Lcb / Lcg

[Mat-C]

Many of the texts I've read suggest that the faster the boat, the further aft the LCG should be. That would in turn suggest that the LCB would be further aft, assuming that the boat floats on her lines.
But just how far aft is far enough? I mean take a boat like the Wally tender, or one of Levi's delta shapes, which they appear to be similar to. How far aft would the LCB be on a boat like that? And if it's say 60 - 70% of the WL aft, then how would that effect the boats ability to plane at lower speeds - at least without excessive bow rise?
Put it anopther way... for a 30-odd foot boat designed to have a top speed of say 30 - 40 knots, just how far aft should the LCB / LCG be?... Especially if you want to also be able to potter around at say 18 or 20 knots without the bow pointing at the sky. And is there a way of determining its optimum location?

[gonzo]

You have to create a design and then find the optimum location for that specific vessel at the target speeds.

[Mat-C]

Thanks Gonzo... but that seems a bit arse-about to me (no disrespect intended!) Surely you would create the design with the optimum location of the LCG / LCB as one of the fundamental aspects of the process. I mean, for the boat to float in its lines, the CG and CB must line up, and the CB is determined by the shape...
All my research suggests that a CG that lies between 50 - 55% of the WL aft of '0' is the best compromise for promoting lower speed planing. Boats with that delta-type shape - like the Wally and an increasing number of others - would appear to have it at least 65% of the WL....
I realise that these boats are made more for zipping about the Med, showing off, rather than good low-speed performance, but still......

[Mat-C]

Ok - so I've been fiddling about with shapes for a few days and everything that I come up with that even remotely resembles the shape of boats like the Wally's or the smaller look-a-like C-Boat C12 ( http://www.c-boat.co.uk/ ) winds up with the LCB about 62% of the WL aft of station '0', with the boat modelled on a level keel. Is this not too far aft? I envisage a boat with excessive angles of trim and very poor low to medium speed attributes. Am I wrong? Is an LCB this far aft perfectly normal and will the boat plane just fine at lower speeds?

[Willallison]

Mat, it's worth considering that most of the true Delta shapes - ie those designed by Levi were generally intended for very high speeds. Most were raceboats, or thinly disguised pleasrure versions of such.
I don't know how their low speed performance would rate, though most were very light, so it was possibly better than one might expect with the centres so fat aft. A number of his boats also suffered - some times quite badly - from porpoising and on a number of oacciaions some pretty drastic measures were taken to try to eliminate this.
I've not seen a Wally up close, so can't comment on their bottom shape - maybe some of our other contributors can comment further on the current crop of delta-like boats that are popular at the moment...

[Mat-C]

Thanks Will,
My understanding is that the best approach is to 'decide' uopn the ideal location for the LCB (how do you do this? I've never seen a text that describes how to find the optimum location for the LCB)
Then you produce the hullshape, do a weight study and juggle things around until the LCG is in line with the LCB.
What happens if you can't?
I mean I know the boat will trim down one way or the other, of course, but say you have the LCB at 58% of the WL and the LCG at 61%, what will the effect be in terms of performance / handling?

[conceptia]

to my knowledge more aft the LCB is the less resistance vessel is going to face. Thus, an engine of lower power can be selected. In short, power efficiency of the hull form increases.
LCG is then brought aft so that the vessel is not going to trim by forward. It is to be noted that LCG should be at or aft of the LCB for better performance.

[Tad]

Originally Posted by Mat-C

Thanks Will,
'decide' uopn the ideal location for the LCB (how do you do this? I've never seen a text that describes how to find the optimum location for the LCB)
Then you produce the hullshape, do a weight study and juggle things around until the LCG is in line with the LCB.
What happens if you can't?
I mean I know the boat will trim down one way or the other, of course, but say you have the LCB at 58% of the WL and the LCG at 61%, what will the effect be in terms of performance / handling?

