In mould waterline markings

Right people

I am about to start building a new plug and subsequent moulds for a sports fishing boat that I have been designing.
My question? and Ive asked many people with no clear answer , is this .....

How do I determine where the waterline will sit as I would like to mark my plug so it comes out in every moulding, taking into account a general loading weight including outboard , fittings etc
and..

if I had to mark it once the first boat has been built and tested-transfer this over to the mould surface.

I have built many, many plugs annd moulds over the years so technically the moulding of the boats is not a problem, but this little question has been annoying the hell out of me for some time and wondered if you guys had any ideas.

Many thanks
regards
Justin

 

Practical:

Take the blug and use it as a test boat. Load it with sand baggs wich are corresponding all the possible weights and their places in the boat. Observe loaded water line.


or "push the button" method (absolute uncertain)

Have an exact 3D surface model of your boat shell. Have a weight analysis table of the weights and their places to find the centre of gravity. Ask your boat design software to analyse hydrostatics.


or do it long way:

Study lines drawing, hydrostatics and weigth analysis.


or short way:

Hire a naval architect (some boat designers can do it as well).

 

I don't think one can add anymore than that, nicely put

Other than, one assumes Justin knows the full load displacement and/or payload to start with?

 

Thanks guys

I hadnt planned on being able to use the plug for testing as its being made in foam and MDF. But on giving it a bit of thought I supose I could just as well make a one off and then test it and iron any hassles out before taking the moulds-probably the safest option.

It would be nice to have a designer do it but I don't have that luxury- this will be done 'old skool' , this may seem a bit naive but Ive built many boats and have studied many underside configurations over the years, so know what makes a basically good usable design(in my opinion).

Many thanks for your time and trouble
regards
Justin

 

Itchy; You are getting off much easier than many questioners before you. Some of our curmudgeonly types are quick (and correct) to pounce wickedly on someone who is "designing" a boat and does not know how to lay out the probable waterlines.

I suggest that a bit of study in Skenes, Gerrs, Chapelles or similar books will get you the information you need. Such study is worthwhile because it adds to your credentials, not to mention your peace of mind. The study is not academicly challenging and some would say it is outright fun.

 

Thanks Mess

Ill have look into that.

regards

Justin

 

Justin, if your choice is to stydy, welcome on board!

You can start reading Larsson & Eliasson: Principles of Yacht Design. It's all (well, almost) there. Have the latest (third) edition.

 

Terhohalme: Yours is one of my favorite tag lines. The sharing of knowledge is one of the most important pillars of civilization.

I am so old that I can recall being a deckhand during the time of the Hanseatic League. I am old but still learning. New information almost invariably comes from another human. Humans who have written a book, conducted a class, or simply participated in forums like this one. Let us raise our glasses in tribute to all those teachers who have gone before us.

 

Yep, one of the first signals of being old is when sharing experience feels more comfortable than increasing it.

 

Relative to the original question:

1.) Build hull #001 as designed and fit it out as you intend to. Do not use the hull plug for the purpose of determining as-built immersion. Use a hull fabricated the same way as the consecutive parts are intended to be built (laminate schedules, structure, etc).
2.) launch it in ready-for-use condition and record the immersion data and mark the actual waterline.
3.) use the empirical data to design a boot stripe and scratch your new boot into the mold (laser levels are great for this).

Even major production builders (those that paint 'real' boot stripes and not stripes that follow the chine) with Naval Architects and Professional Yacht Designers on staff hold off on scratching the mold until hull #001 of any new model is in the water. That's not implying that they have no confidence in their weight calculations or that they have a poor handle on actual bills of material, it's just that there's no incentive to jump the gun and mark out the mold until you have all the information to do the work properly. And that includes arming yourself with empirical data.

Typically, production builders will paint a boot on hull#001 (and subsequnt hulls in-build behind it) based on the calcs and most of the time they are right on the money, but there's no need to permanently mark the mold until the calcs are confirmed when during prototype trials. Be smart, launch first, scratch second.