Low in the water - buoyancy solutions

[marmaduke]

When I bought my 36ft narrowboat, the surveyor recommended a brand new steel base plate to be welded to the bottom. This was because of the sample readings of the hull thickness he'd taken. If I remember correctly the thinnest reading was about 4mm. At the time I was new to boating and trusted the surveyor without question. In hindsight I realise that perhaps it wasn't such a necessity to have a brand new baseplate welded to the bottom of the hull. The surveyor was new to the job too and was probably being over cautious. As most will know too, in order to get insurance, you have to satisfy the surveyor's Essential Points in their report. So I didn't have a lot of choice even if I had wanted to forgo this steel plate.

 

4 years on, I'm now dealing with a very heavy boat due to this extra thickness of hull, along with personal effects and a fit out. The water tank is at the front of the boat under the front deck. When the water tank is full, the drainage holes on the front deck now come right down to the waterline. When the water tank is empty, the weight is all at the back, and my weed hatch starts leaking from the pressure. Not ideal...especially if I need to use that weed hatch when I have run out of water. I'm fairly in tune with my vessel and know through instinct what state she is in, and keep the weight balanced to avert potential disaster.

 

However, it would be nice to come up with a more satisfying solution to this problem. I've removed as much ballast and personal things as I can get away with. A solution I've been pondering recently is some sort of buoyancy aid. I was thinking I could put some empty large water bottles with their caps tightened firmly in the watertank. As the roof of the water tank is the floor of the front deck, which is at the waterline, I believe this should raise the bow. Obviously it will leave me with less volume in the water tank too.

 

My questions are then as follows:

 

- is this a reasonable solution?

- will it make much of a difference?

- if empty water bottles in the water tank will raise the bow, how can I apply this to the stern?

- any other ideas?

 

Any comments are much appreciated.

 

M

[David Mack]

Why not just put less water in the water tank?

 

The top of your weed hatch should be above water level at all times. If it really does go below water level at certain states of trim, then it needs extending upwards.

Edited June 25, 2014 by David Mack

[nicknorman]

The bottles would have the same effect as not fully filling the tank - the latter solution seems simpler! Have you removed all the ballast from under the floor?

[marmaduke]

Unless I filled them with helium...

 

Damn. The answer is not the answer I was hoping for. Such is boating life. Cheers

[john6767]

I would say that all that would do is to reduce the capacity of your water tank, same as not filling it. What you need to do to get the boat higher in the water is to remove some ballast, with an additional baseplate you may not actually need any ballast at all.

[Paul C]

I bet all your hull fittings for kitchen sink, bathroom sink, shower drain outlet, gas locker vent etc are all below their design height from the waterline (is the gas locker vent now underwater?) too. The only way is to re-balance the boat to get the weight the same as before. If removing ballast doesn't do it, then the only proper way to solve it is to remove the welded on baseplate and instead of overplating, re-plate it to the original thickness.

[marmaduke]

Gas locker vents are still high above the water, as are the drain holes. Looks like I'll be taking up the floor then...

[IanM]

It does seem obvious. If you add more weight to the hull and want the boat to sit the same in the water you have to reduce the weight of what's in the boat by the same amount ie taking out ballest.

[MtB]

I'll be staggered if there isn't a ton at least of ballast still in the boat somewhere.

 

Finding it and removing it is the only solution. Messing about with containers of helium will only improve buoyancy forces by a Kg or two - background noise!

Another approach which you won't like, would be to chop off the extra baseplate and get third party only insurance which doesn't require a survey.

 

MtB

[Paul's Nulife4-2]

Hi ya,

Can I ask, What is the waterline at the Stern like, when the Bow Tank is Full ?.

It may just be a case of Re-Trimming the attitude ofthe boat !.

Any Pictures of the way the boat lays with Full & Empty water tank.

Ithink putting Plastic bottles in will produce thumping and knocking that you couldn't then do anything about as the water sloshed about.

[David Mack]

I think there is something odd going on here Steel is about 7.85 times the density of water, so adding a 6mm plate to the bottom of the boat will increase the draft by about 47mm (or 10mm plate will add 79mm). If your vents and weedhatch are so low now, with the boat drawing an extra 47 or 79 mm, then it sounds as if they were pretty low before the bottom was added, which is something your surveyor should have pointed out at the time.

