How Heavy is Your Boat?

justsomeguy · Apr 16, 2020

Sailgunner said:
Anyone else have similar numbers?

I've certainly got a shload of extra weight aboard my boat.
Spare chain x 2
Spare anchors
Engine parts 'n tools
Wife

et cetera, et cetera, et cetera.

Never had it weighed though, but dry it's supposedly 9 tons. Prolly an extra 2.5 tons.

jon hansen · Apr 16, 2020

like a ship, as you load a vessel it sinks lower in the water. is your waterline boot a foot underwater? prolly not.
my guess? the scale is off, way off

Capt Robbie · Apr 16, 2020

My 1983 Hunter 34 displacement is rated a 11920 lbs. I never looked at the lift scale, but most of us sailors tend to become pack rats. This became apparent to me after my boat was damaged by hurricane Sandy. Before I could start my boats storm damage restoration project I had to remove all of my damaged gear. At the end of three days, I had 57 completely full contractor size garbage bags of stuff I had accumulated over 30 years. I had so many spare parts and duplicates of spare parts. Some items I had packed away and totally forgot I even had. I found tree anchors that I had not used in over 20 years. Absolutely was amazed by the amount of item I had acquired over the years that I never needed.
All the items I had trashed could have easily been over 8,000 lbs.
After the storm damage restoration the boat water line now sits about two inches higher and much faster.

jon hansen · Apr 16, 2020

a contractor bag is 55 gallons = 55 gallon drum. 57- 55 gallon drums on a 34' sailboat that weigh over 8,000 lbs that raised the boat only 2' on the waterline.

Capt Robbie, what you been smoking?

Terry Cox · Apr 17, 2020

Our boat has a spec weight of 24,000 pounds. The missing part is what they include in this weight. Mast, sails, anchor system, etc. On the travel lift, cruise ready with full tanks and provisions, she weighs a hefty 35,000 pounds.

jon hansen · Apr 17, 2020

displacement 'DOES NOT' mean weight. displacement is the amount of water displaced when the vessel is on it's draft marks.
if you load 5 1/2 tons of stone into the hull of a 38' lwl hull it will sink deep below it's marks, dangerously deep. and of course that load will have to be down in the bottom of the hull for stability purposes.
putting the weight of 55 grown men on a vessel with a 38' lwl will sink it with it's boot well underwater. forget the lift scales, read your waterline.
P.S. load those men deep down in the hull or you will risk rolling over at the dock

Benny17441 · Apr 17, 2020

How about counting the many gallons of water trapped in the hull stringers below the sole, anchors, optional equipment.

shemandr · Apr 17, 2020

One good thing about racing now and then is that you tend to fight that pack rat thing. I don't like clutter so, at least on the 356, most the stowage was empty. When I looked at a J30 five years ago there was so much crap in every cabinet I just couldn't warm up to the boat.

Benny17441 · Apr 17, 2020

Archimedes Principle; Displacement is the weight of the water being displaced by the weight of the boat and both the weight of the water and the weight of the entire boat are the same.

jon hansen · Apr 17, 2020

Benny17441 said:
Archimedes Principle; Displacement is the weight of the water being displaced by the weight of the boat and both the weight of the water and the weight of the entire boat are the same.

that is correct. that said, when a designer gives the displacement, the "design displacement", the number that one might read on sailboatdata.com, those numbers do not reflect the actual weight of the boat. they do represent the displacement of a loaded, ready to go weight.
if a 38' lwl vessel was displacing 5 1/5 tons over it's "design displacement" it would be way deep in the water below it's marks. the 'boot' on our vessels are the the load marks. the boot is out of the water when properly loaded.

Will Gilmore · Apr 17, 2020

I have read several places recently about the difference between design displacement, built and rigged, in the water displacement, loaded, racing/cruising displacement and none of those numbers agree with the marketing displacement.

I suspect the lift scales are a little off, but it is also likely the boat is significantly heavier than the advertised specs.

-Will (Dragonfly)

jon hansen · Apr 17, 2020

the boot on our sailboats is not there for looks. its our loading gauge just as the primsol marks are used on a ship

smokey73 · Apr 17, 2020

I have to agree with jon. First a 55 gal contractor bag can't hold the weight that you imagine. First, you have to load it, get it up the companion way ladder, off the boat and to whatever you are carrying it in. I'm not at all sure where you could even put 57 bags that big full of stuff filled it to capacity on a 34 foot boat? I guess you never went below. Besides with heavy stuff it would split. (Yes, I'm an engineer who has done calcs so here goes).
Assume:
Each 55 Gallon bag is filled with mixed "stuff" at an average weight fo 60 lbs (probably a high average since some bags would have very light stuff too like soft goods)
57X60lb = 3,420 lbs in bags (most likey a very high estimate)
Since these were anchors you didn't know had, they couldn't have all been big heavy primary anchors so 3 at 40lbs each
3x40 lbs = 120 lbs for anchors
Assume 6 25 pound tool bags (you have to lift the tool bag to use it) A 25 lb bag is pretty big.
6X25lbs = 150 lbs for tools
This totals 3690 lbs for bags of stuff taken off, anchors, and tools
This is still 4,310 lbs short of the 8000lbs taken off? Even if my estimates are off, they are not off by 4000lbs.
By the way, 2 +/- change in draft sounds about right for 3500lbs on a 34 foot boat.

On my 40 foot boat, the CE Certification page lists:
Light Ship Displacement 19,775 #
Full Load Displacement 24,970 # (a full load as 5,190 #)
Sink @ Full Load Disp 2.67"

My numbers are very consistant with the numbers from NIgel Canter's Cruising Handbook which says for a heavily equiped blue water cruising boat, the fully loaded displacement should be about 5000 lbs above the light load displacement. Now I don't consider my boat a Blue Water boat (so lets not argue about that point) but the numbers are consistant with the concept.

