Lcb / Lcf

What is the significance of the Longitudinal Centre of Buoyancy (LCB) and the Longitudinal Centre of Flotation (LCF)? Should they be at the same location? Is one supposed to be ahead or behind the other?

 

Hi,

It is becoming a very common question here. If you use the "search" function you'll discover many threads covering this topic, the most informative being the one where Eric Sponberg has written a guide about boat design ratios and coefficients.

So I'd really invite you to use the search function, which is the most powerful ad useful tool this forum can give to both amateurs and professionals.

There's a knowledge database equivalent to a complete university and professional course in boat and ship design, construction, maintainance and handling in this forum, you just have to look after it.

Cheers

 

SP, your answer starts on page 3......

 

Doug,
That was generous, but I personally would have preferred SP (and others) to exercise using the search function, which is imho really useful.
Now we have yet another copy of the same pdf file occupying the space on the server.
But that's just my point of view...
Cheers!

 

Search

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Slavi, I understand your point but it seems to me that SP and others would be more likely to learn if they are given a direct reference in answer to a specific question like this. It is very true, in some cases, that learning-or being willing to take the time- to learn how the boatdesign search function works is highly beneficial. But I've found sometimes that it's very difficult to get your search terms just right so that the machine understands the search you're trying to make.
--
I am a big advocate of people that post their own design ideas on the forum
learning to, and being willing to, post the relevant design ratios as a benefit to all of us, so I asked Eric a while back if I could post his Design Ratio's PDF
in circumstances where I felt it might help spread the message.
--
I've been thinking for some time now if an index of important threads and their contents might be beneficial and I even started one for the "Foiler Design" thread but before I go any further I guess I need to see what Jeffs reaction would be because it is really labor intensive.

 

Good news about LCB and LCF lines, plotted as function of draft d, i.e. hydrostatic curves LCB=f(d), LCB=f(d). They always intersect in point where LCB has maximum.

Just useful guide to check calculations.

 

I agree, that would be helpful.

There is another tool, the use of which should be encouraged whenever possible, imho. I'm talking about the "rate thread" button.
If we had a "sticky" updated list of the most voted threads (divided by forum topics), that would imho be a big help in finding out the most informative dicussions which have unjustly fallen into oblivion over the years.
cheers!

 

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I've had a negative experience with the "rate thread" thing: it can be used by people who don't care about the subject matter to dis a thread because of
their own lack of interest in the material or because of personal animosity. When my personal stalker was still here three of my threads were de-rated in a one week period. Jeff says that if you put the cursor over over the :star(s) you can see how many people contributed to the rating. I agree with your sentiment if, before the rating was shown, a certain minimum number of ratings-like 5 or 10 would have to be received. That would make it more meaningfull and lessen the effect of people just out to sabotage a thread.

 

Doug, thanks for the Eric Sponberg document. You are a scholar.

 

I've read the Eric Sponberg article and I'm just over 10% into the "center of flotation calculation and implications?" thread at the Boat Design folder. So far, there has been a pretty good description/definition of LCF. But I'm waiting for the punch line.

Is there a preferred location for LCF? Is there a preferred location for LCB? Is there a preferred spread between the 2?

Instead of making me watch the entire 'chick flick' can someone please tell me if my boat dies of leukemia in the end?

 

One does not need to start desiging boat from LCB abd LCF.

Yes, there is optimum LCB for certain speed, but it is a range of values, not a particular value. LCB is much effected by load distribution and LCB for particular design is a compromise.

For LCF - we never check it specially, at least for hull shapes we play with. The only good thing about LCF is that taking load in CF point does not change the trim, so it should be considered when taking payload.

 

I'm about 1/2 way through the Sponberg thread. Lot's of good information, but I'll probably have to go beyond to find the LCB/LCF info.

On a small boat, once a 65 lb outboard is hung from the transom and a gas tank and battery are placed in the lazarette and 2 crew are posted in the cockpit, the LCB and LCF may no longer be where the designer intended them. An understanding of LCB and LCF could lead to decisions about locating the battery forward or relocating the gas tank to the anchor locker or keeping the beer cooler in the vee-berth . . .

 

SuperPiper, I think you will find that there is no hard and fast rule as to where the LCF should be in relation to the LCB, although in most modern designs (say of the last 100 years or so), the LCF is typically aft of the LCB. How much depends on the hull shape and what the design is meant to do. For example, an Open Class boat (40', 50' or 60') may have the LCF way aft of the LCB, but a much narrower and more conventional cruising sailboat will have them much closer together. On the other hand, some multihull amas may have the LCF forward of the LCB. It all depends. You put them where they will do the most good in consideration of the purpose of the design.

If you are designing a small boat with the outboard, gas tank, and battery on board as normal gear, then the hull shape should be such as to account for this weight and its location. Of course, the weight of the crew/passengers will be significant, too, and they should also be accounted for. Add them into the weight estimate and make sure they can be carried so that the boat will still be at level trim, or whatever trim is necessary for the best performance. That is the total trick of boat design--estimate the weights and their centers, and then shape the hull accordingly for best performance.

Eric

 

Damn! How long is a piece of string?

Eric, thanks for the response. No, I'm not trying to design the boat for optimum performance. I have a harder task. I'm trying to sail the boat for optimum performance.

She's a micro cruiser and she sails like a heavy dinghy. I can affect the trim depending where I set down my sandwich. There is a story within the fleet of a racer with a 50lb stone in his anchor locker. He sailed past boats with 5-6' more LWL.

In a related question, how important is it to keep the transom dry? When she gets way on, the bottom 2-3" of the transom is usually submerged in the stern wave. I think that the transom should be dry and that the wake should peel off exactly at the hull-transom junction.

 

There is more going on with sailing your boat than worrying about the LCF--that is only a symptom, not a cause. Trial and error with the placement of your weight on board, as I am sure you are doing, will eventually reveal the best attitude to sail your boat. Your performance has mostly has to do with hull shape in 3D, length-to-beam ratio, and that you have a fairly heavy boat for its length. If you can get the transom out of the water, so much the better.

Eric