Motion Control

[skipmac]

Thought it might be interesting to add an admittedly anecdotal comment to this thread.

Old friend of mine used to go out frequently with his father for the weekend yacht club races. Not exactly top level but they had whatever computers and gauges for real and apparent wind, speed, etc that were available at the time.

They did fairly well but never seemed to win races. One summer they talked mom into joining the team for the season. First race out she started complaining about the bashing, crashing and too much heel. So to keep mom on the team they started reefing much sooner to keep her more comfortable, and they started winning races. The gauges and computers didn't show any significant differences (other than the obvious lower heel angle) but the results were there and everyone was more comfortable.

[Tempest245]

Originally Posted by cal40john

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I have also done the jib reach test on a chartered Privilege 39, more sail resulted in higher speeds even though the helm was bad. How bad was it? It was so bad something was binding in the steering system. At first we thought we had it hard over to the lock, but as we got off of each wave and the load eased up, the rudder had more to go. We decided this was a bad idea and tucked the reef back in.

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John

John, I experienced a similar experience on a sail last season. I was close hauled with everything up, sailing nicely in a steady wind, when I noticed that the wheel was pegged and I could not tack or even stall the boat.
Very weird. Once I eased the jib a little, I had control again. I had never experienced that before on this boat....The boat seemed happy....but I was running out of water...

[Bash]

Originally Posted by Tempest245

Once I eased the jib a little, I had control again.

...try easing the traveler rather than the jib. Not only will you get your helm back without losing the ability to point, but you'll be in less danger of rounding up should a gust hit and the rudder be overpowered.

[Tempest245]

Bash, Thanks, Agreed, that's usually my 1st instinct.....This was strange since I didn't feel overpowered, no weather helm, not heeling excessively, I didn't even realize the wheel was so far over, until I went to tack.......I went for the jib this time because the sheet was reachable. The traveler is on the cabin top as well as the main sheet...I was solo....

[bob perry]

Tempest:
I'm sure lowering your board would have a major effect on motion. It's a lot of lateral plane and I think it would have to slow the motion down. I suspect it's a neutral bouyancy board so in lowering it you are not effectively lowering the VCG. Right?

The old PNW salmon fishing boats used to add log "bilge keels" to dampen their motion.

[Tempest245]

Bob, Yes neutral bouyancy and as such would not lower the VCG. It was the lateral plane, I was thinking about.

The boat was sailing fine downwind, with a tiny jib...in the 6 to 8's that I was refering to; but every 8th wave or so came from a different angle, a little abaft the beam and set me to rocking pretty good..side to side..for about 15 to seconds.....( which seems longer when you're doing it ) Once, I got used to it, and saw the boat was still sailing fine...I just rocked with it. But I've always wondered if lowering the board would have made it worse...or dampened this rocking effect...

If I recall we pulled the board up in middle the night, as it was making a racket.

I guess I'll just have to experment next time

[Amgine]

My navy time was aboard an LCC, which looks like a 300' long rowboat with sponsons. And rolls unbelievably. The destroyer guys would come on board looking smug and confident, and soon quite green because the motion was so unpredictable. The lack of any form stability seemed to me to be the issue.

That said, I have to say I prefer a bit of motion. I sleep better under way if there's a slow, steady rocking than when I'm ghosting along on glass. It may be needing a bit of wind in the sails, rather than the seas, or maybe just a personal weirdity.

[Sandero]

Does anyone have a feel for how much weight their boat was designed to carry in stores and occupants.

It's fair common to have to raise your waterline when you fill up the boat for cruising. This seems to indicate that this is exceeding the design load of the vessel.

How does one tell how much they have aboard in weight and how do we know what the optimum weight is?

Don't designers EXPECT added weight and figure that in as a "requirement" for the boat to trim correctly?

Any thoughts?

[Amgine]

There's a difference between the Designed Water Line and the actual water line, usually a couple inches. The mass to sink the boat an inch can be determined from the water line plane, so if you have 2" to the DWL you could carry the mass of 2" x the area of the water line plane x the mass of seawater.

[Tempest245]

I was wondering about myself that earlier.

