A formula to estimate boat weight.

[Old Ironmaker]

Because I don't do what I am supposed to do during the day I find I have time on my hands because it's too windy to fish. The most common question I get and read here is " How much does my boat weigh?" I have been trying to come up with a formula for weight based on length, beam and draught. Both glass and Aluminum. Of course not all are of the same construction but close I believe. Am I spinning my wheels because of so many variables or should I repair the deck? Any suggestions. Who wants in?

 

[QBhoy]

Haha. Interesting but surely the inputs required to the equation are too varied ?
Best just to google, haha.

 

[QBhoy]

Not to mention the confusion caused by displacement, gross and dry weight figures !

 

[southkogs]

Well ... boats float in water, so what else floats? A duck! Perhaps if we can figure the weight of the duck ...

... oh, never mind :lol:

 

[roscoe]

The info needed by the boat owner, would be infinitely more complex and more confusing than either weighing the boat, or identifying the boat and looking it up online.

So yes, lets do this, and confuse the heck out of anyone that asks the question.

We will need to include a multiplier for BIA hp ratings, which would be different from the multiplier used for boats with a USCG hp rating placard.

 

[ondarvr]

Too many variables, two boats made on the same day can be very different due to the crew making them, let alone all the differences in design.

 

[Scott Danforth]

from first-hand knowledge........ two brand new boats, side by side, both exact same width, both exact same length, both exact same transom height, both exact same layout, both with exact same motor. both fiberglass. only difference is who manufactured them

one weighs 1100#, one weighs 1900# The extra 800# was extra resin, cloth, filler, adhesive, and other things under the deck

it is mathematically impossible to get the weight of a boat from its lenght, width, depth, etc. you would need to know the thickness of all materials and the density of all materials as well.

so man invented a scale.....

and iboats has a database..... http://boatspecs.iboats.com/

 

[Ned L]

Well,..... (It's beyond what people here would want to do, but,.....) If you calculate the displacement you know the exact weight without having to know anything the construction method or anything other than the shape.
N/A's do this every day.

 

[ondarvr]

Well,..... (It's beyond what people here would want to do, but,.....) If you calculate the displacement you know the exact weight without having to know anything the construction method or anything other than the shape.
N/A's do this every day.

Please explain how you are going to calculate the displacement.

 

[Maclin]

So, take the boat to the water, get the waterline marked, then take it back out and measure the area from that lines shape, and then....

 

[Maclin]

Suunds harder than going to a cooperating scale. And no good method without the mfg's BOM to know exactly what each option along the way added.

 

[roscoe]

Because I don't do what I am supposed to do during the day I find I have time on my hands because it's too windy to fish. The most common question I get and read here is " How much does my boat weigh?" I have been trying to come up with a formula for weight based on length, beam and draught. Both glass and Aluminum. Of course not all are of the same construction but close I believe. Am I spinning my wheels because of so many variables or should I repair the deck? Any suggestions. Who wants in?

I think everyone but southkogs is missing the point.

Perhaps you should have a deck repair party event. And when you are done, bring the crew over here, for a deck replacement party, as mine is beyond repair.

 

[Silvertip]

Or you could do a "Sheldon" thing. Build a tank big enough to float your boat. Add the boat. Add water until the boat floats. Mark the water level. Now remove the boat and again mark the water level. Now calculate the displacement. Then you begin a spread sheet that includes EVERY detail, every option, every power plant variation and every accessory available and repeat the process. By the time you die you will have covered perhaps 1% of the boats and there would then be at least 1000 more variations that arrived on the scene including electric, hybrid, and hydrogen powered versions. But as was mentioned, just look at the manufacturers specs or go a scale and weigh the old tub.

 

[Fed]

There is a way to measure the boat without the trailer by measuring the weight on the tow coupling then moving the boat back on the trailer and re-measuring the weight on the coupling again. You have to throw in the distanced moved to the mix as well.
You could also use the same method to get the weight including the trailer which would also then give you the weight of the trailer alone.
Need to use jack stands moving on the trailer frame instead of moving the boat for that.
If you want to pursue it I'll find it for you, it needs someone just like you to tidy it up into a neat package.
I've also seen where you can work it out by measuring the trailer tire footprints & the PSI in the tires & doing the sums.

