Why do boats nosedive?

I was wondering why boats nosedive especially model boats. Is it just the forward moment of the sail which tips forwards?

 

Not enough volume in the bow to keep it from sinking?

A rig placed too forward?

Weight set to forward?

 

Mostly, the force of the wind in the sails, coupled with the problems mholguin notes.
Everything must balance, and a boat is just a series of compromises that sometimes fails to do everything well all at the same time.
Something like that.
Steve "sounding like Sean Herron"

Hi, Sean....

 

Actually, there is too much volume in the stern in modern designs. The designs of Capt. Nat Herreshoff and his son, L. Francis Herreshoff, tended to have very slender sterns which, as the boat heeled, allowed the stern to sink and the bow to rise up, the opposite of more modern designs. This gives the hull a natural tendency to put positive angle of attack on the keel, which is good, whereas fat stern sections tend to put negative angle of attack on the keel as the boat heels (bow down). This concept is a bit of a lost art, but you find it in relatively narrow hull designs. The key to analyzing this is to track the movement of the LCB as the boat heels. If, at 15 deg heel, for example, the LCB moves aft, then the hull buoyancy pushes the bow down (not good). If LCB moves forward, then hull buoyancy pushes the bow up (preferrable). Ideally, you'd like the hull to remain level (LCB stays in same location at heel) or if you want the "Herreshoff effect" adjust the hull shape accordinglywith slenderer aft sections and fuller forward sections.

Eric

 

Is it a forgotten art, or just something that is accepted because a fine bow helps you get upwind well (wave penetration), and a wide stern gives you stability (all else being equal, of course)?????????

 

Nose diving in models

Alex, in models , particularly scale models of bigger boats, scale effect prevents the form stability of the larger boat in both pitch and roll from translating to a model. Thats part of the reason why you see models with such long fins.Righting Moment(form stabilty + lead on the end of a fin keel) on most models with scale rigs or nearly scale rigs is not enough to allow that kind of rig to be used in more than about 5-8 knots of wind. In model multihulls(except in the lightest wind) it's virtualy impossible to use the high beam to length ratio hulls like those used on Tornado's and Orma 60's because they have virtually no pitch resistance at all- unless the boat uses hydrofoils to augment stability.
There are solutions for monohulls such as rudder t-foils, less sail area than a "scale " rig, moving the rig aft and movable ballast. Most rc classes prohibit movable ballast and rudder t- foils so you're options are reduced. In classes like the IOM, Marblehead, 36/600 rc designers make use of multiple rigs--up to five different rigs(in the Marblehead class).
Some models designed to use radio controlled
spinnakers set the spin's up so that they actually lift the bow downwind. But, alas, spinnakers are illegal in many classes.
So,you have to look for solutions within the rules of the model class you're designing/building. And you have to explore the downside of each pitchpole "solution".
How's your boat coming along?

 

It's bit different in models to 'full size' dinghies - the crew in dinghies cam alter for/aft position to try and counteract the rig force that makes the bow go down.

It's the usual compromise between handling and speed - eg low rocker = more speed, but more critical on trim. lots of rocker makes it easy to sail, less nosediving, but a bit slow.


T-foils as in int moth keep the trim angle relatively constant, and nose dives are rare.

 

Thi is of course all true, but is it entirely relevant to the problem of nosediving? I've never found to be much of an issue if the boat is heeled, no matter how wedge shaped the craft.

 

Thanks for the help.
The boat is still being designed i will start building when i go back to school.
I have thought about moving the rig back and that is certainly feasable. I can also use t rudder as someone last year used a t keel. But light winds meant that we didnt see any benefits of it.
The hull will be as narrow as possible for a monohull. Please see my new thread on placing the keel. Alas we are not allowed spinakers and we are not allowed more than 2 control channels.

Thanks

ALex

 

A long thread on a related topic, including a discussion of Admiral Turner's metacentric moment idea, is at:

http://www.rcsailing.net/forum1/showthread.php?t=3436

Cheers,

Earl