Motion Control

[Tempest245]

Originally Posted by Amgine

Here's where I poke the dog.

It's a brave man who pokes a dog, after he's lost the super bowl..

[Amgine]

More likely a fool... <winks> I shouldn't mention that I've yet to figure out what armored men chasing a badly-shaped ball look most like, but it is an occasional distraction.

[Bash]

PPI is basically the weight needed, in pounds, to sink a given boat an inch. A simplistic formula (that doesn't take into consideration true waterplane) is to multiply LWL times 5.33 for saltwater or LWL times 5.2 for fresh.

Unfortunately, this simplistic formula doesn't take into consideration the logarythmic proportionality of waterplane as the boat sinks due to hull flare. Some boats, anyway.

I'm certain there must be a better calculus out there, just as I'm certain that it would be beyond my limited mathematical skills to understand it.

[Hiracer]

Originally Posted by bob perry

Robert:
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.

No Bob I'm not making that mistake because I agree with you that a boat is not a collection of ratios.

Where I differ (the stick poking) is your prior assertain that all boats within normal parameters have similar motion. My personal experience beween two boats says otherwise--and my prior post was trying to suggest that this radical difference in motion is attributable to more than just D/L.

I think it would be more correct to say that most boats within normal parameters have similar motion, Every time I go sailing I know for a fact not all such boats have similar motion.

* * *

Changed the boat's engine coolant today. Very proud of myself. Never done that before. Prior boat was raw water cooled.

Topped off battery water and charged them up. Changed transmission fluid. Fixed some burnt out navigational lights. Cleaned the bilge. Emptied water out of front water tank. Biocide in fuel. Checked fuel filters. Fiddled with spin pole. Started diesel to lube things a bit. Hosed the outside of the boat down. Bought another crab trap at WM. Confirmed that the Perkins has no zincs in the heat exchanger. That was a big deal. Forgot, damn it, to put in a new O-ring to see if the other propane connector will stop leaking. Oh well.

Was a good day, all and all. Told the kid we're good to go and then he tells me next weekend he's got an obligatory birthday party. Rats.

[Robertcateran]

Originally Posted by 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.

Hi Bob,
glad that my explanations made sense. I have been pushing these ideas for the last 30 years after I was playing with the shape of my hand over the side while sailing a small cat and noticed the difference in resistance. I have also spent a lot of time surfing and know how to duck dive. I spent a little time studying hydrodynamics as part of a maths/engineering degree which allowed me to start putting some numbers to the systems. I built a small tri when in PNG and tried out some of the concepts and was pretty happy how it handle the chop of hundreds of miles of 20knot fetch hitting 6 fathoms of water on the bow. I commissioned a 40' sailing fishing cat from Loch Crowther and pushed for lower down bouyancy and a vertical stem but he couldn't bring himself to go reverse
Ideally a vessel should have most of its volume in a torpedo underwater , with a minimal couple of foils to conect this to the upper part of the boat. Ie minimal waterplane. Someting like this will be much more comfortable in a sea. Not particularly suitable for sailing because of the gross movement of centre of bouyancy required for righting moment.

Where I am heading in my next boat is a Harryproa (Harryproa.com) (15m lw hull/ 9m ww hull 6.5m with sailing, collapsible to 4m easily for narrow waterways and marinas and to 3m for trailering, two queens and two cosy doubles, a saloon, shower, galley, Bruce number about 1.7, total weight lightship 1.5 tonnes rigged), because it makes a lot of sense once you really study the engineering. All reports say it is possibly the most comfortable boat of its size in a moderate sea. The comfort is from having a long narrow waterline, high prismatic coefficient, two different hull lengths to avoid setting up resonance with swells and having the cockpit in a dry central area.
regards,
Robert

[bob perry]

Robert:
Interesting post. Thanks. I frequently draw boats with reverse bows but I just like the look. That point about different hull lengths is quite intriguing.

Amgine:
Rm is Ra times displ so any increase in displ will increase the Rm at one degree if Ra stays the same. But if the added weight is above the original VCG then the Ra will decrease and boat will be more tender. You may not notice this at low angles of heel.

Hi:
Have it your way. But in the interest of accuracy would you please point out the post where I said "all boats". I would never say "all boats" about anything.

[Hiracer]

Originally Posted by bob perry

I'll go ahead and poke the dog with a stick on this one.

If we limit ourselves to boats that are "normal" say with D/L's between 150 and 350 and eliminate the extremes of shape I don't think there is to much motion difference. Both boats will be uncomfortable in a seaway.

I was reacting to the above. I think you overstate your case. In general I agree, but know for a fact that there can be substantial differences on occasion. I agree that these boats with significanly 'better' motion might constitute a small minority.

Originally Posted by bob perry

The less initial stability you have the slower your motion but, generally speaking the slower the boat, generally speaking. .

My limited experience has born this to be true, at least with regard to windward work. There is no substitution for Rm. But the loss in windward ability is not as large as I first thought it might be. Of course, I don't have a racing bone in my body.

Originally Posted by bob perry

We all seem to like stiff boats. .

For a while; that's my point.

Originally Posted by bob perry

The way I see it is rolling around slowly for 12 hours more comfortable than rolling around quickly for 8 hours? .

I agree, this goes to the essence of the question.

So, we agree more than disagree. The difference in motion between my two boats has made me a convert and a bit evangelical about boat motion.

* * *

Bob, have you seen Practical Sailor's February issue? They have a large article ranting about how they don't like the 'performance' of modern light beamy boats in gusty conditions (18 - 25 knots). The article gets into why such boats have a tendency to roundup in the gusts, overwhelming the rudder.

