the water surface.

Preventing DIY builders from adding unnecessary weight is a major issue as well as where the weight is added affecting the boats balance.

 

Thank you
I think painting the waterline stripe after launch would be the smart way to go for me at least. Ad Hoc's construction group list seems to work very well for me. My theoretical design PUFFIN 51 which I cannot draw. has the followng dimensions full load: LOA 51ft Lwl 49ft BOA 12ft 11in BWL 12ft 2in draft 4ft 9in D/L = 170 Cb 0.246 MC 0.403 Power is 85hp John Deere for 9.38 kts with 1250usg fuel. 400usg fresh water Freeboard front to rear 7/4.5/5.66. 6 crew/passengers Full load Disp 44500lbs Normal Disp 35920lbs Light ship is about 23186lbs.
I really must try to learn to draw. This design is a long Deisel Duck.
Who needs a thousand word to paint a picture mik.

 

Diadola and Rysling had a paper in Marine Technology a while back with a simple parametric method that works about as well or as badly as any other, to use for a start. It is probably on one of the SNAME small craft CDs.

 

Ad Hocs method is a very good explanation of how it is done. My very first job in the marine industry was in a shipyard calculating weights of structures on the ships we were building. This was long before computers to make it easier. It was very tedious and daunting, and a huge consumer of time, but there are very few things as satisfying as seeing a ship sit on its waterlines just as you predicted. One hot tip. Use the old carpenters rule, measure twice, cut once. Only I did each measurement and calculation three times. Takes a while but eliminates a lot of mistakes. Check, recheck and check again.

 

My first job, i spent the first 3months every day walking around the shipyard with a 20kg spring balance. My job..go weight everything and record it. Tedious but very very instructive. A lesson i've never forgotten. Boats always get heavier...never get lighter. Thus a good weight estimate is the key to everything.

 

I would say the worst mistake the boat designer could make is to give optimistic weight estimates. Though it might not look attractive, but we need to know realistic weights and add the margins, and come up with real figures. This is different from 'out of the box thinkers' - boat designers without engineering background who propose very light 'eye catching' and 'edgy' concepts which will result much heavier once built, and even more heavier once in use.

As an example, we had a (sailboat!) client recently complaining about the design displacement 'too heavy', but the guy never read the ISO8666 and has no clew what is to be included in loaded displacement condition. There is full load of persons to be included, and for 60' boat it is about 14 people (or as much as one wants to take on board for day trip) with personal effects, minimum safety equipment and possible options. I think such customer has to learn his lesson first working with amateur 'out of the box - ignore the rules' boat designer and resulting overweight built, then coming back to us for the good one

 

A little hard to do it that way with a 570 ft, 17,000 ton ship. (LOL)

 

With a sufficiently sized lever, he could get the whole ship or even the world, if a reasonable fulcrum could be gathered.

 

Hmmmm...see your point..hahaha!

Well, it is a lot easier weighing bollards, hatch covers, nav lights piping motors, lifting eyes, seats of 2 3 and 4 man etc etc..with scales and a helpful "heavy gang" on smaller high speed boats that your beast

 

It works. That was also my first job. The boat we were building was heavy as it was always floating below the designed waterline and we could not get her up to predicted speed.

We weighted everything that goes into the boat, including the engine and all of its accessories. Fortunately, it was only a 12 m cat with FRP hull.

For the hull, we weighted all the resin and fiberglass that was used then summed it up. May seem crazy but that was part of the quality control.

For the furnitures, we weigh the finished part or the wood that was used to build it, including the paint and varnishes.

With the sum of everything, we were able to predict how the boat will weigh and we were quite surprised. It was really heavier than predicted.

With a couple of spreadsheets, we were able to analyze which needs re engineering including the retraining of the laminators. It turned out that some habits are hard to break. When the glass would not lay flat or have bubbles, they just add more goo in it.

Nothing much we can do with the engine weight but with a tighter control and some re engineering, the predicted weight and the power needed to attain design speed was realized.

It may seem an overkill but we were able to build a database and subsequent design was built using the database. The only thing we have to weigh is what is not in the database.

 

So I can constitute that good naval architects start as weightmen! Important stage missed by 'edgy concept' designers