lead and foam question Revell GATO

[beeryboats]

This will drive you to drink. On one try it wants to go to the bottom and on the next test it floats with the conning tower out of the water.AAArrrrggg! I thought maybe I had an air bubble somewhere, but I held it under water and shook it back and forth and no air came out. Then I blew the tank and she promptly wanted to fall over once surfaced. I know, more lead and more foam right? I may be closer to being in trim than it looks. It is so sensitive to where and how much foam you hang on the outside. My surface trim would be great if it would stay upright. What's more effective, adding lead to one side or foam to the other?

[He Who Shall Not Be Named]

Well, that depends on how well you've secured the deck, doesn't it?!


Just keep cramming in the foam and weight till the boats stable. How hard can that be????.....

Distribute the weight so the longitudinal CG (where it balances on an even keel) is about half-way along the boats length.


There is only one center of gravity, but it has three components to it: where it resides vertically, laterally, and longitudinally. We want that CG as low as possible; smack-dab in the middle laterally; and about at the center of the boat longitudinally.


There is only one center of buoyancy -- the point in the boat where all the buoyant forces merge into a fixed point. We want the CB as high as possible and directly over the CG longitudinally and laterally.


The greater the distance between the CG and CB, the more statically stable the boat becomes. The vertical distance between the two points of force is described as the metacentric height. The greater the metacentric height, the more stable the boat.

Your boat, as you describe it, has the CG and CB too closely coupled -- the moment arm between them is too short; exerts too little force to overcome disturbing influences in a timely fashion. Increase the vertical distance (metacentric height) by adding fixed ballast weight as low as you can, and adding compensating foam as high as you can, but under the boats surfaced design water line.



David

[He Who Shall Not Be Named]

Just keep jaming in the foam and weight till the boat is stable.

David

[beeryboats]

Well it floats, and sinks, and almost tips over. I measured how much lead I added, about 6oz. I added another 8oz, mostly at the bow, and added a couple strips of foam aft. It's still unstable, specially surfaced. I'm going to add 10oz of lead and more foam and try again later today. Can you add much foam to the upper deck below the water line without pulling the deck off? I was thinking of a thin strip along both sides.

[trout]

David wrote somewhere that to get the tank filled the most, do a short blast and then purge the gas. Then immediately fill again with a longer fill time. The purging will cause the tank to become very cold and able to receive a full charge.

[vital.spark]

Actually you only need a little amount of propellant in the tank. A short blast on the blow should bring your sub up enough for you to use your LP Blower. You will get quite a few blows with the tank partly full this way. I would chek this out in bath trying to fill the tank with 2 seconds of gas first and if you think you need more go for 3 seconds next charge etc. When I was living in New Zealand the cost of a can of airbrush propellant was about US$25!!! so I was very carefull on how I used it.

I've just last month moved back to Hong Kong where I can get car A/C refridgerant for about US$ 6.00 per can. They have a special valve that you can buy so that you can turn the can on and off as the cans are actually designed for a one shot use only. Which means you must leave the valve on the can as long as there is gas in it.

[beeryboats]

Thanks guys. I started by adding about 8oz of lead right down the keel but concentrating in the center. Then I glued two chunks of foam to the hull sides opposite the balast tank. I figured that would be safe starting point. Tomorrow I'll toss it in the pool and see what I get. How does one know when the Propel tank is full?

[Reckless]

also remember you have 2 different trims to do in one boat... surface, and submerged...

personally I did my type7 this way -

1. bugged Dave for an aproximate amount of lead in the keel ... added said suggested amount
2. carved foam to fill hull *saddles* .. install in hull
3. install SD and all pushrods, shafts, etc etc... aka made it ready to go
4. put 4 rubber bands on it 2 near the bow and stern, and the other 2 closer to the tower
5. throw sub in tub (fill tub first....)
6. using precut foam chunks/strips and the 4 bands attach foam on outside of hull ... bring it to waterline (using the bands on the outside allows WAY faster changes on the fly in the tub
7. fill balast tank ... place more foam above waterline and/or move foam to put sub at submerged trim ...

8. play with your ballast making your sub go up and down .. watch how it rises and sinks.. settles.. etc and move foam around as needed as it's readily accessible on the outside of the hull

lather, rinse repeat as nessasary ;)
6.

[trout]

Jay,
i did my first trim with just enough foam and weight to get it to submerged level (semi). Installed all the foam and weights and let it dry. Then re-tested the sub both submerged and surfaced. Did some adjustments. Needed to add more weight because of a listing issue, but I need to add foam to counter act the weight. By adding more weight, the sub becames more stable, it takes more to move the weight.
I have repeated the trimming of my sub many times and will do so again since I have added do-dads. Very finicky.
Peace,
tom

[beeryboats]

[QUOTE=KevinMc;20790]Hi Jay,

. Now blow the tank dry and re-check that all the foam you added is still below the current waterline - what ever waterline you get now is all you're going to get, and the only way to get the boat any higher is to make the ballast tank bigger, take some weight off the "above waterline" portions of your boat, or turn it into a suface runner. (Ugh!)

I guess that's the point I'm having a hard time getting through my head. Seems to me that if you add more foam below the waterline it would add more bouyancy and lift the boat higher. Otherwise why do you need it at all. Just add lead until you get near neutral. I can see where you would need it above the waterline for stability.
Jay

[KevinMc]

Hi Jay,

Here's the deal on ballast, foam and lead - Per the instructions, start with the ballast tank full of water: add lead (to the keel) and foam (just below the waterline) as required to get the boat either barely positive or barely negative to your preference. Now blow the tank dry and re-check that all the foam you added is still below the current waterline - what ever waterline you get now is all you're going to get, and the only way to get the boat any higher is to make the ballast tank bigger, take some weight off the "above waterline" portions of your boat, or turn it into a suface runner. (Ugh!)

If the boat rises so high that it sits above the scale waterline you've got some options - 1.) add more lead to the keel and an equivalent amount of foam above the waterline to get more stability when submerged, or 2.) move some of your foam into the ballast tank to reduce its dynamic volume, or 3.) just cut the ballast tank size down. (None of these are likely to be necessary with one of David's upgrade kits, but hopefully this will be useful when building your own ballast system some day.)

[vital.spark]

If you don't drink, you soon will!!!

[beeryboats]

I'm in the final stages of my GATO build and have been reading and re-reading Davids instructions on trimming the boat. Sounds to me like you need to get as much foam along the water line as you can fit and then add lead until your level at your submerged trim. But then how do you get it to surface near the scale waterline? Add foam? Then add more lead? Sounds a bit like a Catch22.
Thanks,
Jay