If the load cell on the travel lift is lke most OSHA cranes, it is also measureing the lifting equipment itself (hooks, cables, straps, etc) since that is what the cell is seeing and the purpose of the cell is to prevent overloading the crane. It may have some built in bias and also may not have been calibrated in awhile. I just don't believe those travel lift weights.

Kings Gambit · Apr 17, 2020

I get a weight check now whenever a travel lift is used. Two different lifts, three measurements over the past maybe 8-9 years, have read consistently near 15,500#, or within about 2- 3% of the design displacement number. The boat has not been heavily loaded during these lifts.

Don S/V ILLusion · Apr 17, 2020

There is no correlation between boat weight specs and scale weight because, among other things:

Travel lift scales are not precise.

Hunter weight specs exclude mast, rigging, etc.. as other suggested previously.

Displacement vs. weight distinctions are a moot point in this discussion.

Most of us have nothing better to do than debate this.

Claude L.-Auger · Apr 17, 2020

Like Capt Robbie my 34 has a Hunter posted displacement of 11920 pounds. I do not carry 57 drums of stuff but having cruised long range every season with her for the last 20 years, I do carry lots of tools and spares besides food and liquids. Pretty much always the same stuff year year out. Every winter on the hard (save for that year in Florida and Bahamas) and it's always a joke at launch and haul-out time when the crane operator call the weight. Septic tank empty but other tanks almost full. Lowest weight ever recorded was 14500 and highest 16250. Always lighter in Spring than Fall. Really no measurable difference on the water line level (set by Hunter) from start to end of the season. So yes the scale on the cranes are probably not 100% correct, but I believe several factors come into play:

1 - Is displacement supplied by Hunter a calculated one from drawings ? I have visited the plant and did not see anywhere where they could fill a hull with water and weight it. And when displacement is calculated on ships, it is always done with full tanks. On my boat, +/- 25 gallons of fuel and +/- 65 gallons of water = +/- 725 pounds. Somehow I doubt Hunter includes this in their displacement figures.
2 - Hunter specs probabably does not figure mast, rigging, sails etc...
3 - Boat on the hard for months becomes completely dry. After some months in the water, while limited,there is some water that has accumulated in the boat.
4 - Over the years I have installed a bunch of equipment that were not part of the boat fresh out of the factory.

If I include the fuel and water, the figure is closer to 12650. Adding rigging and spars (800 lbs) would probably bring it close to 13500. So everything considered the added weight of all my stuff and gear + the scale's difference does not seem all that bad considering the boat is riding where it should based on the designed waterline height.

jon hansen · Apr 17, 2020

........ plus it is snowing here. 4" expected today.

smokey73 · Apr 17, 2020

Terry Cox said:
Our boat has a spec weight of 24,000 pounds. The missing part is what they include in this weight. Mast, sails, anchor system, etc. On the travel lift, cruise ready with full tanks and provisions, she weighs a hefty 35,000 pounds.

Terry, that means that your "as measured" displacement is 11,000 lbs over your "spec weight". Not sure if your spec weight is Light Load Displacement or Displacement at full load. But this is 46% of your spec weight

. I just can't fathom that as a reasonable number for a sail boat that has to maintain stability. That's also probably more than our keel weight! My keel is 7000 lbs. I am pretty sure your 42 has the same basic hull shape as my 40.5 and a keel weight of 7,700 lbs. I can't imagine that the load allowed would exceed the weight of the keel but I may be wrong in that assumption. Where did you get the weight of 35,000 lbs from? What does the CE certificate in the manual say?

Now don't get me wrong, I know the manufacturers sometimes take very "factory favorable" numbers for marketing so the boat they are marketing "seems light" and thus faster if they are saying its fast compared to it competetors and say it is "heavy" if it is being compared to offshore so called "blue water" stable boats. But that 11,000 lbs at 46% higher than your spec weight seems way out of wack. Light load displacement should include the mast, boom, engine, and fixed weights but not the fuel, water, etc. It probably won't include the anchor and rode, electrical equipment, generator if you have one, etc. Whether or not it includes the sails you probably can't tell unless you have inside knowledge.

With a similar hull shape and length, you should be about 2 X 2.5 inches (using my certificate numbers) below your light load draft or about 5 inches down

. That should probably put you above the bottom paint unless you had that extended or just about at the bottom of the boot stripe or close to it. I doubt you are there so I doubt that you are really 11,000 pounds over your spec weight. Just a guess though. "Only the Shadow knows for sure"

Kings Gambit · Apr 17, 2020

Don S/V ILLusion said:
There is no correlation between boat weight specs and scale weight because, among other things:

Travel lift scales are not precise.

Hunter weight specs exclude mast, rigging, etc.. as other suggested previously.

Displacement vs. weight distinctions are a moot point in this discussion.

Most of us have nothing better to do than debate this.

It may be true that we have nothing better to do, but it’s not true that displacement (boat weight specs) and measured weight are uncorrelated. Of course they are correlated, at least for anything that floats, which is ultimately what the Archimedes Principle refers to in this discussion, i.e. the buoyant forces. Clearly, a vessel that floats with a rated displacement of 20,000# is not going to tip the scale at 10,000#, or vice-versa, even on a uncalibrated travel lift. A loaded boat is heavier than an empty one, so sits lower in the water. It displaces more water, therefore, and that water has weight, etc. So, there is a correlation. Lots of shipping here through the Ports of Los Angeles and Long Beach. See boats riding high (empty) and low (loaded).