wood speargun shapes and buoyancy

[doyenofcastle]

Hi guys,

hour ago ,I was trying to ballasting a new teak gun and I am little confused ,
I have built two teak same wood piece but one have tapered shape 50mm to 30 mm other was similar to riffe rectangular shape no thickness change ,both almost same in weight little more by grams for riffe shape ,tapered one doesn't need more than 15 gram to be added to spear/trigger to sink slowly ,other one I was shocked when it need almost 4 ounce !!!anyone can explain why is that ?

[popgun pete]

Four ounces is about 113 grams, so about 22 times the weight if the other only needed 5 grams. The stocks may weigh the same, but volume must be different. Mark the water level in the tub and then put the stock in and hold it under with a large weight and check the water level in the tub. Remove the stock, get back to the same initial water level and then sink the other stock with the same weight and observe the water level in the tub. A larger volume stock then the greater the water displacement. The weight used has the same effect on displacement in each case. Otherwise the density is different in the two pieces of wood.

[Spear One]

Originally Posted by doyenofcastle

Hi guys,

hour ago ,I was trying to ballasting a new teak gun and I am little confused ,
I have built two teak same wood piece but one have tapered shape 50mm to 30 mm other was similar to riffe rectangular shape no thickness change ,both almost same in weight little more by grams for riffe shape ,tapered one doesn't need more than 15 gram to be added to spear/trigger to sink slowly ,other one I was shocked when it need almost 4 ounce !!!anyone can explain why is that ?

Even though the weight of the guns may be the same, the density of the wood can be quite different due to where in the tree the wood came from. Wood that comes from the center of the tree is often more dense than wood that comes from the perimeter of the tree. The more dense the wood the less bouyant that wood will be. Therefore it is likely that one of your guns has more "heart" wood than the other one and that would account for the ballast difference.

[Tyson Brown]

Shape has nothing at all to do with buoyancy. It's simply density and displacement.

The bouyant force is equal to the weight of the gun subtracted from the weight of the water displaced by the gun.

If your stocks are roughly the same weight, then the one that floats better has less dense wood / occupies more volume / displaces more water.

[doyenofcastle]

the tapered one have big rounded corners but the flat one is more rectangular with small radius round corners ,I also notice the gun float with spear up not same other tapered gun the spear was heavy enough to flip the up dawn ,I used same spear for both gun as both can use the same spear length but one is 120 and other 130(tapered),I think what I need to check is the contact area as I remembered in physic I heard my teacher saying something about increasing area contact can do difference for floating as in steel ship ,need to check again it was so long since i was in school

[popgun pete]

The answers given to you above by all posters are correct, anything else is down to imperfect memory on your part if you think that there is anything else to it. Only a trapped air pocket could otherwise influence the buoyancy, which is why I told you to sink the gun in both cases, if fact the sinking weight could be the same dive belt weight tied to the gun for the purpose of the experiment.

[doyenofcastle]

Originally Posted by popgun pete

The answers given to you above by all posters are correct, anything else is down to imperfect memory on your part if you think that there is anything else to it. Only a trapped air pocket could otherwise influence the buoyancy, which is why I told you to sink the gun in both cases, if fact the sinking weight could be the same dive belt weight tied to the gun for the purpose of the experiment.

actually i ll do that experiment if I have a bathtub but I don't .

[popgun pete]

The smaller the tub the better as otherwise the difference will be hard to detect as volume change equals surface area of container times the depth change (if the tub sides are relatively vertical). For example in a swimming pool the change would be virtually undetectable as the surface area of a swimming pool is just too big. A long narrow tank or vertical barrel that can swallow the gun is what you need as you will be sinking the gun with the weight.

[Divedog]

Originally Posted by popgun pete

The smaller the tub the better as otherwise the difference will be hard to detect as volume change equals surface area of container times the depth change (if the tub sides are relatively vertical). For example in a swimming pool the change would be virtually undetectable as the surface area of a swimming pool is just too big. A long narrow tank or vertical barrel that can swallow the gun is what you need as you will be sinking the gun with the weight.

Do you think maybe a 4"or 5" pvc or abs sewer pipe would work if you capped the bottom end?

[popgun pete]

Maybe, but the column of water will be really something to hold up! A fish tank would do if it was a long one, the finny inhabitants may like seeing the guns, however it is a lot of trouble to organize whenever since Archimedes' time we know the answer. According to mythology he compared crowns of equal weight to see which was pure gold and which was contaminated with base metal in an alloy. To weigh the same the fake crown had to be thicker and thus occupy a greater volume which was revealed when they went separately into his bath tub. Alternatively he weighed them with a balance, the upthrust by the displaced water being different for the two crowns. Naturally being metal, both crowns sank.

[Mike N]

Yes, but the handle might not fit even if not installed. Make the tube long enough to hold both water and a totally sunk gun. Mark the water level with the gun completely submerged. Then remove the gun and mark the water level. Determine the volume of water displaced by the gun. Weigh the gun. Divide the weight of the gun by the displaced volume and viola, you know the average density of the gun. Make sure you use the correct units. kg/liter, or pounds/cubic foot. Bernoulli is your friend.

MN

Originally Posted by Divedog

Do you think maybe a 4"or 5" pvc or abs sewer pipe would work if you capped the bottom end?

[doyenofcastle]

I just put SS piece near the muzzle and before band slot ,when we drill a slot in that area does it weaken the wood significantly or main load on center of the stock and nothing to be worried about ?

[popgun pete]

No (as long as slot is longitudinal and not too deep) and yes respectively. New question, new thread?

[doyenofcastle]

I just ask many questions so others who read thread in future will have answers for every thing

[doyenofcastle]

first shot was accurate enough to catch that king but recoil is high as it kicked my wrist strong