Maximum beam ?

I did not see that the thread went on a page, so my replied to page 1.

After reading page 2, I honoustly have no comment. You probably all know well.

I do have something to say though. I sailed a catamaran for a few years. For people who know, it was a Nacra Inter 18 (built in '96). This cat was extremely wide in the bow, and it kept the same with going further back.
The cat's length is 5.52 metres (18 ft.) The widest point was about 50 cm from the bow (nearly 2 ft.). The cat kept its width to approx 1/3 of the stem.
This cat sailed magnificent and was really one of the faster cats around. I liked the wide hulls; you have much bouyancy if anything goes wrong upwind or downwind with the gennaker. And when sailing in waves, the boat never went through a wave but always over it.

In my opinion the widest point is where you like it.

(The yacht I am willing to design will have the widest point further back though. I would like the yacht to plane if possible and for that you need a lot of surface aft).

 

That's exactly what I did, before this question. ...and when the hull of intentions took form I realised that the max beam was ~65-70% from front.

It was may aim to push max beam backwords... ackording to my knowledge from aircrafts.

Keeping the laminar streaming as far back as possible and behind of that handle the turbulence.
...I'm not a dynamics pro and really don't know if this works in practice.

When it comes to thinking in general I've come to the conclusion that "cutting it fine" very often result in unexpected insights... but one must be able to tak it for fun... wich isn't the same as joking.

 

Coen,

A station is a "design frame" (transl: ontwerp spant). Normally spaced in equal distences from each other.
There are basically two being used systems:

- starting with station 0 on the centre line of the rudder stock (most common in commercial shipbuilding)
- starting with station 0 at the intersection of the dwl and the stem.

The amount of stations depends on the size of the boat.

(right, sorenfdk!?)

 

Max beam

It’s well and good to discuss this in the abstract sense, but is any one thinking about the real world effect of pushing such an extreme beam aft so much. When you roll the boat the bow is depressed, the stern rises and the center of effort will move forward because of the change in attitude of the mast. You may even roll the rudder entirely out of the water. This will lead to directional instability and loss of control in extreme cases. For the cruising boat such an unbalanced shape will guarantee that the boat cannot self-steer without some type of added machinery. Take as an example the Carl Alberg designed 22-foot Sea Sprite. This is the first boat that I took Trans-Atlantic, so I am very familiar with her design. Her hull has a distribution of volume that is evenly split between the forward and aft half of the boat. She will balance and sail herself without any added gear. In fact I took her across single-handed with out any self-steering gear at all. The other example that I offer is closer to the modern extreme, but not as extreme. That would be my Carter designed 2-Ton racer “Juno TT”. She had an unbalanced shape and all the problems that went with it. Does the modern racing boat get any advantage under the measurement rule to justify the extremes that are taken to bring the beam aft? We may have gone to far in this trend and I think it may be better to take a more moderate approach to design in all the parameters. At the very least the cruising boats that are designed today are designed to look like the racing boats and the result is not at all suitable for cruising. Of course this is partly marketing and style not just engineering, so my opinion is only worth something to others of similar thought.

 

Thanks for your reply Peter, now I understand what you mean with a 'station'

If we look at the Vendee Globe, we know that these are unbalanced designs as you call them. If you see such a hull onder 45 degree of transverse trim (heel?), you see the bow nearly dipping in the water, because the centre of bouyancy comes back so far aft and the centre of gravity, off course, stays in place. So you have a lot of longitudinal trim due to the transverse trim. A single rudder would be ventilating far too much by now or will even be lifted out of the water.

There fore, I think it is important to try to lay the Bmax as far FORWARD as possible. That is; something around 50 % of the waterline. IN this case, the boat would not trim (longitudinal) and it therefore is able to sail in one straight line.
As I said, this is my vision. Comment (argumented ) is welcome!

 

Ah...

Hello...

Thanks Robert Gainer - Alberg - ooooh - I so want an Alberg 22...

Time to bow out of this one and do something productive, like securing my batteries on the centerline by driving three inch grade 02 stainless screws thru the the hull and then walking away....

This is beginning to sound like a bunch a grumpy drunks trying to keep warm by a barrel outside the liqour store...

SH.

 

What you are saying is... from my understanding.... that CB must be in center for a cruiser if it should sail smothly without problems...

Volume as you say must be the buoyance (CB), isn't it?

...and IF that should allow max beam to be far back it would be okay?

 

You're close Slowmo.

If your boat would always sail upright, so without any trim (transverse) it would be better to have the CB in the middle and the force produced by the power of the sails right above it. Then it would sail in one straight line. But because you get a transverse trim your boat would turn its bow into the wind. Therefore your CB should be a bit aft. These guys can tell you better than me where it should be.

