Subject: Re: [harryproa] bow down attitude
From: Rick Willoughby
Date: 10/14/2012, 5:49 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Ben

There is not much history with Harryproa but you do have to ask how many have experienced pitchpole?

The 18m proa I have sailed on has the worst possible shaped bow for pitchpoling - round bottom with zero rocker produces little dynamic lift; snub nose LIFTS significant  volumes of water about 4m into the air at speed and flat, rising foredeck will provide dynamic dive if ever submerged.

If the 18m proa ever gets over 20kts I would be contemplating pitchpole as a possibility.  

Compared to most other boats the height of the rig in relation to the length of the lw hull looks a joke.  Have a look at some of the side-on videos that have been posted.  The rig looks puny compared with the length of the lw hull.

I did state that dynamic lift is possible with flat entry.  However you did not repeat the most important aspect I offered - all hulls have a tendency to trim bow up once moving that is controlled solely by buoyancy.  This is due to the bow rising on the bow wave and the stern sinking in the aft wave trough.  With typical sail plan and lw hull length harryproas will trim bow up to about 15kts - depending on overall size.

Pitchpoles I have observed end up being quite slow motion.  It is a condition most commonly off the wind where the bow is slowed at it is pressed into a wave and there is increased drive in the sail due to increasing apparent wind.  The drive moment overcomes the countering moment arm of the boat weight and the distance to the now forward CoB.  There is limited ability to reduce drive by releasing the mainsheet.  There is also some influence from the momentum in the rising CoG carrying the boat over.   There is limited ability to reduce drive due to short time and limitation of the rig.  By contrast releasing the sheet on a aerorig will completely depower the entire rig at any stage.  Still a matter of having enough time and presence of mind of course.

This brings up another factor in the design, being to keep the CoG as low as practical - inconsistent with adding mass to the boom.

Rick  

On 15/10/2012, at 1:23 AM, bjarthur123 wrote:

 

stuck at home sick on a beautifully warm sunny day :( perhaps one of the last of the sailing season. sigh. perfect time to ask the folks here a question.

for awhile now i've had my eyes on a harry proa. more precisely, should i ever buy a big boat, which might or might not happen, that's definitely what i'd get.

my main problem though is that, without exception, every single video i have ever seen has shown them pitching down. unacceptably unsafe in my opinion. what can be done about it?

1. rob has suggested to minimize the pitch by putting more buoyancy in the ends of the leeward hull. but yet, the pitch is still going to be negative, albeit smaller. and how would that effect hull drag at low speeds and weight?

2. rick has suggested to shape the hulls to provide dynamic lift at speed. i think a static method would be safer. and again, drag and weight?

3. one could pantograph the windward hull. oh, but nasty moving parts puts one at risk for a huge catastrophe. damn it! i really liked this idea.

being primarily a dinghy sailer i am very much used to the idea of weight shift. my main question for you all (yes, i spent some time in texas), is how much weight needs to be shifted?

the lift of the sail times the height of the COE is a pitch moment. yes, it varies with the wind strength and a gazillion other factors, blah blah blah. order of magnitude please. how much weight would be needed on the aft beam to compensate? is it more or less than the weight of the motor, fuel, batteries, and fresh water (and maybe crew)?

here are two ideas:

4. put a sled just inboard of the leeward hull between the beams on which all the moveable weight is placed. slide it aft with each shunt.

5. better yet (thinking way way outside the box here as usual) put all this moveable weight IN THE BOOM. there is already some weight shift on a una-rig just from the sail and boom alone. but if the boom had internal compartments for batteries, fuel, and fresh water, and if the engine when not in use could be hoisted and mounted on the end of the boom... no need to slide a sled here, it's automatic!

tear it apart! be constructive!! give me a reason to start saving for a harryproa!!!

ben arthur
weta #358, "gray matter"
chesapeake bay


Rick Willoughby




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