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Old 24-01-2010, 08:41   #1
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Pitchpoling and Rollovers

I own a 49.5 Privilege cat. I bought it in England in 2007, sailed across to the Caribbean and have sailed there since. I learned something about sailing that may be of some value to newbies. I think it applies to all cats. Every cat sailor worries about tipping their cat. While I crossed the Atlantic from Europe I found my boat surfing on the waves sometimes. Once we even got up to something like 18 knots. My crew was revelling in the excitement. I spoiled the party by telling them that when anything 1/2 way close to that starts to happen it was time to make adjustments to slow things down.

Catamarans have 2 hulls in the water. The friction of water on the hulls increases exponentially with speed. While my math may not be exact I will give these examples.....

5 mph vs. 10 mph. Lets give 5 mph the base line number of friction as 1.
The friction on the hulls increases to a value 4X as much at 10 mph. Then take it to 15 and I think it goes up another 4X = value 16 etc.

Now take a multi-hull with two hulls spaced apart surfing down a wave. The waves are usually not directly behind you so one hull ends up deeper in the water than the other as the wave picks you up. As your surf down the wave one hull has way more friction resisting the increasing speed than the other one because it is deeper into the water than the other hull. You begin to slide in an arc rather than a stright line down the wave. The arc always takes you broadside to the wave and the wind. This increases the likelyhood of rolling over. There is also the effect of the centrifugal force of sailing the arc.

The centrifugal force is what I noticed first and began to put my mind around it because of this. I began to see stuff that was normally resting on the table begin sliding across it during the arc slide. I began to think about that big mast and sail up there wanting to do the same thing with considerable mechanical advantage.

Anyway my point is that when you find yourself having fun surfing on waves think again! When stuff starts sliding sideways you ane now increasing your risk taking considerably!
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Old 24-01-2010, 14:15   #2
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Ken

Did you have a reef or two in at the time? If you did what was your next actoin?
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Old 25-01-2010, 04:43   #3
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On the days when we noticed this the helmsperson was instructed to stay at the helm. I instructed him/her to quickly steer down the wave more directly than to allow the boat to go into the arc whenever the boat speed got over 12 knots surfing. The waves in the Atlantic can be far between even if they are big so a good helmsperson can sense what is happening and steer down the wave. Frankly our crossing was more like a milk run. It was in Feb. and we went from Cape Verde to St. Maarten in 10 days (aprox. 2000 miles).
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Old 25-01-2010, 06:12   #4
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Ken -
I agree weight aloft is a negative stability factor but it can also work to your advantage (to apply down-force to the windward hull) if you are turning from a reach to downwind.

I think most multihulls are generally less likely to turn and broach when surfing down a wave than most monohulls. Were those waves so steep they would have capsized you if broadside on a reach?

I think a big worry going downwind is if you carry enough sail to be powered-up enough that slowing the boat would cause you to be overpowered. Going beam-to or stuffing a bow in the back of the next wave, would have the effect of suddenly slowing the boat and increasing apparent wind.

Did you deploy a drogue? Did you shift movable weight aft to keep bows from submerging? Did you drop the main and leave a bit of jib to keep your sail's center of effort (CE) forward of center of lateral resistance (CLR)?
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Old 25-01-2010, 08:48   #5
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Actually, weight aloft helps avoid pitchpoling and rollovers--at least in the case of breaking waves. The lever arm of a mast acts similarly to keel balast in resisting the torque applied by a breaking wave. It's not as heavy as ballast, but the lever arm is much longer. That's why dismasted monohulls are easier to roll.

But that doesn't address the circumstances Ken had. How big does a non-breaking wave have to get before a 50' cat is in danger of tripping itself?

Brett
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Old 25-01-2010, 08:58   #6
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How big does a non-breaking wave have to get before a 50' cat is in danger of tripping itself?

Brett
I think that totally depends on the design of the cat.
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Old 25-01-2010, 09:12   #7
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[QUOTE=SailFastTri;393441]Ken -
Were those waves so steep they would have capsized you if broadside on a reach?

I think a big worry going downwind is if you carry enough sail to be powered-up enough that slowing the boat would cause you to be overpowered.

Did you deploy a drogue? Did you shift movable weight aft to keep bows from submerging? Did you drop the main and leave a bit of jib to keep your sail's center of effort (CE) forward of center of lateral resistance (CLR)?[/QUOT

To be frank I was a newbie at the time. I have no idea if we were really close to the edge. I didn't like the feeling of the starboard hull rising above the port in a significant way. Most or all of the lift was likely the wave being higher on that side than the other as we swung to 90 deg. but the centrifugal force made me feel really uneasy. Those sorts of conditions were synonymous with 20 to 25 knot winds at 30-40 deg. aft stbd. and 15-20 ft. rollers that were at least 100 yards or more apart.