Bob P. is no doubt the best person to answer that.
He mentions in his book, about all the gear people load that set the boat below the DWL, But I don't recall if there was mention of the anticpated loads.
He also talks about the difference between design criteria...and what I would call
"As-builts."...did the builder use heaver than spec'd materials during construction?

I have to imagine that the tankage is allowed for. The designed number of batteries.
Anchors, chain ..etc...and that the boat doesn't sail itself...(some people..weight)

But then we add electronics, radar, wind, solar, wire, food, beer, rum, dinghys, motors. xtra fuel, etc.etc....that's got to add up to 1.000 lbs or more quickly. Then add a few hefty people with their gear and we could be talking another 1,000 lbs...
It's a wonder we get away from the dock...lol

I think the old, Capacity plates used to calculate the number of persons a small boat could carry.....as I recall they used a 150 lb as the average weight...per person....I haven't been 150 lbs in awhile.....I believe the new plates just give you total weight....

It would be interesting to know what the anticpated design loads were.

[Sandero]

I understand the concept of computing the amount of weight by the volume displaced, but I don't actually know where my waterline was supposed to be. I know where it is! And that's with the boat loaded as it is.

I am not even sure how to "re establish" the DWL.

I suppose I could get some drawings of the boat and measure from the deck to the DWL and then compare that with what I have. If I am loaded more than the distance to the deck will be less.

Once I establish the difference between the actual WL and the DWL I can then estimate the weight of the "extra" stuff. But figuring the area of the WL plane is not easy. The drawings were not done in CAD.

Does this seem like the way to do it?

[Robertcateran]

Originally Posted by bob perry

Robert:
Could you please take the time to exlain that last part about the effects of reverse sheer. You have lost me there.

Hi:
You are making a mistake of thinking the seas act on a boat as functions of indiovidual components and ratios. It does NOT work that way. Rm alone takes ALL the things you mention into consideration.The sea see's the whole boat. Of course, every ratio plays a part in how a boat behaves. A boat is not a collection of individual ratios it is one thing.

Hi bob,
consider an overhanging bow meeting a wave. The time of impact is short as the bows profile matches the waves. Consider reverse sheer, the lower part of the bow meets the lower part of the wave nice and early, and the upper part of the bow meets the upper part of the wave a bit later. This allows an earlier gentler rise. This greatly reduces the vertical acceleration of the bow, the subsequent rocking of the boat and disturbance to the drive of the sails and loss of forward motion. This is not new technology; Phoenicians and Greeks knew this thousands of years ago, but it is just being relearned as is seen by some of the Scandinavian ships beiing built recently and the latest oceanic multis.

[Tempest245]

Originally Posted by defjef

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I suppose I could get some drawings of the boat and measure from the deck to the DWL and then compare that with what I have. If I am loaded more than the distance to the deck will be less.

Does this seem like the way to do it?

Seems reasonable, if you can obtain a drawing that gives you that vertical number.

[bob perry]

Robert:
thanks for that cogent explanation. Verrrrrrrrrrrrrrrrrrrrry interesting and something I had never considered. It sure is fun to learn new things. Makes sense to me. But those Greeks had some weird ideas.

I'm an old Ashfield boy. Same school as Angus and his brother.

The rest of you:
Quit obsessing about actual DWL. Your DWL is what you would actually measure at your current flotation plane. "DWL" as a designer uses it is used to decribe the designed flotation plane. It is a benchmark to base the calculations on and one would hope the boat would float close to this plane. In reality your's will probably be different.

Lbs. per inch immersion is a function of waterplane. (NOT displacement)

Lbs. per inch immers.=waterplane (64) /12
The more you sink the greater your water plane.
The greater your waterplane the greater your Lbs per inch immers.

My team lost the Superbowl. Damn!

Forget these numbers.
Enjoy your boat.

[Amgine]

Here's where I poke the dog.

Once you start sinking that waterline don't you a) tend to increase form stability b) increase total moment force and c) usually (as you noted previously) raise VCG / decrease initial stability? All but the latter of which tend to increase loads on the rigging and fittings?

I'm sayin' this as someone whose boat, after being completely emptied (with a small mountain on the dock) except the fuel tank, had about 1/4" of bottom paint showing below the mussel-fringed boot strip. Yah, I raised the bottom paint, but it was mighty painful doing so.