Specification question

[capt jake]

Displacement:

I have been lead to believe two different meanings;

1 - The weight of the water that the boat displaces

2 - The actual weight of the boat.

Which is correct? Some plans only give displacement; others give the finished weight of the actual boat. Others still, give the weight of the hull only.

[Charlie Jones]

Displacement is the weight of water that the boat actually displaces.

Among the best designers, it is normally calc'd using an empty boat, with half fuel and half water capacity aboard, but nothing else.

And mostly it's a guess cause the guy who designs the boat can't control how much extra ( or less) weight the builder puts into the boat.

[Tom Lathrop]

One of the main things you need to know about a boat is how much water it displaces in operating form. Only in this condition can the critical characteristics be evaluated. So what is needed is the weight of the hull plus whatever is needed to operate it under normal conditions. This includes engine, some fuel and water, some stores and normal crew. Too many give the hull weight and call that the displacement. The only way to know is to ask the right questions of the people with the knowlege.

Sail area to displacement ratios mean nothing if these things are not included. In small boats, the extras often dominate the true displacement.

[umop_apisdn]

Displacement:

1 - The weight of the water that the boat displaces

2 - The actual weight of the boat.

Which is correct?

Ideally, these will be the same thing, as that indicates that your boat is currently floating. When the weight of the boat exceeds its displacement, you start swimmming. :-D

Now, when you throw the anchor out, does the water level in the lake rise, fall, or stay the same?

Noel

[capt jake]

All good information. The reason for my question, It would be nice to know the weight of the actual boat in many cases.

[Charlie Jones]

I strongly suspect that the ONLY way you'll ever REALLY know the actual weight of ANY boat is to weigh it.

Way too many variables involved. Even boats coming from the same molds , from the same factory, today, have slightly differing weights.

[capt jake]

Good point, Charlie. I would agree that they will vary from one layup to the next. In looking at different plans, they all weigh just a few ounces. ;)

[Greg Luckett]

We weigh boats here in Michigan at the same certified scales the big trucks used. To get a trailer license here the empty trailer must be weighed this way too.

If you ever have a big boat shipped/trucked, the shipper should be able to tell you the nearly exact weight of the boat too.

[Phil]

We do it here at the landfill they weigh you going in then after you dump and charge you by the pound :evil:

They will do your boat for free . See the gov's not all bad.

[JeffM]

Stays the same! uh...I think. Geez, I'd better be sure--I've been assigned to teach 9th grade physics next year!

[Ken_Potts]

Noel,

Once the anchor has come to rest on the bottom the lake level will have dropped because your boat will have risen by the amount of water displaced by the weight of the anchor. Since the anchor is denser than water it is now supported by the bottom of the lake and it will be displacing water by volume instead of by weight as when it was aboard the boat.

Somebody please direct me to professional help.

So if everybody in Lake Powell throws their anchor overboard at the same time would the lake dry up?

[umop_apisdn]

Jeff, I think you should make that an extra credit question for your students. Have them build a Weekender and test it out as a lab assignment.

How to weigh your boat if it sits on a single axel trailer:

redistribute the weight in the boat until your trailer tongue has essentially no weight on it.

Take a 5 foot long board, and mark it off in feet.

Jack up one wheel, put it on the 1 foot mark.

Put a brick under the 0 foot mark.

Put a bathroom scale under the 5 foot mark.

Read the scale and multiply by 10.

Subtract tare weight of the trailer (which should be on the VIN tag? or at least on the title? If not, take the scale with you when you go sailing, and weigh one wheel of the trailer directly, and multiply by 3)

Noel

[JeffM]

Got it, Noel! I WILL put that one in my "resources" folder.

[Stump]

:wink: :shock: WOW :shock: :wink: Just like McGuyver (sp)

8)

[Frank Hagan]

Here's another boat weighing method that James Manning posted way back in 2002 that doesn't require deducting the weight of the trailer:

1. Do this on level ground. With the boat in the most forward position on the trailer, slip a bathrrom scale under the trailer tongue at its contact point with the ground. Call this W2 and write it down (measure it in pounds).

2. Now move the boat back on the trailer exactly one foot. Again, read the bathroom scale and call the weight in pounds W1.

3. Then, measure accurately the distance from the center fo the trailer axle to the contact poiont of the trailer tongue with the ground. Measure in feet and decimals (example: 15.75 feet, not 15 feet, 9 inches). Call this distance L.

4. Boat weight = L x (W2 - W1)

[Ken_Potts]

And another one. Find a truck scale - Our county dump has one or a weigh station on the highway or some private businesses have them too. Head over with an empty trailer and fill your gas tank on the way. Talk real nice like to the scale operator and maybe he'll agree to the experiment. Weigh the whole rig (truck and trailer combined) then go home and get the boat, stop for gas at the same place on your way back to the scale and weigh the whole rig again.

Wait, I think I like everybody else's method more than this one...

[Greg Luckett]

Just drive the empty trailer over to the truck scale and weigh just the trailer, then do the same again with the boat on the trailer. There is no need to weigh the truck.

[Ken_Potts]

If you don't weigh the truck you'll have to also check the difference in trailer tongue weight and add it to the axle weight. That might still be easier than making all those gas stops.

[Greg Luckett]

At the truck scales here we just lower the tongue wheel and make sure it is on the scale, then remove the trailer from the truck hitch ball, making sure the truck's back wheels are off of the scale. The weighing is accurate then. I think it cost me about $10 the last time I did one....last year.

[Guest Oyster]

I weigh all of my hulls at the concrete plant for free. They have scales for loading dump trucks with sand and gravel.