Is there a laymans method of determining the Longitudinal Center of Buoyancy of a boat? I can determine the CG by hanging the boat from a string a couple of times. Is there a similar simple way of doing the LCB?

Don, couldnt you just set the hull in the water and then use the edge of a straight edge( or dowel) across the beam , sliding it( rolling it) back and forth while pushing down into the water? Or pushing at a point in the bottom inside of the hull.

When it goes straight down at the waterline, without tipping , where that vertical line intersects the water line ,I think thats it.

I think it would be easier/ more accurate, if your using a hull, to remove the fin/ bulb and rig .
I should have asked, what are you doing?

By definition the LCB falls on the same vertical plane as LCG. However, this is not necessarily very helpful since the boat will trim bow or stern down until this becomes true. Presumably you want the data for a given trim.

This is actually quite hard and I am not sure how to do it. We can adjust position of LCB and LCG to arrive at a given trim by addig weight in the right place or pushing the boat down with simw sort or probe. The latter is much more complicated in that we need to measre the thrust on the probe. This is effectively ‘weight’.

The problem is that if the weight added is in excess of the designed displacement, the boat will sink bodily in the water. Depending on the hull form this will change the numbers we are trying to measure to a greater or lesser extent.

I hope this helps,


Thanks guys
I am still thinking about the difference between the Venom’s attitude(nice and level when heeled) and my modified Mistral (bow down when heeled) and thought that it might be that the Venom’s LCB is farther forward than mine. If there was an easy way to compare them it would be interesting. I didn’t get a chance today but I am going to hang a bunch of weight off my mast to heel it to about 35 degrees and see if it goes nose down when it isn’t moving. I don’t think it should. Comments please.

model both hulls in software(hullform??) then heel them to see the trim changes.

:DI think the venom sections are secret.

I have tried to use hullform with absolutely no luck:mad::mad:. Maybe this winter I’ll give it another shot.

I have the mistral lines,will post some pics of the boat upright and heeled when i get home.

Here is the mistral hull at zero heel,30 degrees and 45 degrees heel.
she rises out of the water and pitches a little.
This hullform file is not mine so cannot comment on the accuracy of the lines although I do trust the original source.

Here is a more balanced US1m so you can see the difference

Long thread on the topic of hull balance here:



Thanks Brett
I think thats what I’m seeing. Can you tell us what the “more balanced” hull is?

I followed that thread as it was written, I think I learned a lot but a lot was over my head. I go back once in a while and re-read these big threads just to see if I understand a little more.

I put the modified Mistral in the test tank(hot tub) and hung some lead off the top of the mast. It took 12 oz. to heel it 40 degrees. The bow waterline was about the same as 0 degrees and the bottom of the transom was just touching the water. There didn’t seem to be any pitching at all. Now I will try the unmodified Mistral. Back in a minute or so.

I’m back. The normal Mistral looks about the same althought it took 1.5 oz less(10.5 oz) to heel it to 40 degrees. I used the same bulb for both tests. I’m not sure if I proved anything or not. I do know that when they’re sailing with that much heel they pitch forward. Does that mean it’s the sails pushing the bow down? What else could it be? Does that also mean that in a static test it should pitch back? Comments?

Am I using “pitch” correctly or does pitch mean bow down and there is another term for bow up?

‘Pitch’ usually involves movement. A ‘sttic’ condition is usually calleed ‘trim’. So the boat is not trimming bow down (or trimming by the head) when stationary but trimming bow down when sailing. When it meets a wave and the bow lifts or sinks, it is pitching.

This is good English and i m pretty sure it is good American.

Just knocked it up in hullform to show the difference…

I was looking at a Hullform table and they showed pitch as a positive or negative number. Which one would be bow down? I tried to Google it but all I got was propellers and oars.

Hi Don,

I don’t know what you are after here, but try a phrase like

pitching moment of inertia

The earlier post had it correct. Trim is the natural position in flat water. It can be static, as in your tank, or dynamic as in while sailing. Pitching is movement caused by waves.

I would have thought ‘Moment to trim 1 [unit - mm, inch, foot…]’

The positive figure will give a bow down trim (unless Mr. Hullform is thinking about things the other way round from me!)

Mr Hullform does think the other way,Negative values show bow down trim.
Hullform references a pre determined "baseline"from which some calculations are done from,“trim” being one of them.
Hullform is unusual in many respects but I am yet to see software that treats heeled calculations better.