Determining draft

[ted655]

What is the method of finding draft on a flat bottomed boat?
area-displacement. ?????

[SailDesign]

Make mark on hull at waterline when floating in calm water - haul boat and measure to bottom.
Simple! ;-)

[ted655]

OK, I asked for that one. How about before the boat is built. How do I get a draft spec.

[Paul Browne]

Hi Ted,

I think many are unsure as to just what you are after. Do you just need to estimate how much the boat will draw, given that you know its shape and gross weight?


Paul Browne
New stuff up at Geezer Boatworks

[ted655]

Exactly. I know the dimentions, gross weight and displacement. I was looking for a formula that says; Given "blah" times "blah" divided by "blah," the hull will sink "x" inches below the water. Something like that. I see all these new boat specifications and draft or freeboard is always given. I'm not the brightest bulb on the tree, but I know they didn't put them in the water and draw a line. They know in advance where to paint the boot stripe.

[terhohalme]

Look here:

http://web.nps.navy.mil/~me/tsse/Nav...le3/basics.htm

Terho

[mmd]

Just remember, you asked!

1.) Calculate longitudinal centre of gravity of boat & contents (LCG).
2.) Calculate draft for calculated displacement, adjusting for salt or fresh water.
3.) Calculate waterplane area, longetudinal centre of floatation (LCF), and longitudinal second moment of area about midships (I(midship)) at displacement waterline.
4.) Calculate longitudinal centre of buoyancy (LCB) of immersed hull volume.
5.) Calculate longitudinal metacentric height (GM) of hull at given displacement.
6.) Calculate moment to change trim (MCT) of hull at given displacement.
7.) Derive trimming moment from LCG and LCB.
8.) Calculate total trim (T) using formula T = (trimming moment) / (MCT)
9.) Calculate trim aft and trim forward using formula

t(a) = T*(0.5 - (x/L))
t(f) = T*(0.5 + (x/L))
where x = trimming moment, L = waterline length, and LCF lies aft of midships

If LCF lies forward of midships, then the formulae are:

t(a) = T*(0.5 + (x/L))
t(f) = T*(0.5 - (x/L))


A basic naval architecture book such as Barnaby or Derrett gives a full explanation of this gobbledygook.

Sometimes it is easier to float the boat first and scribe the waterline en situ!

[Paul Browne]

What's your boat look like Ted? Let's go from there. This is probably a lot easier than you might be thinking.

Cheers,

Paul Browne
www.geocities.com/geezerboat

[ted655]

Ok Paul. The firewall to the above link, kept me from going to that site. I'm still trying to decide if MY boat has all those measurements I'd need for the various formulas above. In fact, I think step #2 was what I wanted in the first place.
.
The hull is 8' wide, 24' long and 1' deep. Flat bottomed, with a short upturn on the bow, and solid flush deck. A barge for fresh water. Thanks ted.

[duluthboats]

Add all the weights of your boat and everything that will be in it, you included. Divide that by 62.5, this will give you the cu ft displacement. Your boat is a big box, each inch displaces 16 cu ft. Divide total cu ft displacement by 16, this = inches from bottom. I know there should be some X’s and Y’s here but I’m no pro.

Gary

[mmd]

I'm sorry to be such a pain the the you-know-where, and Gary is correct up to a point. The numbers arrived at using his method will certainly give you the draft if the boat is floating on an even keel. However, if the motor, fuel tank, and other semi-permanent boat stuff are sitting on or near the transom, and there is precious little else in the boat, she will trim down by the stern. This becomes an issue if finding the waterline at rest dictates the application of anti-fouling paint. If in such a situation, the stern would be submerged below the line of the anti-fouling when at a mooring, allowing marine growth and boring critters access to the unprotected hull.

I certainly am not advocating doing all those esoteric calculations for "a barge for fresh water", unless you tend to be an anal-retentive designer-type like myself and you crave punishment. Rather, I'd suggest painting the entire hull with the topsides paint and launching it. Fit out the motor, hardware, regular in-boat stuff, and use it a bit. One day, while you have a few moments at the launch ramp, wade in the water and make a few marks along the length of the boat at the waterline on both sides, and then haul her out. When you get home and she dries off, take a batten and "connect the dots" with a scribed line to define her at-rest waterline. Scribe a second line the width of the desired boottop, and paint on the boottop and anti-fouling.

By the way, a good method of scribing the waterline while afloat is to float a piece of 3/4" thick softwood beside the hull and mark to the top of this. That way you will both have an easy, dry reference surface to mark, and the resulting line (top of anti-fouling) will be about 1/4" above the actual waterline, which will prevent scum from forming on your nice bright boottop.

[ted655]

Thanks for the help and tip about the floating board, everyone.
I was sitting and playing with my calculator...8x24x1x62 came out to about 11,904??. this is the weight of the water it would take to subnerge the hull completly. So...divide that by 12 (number of inches of depth of hull). I came up with 970?? (roughly). I reckon that for every 970?? pounds of weight, my hull will sink 1 inch/?Does that make sence?

[mmd]

Yes. That, in naval architecture circles, is known as the tons-per-inch immersion, or TPI.

[SailDesign]

mmd - haven't we got back to where I started him out here? Took you guys long enough... ;-P

[mmd]

Yes, but now he knows why .