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 Boat Design Forums Displacement vs Actual Wts?

#1
07-12-2012, 04:17 PM
 dem45133 Junior Member Join Date: Aug 2006 Rep: 22 Posts: 38 Location: ohio
Displacement vs Actual Wts?

When a spec sheet states a displacement of x, thats the amount of water in x's units to float. That doesn't equate to a fully out of water weight does it?

If that is right, what is a generic multiplier to get close to actual weight for a typical 32 ft or so sailboat?

Thanks,

Dave
#2
07-12-2012, 08:51 PM
 PAR Yacht Designer/Builder Join Date: Nov 2003 Rep: 3465 Posts: 11,645 Location: Eustis, FL
There is no generic multiplier for published displacement figures. Each design is different as are the designers. Some try to make their product sound very light and offer just a shipping weight of a stripped boat, while others intelligently offer a cruising displacement, with partial or full tanks and a crew, with stores, equipment and supplies. As to which the figure might be, well that's a guess, unless familiar with the designer's efforts.

Which boat are you interested in?
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#3
07-12-2012, 08:52 PM
 Ad Hoc Naval Architect Join Date: Oct 2008 Rep: 2188 Posts: 3,726 Location: Japan
Ok, think of it this way.

A boat is in the water, floating. You then freeze the water solid. You take out the boat and left behind is a recess or "hole" in the ice. Now fill the hole with water to the top level of the ice.

If you then take out all that water you have just poured into the hole (left by the boat when removed) and put it onto a balance scale. On the other side of the scales, you put the boat.

The weight of the water equals exactly the weight of the boat...the scales are perfectly balanced.

So, if the boat weighed 1.0 tonne or 1000kg....and you lifted it out of the frozen ice, and then the water you poured into the hole left and weighed that water it would also weigh 1000kg. (assuming fresh water).

From this we can get that the density x volume = mass

The density of the water you poured in (assuming fresh water) is 1000kg/m^3. If the boat when floating has a volume of 1.0m^3, the mass = 1000kg/m^3 x 1.0m^3 = 1000kg

The amount of water that is displaced is equal to the weight of the boat.

In simple terms, does this help?
#4
07-12-2012, 09:48 PM
 dem45133 Junior Member Join Date: Aug 2006 Rep: 22 Posts: 38 Location: ohio
Thanks gentlemen...

I take the advertised displacement as an empty rigged weight. add another 1500 or so for water, fuel, gear etc.

I understand that the boat will "displace" x amount of water in like units. But here is my question.... when items that are below the waterline that are more and less dense wouldn't that change its actual weight in air... i.e on a trailer;

i.e I weigh 155 lbs and am slightly less dense than water.. thus I float... now did I "displace" exactly 155 lbs of water... I think no, I displaced something slightly different as I was positively buoyant, but I still weigh 155.

I will be pulling a max gross load down the dept of trans highways. I fully expect they will bitch and moan if I'm a lb over Chrysler corps 21,000 lbs CGVW (which is not driven by a safety need, but rather a warranty driver based one many factors in the drive line). With an engine retarder system, 4:10 gears Cummins HO and 6 speed and a goose necked or fith wheel quad axle trailer, all axles braked... I KNOW I can safely gross 24 or 25k with the same truck without overloading anything... tires, brakes, driveline, or frame!

I also do not want to go to a greater than 26k rig as then one needs a CDL EVEN though one is not doing commerce or commercial hauling. I didn't need one for the 600000 miles when I weighed 80k, 80 ft long and 13'4" high in the 80s... just a chauffeur's.

Dave
#5
07-12-2012, 10:21 PM
 gonzo Senior Member Join Date: Aug 2002 Rep: 1732 Posts: 8,377 Location: Milwaukee, WI
The displacement is the water displaced by the weight of the boat. It weighs the same in or out of the water.
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#6
07-12-2012, 10:35 PM
 Ad Hoc Naval Architect Join Date: Oct 2008 Rep: 2188 Posts: 3,726 Location: Japan
Dave

Slightly mixed messages you're giving. Thus as I understand it, the thrust of your question is, what is the weight of the boat, and does it exceed the limits you're legally allowed to trailer...is that correct?

