Looking for stability to a heavy cruiser (Northampton). Little tips and tricks to help reduce the roll. So far I have added 1/2 fiberglass bilge keels. Gutted the boat and ready to put it all back in again, do I move the weight closer to the outside of the hull (away from the centerline)? Weight will be as low as possible in the hull. Do I run max weight (fast gun MWC)? Gear: (2) Kips and aluminum tanks Servo M class LIFEPO4 batteries 3 ounce tank and lightweight strike regulator mtronics speed control small BC pump 390 motors (already moved away from center so the guns would fit in between)
Ballast it with lead sheet on the bottom of the hull. Do everything humanly possible to reduce topweight. Did you do your 'cast in foam SS' trick already? You know you want to, your inner artiste is calling... The biggest improvement to Lil Scharnie's stability was when I cut weight topside (foam stacks and ABS sheet SS, vice wood parts for all of thes above) and added weight down low. The best place for the ballast is centerline and bottom of the hull (hence my suggestion for lead sheet). An alternative might be lead shot poured into the water channeling, close to the centerline near the middle of the ship fore&aft-wise.
Sounds like you are on the right track. I have found that super structure weight on cruisers is a big issue. Try to get that as light as practical. Runinng max weight for the ship will help but dont go crazy worrying about that just yet. Get everything in her and then add more ballast if needed. Also water channeling is really important in cruisers as a little water trapped on the side can keep you healed over.
I'd be very careful about adding ballast. You usually don't have a very large margin over the necessary items like bottle, guns, batteries, etc. You want your heaviest items as close to the center of your floatation both fore/aft & centerline as possible, and as low in the boat as possible. Weigh your bottle/regulator combo, compare to the weight of your batteries. Whichever is heavier goes dead amidships. Put your pump just aft of this. Next put your guns in place (since they will be up high & toward the ends of the boat, they have a more significant impact on trim than you'd expect from their weight). Also put in your kips / accumulators at this time. They sit as low as you can get them, as close to their associated gun as possible. Now put in whichever of bottle or battery hasn't already been placed. Adjust to balance out versus gun systems. Add your radio box or electronics. All systems should now be in. Place your ultra-light superstructure on & weigh the total. How much margin do you have? If it's p> BTW, bilge keels are a great way to help w/ stability without adding weight. Great that you're using them Hope this helps
I know its counter-intuitive, but putting weight on the centreline will not assist with roll prevention as much as the same weight distributed into the bilges.
Putting weight further out from centerline may help with the rolling, but if you do roll, it'll be much harder to recover from it.
Thanks for the replies guys. I mark the center point on the shift (bow to aft) and ballast slightly aft to aid in sending water to the pump. To accomplish this I put a rod on the center mark, acts like a pendulum. This is a cruiser, so it will roll no matter what. The super is made out of lightweight expanding foam, that is as light an pretty as I want to get it. The batteries are the M class LIFEP04, nice big cylinders. Still not sure from above on the "best" way to mount them. 1. Side by side at the center-line of the ship 2. Side by side, but pushed out to the bilge (space between the batteries so that the center water channel is more exposed) 3. Inline, so that battery A is in the center-line of the water-channeling and battery B is behind it (Aft)
Putting weight out to the sides reduces the rate of rolling motion (slows the side to side rocking speed but doesn't stop it). This changes the moment of inertia of the ship. Putting weight down low increases the righting moment of the ship (makes it want to flip the right way up). These are two different things that do not conflict with each other. Assuming that the mass is balanced side to side, it wont make it harder to roll upright. But placing the mass as low as possible will make the ship want to stay up right. Do both and you have a ship that wants to flip up right but will do so at a slower rate (this isn't really a bad thing as too fast and you tip past upright).
I'm not a marine engineer or into fluid dynamics, I could be completely wrong, feel free to disagree and if you can explain why I am wrong, I'm happy to admit it - every day is a learning opportunity. As I recall my wonderful physics courses of yore, given equal masses attached to levers from a pivot point, and the same force acting on those masses (gravity), the mass on the longer lever will win. Initially, when the ship is upright, the masses being equal and in equal position relative to the centerline of the ship, things will remain in balance. But, if you heel the ship over 20degrees to stbd, the lever arm upon which the mass is attached becomes effectively longer relative to its distance in-plane from the centerline, while the mass on port has a shorter lever from the perspective of gravity. As I understand things, a ship rights because gravity wants to pull down on the bits, and the water wants to push up on the intruding hull (because the centre of bouyancy shifts with the roll). If your mass is on the end of a long enough arm, I would expect gravity to win. With all that said, I don't think that most cruisers are wide enough that splitting your weight out to the edges of the hulls will cause you to capsize by itself while rolling assuming the distribution is equal and it is kept low. The 'free' ballast sloshing around in your hull during this is another matter..
If you have enough beam to get away with it, I'd put the battery cylinders at the CG & outboard, that way you can put something else in between (like the pump maybe?). You aren't ever going to be a spinner boat with your length, but the less mass out toward the ends, the better your turn rate. Here's a thought. You have 4 shafts, 1 rudder. I'm assuming your driving the inner two and dragging the outer ones. Are you going two motors, or one w/ a twin output gearbox? Depending on space available, you may want to extend the driveshafts forward a bit past the after turrets. That way you have room for your guns, and can put your electronics box aft. Put all your "stuff" in the hull & move it around. See what configuration makes best sense for you. Keep in mind where your access hatches (removable sections of deck) will be. Most times, the only things that need to be accessed are the CO2 bottle, batteries, and the fill tubes for your guns. Try and position those items so they can be easily accessed from one place.
Height does matter. Get your weight low! and then worry bout getting it to the sides. For the purpose of side to side weights vs center line, actually its the horizontal distance from the center of buoyancy that matters as the ship heels over. You can multiply this distance times the acceleration from gravity to get the force applied by each mass and if you knew all the forces acting on the ship you could find the righting moment. If they both started at the same length and height, they will both shorten at basically the same rate, because the center of buoyancy is moving. It all cancels out. But by moving the weight outward the moment of the ship does increase and this reduces the rate of roll (the speed). Of course other things are at play here when we are talking about stability. Usually you calculate the center of mass and compare that to the center of buoyancy to determine the righting moment. Assuming symmetry, side to side distance makes no real difference here as the center of mass doesnt change.
So, I added on fiberglass (1/2) keels, will see if they do more than make it easier to pick up the ship. Also moved the internals towards the middle, will do stability tests this weekend with the camera.
So, here is the reason I am trying to reduce roll in the ship. I have mounted a camera above the stern guns and after the first battle the video is fantastic. 1080 hi-definition goodness all wrapped up in 4.3 ounces. To top that off, I can use my G5 phone and see the video in realtime, as well as review footage at lakeside. Very cool! Black material around the camera is a silicone sleeve, the lens is exposed so I have a backup ready to install in case of a hit. This camera is waterproof to 30 feet. http://www.amazon.com/Polaroid-XS10...F8&qid=1402314779&sr=8-9&keywords=polaroid+xs
Should be fun video. Rolling with an onboard camera isn't as nauseating as you might think if the roll is slow enough. I had one swaying atop a little cargo ship for a while. I am curious though as to why it looks like your guns are coming out of the barbette area on the turret, seems the turret cover should have plenty of room to house the guns?
