Displacement

Hi, i am trying to figure something out. i've been told that my boat is very lite and will not handle much weight i know their are other boats the same size that have know problem with weight so let me ask the question this way- if we have two boats the same length and beam, boat A and boat B filled halfway (thats how they measure displacement)then we add say another two hundred lbs. to each boat and boat A sinks deeper on its lines than boat B what would be the possible reason for this? I know this sounds like a crazy question but it really is important for me to understand this so i can refit my boat,so I'd be thankful for any help.Thanks Robertabc

 

Without knowing the lines of your boat(s) it's hard yo say.. But you can make a test to find out what's reasonable. Have someone with more experience with..
Remember there's difference with can do and can do safely

 

Length and beam aren't the only factors...prismatic coefficient, slack or firm bilges, draft, buttox shape, diagonal section shapes, waterline section shapes, rocker etc all have a factor in the overall displacement of a hull. Construction also plays a significant part as material weights contribute to the overall displacement and carrying capacity of a specific design. We would need to see the specific boats being compared in order to give you a reasonable opinion...along with a spec sheet for each.

 

Put in the simplest of terms , ITS SHAPE.

One really striking example is a catamaran versus a monohull.

Think about it.

Apples versus oranges.

 

Robert,
IF all else is equal then you can use what's called "Moment to trim 1 inch". This is the weight required to sink a vessel 1" lower than her designed water line. This amount will change (usually increase) as the vessel gets deeper and deeper, as the typical flare of a hull will increase its waterplane area as it gets lower and lower.
So, to your question, the reason boat A will sink deeper than boat B given the addition of the same amount of weight has nothing to do with the actual weight of the vessel. The only thing which affects this reaction to added weight is the area of the waterplane at any given point. Picture the boat being sliced horizontally by the plane of the water. The larger the area of this resultant shape the more weight it will require to sink the boat 1". Or, conversely, the less it will sink given the same amount of weight (boat B vs. boat A). Elmo's diagram above illustrates this very well.
Here's a more detailed scenario. The hull forms are not practical, but rather exaggerated for effect.



You can see that with the same overall length and beam the waterplane areas are drastically different, and so therefore are their load carrying capacities.
If you want to determine your own capacity, just get a rough measurement of the waterplane area in square feet, divide it by 12 (this takes the square feet and converts it the cubic feet volume of this area x 1"). Then multiply this resultant trim change (read displacement) volume by 64 #/sq.ft. for salt water and this is the weight it will take to sink that vessel by 1".

 

One boat sinks deeper because it increases length and/or width faster as it is loaded. The waterplane (the shape a steam iron burns into clothes) doesn't increase much if the sides are nearly vertical. A bowl-shaped hull increases waterplane dramatically as it is loaded.
This and this only changes the load-carrying capacity of two boats of identical displacement.

 

Thanks, so basiclly the shape and the wetted area due to the shape dictates the amount of weight we can put on the boat and without changeing the overall shape nothing can be done to change that. it may not seem like much but that has helped a great deal. Robert

 

Correct. Shape dictates the volume to be displaced.

 

Not quite right.

The amount of weight required to sink the vessel is "Pounds Per Inch Immersion". PPII does change as the displacement changes for any hull.


"Moment to Trim 1 Inch" is the weight x distance required to change the fore/aft trim by 1". This 1" is the sum of the bow down (or up) and the stern up (or down).

 

Aaarrrgh!!! Thanks Paul. I KNEW I should have looked ahead in my student manuals before I assumed that that was what was meant by "Moment to trim 1 inch". Of course! Moment implies the distance, doesn't it? Shoot. Sorry about that Robert. But just for your info, EVERYTHING ELSE I SAID WAS 100% CORRECT (i hope)!!!! I just goofed up the term I used to describe the behaviour of the hull. Everything else applies, just substitute PPII for MTT1I.
That's my Mulligan for the round.