If so, then you need to get the fully finished outfitted lightship weight of the boat. And not just "their word" for it, if you're concerned. Make sure you get a piece of paper saying it from them. Just to cover your arse when they bitch and moan.
#7
07-12-2012, 10:44 PM
 gonzo Senior Member Join Date: Aug 2002 Rep: 1732 Posts: 8,377 Location: Milwaukee, WI
You are buoyant because the density of water is higher than your body's. Therefore 155 lbs of water have a smaller volume than your body. The difference in volume is what float above the waterline.
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#8
07-12-2012, 10:54 PM
 bntii Senior Member Join Date: Jun 2006 Rep: 1324 Posts: 793 Location: MD
Quote:
 Originally Posted by dem45133 i.e I weigh 155 lbs and am slightly less dense than water.. thus I float... now did I "displace" exactly 155 lbs of water... I think no, I displaced something slightly different as I was positively buoyant, but I still weigh 155. Dave
Nope

The volume of water the submerged portion of your body 'displaces' is exactly equal in weight to your 155 lbs.

http://en.wikipedia.org/wiki/Archimedes%27_principle

#9
07-12-2012, 10:55 PM
 PAR Yacht Designer/Builder Join Date: Nov 2003 Rep: 3465 Posts: 11,645 Location: Eustis, FL
John is correct, you need to contact the manufacture and get delivery documentation. You're not the first that had to haul a boat and the same rules apply to them.

If you can tell us the make, model and year, we can narrow down you vessel weight to a few hundred pounds (assuming a production boat). A typical production 32' sailboat of modern proportions, will be in the 9,000 to 12,000 pound range and well within your limitations. A motorsailor may be a little heavier, but 15k would be a fat 32'er by modern standards.
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#10
07-12-2012, 11:18 PM
 Petros Senior Member Join Date: Oct 2007 Rep: 1260 Posts: 1,774 Location: Arlington, WA-USA
Tow weight and max displacement are different issues. Your towing weight will also include the trailer weight, which would not be part of your displacement. But food, water, gear and crew are part of your displacement weight.

Consider that there will not be crew on the boat when going down the hwy, and most likely your personnel gear, food, drinking water, etc. will be in your tow vehicle and not in the boat. It is possible your tow weight will be less than the displacement weight.

You need to know the weight of your trailer, plus the empty tow weight of the boat, to determine your tow weight.
#11
07-13-2012, 07:58 AM
 hoytedow Senior Member Join Date: Sep 2009 Rep: 2142 Posts: 4,225 Location: North of Cuba
Quote:
 Originally Posted by dem45133 Thanks gentlemen... I take the advertised displacement as an empty rigged weight. add another 1500 or so for water, fuel, gear etc. I understand that the boat will "displace" x amount of water in like units. But here is my question.... when items that are below the waterline that are more and less dense wouldn't that change its actual weight in air... i.e on a trailer; i.e I weigh 155 lbs and am slightly less dense than water.. thus I float... now did I "displace" exactly 155 lbs of water... I think no, I displaced something slightly different as I was positively buoyant, but I still weigh 155. I will be pulling a max gross load down the dept of trans highways. I fully expect they will bitch and moan if I'm a lb over Chrysler corps 21,000 lbs CGVW (which is not driven by a safety need, but rather a warranty driver based one many factors in the drive line). With an engine retarder system, 4:10 gears Cummins HO and 6 speed and a goose necked or fith wheel quad axle trailer, all axles braked... I KNOW I can safely gross 24 or 25k with the same truck without overloading anything... tires, brakes, driveline, or frame! I also do not want to go to a greater than 26k rig as then one needs a CDL EVEN though one is not doing commerce or commercial hauling. I didn't need one for the 600000 miles when I weighed 80k, 80 ft long and 13'4" high in the 80s... just a chauffeur's. Dave
What Gonzo said in other words: You have positive buoyancy yes, but you still displace 155 lbs of water which being more dense has less volume than you so parts of you will still stick out of the water.
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#12
07-13-2012, 09:28 AM
 dem45133 Junior Member Join Date: Aug 2006 Rep: 22 Posts: 38 Location: ohio
Thank you all

OK... displacement = light boat weight on the trailer. I'll allow for another 1000 for gear and stuff.

Trailer weighs 2550 now but will gain a few pounds when I mod for a non-shoal keel. Right now its a pull behind on a 10,000 lb 2-5/16 ball, 14,000 cap receiver hitch. Can't really go goose neck or fifth wheel as it does not lend itself to an extendable/retractable extension for launch. I could go a pintle hitch though... they have a 15 or 20k capacity.

curb weight of the 1 ton is 7200 I think (01 dodge 3500 4x4 ext cab long bed dually, cummins, 6 speed)

So looks like I'm limited to not greater than 10k on displacement. That going to be hard to attain in the 33 to 35 ft class which is really where I wanted to get to. Maybe go 30 or 31 now... while it does have to live at home. Then in few years... go clear to the 40 ft class and live on it in my mid 60s.

Thanks all.

So, Which older 30 31 or light 32 are under 10k?

