Proa questions...

I have never given Proa's a second thought. It was just too strange for me. However, some recent issues brought it back to the forefront of my awareness. And, I now desire to look at it with a babe's unbiased eyes. I have been reading several of the major threads on the subject here and beginning to recognize the key “players”. I have a few questions. Please forgive me as some of them are pretty naive.

1) What is "aback". I keep reading it and hearing that it might be the only vice that might cause damage. Does it have something to do with the shunting maneuver?

2) It seems to me that the required double ended nature of the beast makes it behave like a Wharram catamaran. Is hobby horsing an issue (or why not)?

3) I have yet to find any information on its trans ocean merit. Are there issues?

4) Shunting. I get the how’s and why’s. And I see it being a moot point for any ocean sailing/cruising. BUT:

5) If I’m on a lake… I’m used to tacking pretty close to the shoreline (its only several miles across) with my current monohull. I can make the decision to tack based of the depth I see NOW. With shunting, it appears that I must make the decision based on where I’ll be when it finally stops and reverses. And with the speed potential, this is beyond my vision in the murky water. Does it become second nature “knowing” when to shunt in a confined space at different speeds to predict my end point? And is there any way to botch it and run way over where you've predicted. Right now, if I botch it the worst that can happen is I'm in irons... which appears better than being on the rocks.

6) Say I’m in a race and a malicious or ignorant catamaran driver in on my outside *** and we are coming onto this bank again. I can’t shunt first or I drive in front of him and he can’t tack first without kissing my ***. Besides the colorful adjectives that will be flying, what are my options?

7) Again… the wind on the lake routinely changes through 90 degrees. What happens if it goes from the good side to the bad side of the Proa in a heartbeat?


Thanks

 

There seems to be a lot going on in 'the proa world'.....
A lot of disagreement too!!! Many types of proa and all think theirs is the best . Fascinating to watch the re-development.

The yahoo group is difficult to follow - so i like this thread - lets get more going here - nicer/clearer/easier!

 

Welcome to the twilight zone! Answers follow your questions.

Aback is the wind coming from the wrong side of the sails, as in your #7. In traditional proas, and a lot of copies of traditional proas, the mast falls down when this happens. In most proas with stays holding up the mast, the boat can capsize the wrong way, and/or the mast can fall down. In harryproas (which I sail, design and build) the unstayed, balanced rig weathercocks and the boat stops. You then have plenty of time, and no pressure, to get it back on course. Certainly much less effort than with a conventional sloop rigged boat.

Traditional proas and copies have very Wharram like deep V hulls and hobby horse in a chop. Harryproas have rounded hulls, with extremely high prismatic coefficients and no rocker. This makes them much easier to build, much less prone to hobbyhorsing and nose diving and gives a potentially higher top speed.

No issues if the design is right. Traditional proas have been crossing oceans for centuries. Modern ones (Cheers, Jzerro, About Face, Familles Milleurs) for decades. A 12m/40' harryproa (Aroha) crossed the Tasman last year, during which it spent 3 days on a parachute in up to 45 knots. No damage. Afterwards they broke a ring frame, but completed the journey and fixed it.

Shunting does not have to involve bearing away onto a reach. Just release the sheet, the boat stops (pull on the new sheet and it stops immediately), reverse the rudders, sheet on and luff and off you go. You can also do it by luffing head to wind, then shunting. If you do run aground, you dump the sheet and sail back the way you came. Very easy once you get used to it. You don't get in irons as you don't pass through the eye of the wind. A shunt is much less stressful than a tack and can be reversed at any time.

1) Stop and wait. This is possible in a proa without altering course.
2) Stop and reverse.
3) Do a luff head to wind shunt. You will go a lot further into the wind than the cat will, so he can tack behind you. You then shunt and if you do it right, you will end up further upwind and ahead.

 

On point 2 I agree with Rob but would like to add the comment that flat bottoms, suitable for flat panel construction should work well on these slender hulls.

Volume in the ends contributes to hobby horsing. So keep volume of the ends low. A hull that drives through waves with dynamic lift is better than using the buoyancy in the ends to lift it.

The flat sections in the bows will help pitch damping with little tendency to slam because the hulls are slender.

The flat hulls will also provide dynamic lift whereas the round bottoms have little lift or even sink at speed.

Rick W

 

How can't I buy this?

 

The amas on these tris have low buoyancy in their bows:
http://www.youtube.com/watch?v=1GuTs0KM1ZY&NR=1
http://www.youtube.com/watch?v=t_vegHhgy9c&feature=fvsr

You will see these drive through waves reducing the amount of pitching.

The important part of the low buoyancy bow is to ensure they lift rather than dive when submerged. The deck design is playing an important role.

The flat decks on my current boats tend to dive once submerged but I am building a new boat with a deck that will improve lift.

 

What amas of these have low buoyancy bows? Do you have their Cb, Cp aft , Cp fore and LCB information when fully loaded?

Following amas have not low buoyancy bow and flat bottoms, have they?

http://www.youtube.com/watch?v=oh4q2bt61EY

What reduces then her pitching, I can't see any?

