How Important Is Sugar Scoop Design When Lying to a Parachute Sea Anchor?

[Maxingout]

How important is the size of the sugar scoops on the back of your hulls when lying to a parachute sea anchor?

I don't know the answer to this question, but I have heard people say that the larger the sugar scoop, the more dangerous it is to lie to a parachute sea anchor. They say that if your catamaran gets pushed backwards by a wave strike on the bows, larger sugar scoops will dig in to the seas, and you will experience a backward pitchpole, bow over stern. On the other hand, larger sugar scoops create more reserve bouyance in the sterns, perhaps making it less likely that they will submerge, dig in, and become the pivot point for a backwards capsize.

We use 500 feet of Samson 1 inch double braided nylon for our parachute tether on Exit Only.

I have been told that this double braid has up to about ten percent stretch, which means that 500 feet of double braid could stretch as much as fifty feet. A wave strike on the bow could possibly push my boat backward as much as fifty feet.

That's the stretch part of the theory, and there is probably some merit to it. However, when you are lying to a parachute, you are already taking some stretch on the double braid (there's catenary to think about as well). The truth is, I don't know how far backwards my boat could be shoved by a wave strike on the bow, but it could be substantial.

When we lay to our parachute in a storm north of New Zealand, the ride was amazingly smooth. We could have cooked and eaten a three course meal without a problem.

One of our friends in a Privilege 48 was caught in a different storm north of New Zealand, and their ride was so smooth on the parachute sea anchor that they baked a birthday cake for one of the crew members who was celebrating his birthday in the gale.

The pull on the bridle and 500 foot double braid tether was constant and even. We didn't feel like we were riding on a bungee while attached to the sea anchor. Absolutely zero jerking, and we had no appreciable movement aft. Our boat did move half to three quarters of a mile while lying to the parachute for seventeen hours.

I admit that we didn't have any massive wave strikes on the bows, but I suspect that in more severe conditions, the catenary in the tether and the stretch in the double braid would incremently stabilize the motion of the catamaran so the the boat would not get shoved huge distances backwards if there was a wave strike on the bows.

I'm not a mythbuster, but my bet is that sugar scoop design isn't that critical as long as you have a sufficiently large parachute and your tether and bridle are in good condition. On the other hand, if your parachute should fail in a wave strike on the bow, or if the tether or bridle broke, then larger scoops might pose a greater risk of digging in creating a backward pitch pole as compared to a cat with smaller sugar scoops.

What's your opinion? Is sugar scoop design an important consideration when lying to a parachute sea anchor? Are bigger scoops better or worse? Are smaller scoops safer?

[ForumAdmin]

My only experience of storms is in a large power planing cruising boat and a lot of theory from many books on the subject. I have also studied the weather for years because I quickly found out when I started boating that I could not trust any single weather report.

All I am saying is that I speak from zero experience as regards cats in storms.

Looking at the question of sugar scoops - the extra stern buoyancy added to the St Francis over the 48 by having the sugar scoops is great - so great it allows you to have a super sized 400/500kg dinghy at the back. We are not having such a dinghy - we are having a super light one with wheels, so we will have a lot of spare bouyancy at the back.

I cannot see how extra buoyancy at the back is a bad thing in a storm - only a good thing surely?

One of the videos we are shooting over the years is a safety at sea video. This video will amongst many issues show us trying out a parachute and rehearsing it warts and all. The video is based on what really happens in many situations rather than a sanitised how to approach.

.

[Maxingout]

Looking at the question of sugar scoops - the extra stern buoyancy added to the St Francis over the 48 by having the sugar scoops is great - so great it allows you to have a super sized 400/500kg dinghy at the back. We are not having such a dinghy - we are having a super light one with wheels, so we will have a lot of spare bouyancy at the back.

I cannot see how extra buoyancy at the back is a bad thing in a storm - only a good thing surely?

One of the videos we are shooting over the years is a safety at sea video. This video will amongst many issues show us trying out a parachute and rehearsing it warts and all. The video is based on what really happens in many situations rather than a sanitised how to approach.

