Longitudal stability of sailboats....

Is there a rule of thumb for longitudal stability/sail area?
It must be desirable to design the fwd stations so that the entry angle of the WL is as narrow as possible (for race boats that is), but where is the limit? If the bow is flared, the displacement will increase quicker, what are the advantages and disadvantages?
Also, this must be dependent on the sea conditions...

/Henrik

 

Henrik,
Skinny bows may "pierce" the water better, but don't forget that lovely moment from the sails trying to bury the bow. Some of the most successful planing boats have very bluff bows (comparatively) to keep enough volume up there so that the nose doesn't try to play submarine from all the power aloft.
Just a thought...

Steve

 

An insight that has informed Bruce Farr's work is that boats with lower displacement/length ratios require less reserve bouyancy forward. I'd suggest taking the longitudinal moment of inertia of the waterplane at 5% of DWL above DWL. Perhaps consider only station 5 forward, as though you joined two bows at midships to form a canoe shape planform, and take the I(L) of that shape a bit above DWL. Then compare the fourth root to the heeling arm (or the height of the CE above DWL, or perhaps the average of the two).

 

Just thinking - Haven't done this before but if you have a stability program you should be able to apply the sail plan heeling moment by turning the hull at 90 deg and running an analysis to check the resultant trim. Admittedly, easier done on some programs than others. - Brett

 

It depends largely on the size of the boat. For example, a dinghy, with the wieght of the crew is very easy to trim. A ship has to be designed and built to float on her lines. Also, clipper bows work well on boats 60' and longer. On shorter boats it is a disaster, because it causes violent pitching. Another very important subject, is the difference between a fast boat and a racing boat. Fast boats go fast. It may sound like an oximoron, but racing boats are designed to beat a rule even though they are slow. Consider a 12 meter which costs tens of millions of dollars but is slower than a Hobie Cat which costs $5,000.

 

got wings

 

Interesting. Anyone care to try to explain radius of gyration ("gyradius")? That bears on this issue as well.

 

good gyradius

 

Stephen,
Radius of Gyration really only has bearing on the inital acceleration. Once the "system" is up and running, it all evens out (like you car when you ease off the throttle and the front wheels hit the ground again). It does affect wave-induced pitch to an extent, of course, but that's moderately OT, I suppose. I am stilla dinosaur when it comes to weight in the ends - I like a bit of mass to stop the boat pitching so the rig can keep working. In older, slower boats it was more importamnt to keep the hull moving, so pitching to waves was important, but I feel it is less so with "sport-boat" types.
I know, I know....

Steve

 

Steve (SailDesign):
I didn't follow that well. Could you explain it again, real slow?
-Shu

 

Shu,
Radius of gyration is kinda like a rotative moment of inertia. it is a measure of how resistant the body id to rotation in the pitch mode.
Hence, if you have a small gyradius, and apply a large load suddenly from on high (like the rig suddenly getting wind) then the boat will pitch a lot as that happens. When the boat has achieved speed, and there is no out-of-balance force to cause opitcching, it will "settle down". Like accelerating with a powerful rear-wheel drive car, only pitching forward not aft. ;-)
A large gyradius will lessen the initial pitch, and will be less affected by other forces such as wave impacts.
Steve "the other one" Baker

 

Where I'm going is that a boat with a low gyradius and a low D/L can also have a fine, straight, slab sided bow without burying it. Going to windward a fine bow will not initiating as much pitching as a full one, which Steve Baker is indicating could be a problem on a boat with a small gyradius when he says "A large gyradius will lessen the initial pitch". I'm suggesting addressing that problem by fining up the bow, not by spreading weight away from midship. Downwind a low D/L should mean a lower ratio of sail area to length, hence less need for a full bow shape.

 

Steve and Steve, Thanks.
-Steve
(I just had to do that)

 

Stephen D, yes, a slim bow will have that effect. As far as the low D/L and less sail area, well... Let's just say that I prefer the "more sail, fuller bow" approach to speed ;-)

Steve B

 

Shuttleworth has a stability index for mulithulls that covers longitudinal stability - see "Multihull Considerations for Seaworthiness" in Adlard Coles' Heavy Weather Sailing. (also on the Shuttleworth web site, http://www.steamradio.com/JSYD/Articles.html , but he seems to be having server problems at the moment)

One way to get a handle on longitudinal vs lateral stability is to look at a "footprint" plot of the center of buoyancy as a function of the applied loads. Stability indices are really expressions for maximum moments, and knowing the weight of the boat can be turned into location of the center of buoyancy, too.

Here's an application of Shuttleworth's indices to a 36' trimaran design:

The green lines are the location of the center of buoyancy that would be required to meet Shuttleworth's criteria for a trimaran. The blue lines are the location of the center of buoyancy for various combinations of heel and pitch trim. The red line is the location of the center of buoyancy when the lee bow is just immersed. It's evident that this design nearly meets Shuttleworth's stability index for 25 kt apparent wind, and probably will meet it if the ama is given a little more sheer so as to keep the bow above water when pitched down 6 to 7 degrees.

I think you could do the same with a ballasted monohull. As the boat is heeled, the actual c.g. would be moving to windward while the center of buoyancy would be moving to leeward. So the lines would be more compressed in the lateral direction. Something similar would happen in the longitudinal direction, but not as severely because the range of pitch attitudes is much less than than for heel.

An equivalent approach would be to decide what the limiting pitch attitude and minimum bow freeboard would be acceptable, and design to that.