Boat Balance

[BonesD]

How is the balance point of a boat determined? (Aft to rear of course).
In my case a 1977 Schucker 436 motorsailer.

I’m sure some will say go by the stripe at the water line but who knows who applied it and what did they use to decide it’s location?

I’m imagine too much weight one direction or the other could have adverse effects in the way a boat handles.
Is this something that would be provided in the original paperwork of a new craft?

Thanks

 

[fgarriso]

Are you referring to gravity of gravity or center of buoyancy.
They both have a major impact on hull stability.
In both cases they are initially determined by the Naval Architects

 

[BonesD]

I don’t know what gravity of gravity refers to
I’m guessing center of buoyancy is my concern.
For instance stowing to much weight forward or rearward. I can’t change the design of the boat but I do have the ability to shift weight.
Like an airplane, too much weight behind the correct center of gravity can make a plane stall faster, up to a point it can also make it quicker to respond and that can be desirable but also kill you quicker. Too much weight forward of the preferred CG will make the plane less responsive to control inputs.
I’m not an expert in any of these things.

Let’s say you were provisioning for a trip, would you not try to distribute the supplies in such a way as to keep the boats attitude the same as if it were empty?

 

[MYTraveler]

The trim of my boat, which has a 700 gallon fuel tank far aft (the two main tanks are midship), as well as a 170 gallon bait tank on the transom and two 1-ton fish holds under the swim platform (integral hull) varies enough to noticeably affect the boat's handling in following seas. I find that, especially when I am not fishing (so the bait tank and fish holds are empty), the aft tank should be emptied last. It is hard to notice the trim change just by looking at the waterline.

 

[psneeld]

I may be off, but this is my understanding....


To the OP........when the boat was designed, the designer hopefully roughly guessed (maybe accurately calculated) that the immersed section would float where he /she wanted it, so the center of gravity was vertically aligned with the center of buoyancy. When weight is moved and the CG moves horizontally away from the center of buoyancy...the trim changes and the boat is no longer said to be "floating on her lines".


So yes...you would need that original set of lines to find the "designed waterline" versus where it has wound up since the design left the drawing board (computer).


There may be an easy way, if you found out that the main deck was supposed to be perfectly level, than that could be used to establish what "trim" you have and could move weight or ballast to correct.


Unless you know that a design runs better at a different trim than as designed (hard to establish for anything other than a full displacement boats as speed greatly varies trim ....unless tabs adjust it back).... then yes, load to keep trim the same.

 

[fgarriso]

Are you referring to gravity of gravity or center of buoyancy.
They both have a major impact on hull stability.
In both cases they are initially determined by the Naval Architects

center of gravity or center of buoyancy

2AM does that to me at times

 

[BonesD]

Pretty much as I figured.
Trying to find a level spot in a boat that is designed to shed water one direction or another may be impossible.
Maybe I can get lucky enough to get another Schucker owner to respond with some information on their boat.
I don’t know about all of these boats but I do know that on at least my boat the interior was fitted by the original owner and the choice and quantity of materials was left to his needs and wallet.
Maybe the mast is supposed to be perpendicular to the water surface. That’s assuming the mast is symmetrical. That would be easy enough to check.

 

[psneeld]

That would assume the mast was designed that way and is properly stepped.

 

[BonesD]

Lord only knows. The mast looks symmetrical and does show to be about an 1/8 of a bubble pitched forward to the earth. The horizontal plate that the mast is stepped on on deck shows about the same angle. Nose down.
I’m curious about this for a couple reasons
Steering has some wonky issues and I nderrstand that a nose heavy boat can show up as problems at the helm.
And
I have 2 spots on the boat of concern
The shower floor drain in the bow does not drain completely as the drain is in the aft part of the shower area so water pools slightly toward the bowt
And
The lazzarette lid recess has two holes drilled in it toward the stern to allow water that collects to drain.
The water however runs to the front of this recess. Once again just slightly but enough to make me wonder about the boats attitude.

I don’t know that this information helps me at all but there it is.

 

[markbarendt]

The fore and aft balance is always effected by how the boat's cargo & equipment is loaded/installed. Since 1977 a few things may have been moved, added, removed.

If, for example, a choice was made to add all chain anchor rode in place of a mostly rope rode, that change would tip the bow down.

If the fuel or water tanks are aft, and they aren't full, again the boat would naturally tip forward.

There are lots of these examples including mundane stuff like where you pack your canned goods.

On my boat, before I got it, the generator was removed, I'm sure that tilted the boat a bit bow up. I moved an 8D battery from the engine room to the bow, that change will bring the bow down a bit.

In a practical sense it's easy to test on your boat, rearrange the cargo to balance/level the boat. If needed use sand bags in the aft lazarets.