.

I think the extra buoyancy of broad sugar scoops is a good thing.

I have seen some cat designs with narrow long sugar scoops that don't have much reserve buoyancy. Those types of sugar scoops may have a higher risk of digging in if the boat gets pushed backwards by heavy seas.

I met a skipper in Whangarei, New Zealand who had long narrow sugar scoops, and he told me that he runs off in storms rather than lying to a sea anchor because he has the narrow long sugar scoops.

Probably many different factors are at work in this area. Reserve buoyancy, beam, volume of the sugar scoops, and length of sugar scoops could all be important. Large reserve buoyancy would seem to add safety and decrease the likelihood of the scoops digging in. Scoops that are excessively long - (platform-like in their construction) could easily penetrate large waves behind the yacht if the boat was pushed backwards, and then could become a pivot point for a backward pitchpole.

I once read an account of a catamaran in the Mediterranean who was lying to a parachute sea anchor in a storm. The skipper had attached his parachute bridle to the crossbeam of the cat. A wave strike on the bow ripped the crossbeam out of the catamaran, and the cat pitchpoled backwards tripping on the sugar scoops.

I don't know the answer to the sugar scoop question, and that's why I put the subject up for discussion. This would probably be a good area for tank testing with scale models.

[Talbot]

I would be less concerned about the stern digging in, than in rudder damage in these stretch conditions, however, I suspect that a lot of the stretch happens on the forward part of the wave, and that the catamaran's motion relative to the water on the wave front is so close to zero as to be totally disregarded. The stretch in the line finishes when the cat is pulled up to the top of the wave front and over onto the back of the wave. Thus the only time this becomes a significant problem is if the parachute and or its rigging fails, or the deck connection pulls out. Then You are definitely going to be in a world of hurt. If the boat does not trip (and the reserve bouancy should help that versus a straight transom) then it is very likely that the rudders will be destroyed.

The area that concerns me more about scoops is when lying to series drogues. a breaking wave could possibly climb up the steps and give you an early bath. However, the additional bouancy here is definitely a good thing, and the very design of the series drogue helps to mitigate the risk.

Thus for me, the risk factor for scoops in a storm has a very low possibility, albeit high consequence. I am more than prepared to accept that factor for the significantly increased capability to rescue a man overboard, plus the sheer convenience of life when using a tender. The additional storage is only viable for lightweight stuff, but is a nice to have as well!

[TYRNTLZRDKING]

We use 500 feet of Samson 1 inch double braided nylon for our parachute tether on Exit Only.

I have been told that this double braid has up to about ten percent stretch, which means that 500 feet of double braid could stretch as much as fifty feet. A wave strike on the bow could possibly push my boat backward as much as fifty feet.[/B]

Do you have to use a tether which stretches ten percent?
Could you use a tether with less stretch which would stop the quick backward movements, or would this cause a different problem?

Jeff

[Maxingout]

I would be less concerned about the stern digging in, than in rudder damage in these stretch conditions, however, I suspect that a lot of the stretch happens on the forward part of the wave, and that the catamaran's motion relative to the water on the wave front is so close to zero as to be totally disregarded. The stretch in the line finishes when the cat is pulled up to the top of the wave front and over onto the back of the wave. Thus the only time this becomes a significant problem is if the parachute and or its rigging fails, or the deck connection pulls out. Then You are definitely going to be in a world of hurt. If the boat does not trip (and the reserve bouancy should help that versus a straight transom) then it is very likely that the rudders will be destroyed.

The area that concerns me more about scoops is when lying to series drogues. a breaking wave could possibly climb up the steps and give you an early bath. However, the additional bouancy here is definitely a good thing, and the very design of the series drogue helps to mitigate the risk.

Thus for me, the risk factor for scoops in a storm has a very low possibility, albeit high consequence. I am more than prepared to accept that factor for the significantly increased capability to rescue a man overboard, plus the sheer convenience of life when using a tender. The additional storage is only viable for lightweight stuff, but is a nice to have as well!

