What is wrong with the BOM?

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jacquesmm
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What is wrong with the BOM?

Post by jacquesmm » Tue Jul 23, 2013 11:24 am

A builder posted this:
I know the study plans usually describe the bare minimum of material (and labor), but the OD-18 says it takes 13 sheets of plywood and C.Larry used 22! 8O I guess it adds up- console, side decks, compartments . . . but 22!!?
The BOM's are correct, you can check against the plans. It's simple math done on a spreadsheet, not much guessing involved.

For the plywood, check the nesting. It's all there, you can build the boat with what I specify BUT I do not include a console or customization. For the OD18 for example, I do not include the gunwale that many add: it's not on the plans, we mention it as an option.
In this case, the builder also used plywood for the stringers instead of a pair of 2x6's. It is a valid option mentioned in the plans but not part of the standard list of materials.
For this boat, the nesting of the overlapping side panels is tight. Some builders prefer to cut each panel from a new sheet.

The same applies to the resin plus a factor that has to do with builder's experience.
Two factors will make you deviate from the BOM: fiberglass content and customization.
Customization is same as above: we do not include fiberglass and resin for options like the console or the gunwale.


Glass content is the most important factor.
Let's say that a boat uses 30 lbs of fiberglass (that is easy to calculate and very accurate). If the builder is experienced, he will end up with a 50% glass content and use 30 lbs of resin BUT if he is a beginner he will have a 33% glass content that means in the example, 60 lbs of resin instead of 30.
Double the resin!

For example, a BOM figure of 6 gallons becomes 12 gallons.
Now, add to that some customization like the gunwales and the tendency of many builders to add glass were it is not necessary like all the way to the gunnel for some designs or inside topside or just add a layer here and there: my BOM of 6 gallon that became 12 gallons is now suddenly at 15 or 20 gallons!
We always add a 15% waste factor but some waste more.

As the designer, I must make choices. If I expect the boat to be built by beginners, I often calculate with 33% glass content but if I estimate that most builders will have some experience, I go for 50%.
If I based my BOM for all boats at 33%, many builders would buy too much resin and complain.
Conclusion: be aware of the points above and there will be no surprise.

One more point: less resin means stronger. This is counter intuitive but a laminate that contains less resin will be stronger.


Jacques Mertens - Designer
http://bateau.com

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