See this paper for selection of optimum LCB for speed.....http://www.tadroberts.ca/about/pdf/p...d-function.pdf

1) Decide on length.....
2) Decide on weight....
3) decide on speed or power....one gives you the other
4) go back and re-evaluate weight.....
5) Select LCB and chine beam from speed.....
6) design hull
7) juggle weights to achieve chosen LCG

You can only get the LCG so far aft....put the engines up aginst the transom and the fuel directly forward of them. You've seen the go-fast guys with half the boat (forward half) out of water at idle?

Concerning your question of what will happen when LCB is at station 5.8 and LCG is at 6.1.......when you put the boat in the water LCB follows LCG, so both are at 6.1.......boat trims by the stern...What the LCB at 5.8 tells us is that this hull was aimed at a lower optimum speed....there is less volume aft for lower resistance at lower speed. Generally it's not a problem to move weights forward.

[Mat-C]

Thanks Tad. I've read your excellent article before... I should have thought to go back and check it again....
Of course, answers always promote questions...

How do you determine the ideal chine beam?

I understand that at rest the vessel will trim to bring LCG and LCB into line, but what happens at speed if they are not aligned? Yes, I've seen many a go-fast with its stern almost underwater as a result of a way-back LCG. So the designer obiously drew the boat at level trim, knowing that when it hit the water it would trim down by the stern. The aft location of the LCG is good for higher speeds, but what about lower ones - S/L's of say 2.5 to 3.5...?
Will trim be higher than if the LCG was further fwd... closer to the at rest LCB?

[Tad]

Jeezz....you guys want all my secrets.....

See the 3 figures below.....It's all about planing surface area, and the aspect ratio of that area.....for our purposes the planing area is bounded by the outer chine and the transom. The Displacement Trends diagram illustrates bottom area/disp vs hull size and overall length.

Ap = projected bottom area (bounded by chine and transom)

The second two figures illustrate minimum resistance per ton for Volume Froude numbers....

Lp = Chine length
Bpx = maximum chine beam





Figures by Donald Blount

[Willallison]

As usual Tad, your generosity knows no bounds
Which paper was that from?

[Mat-C]

OK, so I've been thinking about this some more....
Logic (which isn't always my strong suit! ) suggests to me that as the LCB moves aft, then so too does the proportion of the waterplane area. Which in turn ought to provide greater surface area for dynamic lift towards the rear...reducing trim.
Why is the opposite the case?

[Wayne Grabow]

Great discussion. I really appreciate the comments from our resident NA experts. I am building a 18' runabout-style hull and getting to the point of determining weight distribution. I realize that the weight distribution, and resulting trim, needs to be different for higher speed than slow cruising. The approx. weights are 750 lb. bare hull, 400 lb. engine-battery-fuel, 300 to 700 lb for two to four passengers. With a high proportion of movable weight (fuel tank, battery, people), I was thinking that with some combination of weight shifting and propellor trim adjustment the boat could perform efficiently in both speed ranges. From the comments here, it appears that I should just choose a desired speed and stop trying to achieve "the perfect boat".

[Tad]

I see I'm remiss.......

Will.....I believe the first sheet "Trends" is un-published.....

The other two are from "Reflections On Planing Hull Technology" 1993 Fifth Biennial Power Boat Symposium.

Matt....lost me! Typical planing hull behavior is increasing trim as speed increases.....until the hull overcomes it's particular "hump" when it will often see maximum trim. Once through or over the hump, drag drops as does trim (typically) and the boat seems to leap forward......after which drag increases again along with trim.

You can see the resistance curve of the planing hull below...



Wayne....

Hull form and LCG must both be right for the speed.....one right and the other not will never be "perfect". "perform efficiently" is a fairly imprecise measurement.....against what measure?

[Mat-C]

Tad - wouldn't be the first time I've failed to explain myself clearly!
I'm thinking primarily of the hump speed and low planing speeds.
In a typical planing hull, if the LCB is moved aft, then it stands to reason that the centroid of the waterplane area will also move aft. Which in turn would suggest that there is more area towards the rear to lift the aft sections. This, one would expect to reduce the trim angles at these lower speeds...and indeed at higher speeds too
My understanding however, is that the opposite is the case - the further forward the LCB is (within reason, of course) the better the low speed performance....