Dave
#13
07-13-2012, 11:34 AM
 jehardiman Senior Member Join Date: Aug 2004 Rep: 1909 Posts: 1,748 Location: Port Orchard, Washington, USA
Quote:
 Originally Posted by dem45133 So, Which older 30 31 or light 32 are under 10k? Dave
Do you want to launch from the trailer or just store the boat on the trailer seasonaly? That makes a big difference! The are several older lightweight racing boats (i.e. J 30's, Olsen 30's) of that size range but most are fixed keel and cannot be launched from the trailer (i.e. they need a travel lift or crane to get them off the trailer... a common dry sailed condition at many west coast yacht clubs...or you only launch in the Bay of Fundy). There are some newer swing/retractable keel boats (i.e. Seaward 32, Jeanneau 32) of that size range that can be launched from a trailer (i.e. off a ramp). There are not many older boats that size that can be trailer towed/launched. As said before, most older (70's-80's) racer/crusiers of that size weighed in 12k-15k lightship.

What exactly are you looking for?
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#14
07-13-2012, 12:16 PM
 dem45133 Junior Member Join Date: Aug 2006 Rep: 22 Posts: 38 Location: ohio
To clarify... I am not your normal sailor

Road-able and launch-able both... but before you say one can't one has to consider who I am and what I can, and do, build. Most sailboat ramps (not regular ramps) are designed deep. At our local lake where we slip, a sliding tongue extension wasn't necessary for this shoal draft 27 Buccaneer... and it floats on/off just fine. I also designed up a singled handed mast raising and lowering system that uses the trailer's winch.

Now a non-shoal draft will require at least 7 or 8 ft of water to float off the trailer. That is reachable on most all sailboat designed ramps and many regular ramps that are a bit steep with a telescoping 12 or 18 ft tongue ramp extension (5 and 6" 1/4 wall sq tube that slide under the main frame and inside each other. They lock in place collapsed or extended with grade 8 pins, hitch never unhooked). This is entirely doable. Just have to get the material and build it. Truck tires stays dry which is critical when pulling up loaded. My 2 WD 3/4 gas V8 automatic couldn't do it at 8500 lb trailer weight even at 95% throttle. at 100% it barely started climbing... it wouldn't have held that long... trying to cook the torque converter. The next year we brought my wife's bronco too and chained up to give it a hand pulling out. While the 3/4 yanked it home 700 miles from Green Bay... without issues... it was not designed for long grades and the resulting heat generated. Thus the replacement.. the 2001 1 ton 4x4 Cummns powered manual. In 4 wheel low range and in Granny on the 6 speed it will idle out without any problems.

Most of the inland bodies of water I'd haul to for a weekend or vacation will have sailboat ramps. If I get somewhere where they do not have a suitable ramp I'll have them lift it off. I attached a picture of the Buc when we pulled out a few years ago. Never did paint the trailer. I built that trailer in 3.5 weeks using a \$300 1944 16ft tandem donor... totally redesigned and extended to 25 ft bed area with wrapping 7 ft tongue and stiffened the 6" in C channels with 1/2x2" bar on 4" risers. I also built up a sliding triple axle group. Tows just fine. Just didn't have time to paint and get to Green Bay to haul it home before winter weather in Wisconsin in October. I may have to shorten the frame behind the axles as it does tend to bottom out on the flat below the ramp and prevent further backing. I have it there to protect the boat in case someone underestimates its over hand on the trailer coming up behind. Its easier to fix steel than fiberglass. I could make that telescoping too for that matter. But the more complicated the system the more trailer weight.

I may not have mentioned either in this thread that as long as it doesn't require an escort vehicle... I don't care if I have to get a wide load permit. Figure 10 ft beam max.

Edit... I attached a few other pictures of the trailer build including the donor. Built it without setting eyes on the boat yet. Fit like a glove (which was NOT by accident, I had a scaled drawing of the boat, and just needed a couple measurements from the marina it was at).

Thanks, Dave
Attached Thumbnails

#15
07-13-2012, 10:13 PM
 Petros Senior Member Join Date: Oct 2007 Rep: 1260 Posts: 1,774 Location: Arlington, WA-USA
nice work, think you can make me a trailer like that for a fixed keel sailboat?

Not really ready for one yet, just wishing I can afford it.

BTW, I think you are still not quite getting the displacement weight thing: most manufacturers consider the displacement weight the weight of the boat and all cargo, passengers and gear (the max total displacement the boat was designed for). Some will also list Max weight, and design weight where they are showing performance figures for a less than full boat. What you need to find out is the empty transport weight of the boat from the manufacturer. The "shipping weight", plus the weight of the trailer, and any gear you intend to have in it when you tow it.