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?

Before going to all the trouble, I would verify the bridge heights. Most governments choose to be conservative on clearances to avoid lawsuits.

Ted

Buiz said:

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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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.

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.

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.

Can you simply remove the wind screen?

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)
Peter

The windscreen is about 11 feet off water.
You must be going under a not normal navigable bridge.

His avatar says he is in Rotterdam, they may have some very low clearances there.

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.

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.

mvweebles said:

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

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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!

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!

If this is a one time event, get about 6 or 8 of your friends to go fwd.

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?

Cigatoo said:

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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10,000 pounds would require between 50-100 people. Might get a little crowded.

BlueMoon1975 said:

10,000 pounds would require between 50-100 people. Might get a little crowded.

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So you had to do the math.

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.

Buiz said:

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.

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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?

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.

Buiz said:

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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"That means 4.6 metric tons (abt 10,000 pounds) to get the 10cm (abt 4 inches) I need."
I believe that comes out to about 1,200 gallons which would be about 160 cubic feet of water.
Does it seem like your 36' boat really has 480 square feet of boat footprint facing the bottom at the waterline?

I have to agree with BandB that it doesn't sound wise to plan a trip where you are relying on clearances in the cm range. Even if the heights are accurate, there are many variables that could dynamically change things by more than a cm or 2 and that could get ugly and potentially expensive.

I had a look at my GB36 windscreen. Suggestion.
Remove the screen, the supports are low enough. If you must have a screen, get lower profile cut.

Are those calcs right? Many owners have removed way less than that, selling or cleaning up, the resulting boat rise is noticeable.
Can French canal water heights vary? A modest rise, you`d be in trouble.
(PS. Been to Rotterdam,loved the maritime museum.)

Did anyone confirm the details of what he is doing? How often, measured height of his boat, bridge or bridges at H/L tides?

backinblue said:

I have to agree with BandB that it doesn't sound wise to plan a trip where you are relying on clearances in the cm range. Even if the heights are accurate, there are many variables that could dynamically change things by more than a cm or 2 and that could get ugly and potentially expensive.

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That's how we roll on European canals

Thanks for all the input. As I told backinblue, tight clearance is par for the course on European canals. New boats that are built here with canal cruising in mind are built to standard spec of 3.45m height. I'm 3.55m and will ballast down by -10cm so I'll achieve the standard spec height.

It's 349 bridges over some distance so getting people onboard is not practical. I have removed the windshield, the compass (that otherwise would poke up) and even a railing or two. There's nothing more to remove unless if I take a saw to the polyester, which I've decided against.

I will have to devise a way to regularly and accurately check my actual height. That's more tricky than you'd expect. I have been messing with horizontal poles with strings attached with a weight but it's hard to do this accurately - any suggestions are welcome. I have also been given the suggestion of installing an "air draft stick" at the bow of the ship to to get an early warning in case a bridge is lower than advertised - that will however only make sense if I can install it accurately and securely.

There was a great comment about keeping tanks topped off. I'll have to take that into account as I progress through this area and either top off tanks (water no problem, fuel more so) or add to the ballast.

I also agree that if I were to add 4.6 metric tons of ballast, which Archimedes tells us would be needed to weigh down the whole boat with the needed 10cm, then it's maybe not doable. I only need to weigh down the bow, however, so I expect to need half the ballast or less. That's about 10 of these drums (https://www.sdpack.fr/futs-adr-meta...econditionne-220l-a-bondes-3173218532988.html) which I believe I should be able to fit in the bow area. A Grand Banks is a sturdy boat so I expect the deck and the roof to be able to carry this weight. It won't be pretty and I'll have to anticipate some trouble when mooring etc, but it should be possible. Once I'm good on the clearance and my ballast is safe and secured then I shouldn't have to mess with it before I empty the tanks and sell off the barrels when I've cleared all bridges.

I'm with BandB, tungsten is the cleanest, easiest, most expensive way to weigh the boat down. You could always sell the tungsten cubes when your done. Getting them in and out would be a chore. But you might get lucky and make money on the cubes, they are getting popular.

If you are trying to lower the fore part of your vessel, full tanks may NOT be your friend....for lowering the overall vessel sure....but that may just require you to use more ballast forward to get a significant downward trim foreard.

It depends on where your tanks are.

I think the suggestion to add people was only as a trial
to see how much a measured weight on the bow will push it down....
then with that information you can better extrapolate how much weight you might need in terms of water weight, etc... for the actual trimming.

drop a plumb bob off the bow to the waterline. Then invite several folks from the marina willing to tell you how much they weigh (so men, not ladies) over to stand on the bow...

it could be a drinking game....maybe buy some beer as bait to get them to come over....
the game...how many 200 pound men does it take for the desired change in height.....
make a video and post on youtube....maybe worth a few laughs