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Old 10-30-2021, 02:14 PM   #1
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Reducing boat height by 5cm (two inches) by ballast

I am going to need to do above on my Grand Banks 36. The boat is the highest at the front of the flybridge, so I need to weigh down the bow by 5-7cm. I guestimate that I will need a ballast of 2000kg or so to be placed as far forward as possible on the boat. My best idea so far is to buy barrels and bladder tanks and fill them with water. Sand or cement will have higher density and thus take less space but it'll be cumbersome to get the material and discard it again when I've passed the area with low bridges that I need to pass. Anyone has any experience with this kind of ballast and any tips and tricks?
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Old 10-30-2021, 02:43 PM   #2
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Before going to all the trouble, I would verify the bridge heights. Most governments choose to be conservative on clearances to avoid lawsuits.

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Old 10-30-2021, 02:49 PM   #3
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I am going to need to do above on my Grand Banks 36. The boat is the highest at the front of the flybridge, so I need to weigh down the bow by 5-7cm. I guestimate that I will need a ballast of 2000kg or so to be placed as far forward as possible on the boat. My best idea so far is to buy barrels and bladder tanks and fill them with water. Sand or cement will have higher density and thus take less space but it'll be cumbersome to get the material and discard it again when I've passed the area with low bridges that I need to pass. Anyone has any experience with this kind of ballast and any tips and tricks?
Is this something you need daily or for a one time trip? If one time, you can fill all tanks and add ballast and be fine, but a horrible way to have to do things daily or weekly.
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Old 10-30-2021, 02:59 PM   #4
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If it is a one time thing and your bilges are clean maybe just turn off the bilge pumps and let water into the bilge. Then pump it out after the transit.
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Old 10-30-2021, 04:43 PM   #5
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Even though the GPS have fairly full forward sections, not sure you will need that much weight if you can get it far enough forward.

Go stand on your bowsprit, then jump off the boat and see how much of the waterline is wet.

My boat easily goes down an inch plus with just 200 pounds extra on the sprit.
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Old 10-30-2021, 04:48 PM   #6
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Can you simply remove the wind screen?
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Old 10-30-2021, 05:33 PM   #7
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Going across Lake Okeechobee, there is a 49-foot bridge. There's a guy who 'tips' sailboats to get under it using 55-gal barrels filled with water along the gunwale.

Here's one couple's experience (warning, quality is lousy)

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Old 10-30-2021, 05:46 PM   #8
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The windscreen is about 11 feet off water.
You must be going under a not normal navigable bridge.
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Old 10-30-2021, 06:54 PM   #9
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His avatar says he is in Rotterdam, they may have some very low clearances there.
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Old 10-30-2021, 07:02 PM   #10
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I am kinda thinking like PSneeld. I’d bet that a 50 gallon bladder or even ten 5 gallon Home Depot pails of water with lids would give you the 5 cm (two inches) on the nose. The “per inch immersion” formula on that boat ought to be around 1600 lbs or so I think. Leveraging the bow down should take a lot less than the stern. Of course it would be good to have a solid deck in that area.
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Old 10-30-2021, 07:07 PM   #11
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It is so hard to judge vertical distances. ... Reminds me when going under the eastern end of the San Francisco Oakland Bridge when the windvane atop our sailboat's mast knocked into the cockpit.
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Old 10-30-2021, 07:42 PM   #12
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Quote:
Originally Posted by mvweebles View Post
Going across Lake Okeechobee, there is a 49-foot bridge. There's a guy who 'tips' sailboats to get under it using 55-gal barrels filled with water along the gunwale.

Peter
This technique has worked for me more than once, including at the Port Mayaca railroad bridge. There I used my sailboat's main boom to hoist a partially water-filled dinghy just enough to introduce the necessary amount of "heel." Fun!
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Old 10-31-2021, 04:11 AM   #13
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As Comodave pointed out, Im in Europe. The route I'm going to take through France has 349 bridges with a clearance height of 3.5m. We can assume that this is precise, these are well-travelled canals.

With all movable stuff (and then some) removed and full tanks in fresh water, I'm at 3.55m above the water line.

