buoyancy, what will happen if i add buoyancy to hull

[jurgenscraft]

I HAVE ALMOST FINISHED GETTING MY BOAT SEA WORTHY AND I AM JUST ABOUT TO ADD BUOYANCY TO THE HULL, ALL THE COMPARTMENTS UNDER THE DECK ARE ACCESIBLE FROM THE DECK AND CAN BE FILLED WITH CLOSED, 1 LITRE PLASTIC BOTTLES. I HAVE BEEN TOLD THAT THE BOAT MIGHT SIT HIGHER IN THE WATER,???????.HOW CAN THIS BE, THE VOLUME BETWEEN THE HULL AND THE DECK REMAINS EXACTLY THE SAME, ONLY IN THE EVENT OF THE HULL BEING HOLED, WILL THE ADDED BUOYANCY ASSIST IN KEEPING THE BOAT AFLOAT. I DO NOT SEE HOW THE ADDITION OF BUOYANCY CAN EFFECT THE HEIGHT THAT THE BOAT SITS IN THE WATER, MAYBE I AM MISSING SOMETHING.KIND REGARDS WILLIAM WRIGHT.

 

[Boomyal]

Re: buoyancy, what will happen if i add buoyancy to hull

WW, PLEASE STOP SCREAMING. IT'S HARD TO HEAR WHAT YOUR SAYING.<br /><br />You are correct in your assessment of the effects of added floation. If anything, technically, it would make the boat sit lower in the water by virtue of the floatation's added weight.

 

[umblecumbuz]

Re: buoyancy, what will happen if i add buoyancy to hull

Hi JC,<br /><br />Better find another expert. The guy who told you it would sit higher is wrong. Your conclusions are right. it won't make any noticeable difference to the freeboard.<br /><br />Mind you, when you hole the boat and she starts to sink, she might turn turtle because all the buoyancy is in the bilges and there is none higher up! <br /><br />This is often overlooked when considering flotation foam. Placement of buoyancy is critical to the boat's attitude and stability when swamped.

 

[18rabbit]

Re: buoyancy, what will happen if i add buoyancy to hull

JC – if you want to do this the correct way, you need to strategically place the floatation such that the passenger compartment areas remain afloat within a certain angle/tolerance so that all passengers can remain in the vessel without anyone getting dumped into the water. The specs on what you should be trying for are somewhere in the US Code of Federal Regulations. I forget what section (46? 49?) … maybe someone else knows. It is a requirement here in the US, not an issue for you in SA but it is certainly an excellent guideline you may want to consider.

 

[JasonJ]

Re: buoyancy, what will happen if i add buoyancy to hull

Adding floatation does not cause the boat to sit higher, and the odds of turtling is minimal enough to not even consider. Like 18 rabbit said, it needs to be done correctly, with floatation foam placed to keep the boat level and at the surface of the water. You also need to have the proper amount of floatation. In order to figure that, you need to know how much the boat weighs, and the theoretical amount of water the boat would displace if it were completely submerged.<br /><br /> Everything about the boat displaces water, the seats, the motor, everything. A boat that weighs 1000 pounds on land may only weigh 6-700 pounds under water. So you have to figure out how much foam would it take to displace enough water to equal that 3-400 pounds left over. A cubic foot of water weighs 65-ish pounds, so you know that you would need to displace x amount of water to keep the boat at the surface. It can be a bit confusing but it does make sense if you think about it and know anything about positive, neutral, and negative bouyancy.<br /><br />Positive bouyancy means an object weighs less that water that is the same dimension as the object. Neutral bouyancy means the object weighs the same as the water it is equal to in dimension, and negative bouyancy means the object weighs more than the water it is equal to in dimension. Example: 1 cubic foot of foam may weigh one pound, 1 cubic foot of water weighs 65-ish pounds. The foam is positively bouyant because it will displace a lot less than 65 pounds of water, so it floats. A cubic foot of lead may weigh 300 pounds, a lot more than the cubic foot of water, so it is negatively bouyant (it sinks). A cubic foot jug of water will weigh the same as a cubic foot of water, so it will be neutraly bouyant (it just sits below the surface). That is how submarines work, they take on water in ballast tanks to change their bouyancy, thus changing depth.<br /><br />On an average 16 foot boat, you are fine with 9-10 cubic feet of floatation. The soda bottles are probably not going to give you enough floatation because of the voids around them. My recommendation is to use the proper 2 part foam under the floor and be done with it. Good luck...

 

[umblecumbuz]

Re: buoyancy, what will happen if i add buoyancy to hull

You said:WILL THE ADDED BUOYANCY ASSIST IN KEEPING THE BOAT AFLOAT

Short answer is YES.<br /><br />Local fishermen here who can't afford to have foam injected have used bottles just like yours for years, and the few times they've got swamped have shown that this makes a difference. They make sure that the deck hatches are solid closed so that the buoyancy of the bottles in a swamped boat does not force the hatches, allowing the bottles to escape.<br /><br />It sometimes takes poverty to develop ingenuity.