I thought the article was very thought provoking. Although not directly on point to Don's thread here and elsewhere, it is somewhat related (to my way of thinking) because a boat difficult to sail when condition are getting really fun is not my idea of a comfortable boat. At the very least, those boats are more work to sail in gusty conditions.

The article introduced a new ratio (to me) of sail-to-keel area (or maybe I got that backwards, keel-to-sail area), something Gerr uses with his students at Westlawn.

Going from memory (I don't have the article in front of me), the three principal causes for the poor behavior of modern designs are 1) fat beam brought all the way to the stern which when heeled even at moderate angles tends to cause a distortion of the underwater shape, rotating the direction of the hull into the wind, 2) small lead between CE and CLR which facilitates the rounding up, and 3) smaller keel areas, which helps the rudder to be more easily overwhelmed. They also talk about how the modern canoe body increases the job that keels (and rudders) must undertake, relative to the older designs when more hull was in the water.

I would assume that the larger SA/Ds of today is a factor too.

I can send you a copy if you wish. I think it has Gerr's fingerprints on it. Just guessing, of course.

[bob perry]

Hi:

Still waiting for that "quote".

Again, I do not generalize. There are modern, fat, light boats that are peaches to drive in a breeze. I have sailed them not just read about them. As you say the modern, light, high performance boat has a much higher SA/D than your typical Mom and Pop racer/cruiser. No question a boat with a SA/D of 30 will require more attention in a breeze than a boat with a SA/D of 18.5. Why would anyone expect otherwise? I have long favored narrow boats. One of the first questions I ask myself when starting a new design is "How narow can I make this boat?"

On Saturday we are turning over one of my newest designs. It is 116" LOA and 6.5' beam. Narrow is good.

[Amgine]

Originally Posted by bob perry

On Saturday we are turning over one of my newest designs. It is 116" LOA and 6.5' beam. Narrow is good.

You canNOT get away with saying something like that and not give us links to information about the project!

116 feet or inches? (suspecting a typo)

Narrow can be very good. Now tell us what this [freak? extreme design?] is. It sounds fascinating! Is the general public invited to the rollover?

[Robertcateran]

The most uncomfortable boat I ever sailed on was a sail training ship. While motoring, the bowsprit would accelerate downwards at 2gs and upwards at 4gs in any sort of swell. Not so bad with sails up but the whippy motion distracted from the sail working. not sure where the problems lay but suspect it was excessive stiffness and expanding water plane in the bow

[Robertcateran]

Originally Posted by Amgine

You canNOT get away with saying something like that and not give us links to information about the project!

116 feet or inches? (suspecting a typo)

Narrow can be very good. Now tell us what this [freak? extreme design?] is. It sounds fascinating! Is the general public invited to the rollover?

Had a look at the site. I noticed the low down bouyancy on the bow of the racing canoe. It looks spectacular. They must be a sight to watch when racing.

[bob perry]

Robert:

You can see the 116' by 6.5' Fautassi on my web site perryboat.com. No typo.

Yes, the Fautassi are powered by 43 Samoans averaging 225 lbs. each. They use carbon fiber sweeps not paddles. The cox is usually a villiage chief and will weigh well in excess of 225 lbs. The boats can do 14 knots and they race over a 5 mile course starting outside the harbor. I have been to American Samoa and it is a fabulous place with great people. Saturday morning we are turning the new boat over
Fa'a Samoa style. That means lots of big bodies. It should be fun.

I look forward to going back to Samoa for the races. I also look forward to attending church there. The singing is out of this world.

Amgine:
I think I could get you into to the rollover if you come down. The yard is in Tacoma.

[Hiracer]

Originally Posted by bob perry

As you say the modern, light, high performance boat has a much higher SA/D than your typical Mom and Pop racer/cruiser. No question a boat with a SA/D of 30 will require more attention in a breeze than a boat with a SA/D of 18.5. Why would anyone expect otherwise?

I never said that, but could have because I agree with it. Nor did the Practical Sailor article say that.

The Practical Sailor article concerned itself with modern Mom and Pop racer/cruisers, complaining that many (not all) were more difficult to handle in gusty conditions than comparable Mom and Pop racer/cruisers from the 1970s and 1980s. Guess I failed to clarify that first time around.

* * *

Still waiting for you to admit that some boat within normal parameters have remarkably better motion than others.

[bob perry]

Lots of variables at work here but in general I agree.

We were sailing the long and skinny Saga 43 one afternoon. It was blowing about 12 knots and we headed uphill. There was a new Ben 40 ahead of us so we thought a race was in order. The wind eventually built to about 22+ apparent and the Ben had trouble holding it's course. They started rounding up from time to time. We just eased the trav down about 12" on the Saga and drove the boat. We passed the Ben quite easily. I felt good.

Again, maybe the Mom and Pop of today don't sail as well as the Mom and Pop of 30 years ago. Sailors today often do not come up thru years of dinghy racing so they are not aware of all the tools available to get the most out of any boat. This idea, a favorite of mine, gets people angry but I'm sticking to it. The best way to become a good and efficient sailor is to do some racing.

Give me five minutes to get the sand bags back in place.

[Hiracer]

No argument from me, even though I don't formally race. I know that's how to learn to sail well, I just don't do it.

I did learn one day, however, that a double headed sloop (my boat) is slower with both staysail and genny up, because I was gaining to the windward in 6 knots of wind on a Caliber 40' (not a fast boat, I know, but it's geniune cruiser like mine) and promptly stopped gaining when I threw up the staysail. Lesson learned.

But there's lots more never learned because I don't formally race; point conceded.