 

Yup - that's right!
I think most yacht designers (and naval architects) use 10 stations, regardless of the size of the boat. It is common to use "half stations" in the ends of the waterline, because the shape of the hull changes quickly there.
So your station numbers will be:
(Aft) 0 - ½ - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 9½ - 10 (Forward ).

 

You must remember to take the movement of the CB as the boat heels into account. If Bmax is far aft, CB will most likely move aft as the boat heels, which will make it trim on its nose.

 

(I'm thinking through keyboard)
Time after time I make the wrong thinking of referencing CB to the "middle". But what is the middle? Yes, really... what is in the middle?

This is in larger terms a great mistake... to make a reference wich is seen by the eye but really doesn't exist.

There is no obvious "the middle".

What "pulls down" is the center of gravity and the difference between CB and center of gravity wich produces rotating movments when they are located differently (as allways).

When a wave hits it hit in more then 1 way.
First it allways hits the bow: In the meaning of that the hull must have enough buoyance compared to it's enert energy in two ways. Absolute Mass and relative movement.

i.e. rotating the hull (acceleration and deaccleration around Z?)
lifting the complete hull

In both cases it is some kind of competition between Gravity and buoyance (of any water line) in each hit station.

Some days ago I sat and looked upon a picture of a viking ship. It have been said that those ships was written about in theire own time as very potent in rough sea.

What characterise them (I think) is that a wave don't lift the ship from the bow*, since it's so sharp downwards. Instead it lifts a "second" later, furter in on the hull. But, then, when it hits also hit higher up at the bow where it is wider. Because of this the time-buoyans-curve doesn't rais at first strike, but very fast a second later and. This lower rotation momentum. Perhaps one can say "first cut and then lift".

The only way not to get the sail CoE to move is to stop "rotation". That must be (still thinking) by taking the lifting force of the wave far in on the hull (more v-form at bow, not flat). Which would be closer to the hull CoG wich will prevent it from "rotation" and instead lift the hole body.

So, what was said earlier about "stupid rolling boats" is naturally true... but perhaps theoretivcly described not accurate enough to make technological sense.

In waves the CoE moves, but so does also CB when waterline is changed at rotation changes around different axis.

This also implies that not only the CoG poistion matters but also how the mass is located in front and stem in comparison to the buoyance in same station. In practice, V-shape. Every station have a relative [mass/buoyance] (not absolute)coefficient momentum when the (dynamic) wave hits.

In calm sea the hull should have a fast profile but as soon a wave hits the hull should be "prepared" by having a wave persuasive shape in "higher waterline".


*does any other type of boat have such an obvious "bow" in the bow?

------------
Please contribute in the thread on designing a Trailer Cruiser.

 

This is more what dinghy sailors do on a boat, that is weight of crew distribution

A Word on Weight

Weight placement in dinghy sailing is key. The ideal combined crew weight is somewhere between 245-275 lbs. Of course, on any given day those limits can be different. In heavier wind a heavier crew may have an advantage over the lighter recommendations. The real key is where you put the weight you do carry. Going upwind the center of gravity should be around the center board, in other words, move forward a bit. The crew should be near the middle of the center board and the skipper should be straddling the traveler bar. Keep in mind that you do not want to bury the bow in the water either, so a balance has to be achieved depending on your weight. Going downwind you would like the hull to plane over the water as much as possible. In turns you can minimize rudder movements by heeling the boat over with weight movements to initiate a tack.

http://hellfire.dusers.drexel.edu/~dusail/notes2-26-01.html

 

This is a quote from a website about the 420 dinghy.
The ideal crew weight depends on the shape of the hull and - of course - the sail area. Different dinghies have different ideal crew weights.

 

I have been reading through this tread and I guess I would like to add my 2 cents ($).....

Where should the maxB be located, that has been illuted to....

Some of the factors that contribute to the ideal location of maxB are:

CE of the sails
CG the hull
CB of the hull
LatCE of the hull

Some questions to ask..., do I want the boat to sail on its own or do I want it to come back and pick me up after I fall off...?.

As the boat heels over the CE of Sails and CG of Hull remain constant, although they change with trim of the boat, but for the sake of argument they will remain constant. The hull shape controls the CB of the hull, overstated... , so you need to decide where you want the CBofHull when the boat is heeling over. The rotational influence of CEofSails, LatCEofHull, and CBofHull will determine which way boat goes, also the feel of the helm, and proformance (related to drag, more effort to maintain course will create more drag). These factors are all dynamic and so the maxB is the consideration of where do you want the boat to go.....? Historical data is always a good reference....., on what you want.

This is the bottom line, this my 2cents worth.....

 

Of course the part in bold is relevant. In a dinghy crew weight is important.

Obviously in cruisers crew weight hardly has any effect. So design of the boat becomes important.

I also was not talking of the crew weight. that is irrelevant in sailing yachts. However the distribution of weight when designing the boat can improve upwind ot downwing performance.