I think that what you are saying about stretch in the tether is probably true. There's a lot of wind and wave energy acting on the tether, and the tether is probably stretching and relaxing in a smooth fahsion all of the time. We never felt any jerking while lying to a parachute. It was totally smooth.

That doesn't suprise me because we used a hundred foot long double braid rope to extract vehicles from the sand when we bogged down in the Empty Quarter of Saudi Arabia.

123

When the Air Force left Arabia after Gulf War I, one of the things they left behind was a hundred foot long section of thick double braid. On of the members of the Rover Club put the rope on a spool, and then mounted the spool on the back of his Defender.

When someone got stuck big time, rather than get out the sand ladders and shovels, we often took the double braid and attached it to the bogged vehicle. The towing vehicle would drive away at 5mph, the double braid would come under stretch and would smoothly and easily extract the stuck vehilce from the sand trap. The stretch was smooth - no jerk at all as the bogged vehicle started moving. That's the way it felt lying to a parachute sea anchor.

I think your assessment is correct. As the boat rides the large seas, there is very little movement backward unless some component of the system fails.

[Maxingout]

Do you have to use a tether which stretches ten percent?
Could you use a tether with less stretch which would stop the quick backward movements, or would this cause a different problem?

Jeff

Actually the stretch is good.

You don't want shock loading of the deck hardware that could pull out the deck hardware.

At the same time, the panels on the chute are constructed of nylon, and shock loading of the panels isn't a good idea either, or you may blow them out.

Modest amounts of stretch protect the components of your parachute sea anchor system from self-destruction.

[mikereed100]

The only way I can imagine that a cat could reverse pitchpole would be if it were surfing backwards and I just don't see that happening if it is tethered to a sea anchor. I would think that a wave would pass beneath the hulls long before the boat could squat enough to bury the sugarscoops.

Mike

[Maxingout]

The only way I can imagine that a cat could reverse pitchpole would be if it were surfing backwards and I just don't see that happening if it is tethered to a sea anchor. I would think that a wave would pass beneath the hulls long before the boat could squat enough to bury the sugarscoops.

Mike

The only catamaran that I know of who did a backward pitchpole was a cat that had the parachute sea anchor bridle attached to the forward crossbeam. During a huge storm, the forward crossbeam failed which allowed the catamaran to rapidly be shoved backwards by oncoming seas, and when the sugar scoops dug in, they made a backwards pitchpole.

The big risk is that some component of the parachute sea anchor system might fail and allow the catamaran to be knocked backwards by oncoming seas.

[Nordic]

The only way I can imagine that a cat could reverse pitchpole would be if it were surfing backwards and I just don't see that happening if it is tethered to a sea anchor. I would think that a wave would pass beneath the hulls long before the boat could squat enough to bury the sugarscoops.

Mike

I agree with you Mike. Waves are cyclic as long as they are not breaking so movement is up and down not backwards and forwards.

Even with a rode that is pretty elastic, we have to remember that it is the windage that creates the load, and even though this will vary between you being in a trough or up high on a wave, the load will not be varying from zero to maximum, so backward movement of the boat should be quite small.

When looking at these big waves offshore (hopefully) they travel fast, so the time it will take a wave to pass from the bow to the stern will be 2-3 seconds, so flipping over backwards is very unlikely. The same goes for the sideways flip IMO, despite all the talk of being able to "slip sideways".


Alan

[mikereed100]

When looking at these big waves offshore (hopefully) they travel fast, so the time it will take a wave to pass from the bow to the stern will be 2-3 seconds, so flipping over backwards is very unlikely. The same goes for the sideways flip IMO, despite all the talk of being able to "slip sideways".


Alan

Derek Kelsall speaks to this in his article on catamaran seaworthiness:
http://www.kelsall.com/images/articles/A6_SeaworthinessAndSafety.pdf

[44C]

I think the extra buoyancy of broad sugar scoops is a good thing.

I have seen some cat designs with narrow long sugar scoops that don't have much reserve buoyancy. Those types of sugar scoops may have a higher risk of digging in if the boat gets pushed backwards by heavy seas.

