Ballast System Structure

I’m looking into upgrading my hard tank ballast system with a much larger fat sack system. I’m planning on adding a pair of rectangular Launch Pad 51x20x20s at 750 lbs each in the rear lockers and stuffing a tube sack under the bow.

I think that I’ve read somewhere that unconstrained fat sacks can put stress on the rear locker side walls. To relive stress on the side walls I was thinking of building an aluminum or stainless box to hold the Launch Pad sack. I’d like to find out if I need to be concerned about the stress a fat sack would put on the hull of my boat. Adding a box should help reduce sidewall stress considerably. By it’s self a box would add weight, but also increase the overall cost of the project.

I have seen the WW article by Trace Trimble on installing ballast systems. Any comments or suggestions would be helpful. http://www.wakeworld.com/Articles/2001/BallastSystem1.asp

Thanks!

you could just make an aluminum tank, if you are going to all of teh trouble of fabbing up a box anyway.

I don't have ready access to a welder, plus I like the idea of a collapsible sack. When the sacks are empty you can store stuff like life jackets in the rear lockers. If you’re on the water and the sacks are full you probably have the life jackets out of the rear lockers.

I guess I want to hear what everyone else is doing.

Ed,

I run factory ballast... so no advice in that department; however, since every boat is built different and every person has a different tolerance for risk... I would run my own little experiment in the garage.

Fill the rectangular sack, outside the boat, and see how much it deforms when unconstrained. Then put it up against a wall and with a piece of plywood... make it rectangular again. That will give you a feel for the kind of force the side walls will have to endure at least under static conditions.

From there... a rigid container (no mods to boat) or even double up and anchor the side walls (mods to boat) should suffice.

I would imagine that the sack will deform more or less and require more or less force to constrain depending on how much you fill it... not sure if you will always fill sack to the max or not.

I will also look back and get a name of a guy with a 22SSV that added a bunch of custom rectangular sacks and got up to +/-2800lbs... he should know about stress on side walls

This is a link to a 22SSV thread... I think the guys name is Richard... shoot him a private message and I'm sure he will let you know what mods (if any) he did to handle the extra weight on the side walls.

http://www.wakeworld.com/MB/Discus/messages/65921/265699.html

Enjoy.

Thanks Kevin.

Some of those setups sound quite impressive. I never seen a 22SSV GG up close, but I’ve heard that Supras have fairly high gunnels (freeboard). The gunnels on my Lightning are probably lower (Avys are higher) and that might figure into how much I can load her up, so I probably won’t go to 2600 lbs like Richard is running.

I’m planning on adding 800 to 1500 in the stern, locker size permitting. I can’t measure my locker size right now because my boat is at the local fair grounds for indoors winter storage . If anyone knows Lightning locker size that would be nice to know but I can wait. I know that the Merc 350 engines are 43 inches long, so the locker must be at least that much. The Launch Pad sack that I’m looking at is 51 inches long, I’ll probably get two of those sacks even if the locker length is as short as 45 inches, it will fill to fit whatever volume is available. Of course if I add that much to trunk I’ll need junk up front too.

Your idea to test a sack is interesting but I think a dynamic test is what I need. What kind of pressure is exerted on the side walls by a 4 inch drop? I’ll see if I can reach Richard and see if he protected his side walls.

Ed,

I just sent you a long winded reply to the PM you sent me. The bottom line is that as long as the water has somewhere to go (an overflow drain) to keep the sac from expanding, you should be fine. If there is not an overflow drain and the sac continues to expand, eventually the pressure from the sac will be greater than the strength of the side wall and the sac will win.

An overflow drain will drain the water out of the sac when the strength of the side wall is greater than the pressure being exerted by the sac. As long as the water has somehwere to go the strength of the wall will be greater than the pressure of the sac.

- Richard