How do you design for the perfect trim?

I have been wondering about the design process for the perfect fore and aft trim. Do you first calculate all the weights of all the components that are going to go into the boat, and then design the shape of the immersed volume so that the center of gravity of the boat is in line with the center of buoyancy? Or do you design the hull to give you a level (horizontal) waterline to start with and then try distribute all other components to maintain that trim. I am talking about a 40 ft sailboat with a fin keel and a ballast ratio of 40%. Since the keel constitutes majority of the weight, positioning it i guess is critical compared to anything else.
The easiest way would be to move the keel a few centimeters back and forth to achieve a final perfect trim but at the same time positioning of the keel is also very important for sail balance.
Alternately, should you start with the hull and keel, then try to distribute everything else around that balance.
What is the correct approach in acheiving this?

 

It will vary from designer to designer. In my practice, I start with the hull shape, and right from the very beginning I have a fairly good idea where the heavy weights will be. Add the keel temporarily where it will go. Design the rig, and make sure the CE and CLP are in correct position to each other. Design the rest of the interior as this will be influenced and restricted by both the placement of the mast and the position of the keel. Start the weight estimate in ever increasing detail. Keep going around the design spiral with small corrections to each item until the design is finished. If you keep everything under control, it will all work out in the end.

Eric

 

Thanks Eric.
Ofcourse it makes sense to get the biggest weights sorted out first.
In this case it would be the hull and the keel. Once this gives you a level trim probably 70% of the total weight is distributed correctly. But the remaining 20-30% is still significant and can be large enough to upset the final outcome.
Following your suggestion,and keeping track of all other additions in ever inceasing detail,do you calculate the effect of any other item going in by recording its position and weight and making sure that they balance around your neutral location? This to me is a very hefty task.
But this also brings another question to mind. Some items in a boat are not in line with the keel but off center. So do you in any case keep record of position of all items anyway to balance the boat athwardly? (not sure if this word is correct)

 

Unless there is something very unusual (and heavy) aboard that is positioned to one side, athwartship balancing is easiest done by "dead reckoning".
This can be done using items that are always onboard like ground tackle, extra hardware, emergency water jugs, and so forth.
fine calculating is nowhere near as efficient as an inclinometer positioned below in still water while you load the boat.

Alan

 

Not doing the athwartships calcs leads to big headaches.

Compare the moment to trim 1deg fore and aft with the moment to trim 1deg athwartship. Walking from the transom to the stem produces a smaller trim change than walking from port to starboard on most sail boats.

Many production boats set on their fore and aft lines, but list due to poor weight management. Then leave the owner to have to compensate by loading the vessel properly to remove the list.

If the homework is done properly during the design stage, you don't have to keep all your ground tackle in a cabin or ask fat uncle fred to sit 18" from the stb gunn'l to trim the boat ...

 

Ultimately, your weight estimate should include all center of gravity positions: Longitudinal, vertical, and transverse, and moment balances for each. The transverse moment is, ideally, zero, so that there is no heel. Unfortunately, we are rarely that perfect in boat design, but you can get it pretty close so that the resulting heel is negligible.

The same is true for longitudinal center of weights--ideally you want it directly over the LCB, but it is not going to be perfect. Just get it as close as you can, and generally, if you err a little bit, it is usually better to err with the LCG just a hair forward of the LCB so that the boat trims down by the bow with no people on board. This is because when sailing, the people usually sit aft, particularly in aft cockpit designs, plus boats always trim by the stern when sailing, so a little bit of bow down trim is desirable when no people are on board.

For VCG, you should be targeting where your VCG should be for the intended stability of the design. The more accurate you are, the better off you are going to be, and you'd like the VCG to be at or lower than your intended value.

When I work out my weight estimates, I start with a weight budget for each category: Hull structure, deck structure, interior joinery, interior fittments, deck equipment, rig, tanks, electrical, mechanical, etc., etc. Each category has an intended percentage of the total displacement and a target CG location divided up for LCG, VCG, and TCG. Each category also has a contingency line for any unknown weights. The success of my weight estimate is revealed in how much contingency I have left over each time around the design spiral. Ideally, all my contingencies in each category will go to zero at the end of the design, but again, that rarely happens. But the job is to make them tend toward zero with the best location for everything. In this way, I can see at a glance where my weights are going and what is causing any imbalance. I may have to move the engine or the batteries or the tanks, for example, in order to get the CG in the proper position. The interior of the boat changes accordingly, and I redesign those parts that need to be to accommodate the shifts in CGs.

It's all a very fluid process. The point is that you have to start with the hull lines and where the center of buoyancy is and design everything else to be in proper relation to it. Certainly, you do not want to specify the weights first, and design the hull to meet the center of those weights--that would likely make for some really unusual boat design.

Eric

 

Perfect trim

Once you have everything or it's weight equivalent aboard, you can put a piece of shaft under the keel at the boat's longitudinal centre of buoyancy, then ballast her. This eliminates any possible chance of a mathematical mistake , especially if you have double checked the balance point with a model.
Do the math to get it close. This is a double check , if you are that worried about where she balances, or how she trims..
Brent

 

what about the tanks, do you calculate with tankage full or empty?

 

Besides the balance when sailing, also consider the balance of the windage when under bare poles.

For example, you could have the sail rig and keel moved aft, and although they would balance properly when sailing, the bow will tend to blow off when trying to raise the main or when coming into the dock with a crosswind.

 

Both,

your tanks should be placed near the LCG so they don't change the trim. During the preliminary design stage I like to determine the LCB at lightship and full dead-weight, this allows you to place the tanks in a location that will change the vessels LCG from one LCB to the other as they fill, meaning you get level trim at both cases.

We do a lot of weight estimate work both on our own boats and for a couple of external design houses. The estimate I'm working on this weekend is for a 80' sloop and has over 2500 individual data points.

In my book, a weight estimate is the most important part of any design process with most "issues" with completed boats being weight and trim based its worth taking the time to do them right. As such a weight estimate needs to be revised as you pass through the design spiral. It will usually start as a total weight based upon experience and research into existing vessels and then be refined as you go. A typical design process is as follows.

Establish Preliminary weight
Design preliminary hull
develop structural midship section
write preliminary weight estimate
develop hull shape
design structure
update weight estimate
modify hull shape if required
update weight estimate

Build hull
weigh boat
update weight estimate (confirm accuracy, advise changes if required)

Add superstructure
weigh boat
update weight estimate (confirm accuracy, advise changes if required)

start fitout
weigh boat
update weight estimate (confirm accuracy, advise changes if required)

Almost ready to launch
weigh boat
update weight estimate (confirm accuracy, advise changes if required)

Launch boat
drink beer.

In the event of **** ups most people will start trying to solve trim issues by moving the keel (sailing boat) or batteries (powerboat).

 

chris, i think that very much depends on if you are doing this on a professional basis or not. if not then the beer drinking stage may come earlier usually followed by great ideas which then lead to fuckups

 

drnick,
LOL, perhaps the most accurate description of the design process ever!

 

mmm beer....

my advice to the home builder/designer is given enough patience anyone can write a weight estimate. This is not the corner to cut.