.

I think it depends on a few different factors. My boat has long narrow hulls, with long narrow sterns on them, but there is very little load in the stern. The hulls extend around 12 feet behind the cockpit, and the last 6 feet or so has no load in it whatever, apart from the rudders. If I simply cut 6 feet off the hulls I'd have much broader transoms, but a less bouyant stern for sure. I would have thought the narrower transom would produce less resistance to being driven backwards, kind of like having bows there.

But I'd say a lot depends on how the boat is loaded, in terms of it's weight distribution.

[mikereed100]

Speaking of sugar scoops....I would not want to slide back on these

[Sandy Daugherty]

In the case of the backwards pitchpole, I wonder if the mast fell backwards when the front crossbar broke, and contributed to the upset?

[Maxingout]

In the case of the backwards pitchpole, I wonder if the mast fell backwards when the front crossbar broke, and contributed to the upset?

I don't know the answer to your question.

On some cats, the loss of the crossbeam would bring the mast down for sure. On our Privilege 39, there is a center nacelle to which the headstay is attached. It would be a major test of the strength of the center nacelle if the crossbeam was gone.

[BigCat]

I agree with you Mike. Waves are cyclic as long as they are not breaking so movement is up and down not backwards and forwards.

Even with a rode that is pretty elastic, we have to remember that it is the windage that creates the load, and even though this will vary between you being in a trough or up high on a wave, the load will not be varying from zero to maximum, so backward movement of the boat should be quite small.

When looking at these big waves offshore (hopefully) they travel fast, so the time it will take a wave to pass from the bow to the stern will be 2-3 seconds, so flipping over backwards is very unlikely. The same goes for the sideways flip IMO, despite all the talk of being able to "slip sideways".


Alan

There is another issue with waves, and that is the effect of steepness. If the wave front is almost vertical, the boat can fall down, as though falling off of a cliff, or the wave can fall on the boat, or the boat can slide down the 'cliff' and bury its bow or stern in the trough and pitchpole as the passing wave front breaks while the bow or stern is buried. I think this last scenario is the one that usually causes capsizes and pitchpoles. I think it also happens occasionally that a steep wave will come from the side, at an angle to the main wave train, and roll a boat over the same way, that is the boat is on the face of a very steep wave that then breaks as it passes the boat.

It's worth pointing out that currents and shoals also increase wave steepness. Some believe that passing over sea mounts or underwater cliffs can lead to breaking waves in storms.

[Talbot]

Even with a rode that is pretty elastic, we have to remember that it is the windage that creates the load, and even though this will vary between you being in a trough or up high on a wave, the load will not be varying from zero to maximum, so backward movement of the boat should be quite small.
I do believe that you have forgotten one element here.

Whilst the wave itself imparts up/down momentum, and yes the largest pressure is the windage, there is another second order effect. When the boat is on the windward side of the wave, the boat will tend to surf towards the parachute due to the natural movement down the wave, whereas, when on the back end of the slope, the boat will tend to move away the parachute. This is a basic principle of momentum from the interaction of the boat on the wave slope, and is caused by the steepness of the slope and the weight of the boat. Thus there will be a cyclic increase and decrease of tension on the rode to the parachute.

[mikereed100]

I do believe that you have forgotten one element here.

Whilst the wave itself imparts up/down momentum, and yes the largest pressure is the windage, there is another second order effect. When the boat is on the windward side of the wave, the boat will tend to surf towards the parachute due to the natural movement down the wave, whereas, when on the back end of the slope, the boat will tend to move away the parachute. This is a basic principle of momentum from the interaction of the boat on the wave slope, and is caused by the steepness of the slope and the weight of the boat. Thus there will be a cyclic increase and decrease of tension on the rode to the parachute.

Good point. I can see where wave slope may cause movement of the boat relative to the parachute, but I don't think this would cause much movement backwards relative to the leeward face of the wave. In the end I don't think it would pose a problem for sugar scoops. I suppose the best way to resolve all this physics is to sit on the back of a cat lying to a sea anchor and watch what happens.

Mike