Author Topic: water ballast tanks  (Read 2154 times)

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windwardcd28

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water ballast tanks
« on: August 09, 2014, 14:05:54 »
I am building Model Slipway's Yorkshireman. I suspect that in any kind of stiff wind she will tend to be cranky because of the high superstructure. I plan on using a 12v 10amp battery wgt 8# placed low as possible in hull and am considering installing ballast tanks to add additional wgt. I have seen a great whaleback model that used this approach to stabilize and trim the boat. The builder uses two pumps (one fill one discharge) to control things. Does anyone have any thoughts as to how I can determine size and placement of the tanks. I want the tanks to be full at all times to avoid the water sloshing. One thought I had is to place removable tanks with three sides in the hull, do a float test and add water to the tanks to get the correct waterline, mark the water level on the tank sides and then cut the sides down to that level and place top side on the tank. Would appreciate ideas.

des

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Re: water ballast tanks
« Reply #1 on: August 09, 2014, 14:13:38 »
For any given amount of space, lead is a lot heavier than water.  Or, put another way, you could put the same weight of lead into much less space than the water would occupy.

My point is that I wouldn't even be considering using water as ballast, due to the issues involved in getting it right, keeping it where you want it, potential stability issues, and risk of leaks.  As well as the added complexity of setting up the two water pumps.

Des.

windwardcd28

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Re: water ballast tanks
« Reply #2 on: August 09, 2014, 14:40:19 »
Thanks for your reply. I agree with the density issue and considered it. I probably should have also mentioned in my post that an important consideration for me is keeping the weight of the boat for transportation and launching to an absolute minimum and to do this without needing to place ballast in the boat at launch time. Model Slipway says Yorkshireman is steady if ballasted low down in the hull, which I will do. She has a lot of hull space and I think I just need a few pounds to act as trim and the water ballast should be heavy enough to perform that function. One gallon gets me 8.5# of added ballast and I think that I can get at least that much in my tanks. That doesn't sound like a lot of wgt, to save but to me it is meaningful. Second reason is I like to tackle new problems. It keeps me thinking.

Volker

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Re: water ballast tanks
« Reply #3 on: August 10, 2014, 00:23:28 »
You should know before how much water ballast you'll need and build the ballast tank so it is really full. Free flowing water surfaces in tanks can decrease the stability of the model.

According to Model Slipway the Yorkshireman will weigh 10 kg (22 lbs) fully loaded.
Their white metal detail parts are quite heavy so make a check how much ballast you really need.
I would go with lead ballast as Des said.
Regards Volker

CGAux26

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Re: water ballast tanks
« Reply #4 on: August 10, 2014, 14:13:54 »
If you must go with water ballast, consider using small floating balls as slosh dampers.  Use enough to cover the free surface of the water.  This is a proven damping system used in real boat fuel tanks.  Prove it to yourself with ice cubes in a glass of water.

But I'm with the rest of the guys:  Use lead ballast.  The CG of the mass of lead will be lower in the boat than water.  I used lead shot in my MS Loyal tender, purchased in a fishing store, and sealed in bags.  Attach it to the hull with Velcro, then you can remove it for transport if you must.  It's easy to adjust the amount of weight with lead shot, too.
Dave

Volker

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Re: water ballast tanks
« Reply #5 on: August 11, 2014, 00:01:06 »
It is not the sloshing leading to eventual problems but the free surface effect when the center of gravity of the water moves as the boat heels.

This at least reduces the effectiveness of the added ballast on the heeling. The small floating balls can't prevent the shifting of the center of gravity.

http://www.bbauv.com/blog/vessel-more-unstable-with-half-filled-tanks/
Regards, Volker