trimaran sea anchor sizes

[earthsail]

I have a Crowther Twiggy Trimaran; LOA 32 x30 beam. I've just completed a total refit & about to set out cruising & want to equip her with drogue & sea anchor. I need some help with selecting a sea anchor size. If their's any Twiggy or other Tri people out there, could you possibly advise.
The sea anchor I will use is from Para-Tech & the question is weather to go with either a 9' dia or 12' dia chute?
ParaTech is recommending the 12'
There guide line lists 25' to 33' LOA with displacement of under 12,000 lbs; use a 12' dia chute. Under 25' & under 8,000 lbs; use a 9' dia chute. Twiggy is no keel boat & weights in at 4400 cruising. Seems to me a 9' dia would do the job. I'm thinking heavy weather storm tactics.
Note; their guide doesn't distinguish between mono-multihull, sail or power
If anyone can shed any light on this it would be most appreciated.
Thanks
PS, As ref; a Corsair 31 would, I think, use the same size sea anchor? being similar in size & a little heavier than a Twiggy.

[Pericles]

"The sea anchor I will use is from Para-Tech & the question is whether (weather) to go with either a 9' dia or 12' dia chute?
ParaTech is recommending the 12'
Their (There) guide line lists 25' to 33' LOA with displacement of under 12,000 lbs; use a 12' dia chute. Under 25' & under 8,000 lbs; use a 9' dia chute."

If "Twiggy is no keel boat & weighs (weights) in at 4400 cruising", why not ask Para-Tech why they recommend the 12' sea anchor? After all, their advice is your assurance that when the chips are down and you survive the storm, you will be grateful that you followed the manufacturer's guidance. It might be that the 9' chute is the correct one for your vessel, but they must make that call.

BTW, get a Rocna. http://www.rocna.com/ Accept no substitutes!

Good luck,

Pericles

[yipster]

large size for multi's, a bridle can point, line 2 waves out, anything can do and exercise
more http://www.seaanchor.com/seaanchor.htm
http://www.multihullpages.com/heavy_weather.html
http://www.google.nl/search?hl=en&q=...e+Search&meta=

[JCD]

Hawdy,

Remember now that an anchor rides on the surface when deployed so a twelve foot crest height for a twelve foot anchor will capture it and collapse it when it is time to head for the trough. For specific conditions, the anchor and vessel must match the "period" of the wave, in other words, they must be on the crest or in the trough at the same time. This is the "theoretical" best position under all circumstances.

Taken with a grain of salt, I remember reading something to the effect that the diameter should equal the beam of the "multi". There are specific ways and lengths of bridle and line that must be set.

I don't believe that those diameters denoted will hold enough volume and offset enough wind force to put the "brakes" on such a light vessel, especially if wind and wave are from the same direction. Para Tech are the experts and should be considered the authoritative word on the subject.

With that said, I would say go with the 12' diameter if you must choose from either since a little more bite can only help.

J

[earthsail]

Thanks everbody for getting back on sea anchor size. I went with the 12' dia chute & also a 48" drogue from Para-Tech.
Earthsail,

[Zack Smith]

I design parachute sea anchors for a living and would use a 9-foot diameter parachute for a 32-foot trimaran. However, Para-tech uses much lighter fabrics than I would ever consider for a parachute anchor, so it's wise to go with their larger 12-footer.

There is much confusion in reference to use and size of a parachute anchor versus boat size. To address these issues, I've completed a large video library that demonstrates how to use a parachute anchor from many different styles of multihulls. That library will be placed on my personal website by the begining of summer and will be linked to para-anchor.com. For a progress report on the free online video check out this link: http://para-anchor.com/faq.zac.html

Best regards--
Zack

[FarmerColin]

If you want to do the sums for yourself you can calculate the drag force on a chute under water using

Force = 0.5 * Density of water * (speed) squared * Chute Area * Drag Coefficient.

In metric units the density of water is 1000Kg per cubic metre, the speed must be in metres/second and the area in square metres. The resulting force is in Newtons (equal to 0.102 Kgf). The Drag Coefficient is about 1.5 for a properly formed chute.

So, at 10 knots (about 5m/s) a 1 sq.metre area chute would generate a drag force = 0.5 * 1000 * 25 * 1.5 Newtons = 18,750 Newtons = 1911 KgF or almost 2 tons!

