LCG & LCB How far apart is acceptable?

I have designed an 18' hull, an when rigged out, the LCG is located between 0.5" and 4" aftward of the LCB. Is this going to create a problem?

Thanks for any help,
CVLNGINEER

 

I forgot to mention that it is a powered, planing hull.

 

By definition, the LCB and the LCG are always in line. Thus, your boat has merely been drawn at the wrong trim angle
What you need to do is to find outr where the boat must trim tro bring CG and CB into line, and see if that is acceptable.
Steve

 

When the boat is put in the water, the LCB will aline itself with the LCG, resulting in trim, how much depends on the shape of your hull. If you have a lines plan you should be able to find the MTI and PPI and answer the question.

 

Thanks for the speedy replies!

With such a small moment arm, I wouldn't think that it would trim very much. At least not compared to a 200 lb person walking around on deck fishing. Is this assumption correct?

Also, I had heard that some trim aftward is desirable in planing power boats, is this true?

 

It may be only a "small" moment arm, but it is applied to the whole weight of the boat and engines, don't forget, and trim is very important to small power craft since they cannot use trim tabs as easily to help get out of the hole.
I would take the design to someone that is familiar with that size and class of boat and see what they think. Not wanting to be rude, or anything, but if you don't know whether this seperation is OK, wouldn't you want the design checked anyhow?
Steve

 

Thanks, Steve, you make a good point, which is what brings me here. I know how to do all of the calculations, and have done so, but I am away from the house at the moment, so I don't have the information readily at hand. I guess what I wanted to know was if there is a margin of error in aligning the LCG and LCB. The boat, fully rigged with gear, gas and three 200 lb. people on board, weighs about 1900 lbs., so the offset of 4" creates a trimming moment of 633 ft-lbs, which is equivalent to a 200 lb person walking 3.2 feet aftward from the LCG. It would require about 100 lbs of positive flotation at the stern or an additional 1.6 cubic feet of displacement. The stern at the DWL is roughly 7' 1" wide. To displace an additional 1.6 cubic feet of water, the stern will have to trim down about 0.8" (rough estimate)

I have never known anyone who has designed any type of boat, so I am on my own in this venture, except for great websites, such as this, and kind people willing to share their expertise.
I can provide more exact information when I get home, but with what I have mentioned above, what margin of error is acceptable? If more information is required, let me know what you need.

Thanks,
cvlngineer

 

cvl,
If you can shift your 3 x 200lb people aft by 1ft, it looks like you would have it made. Will it make a difference? Good question. The best way to find out is undoubtedly to build the boat, keeping in mind that you may have to shift your seating by 1ft in either direction. If this can be accomplished easily, then I would "just do it" (tm)
Steve

 

lets hope the LCB is aft of admidships

 

Yes, the LCB is aft of amidships:

LCB = 30.25 inches aft of amidships

LCG (Rigged with three 200 lb people on board) = 29.26 inches aft of amidships.

LCG (Rigged with one 200 lb person at the helm) = 35.37 inches aft of amidships.

LCG (Rigged with no people on board) = 31.64 inches aft of amidships.

The people tend to throw the trim off a little. Do you normally calculate trim with or without the people?

And while I'm at it, what weight estimates do you use for outboard motors. I will probably power it with somewhere between 70 hp and 90 hp. I estimated the weight of the motor to be 275 lbs.

 

If it were me, I would try to place most of the weight I'm carrying right over the LCB (seating,fuel.....ect.) the CG will always line up over the LCB once the craft is at rest. Now I'm no engineer but I have worked for one builder, we were building 38,42 50 .... ect. footers and that was his advise. with the weight pressing straight down. Now your buddies walking around the deck your going to get trim, roll, pitch,ect......... Are you trying to figure this out cuz you are trying to create small topsides? in a 6 STA. boat your LCB should between STA 4 and 5.

 

I was going to say pretty much what Bruce said. Your human cargo is going to have a much greater influence on the trim than anything else (unless you bring several coolers of beer with you - just kidding) So arrange the seats so that the CG of your human cargo is over the LCB. Any other weight can be distributed around the boat to keep everything in trim.

Think about what you do to keep the boat form heeling to one side. You distribute the weight evenly so that you have as much weight on one side as on the other. Well, the same applies longitudinally. You want to distribute the weight so the LCG is over the LCB. Exactly where each weight is isn't really important as long as the LCG of the total weight is over the LCB.

 

Wdn,

Not really, the freeboard is around 12" at the transom. It is a center console bay boat. It is all flat deck space on top, with only a 3" lip to prevent fishing gear from sliding off into the water. It is 18 feet long, 7' 6" beam at the deck and 7' 1" beam at the DWL. It draws 6.5" at the designed displacement and about 7.15" at max capacity.

If I can ever find any information on how to design a tunnel on it, so I can run in shallow water, I will do that too.

 

One more thing, the books I have, only have rough calculations for determining the Moment to trim 1 inch, and it says that the estimation is for larger displacement hulls. Where can I find information on how to calculate the value for this boat, or is it the same? Using their formulae, I came up with 103.25 ft-lbs and 429.56 ft-lbs. I am assuming the larger value is longitudinal and the smaller is transverse. (their naming convention was a little unclear)

 

As the others have eluded to, the aim is to place as much of the variable weights at or as near as possible to your LCB. That way, as they change, they won't effect the trim. In your case, there are likely to be three major variables: People, Fuel &/or water, and gear.
The first can be organised to some extent by your seating arrangement.
The second, obviously by where you place the tanks.
The last is a little more difficult - you'll probably have to spread it around in various lockers so that the overall weight is at the LCB.

How far aft is the LCB in terms of percentage of the waterline length? If it is in an appropriate place, and you are happy with the trim, then you should move non-variable weights - such as batteries - so that the LCB and LCG are in line