View Full Version : Boat weight

08-04-2017, 08:26 PM
Hi all,

forgive ve me if this should go into the design section...if a mod wants to move it that's totally fine with me. Anyway, how in the heck can one estimate the final weight of a boat? Are there some general guidelines in regard to hull shape/materials? In my early stages of design I have been able to justifiably come up with widely varying figures and I don't trust any of them to use as a design guide.

and on a semi related note, could one use pressure treated wood (say for chines) that might lower the cg and not add to buoyancy? Just some thought while I struggle with this problem.

Its amazing all of the stuff that you never really thought of till you start to design a boat!
Thanks for any insight...have a great weekend...


08-04-2017, 09:13 PM
A boat owner who designs his own boat has a fool for a client. or something like that.

Of course, if you're not going to float people in it, knock yourself out.

Al G
08-04-2017, 11:25 PM
Weigh a sheet of the plywood you're gonna use for planking and bulkheads, weigh a length of lumber you're gonna use for chine logs and framing. Do a rough estimate of what's gonna go into the build and then do the maths. Good luck.

08-05-2017, 06:13 AM

well, thanks for the advice! So it looks like what I was doing is about the only way to do it. Hmmmm, I was hoping there might be a trick or two up some sleeves.

And yes, I am planning on floating some people in it �� and yes, I probably am a fool �� but hey, it's better than my first wild hair which was to sail my pdracer into Lake Michigan. At least this boat will be more up to the challenge, I hope.

More back to the point, are there some guideline on just fastener weight for certain size boats? Or is this the reason behind the "fastener schedule" that I have read so much about? I guess that makes sense now, but geez, what if I change my mind about something halfway through? Or at every step of the build?!

so I guess I should amend the op to not use the word "designing". Mostly just trying to get a rough idea of the boat with some specific parameters I need to hit and then forging ahead. Hahahahah, yes, I am probably a fool...

thanks for for any more boat weight advice all the same.

08-05-2017, 07:45 AM
T-money, I don't know anything about you, so I don't know the level of your competency with such subjects. I don't mean any insult, but this is a serious topic and from your question about how to determine design weight I will assume that you are not well versed in boat design. I apologise in advance if it seems that I am "talking down" to you.

Weights & centres is one of the most critical, and most soul-destroying, processes involved in designing a boat. There are several methods, each with varying degrees of ease and accuracy. But please be aware that if it is easy, it ain't accurate. Also, if you haven't discovered this in your design exercise yet, knowing the centre of gravity is as important (or more) than knowing the weight. Virtually every performance parameter of your boat - speed, stability, handling, etc. - is tied to where the C of G is located within the hull envelope. You haven't mentioned what type of boat you are designing, but please be aware that if this is a boat that is big enough to take your loved ones away into deep water, they are placing their lives in your hands; not just your boat-handling skills or building skills, but your ability to confirm via calculation that the boat will be stable and able to safely operate within the performance parameters that you have determined. If this is a speedboat, the hull shape becomes very important because poor hull form can lead to catastrophic dynamic instabilities at speed. This is not frivolous territory, and should be approached with all due diligence and dour consideration. If you are designing a boat that will be used in deep water with family and friends aboard, I would strongly suggest taking your completed design drawings and calculations to a seasoned naval architect or yacht designer and pay them to review your design. Think of it as insurance for your family.

Getting back to the weights calculation, do you know about the "design spiral"? If not, research it; your design will be better for the discipline that it affords. To calculate weights & centres, set up a spreadsheet on your computer with columns for item, calculated weight, LCG, VCG, TCG, L moment, V moment, and T moment. Then list every component of the boat - hull panels, frames, chines, fittings, engine, fuel, etc. Determine the weight of each item and its centroid from a common point (commonly the intersection of centreline, baseline, and Station 0). Sum the completed weights column to get boat weight, multiply each weight by its L, V, & T distance from datum to get moments, then sum the moments. Divide the sum of the moments by the sum of the weights to obtain C of G.

I apologise if I am being pedantic, but this is serious stuff. Once you know the weight & C of G of your design, do you know how to interpret it? Do you know how to calculate how the boat will float based on that information? Do you know how to calculate whether your speedboat will be prone to porpoising? Do you know how to calculate how stable your sailboat will be when heeled over at fifteen degrees? All of this information is "out there", but (please excuse my bluntness) there isn't a "Boat Design for Dummies" book. There is, however, Skene's Elements of Yacht Design, which is an excellent "starter" book for the amateur designer. If you don't have it, buy it now and read it before you continue with your design efforts.

Finally, with boats size does matter. If you are designing a 10-foot sailing pram to puddle about in off the beach beside the Elmwood Marina, have at it without too much care. If, however, you are designing a 22-foot speedboat with a 200-hp outboard that you want to take the family on fast outings to Old Mission and beyond, be very, very careful. If you make a mistake, people can die.

08-05-2017, 08:21 AM

thank you for the reply. The info you have provided is the sort of thing I have been looking for. I have heard of this design spiral but I will look more into it now. And you are right, I have basically 0 experience with real boat design! And I certainly don't want anyone to die, including myself ��

as as far as the cg and some of the other measurements you mentioned, I have only enough knowledge to be dangerous. I have read a lot and watched a lot of videos and stuff, but I will try and get a copy of the book you mentioned. Knowledge is power!

the design I am basing my build off of is a scow hull sail boat which helps me immensely just due to lack of curves obviously. Also I feel that the inherent stability of that type of boat is working in my favor. the part about calculating how much ballast I'll need and where I know is of great importance without even any experience and so I want to make sure that I am considering these uber important details early on.

I do do have some basic plans to go off of, and some examples of similar boats and their design specs so I am not flying completely blind...but mostly just very near-sighted!

thanks again, and I will look into some of the things you mentioned. I gotta be honest that spreadsheets are often a weak spot for me but I think it might be worth giving that a shot... It might help me just to get a more comprehensive list of materials for each "part" of the boat

08-05-2017, 10:14 AM
Spreadsheets are not necessary; hand calculations are just as valid but usually more confused. Many years ago I did hand-calcs in a bookkeepers financial ledger to keep running and projected total mass and CG for a 150-ft steel hydrographic research catamaran. Computers are easier...

08-05-2017, 10:21 AM
Calculate the immersed volume, and compare that to the weight of materials.