 

Yes very low buoyancy compared with a Wharram cat:
http://www.youtube.com/watch?v=49EyuAQYDJw&feature=related

But not as extreme as BMWO.

Rick W

 

Define bouyancy, please.

Don't you think that waterline length, longitudinal righting moment or gyradius have anything to do with pitching?

 

I thought i was getting somewhere - GYRADIUS?!! Had to google it.....
Seems to me that alot of people have a lot of idea's on pitching - not all of which agree..... i am getting confused.
WHAT CAUSES PITCHING?!?!?!?
All boats do to some extent - what lessens it? What makes it worse?
AND - with 2 dissimilar hulls......

 

See if this sounds good?

May I suggest an explanation... that you're both right.

At first I thought Rick's reply was mistaken... because Rob's designs stress very high Cp (I think I've heard 0.76) which by definition means LOTS of volume in the ends to stop hobby horsing.

If you break up pitching into two different components...
(1) Pitching by moving up and down with NO waves.
(2) Pitching by moving up and down with THE waves.

ONE - I believe the first one is what we describe as hobby horsing. You push down on a bow (IN STILL WATER) and it bobs up and down. Hulls with high Cp (lots of volume in the ends) will dampen out and return to the flat state quicker. Even faster if the front and rear Cp's are different. Wharram catamarans are bad about this because the ends are the same and have narrow V hulls. Rob's, even though symmetric end-to-end, have very high Cp (high volume in the ends) to dampen these out quickly.

TWO - The second is pitching by following the waves. Don't know of a special name for this. Boat with low Cp will tend to pierce the waves instead of riding them up and down. So in this case, low Cp would tend to pitch less.

For the case at hand... or at least the one I'm interested is larger cruising Proas such as Rob's Visionarry. And in that case, Rob's experience trumps all. So, I'd go with his value (Cp > 0.7)

Not that I know anything about your boats, Rick, but I imagine your human powered speed-distance record boats would be optimized with very long hulls with extremely low Cp (well below 0.6) so you can have high speed, but still below planning speeds... like racing shells.

 

The main contributer to hobbyhorsing is reserve buoyancy in the ends. You do not want high flared bows with wide decks like the Wharram hulls if your aim is to drive through waves with little pitching.

The long waterline length for beam is by definition a slender hull. And that is what I was referring to with my comment above.

Flat entry with a little rocker on a slender hull can also be a significant factor in pitch damping. It also provides dynamic lift so reduces tendency to pitchpole once the bow is buried. Avoiding diving requires a deck that will easily lift through the wave rather than providing a down force when underwater.

There are other factors that will affect pitching but reserve buoyancy in the ends is at the top of the list.

I have two hulls that do not pitch in waves having wavelength up to twice their respective lengths. This is the range where most hulls exhibit the highest degree of pitching.

Rick W

 

I am talking about total buoyancy in the ends. It has little to do with the waterplane properties in static trim. You have to consider what is happening to the waterplane when the boat is pitching in response to waves or just driving through waves. The pitching moment is reduced substantially once the ends are submerged. The easier they submerge the less pitching. However you do want dynamic lift at the bow and/or squat at the stern to avoid digging in off a wave.

As a matter of interest the Cp on most of my hulls is much closer to 0.7 than 0.6.

For pitch reduction in waves you must look at what is above the waterline when in static trim. You can do a few little things underwater like flat bottom sections and/or extended submerged bows to help with damping.

The boat pictured had no tendency to pitch in the largest waves I operated in. It was like riding on a trampoline when in waves. I could get it to pitch up at a violent angle by accelerating hard. The nose of the main hull would come right out of the water. The front outrigger was about 3m in the air. I needed to back off to avoid it flipping backwards.

Rick

 

Can you somehow predict the amount of pitching and hobby horsing just by looking the essence of reverse bouyancy in a bow? Well, I can't, so have to look and imagine harder...

How do you predict or calculate the dynamic lift of low bouyancy bows? How do this lift reduse pitching? How can you measure, predict or calculate this pitch reduction?

I must say that many of your observations of pitching are similar than mine, but your explanations of the reasons are not. I can't by them. How in earth can't you realise the length of the waterline being a keaword here?

Make an old test: Drink plenty of coffee and wine and the next morning you feel sick. Next, drink plenty of coffee and whisky and the result is similar. To ensure your result, drink again plenty of coffee and cognac. And the result is clear as the sky: coffee is the reason for your sickness. But, if you study more those other things; wine, whisky and cognac, you'll find that they all contain alcohol, which may be the main factor of the morning sickness. You should study more NA-things before you proclame your illusions as facts.

OK, I know now this is wasting of time. There is a difference between astrology and astronomy.

 

Sorry Alex, just testing Rick. Not passed in "Introduction of boat design".

Pitching is caused mainly by too low longitudinal stability.

See: Larsson & Eliasson: Principles of Yacht desing. It only cost about 1 per mille of your next boat.

....sorry, football is calling.....