 

On a 36 ft narrowboat the area of the bottom plate is perhaps 20 square metres, so a 6mm bottom plate will weigh just under a ton, and a 10mm plate about 1.5 tons. That is how much ballast you need to remove from the boat to get back to its original weight.

[The Bagdad Boatman (waits)]

I had an extention collar made up to the size of my weed hatch bolted it on and solved the problem, re using the existing weed hatch cover.

[John V]

I must say I agree with MtB I think you must have some more ballast lurking away somewhere.

 

If the only problem is the weed hatch and all the other openings in the hull are sufficiently above the waterline then just extend the weed hatch with a short tunnel. You don't want to depend on things like not filling your water tank etc to keep enough freeboard

 

 

edited to say....bah! I type too slow...been beaten to most of it!!!

Edited June 25, 2014 by John V

[MtB]

edited to say....bah! I type too slow...been beaten to most of it!!!

 

 

I find it quicker to use two fingers....

 

 

MtB

[Neil2]

This story doesn't inspire a lot of confidence in surveyors does it?

 

There must be hundreds if not thousands of boats happily cruising the network with less than 4mm between the inside and outside, and countless steel built sea cruisers/yachts will have hull plating of that guage as standard. Why anyone calling themself a professional would recommend overplating on a 4mm base is beyond my comprehension.

 

But, given that the damage is done so to speak, I'd agree the only realistic solution is removal of more ballast.

 

The effect of overplating on the boat suggests that the underwater profile is quite "fine", if this is an old boat she may well have a six foot base, and being quite short obviously the swims fore and aft take up a higher proportion of the overall surface area. So even a modest amount of weight gain could have a significant effect.

 

I doubt very much if the weight of the overplate is anywhere near a ton, but if she has standard concrete slab ballast there could be nearly two tons of movable ballast to play with. So, you don't have to shift a lot of it to improve matters.

 

The problem will be accessing the cabin bilge and ideally you want to remove the ballast from the sides first to keep the boat stable.

 

I saw a 30 foot Piper boat a year or two ago that had exactly this problem. It too had been overplated with 6mm steel, making the boat far too heavy and the water was just about level with the weed hatch top. It's an obvious problem with short, narrow, boats when you think about it.

 

 

[jake_crew]

Extreme solution - if you really have removed all ballast and taken out non-essential items inside, the only thing you can do is to add air ...... so lengthen the boat.

 

Do you have a cast iron stove ? How full and where are the diesel tanks.

 

 

When Jake was rebottomed, we had to take out all the lead ballast at the back end - fortunately the price of lead has doubled up in recent years !

[blackrose]

As others have said, the simplest solution is to get the floor up and get rid of some or all of your ballast. You obviously don't need it now that your boat is ballasted with the new baseplate. Getting the floor up might seem like a load of hassle, but it's not nearly as extreme or expensive as cutting off the new baseplate and having the bottom replated instead, as suggested above.

 

If your boat is fitted over the floor and you can't get the floorboards up without a major refit, then one solution is to look where the screws are in the floor which will show you where the cross members are. You can then use a 10mm drill and jigsaw with a short wood blade to cut sections out of the floor between the cross members. The ballast (assuming it's loose ballast like broken concrete slabs), can then be taken out. You can reach inaccessible ballast with a hooked crowbar. The sections of floor you removed will have to be supported underneath when you put them back.

Edited June 25, 2014 by blackrose

[marmaduke]

Thank you all for a thorough run through of the solutions. I'm pulling up the floor as I type...

[OldGoat]

Thank you all for a thorough run through of the solutions. I'm pulling up the floor as I type...

Remember, before you replace the flooring, to make (any) adjustments to the trim by adding back some of the ballast.....

[Neil2]

Just another thought, if you're going to the trouble of removing flooring/ballast, depending on the state of your plating you might want to take the opportunity of applying some sort of anti rust treatment down there.

 

It would be interesting to hear the outcome of this - do keep us informed.

[marmaduke]

By the way, I cut a hole in the floorboards and removed the ballast...there was some more I could take out after all. She is now listing no more! Hurrah. Thanks all for the advice

[Woodstock]

Good news- glad it worked out

[John V]