Archimedes principle when applied to my boat's precise dimensions comes out with 463kg per 1cm submersion (sorry about the metric terms here guys). That means 4.6 metric tons (abt 10,000 pounds) to get the 10cm (abt 4 inches) I need.

As Heelhustler and PSneeld discuss, it should take a lot less to weigh down just the bow. The highest point of the boat is a fair bit forward so let's assume that we get almost all of the clearance gains of the bow where we need it.

To calculate exactly how much ballast I need on the bow I assume would require that we know the exact hull shape and the longitudinal pivot point of the boat. Let's face it, I'm just going to put increasing weight as far forward on the deck as I can and measure until I'm where I want to be, but some kind of guide as to how much weight I may need would be helpful. Even better, someone who has done this before and has some tips and tricks would be awesome!
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Old 10-31-2021, 05:18 AM   #14
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If this is a one time event, get about 6 or 8 of your friends to go fwd.
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Old 10-31-2021, 09:47 AM   #15
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I like Old Dans idea. Seems like you are overthinking. Just keep adding bodies until you get where you need to be. Easy to move the weight around as necessary. Maybe have to buy a few beers. (After the fact would be better).

Grand Banks says that it should be 3.88 meters to the top of the wind screen so I guess you will be removing that?
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Old 10-31-2021, 11:49 AM   #16
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I like Old Dans idea. Seems like you are overthinking. Just keep adding bodies until you get where you need to be. Easy to move the weight around as necessary. Maybe have to buy a few beers. (After the fact would be better).

Grand Banks says that it should be 3.88 meters to the top of the wind screen so I guess you will be removing that?
10,000 pounds would require between 50-100 people. Might get a little crowded.
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Old 10-31-2021, 12:02 PM   #17
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10,000 pounds would require between 50-100 people. Might get a little crowded.
So you had to do the math.
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Old 10-31-2021, 12:35 PM   #18
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Sinking the whole boat may take 10,000 pounds, my guess is a fraction of that is needed to drop the front half of the boat enough.
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Old 10-31-2021, 01:13 PM   #19
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As Comodave pointed out, Im in Europe. The route I'm going to take through France has 349 bridges with a clearance height of 3.5m. We can assume that this is precise, these are well-travelled canals.

With all movable stuff (and then some) removed and full tanks in fresh water, I'm at 3.55m above the water line.
2" is easy. However, you need far more. Your fuel and water tanks won't remain full. You also might hit small waves from others as you transit.

You can get a good idea of what weight you need just by analyzing what fuel and water did for you. My guess is that full fuel and water vs. 1/4 full or so is about 3". Whether it's 2, 3 or 4" that tells you x pounds gives y inches. Typically full tanks on a semi-displacement boat are 2 to 5 inches.

I can tell you that I'd never make this trip in that boat knowing how close it is unless it was just to transport the boat as it sure would diminish any enjoyment of the cruise. That's worse than 60 or 70 locks on the Erie or somewhere. I don't know what time period you're spreading this over. Are fuel and water readily available so you can continue to top off?

To achieve 10,000 pounds, you need about 1200 gallons of liquid. There is no way to easily achieve that. If you're trying this, you need some real weight, not a bunch of bladders. I don't know if you need that much weight but I don't think liquid is the way of doing it.

That also means a major expenditure to start and then cost of disposal at the other end. Trapshooters actually have lead by the drum and 55 gallons as in a drum weighs 2500-3000 pounds. I can't imagine you even adding this kind of weight without some damage. 10000 pounds would be about 8 to 9 cubic feet of tungsten.

So, all this can be done, but are you really sure you want to do it?
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Old 10-31-2021, 01:23 PM   #20
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As I burn fuel and use water. . My bow sinks a lot.

Knowing your own boat is key.

For my first post, I walked to the stern of my boat. The bow came up way less than an inch. Then to the very bow and it sank nearly 2 inches... at least 4X as much as the aft weight and that was with a 175 gallons of water in tanks all the way aft. Fuel tanks are pretty much on center of everything so no trim change.

On my boat, I bet 200 lbs forward equals nearly 1000 pounds aft. Plus here the OP doest need to increase total draft, just change trim enough.
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