 

[Old Ironmaker]

I think everyone but southkogs is missing the point.

Perhaps you should have a deck repair party event. And when you are done, bring the crew over here, for a deck replacement party, as mine is beyond repair.

You win a Qoupie (sic) Doll. The only problem with those deck repair parties, painting parties or asking 8 friends to move you party by the time you feed and water them hiring a pro would have been cheaper. I remember a move we made in the 80's. If I would have hired a mover by the time I fed those Steelworkers breakfast, Chinese food lunch a finally a steak BBQ at the new place and watered those hooligans with copious amounts of Canuck beer and Rye, some of those scallywags were still there in the AM (no drinking and driving) we would have saved 300 Samolians. But we still talk about it 40 years later, that's priceless and worth the 300 bucks. Plus a box of my classic Playboy magazines disappeared and showed up in a furnace lunchroom the next week.

 

[frantically relaxing]

Or you could do a "Sheldon" thing. Build a tank big enough to float your boat. Add the boat. Add water until the boat floats. Mark the water level. Now remove the boat and again mark the water level. Now calculate the displacement. Then you begin a spread sheet that includes EVERY detail, every option, every power plant variation and every accessory available and repeat the process. By the time you die you will have covered perhaps 1% of the boats and there would then be at least 1000 more variations that arrived on the scene including electric, hybrid, and hydrogen powered versions. But as was mentioned, just look at the manufacturers specs or go a scale and weigh the old tub.

You guys are overthinking the question: the says he's asked how much HIS boat weighs--not every single similar boat like his!

Only 2 good ways to measure a boat; with a weight scale, or calculating water displacement. As for the tank thing, you won't need to measure the boat's waterline, just fill the tank until it it's full, then remove the boat, then measure how much water is missing.

When I had to crane the SkipperLiner out of the water, crane guy wanted to know how much it weighed, so he knew how much counterweight to bring. This boat's relatively easy to calculate because the hull is dead flat in back with a slight V in front...hull is 12'-6" wide, and at 40' forward where the keel rises up, the keel sits 9" deeper than the sides. The waterline was 10" up in front, 14" up in back, and I tried to factor in the added underwater surface area up front best I could, and it worked out to pretty close to 14", or 1.167', average draft.

SO-- the hull's wet area is 12.5' wide, 40' long, drafts 1.167', and water weighs 62.43 lbs per cubic foot...Doing the math: 40' x 12.5' = 500 sq. feet, x 1.167' draft = 583.5 cubic feet, x 62.43 lbs of water, comes to 36,428 lbs... Crane guy told me his scale said just above 36,000.

Figuring out a typical V hull's displacement would be much trickier for sure, but do-able

 

[Silvertip]

Ok then. Boat on trailer weigh the tongue. Then scale under each wheel. Add the three numbers. This is the GROSS combined weight. Now float the boat and repeat the process. The difference is the boat weight. You also now know the trailer weight, combined weight and boat weight based on the extra junk in the boat at the time. You have the knowledge to adjust tongue weight and determine if the trailer is overloaded.

 

[roscoe]

Alright guys, enough.
He knows what his boat weighs. He's a seasoned veteran here.

Point of the thread is to find a diversion, so he doesn't have to go outside and repair the deck.

"Am I spinning my wheels because of so many variables or should I repair the deck? Any suggestions. Who wants in?"

 

[GA_Boater]

Alright guys, enough.
He knows what his boat weighs. He's a seasoned veteran here.

Point of the thread is to find a diversion, so he doesn't have to go outside and repair the deck.

"Am I spinning my wheels because of so many variables or should I repair the deck? Any suggestions. Who wants in?"

Nailed it! LOL

 

[Old Ironmaker]

Not enough time and not enough energy to do much. So I entertain myself all too often. However I do like some of these suggestions. The question does come up often.

I believe Silvertips suggestion is the simplest for sure.