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Old 12-10-2021, 05:00   #1
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Gaining Draught? Tanks Empty?

A quick (silly) question.
My steel Adams 45 weighs about 16.8 tonne when fully rigged and all tanks full (fuel+water@ 300 gallons = 1132 litres or 1.13T) - she draws 2.1m at this weight.

I am splashing it back into the water nearby that is touch and go with regards to depth - I am planning on (need) the biggest tides of the year for this.


If I do not fill my tanks until in deeper water - and by loosing 1.13T I am bringing her weight closer to 15.5T? - therefore this will this bring her up about 160mm and give me a new draught of about 1.93m?

Be gentle on me!
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Old 12-10-2021, 09:20   #2
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Re: Gaining Draught? Tanks Empty?

I think you're assuming a linear relationship between your draft and weight which is not the case because your hull displaces more water at the top than the bottom. So, your draft would change less.
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Old 12-10-2021, 09:57   #3
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Re: Gaining Draught? Tanks Empty?

The amount of change will be a lot less than you are assuming. It can be calculated exactly if you happen to know the "waterline plane area": that is the number of square feet that your hull has right at the water line. Highly unlikely you have that data, but we can come pretty close.

You boat is listed as 45 feet LOA, I'll guess it is actually 39 feet at the load water line, and has a (guessing again) waterline beam of 14.5 feet. Now, it is not a square at the waterline, so we use a number called the "prismatic coefficient" to guess what the actual area is. Your boat is probably about 0.65 or so... so your waterline area is (roughly)

45*14.5*0.66 = 430 square feet

So... pushing the hull down one inch deeper measn the hull has to displace:

430 *1/12 = 35 cubic feet of water.

Water weighes roughly 64 lbs per cubic foot, so to change your draft by 1 inch, you need to change the weight of the boat by

35 * 64 = 2240 lbs.

That's a very typical amount for a boat in your size range. Losing a ton of fuel will change your draft by 1 inch, or 2.5 cm...

You can sharpen the pencil a lot and fix the guesses and assumptions I made, but for a 45 foot boat, one ton = one inch.
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Old 12-10-2021, 14:28   #4
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Re: Gaining Draught? Tanks Empty?

Quote:
Originally Posted by BillKny View Post
45*14.5*0.66 = 430 square feet

Minor correction. You used LOA, not LWL.


39.5 *14.5 * 0.66 = 378 square feet.


or 31.5 cu ft = 2000 lb


But it doesn't make much difference overall to the 1 inch per ton approximation.
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Old 13-10-2021, 00:40   #5
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Re: Gaining Draught? Tanks Empty?

Quote:
Originally Posted by StuM View Post
Minor correction. You used LOA, not LWL.


39.5 *14.5 * 0.66 = 378 square feet.


or 31.5 cu ft = 2000 lb


But it doesn't make much difference overall to the 1 inch per ton approximation.
Thanks Stum. I presumed there were some serious calcs and physics in this. One T one Inch is good to know.
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Old 13-10-2021, 02:33   #6
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Re: Gaining Draught? Tanks Empty?

BTW, it''s not actually "prismatic coefficient", it's "waterplane coefficient" or "coefficient of fineness of the waterplane area"
The former relates to the underwater shape/volume, not the "water area".
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Old 13-10-2021, 05:50   #7
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Re: Gaining Draught? Tanks Empty?

Just went through this myself a few months back. Layed over a few months in a fresh water land locked marina. No tide but wind influence. I had a rough time getting in. Plowed some mud and ran over some sunken trees.

I found a local tide gauge and also got a little gizmo that allowed me to make a “chart” of the channel. Not very good but better than nothing. Going out the warer was a couple of inches lower than coming in. I plowed a LOT of mud. I never ran her up to full power thinking if I get stuck I may need more power in reverse to back off. Took a while, for a good stretch I could only see progress by fixing a shroud on a shore object. But I just let her churn on and she got through. A wee bit nerve wracking as the nearest tow service was 50 miles.

So this location had real soupy mud, the down trees probably cyprus from the up stream swamp. A sand bottom is much different and I have to be more careful. The way our keel is built I can real easy slide up on a sand bar and not be able to back off. You can tell something of the nature of the bottom by the feel of the boat, sand and mud feel real different.

Good thing about steel is you can do stuff like that without fear of damaging the boat, just your nerves.
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Old 13-10-2021, 06:21   #8
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Re: Gaining Draught? Tanks Empty?

Another formula here:

https://www.cruisersforum.com/forums...3&d=1622295530

Do you have a dinghy? could the anchor and chain be lowered into that that and towed until you are safely in deep water?

Pete
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Old 13-10-2021, 07:38   #9
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Re: Gaining Draught? Tanks Empty?

Another possibility, if you are not sailing, but just motoring into deeper water (which I assume), you could ballast one side of the boat with stuff, anchors, chain, water or fuel tanks if located on one side, etc, to change the plane of the keel, and therefore the draft. Might gain you a few inches.

You might also put two "camels" on either side of the boat, small boats lashed tight to the gunnels. After securing fast the one to the side you want lower, ballast it down more with water, or sand. Ballast the one on one on the other side BEFORE you secure fast to the side, then remove the ballast, either pump the water out, or shovel out the sand. You can also secure the main halyard to the ballasted camel and winch it in to assist the list some more. If you are only moving a short distance (several miles) to deeper water, on a calm day, it's worth a try!

Just thinking out of the box! If it worked for Hornblower, it may work for you! (vague reference, some people may get it)
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Old 13-10-2021, 08:47   #10
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Re: Gaining Draught? Tanks Empty?

Quote:
Originally Posted by Pete7 View Post

Essentially the same formula
Just that instead of dividing by 12 to get cu ft per inch of immersion and then multiply by weight of 1 cu ft (64 lbs) to gets lbs/in, he does those two in one step with a "magic number"* - multiplying by 5.3333


*Computer programmers hate "magic numbers" - (unexplained factors) in code. In this case, 5.33333 = 64 lbs/12in
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Old 13-10-2021, 09:52   #11
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Re: Gaining Draught? Tanks Empty?

You could heel the boat over with weights hanging on the spinnaker halyard - as here:
https://youtu.be/WGAzt-8minI

That would reduce draft by quite a bit
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Old 13-10-2021, 10:19   #12
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Re: Gaining Draught? Tanks Empty?

Offloading as much as possible into a towed dinghy makes sense. Also Using camels to reduce your draft is the way to go. Oil drums, old inflatables, dive bags, or the like lashed below waterline before you launch. Depends how desperate you are. How did the boat get to where she is anyway
?
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Old 13-10-2021, 10:23   #13
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Re: Gaining Draught? Tanks Empty?

….. when fully rigged ??? Maybe get the mast etc to deep water separately, then rig the boat? Also Dont forget to take out most of the batteries
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Old 13-10-2021, 12:31   #14
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Re: Gaining Draught? Tanks Empty?

I don't know how you arrived at the figure of 160mm but I suspect a decimal place got lost somewhere as it is more likely to be 16mm... 160mm, even on a vessel as large as yours is a huge difference, that's over 6". Even on a smaller vessel around 33 foot, a fully laden and fully unladen waterline difference is usually around 4"/100mm
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Old 13-10-2021, 13:06   #15
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Re: Gaining Draught? Tanks Empty?

As was pointed out above, it is far from a linear calculation. As a SWAG, I wouldn't anticipate much added draught at all. Not having any specs. on your displacement, I'd say try it and pray for the best. Don't let anybody blow smoke up your a$$ with you gaining much at all.
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