[Paul Howard]

During a passage on our Manta 38 catamaran from Labrador to Iceland we lay to a sea anchor south of Greenland during a northeast gale.
The sea anchor was an old U.S. Navy cargo parachute, stamped 1945. It had been given to us by a couple who had used it during four round trips from Canada to England on their 40' trimaran.
I am not sure how the diameter is classified, as I have no way of measuring the canopy inflated, but laying it out, the very light nylon fabric is about 26' in diameter.
That was way bigger than paratech advised for our boat, but it was free.
It is an anchor - bigger is better. Just as I don't want an anchor in the sand to give a little, I expect the sea anchor to hold the bows into the wind.
In that instance we lay to the sea anchor for 20 hours (ice pellets and freezing rain) and, according to the GPS, lost about eight miles.
I set it up exactly as shown by Victor Shane in his Drag Device Data Base book, page 3.7 & 3.8, except did not use a trip line but an retrieval buoy from the canopy (two fenders, one where the normal float is shown, then another about 50' further on) and used our normal nylon anchor rode three strand twist line with no problems.
It worked a treat. We went below, turned on the heaters, had a meal and slept soundly (one person on watch at all times - crew of four) with very gentle motion.
We only used the sea anchor once during our 22,000 miles on that four year voyage, but it was great.
The pull on the bridle was similar to lying to a normal anchor in a semi-protected anchorage in a gale. The motion was much less than I had thought it would be - the long ocean swells were more gentle than the short chop of an anchorage.
The breaking waves broke at the parachute with a calmer slick running back to our boat and beyond. In the morning there were hundreds of sea birds sitting on the water in the slick behind the boat.
How this relates to your much smaller and lighter boat, I can't tell. I only offer the above experience.
Paul

[Zack Smith]

Hello Paul,

Rode naturally stretches under force until it becomes taut. As force is reduced, rode becomes relaxed. What we want to avoid is too long a period of rode slack because this leaves a vessel swinging beam to, where waves can heavily roll the boat or in more rare circumstance, cause it to fall back on the rudder(s).

Your 26-foot parachute anchor grabs more water that generates additional force on the anchor rode attached to your boat. This removed slack in the rode system. And contrary to popular opinion, the extra windage created by your Manta catamaran also assisted with maintaining continuous tension on your rode. Windage is a good thing.

The only real negative with the larger parachute anchor is the difficulty in retrieval. I've conducted several on-the-water classes with married couples that plan offshore cruising. Not to sound sexist, but the more petit ladies couldn't retrieve any para-anchor that was over 18-feet in diameter and that was under calm conditions.

If you ever decide to use a smaller parachute anchor there are other tricks that I've discovered to remove rode slack: 1. We can pay out shorter lengths of rode; 2. Add a small length of chain next to the parachute; 3. Fly a riding sail to increase vessel windage; and/or 4. Purchase rode with limited stretchiness.

In reference to parachute diameter, measure the parachute canopy from the small vent hole to the bottom of the hem and times it by two. This will give you the uninflated diameter of the parachute anchor. This measurement applies to all aerial and parachute sea anchors, regardless of the manufacturer.

Paul, your experience is relevant because everyone gets to read an unbiased viewpoint about your para-anchor deployment. And, we don't have to pay for it!

Regards,
Zack

[Paul Howard]

Retrieval was not a problem. My wife, 62, and daughter, 26, did the retrieval on the trampoline by catching the second buoy with a boat hook and hauling in the floating line, then the wet nylon in from the loop sewn at the vent of the canopy. The Canopy collapsed and the water went out and it weighed only a few pounds.
Wind at the time of retrieval was down to about 15 knots. I was at the helm and slowly motored up to the second buoy while they advised that the rode had sunk and was nowhere near the keels/props/rudders.
Our parachute is a very light nylon with light line attaching the canopy to the swivel. It did not hold any significant amount of water and slid in over the forward crossbeam. They continued to haul in the hundreds of feet of rode and I had to help with the chain part.
It took longer to sort out the mass of rope and coil it up than to flake it out when we set it.
Paul

[Zack Smith]

Hi Paul,

I'm glad the retrieval went smoothly. Part of it is preparation on your part and it definitely helps to own a multihull. There's a lot of room to work from in the trampoline area. Good thing, especially with all those lines to sort.

As a side note: From 1947 until a couple of years ago, Veteran commercial fisherman, Gerrard Fiorentino, sold a similar style parachute that you used. The parachute is very light, but their life expectancy is uncertain. Sometimes they hold up well and other times they can break quickly. That's why the commercial fisherman usually carried three airman chutes on average. The common 24 and 28-foot aerial parachutes can handle approximately 4 to 5,000 lbs of force before they break.

Regards,
Zack

[kistinie]

Dear earthsail

I have an MKII twiggy
Mine is equipped with a parachute.
This parachute was used on Wingover on the way back to england from the BVI by Alan GW Toone.

http://www.multihullpages.com/heavy_weather.html

If you want i can measure this parachute that is a true 1968 air parachute modified for water use.

Anyway i think a jordan serial drogue is better and i will change it for a serial one

Please contact me to share info about your ship !

Kind regards
François
France

[kistinie]

I will be glad to have a contact with other Crowther Twiggy owner
I am doing a deep refit on mine in france this summer (north side !)