I have an engineering background so I understood centrifugal force really well and while my crew was cheering at 18 knots going down a wave I was frantically hanging on to everything on the table to keep it from flying off with the centrifugal force. To top that off I didn't like the arc effect moving broadside to the rollers and the wind also but I had, and still do not have, any idea when the boat will actually go over. I just don't like even feeling like I may be getting close to the edge or going over it. I have done a lot of sailing since and rarely has the surfing issue been an issue except during the Atlantic crossing where those big long gradual rollers are.

No drogue, just reefing a bit. I don't think a drogue would have stopped the arc as it would normally be placed behind and in the middle so arcing would still happen but at the lower speed it would be much less of a problem and probably easily handled by the auto piolot. The problem was only intermittent so placing a drogue on the port hull would really throw things off during the 99% of the time when things were working like they should.

Like I said I was a newbie. I was really happy with the 10 and 12 knots s.o.g. We had a big crossing so 200 mile days were important so we had a lot of sail up most of the day and reefed a bit at night. The effect was so intermittent the easiest way to counter it was to man the helm at all times when the effects were noticed and simply cut out the auto pilot when the speed got too high on the way down the ocaisional wave and turn the rudder appropriatly to counter the effect.

We did not see any bad weather at all during our crossing!

My real point in posting this is to share my experience with folks and advise people in cats (I don't have any mon hull experience) to beware. I have heard a lot of stories from sailors that enjoyed surfing waves with their boats. I just want to bring balance to that. Surfing may seem like fun but there are danger signs to watch for or bad things can happen if corrective action is not taken.

I never got the sense that the bow would bury into another wave because by the time I got to the bottom of the wave, when I experienced the problem, we were going closer to 90 deg to it due to the friction issues I referred to initially here and the next wave was still a long way behind and ahead of us. Frankly I have been in way worse rough stuff since also and I never had a sense that I was even close to the bow going in.

Perhaps this is one advantage that the Privilege 49.5 has with the nacel (3rd hull) in the middle. It rarely is used for bouyancy but it sure can add a lot of extra lift to keep the bows out of the water when nosing down hard.

Re: moving weight around. This is a really big boat with a lot of storage. We don't have much heavy stuff forward. I could consider moving my anchors and rode but I never did in this instance.
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Old 25-01-2010, 09:32   #8
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But that doesn't address the circumstances Ken had. How big does a non-breaking wave have to get before a 50' cat is in danger of tripping itself?Brett
We have been in 25-30 ft. rough seas and 40 knot winds with breaking waves coming at us from behind at 160 deg. stbd and fwd at 45 deg. other times. I sure got wet a lot but never have I felt like tipping would be a problem. The only time I had that eerie feeling was during the surfing times that I referred to.

We have a big rig with our mast close to 73' above the water.

During our time in the Caribbean I have often not reefed anything until the wind got up to 30 knots (but that depends where it is coming from and how rough the seas are and whether it is day or night and who is at the helm etc. When a big wind is on the beam and I am too chicken to go out to reef the mail I will let the sheet out on it a lot and dump wind that way. We can easily reef the jib from the cockpit. Having said that though we rarely see 12 knots and mostly 8-10 on nice sailing days. We are live aboards and have a lot of stuff so our boat is heavier than most i suspect.

I do take into consideration whether my water tank is full and my fuel tanks are full and which hull they are on and where the wind is etc.
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Old 25-01-2010, 10:52   #9
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Now take a multi-hull with two hulls spaced apart surfing down a wave. The waves are usually not directly behind you so one hull ends up deeper in the water than the other as the wave picks you up. As your surf down the wave one hull has way more friction resisting the increasing speed than the other one because it is deeper into the water than the other hull. You begin to slide in an arc rather than a stright line down the wave. The arc always takes you broadside to the wave and the wind. This increases the likelyhood of rolling over. There is also the effect of the centrifugal force of sailing the arc.
I'm trying to imagine how the asymmetric friction of the two hulls might cause the cat to broach. I would imagine that the downwind and downwave hull has the most friction, because it is buried the deepest. So it should be trying to slow down relative to the upwind, upwave hull. If that is the case wouldn't the cat's preferred orientation be directly down the face of the wave?
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Old 25-01-2010, 12:37   #10
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I'm trying to imagine how the asymmetric friction of the two hulls might cause the cat to broach. I would imagine that the downwind and downwave hull has the most friction, because it is buried the deepest. So it should be trying to slow down relative to the upwind, upwave hull. If that is the case wouldn't the cat's preferred orientation be directly down the face of the wave?
I have thought about that but it does not explain to me why the boat arcs towards the wind while surfing down the wave. Mby I am too novice but the effect is the arc is towards the hull that is highest on the wave. The leading edge of that hull is perhaps deeper in the water I suspect but who knows. The solution to the problem was certainly to manual steer the rudder to port against the arc to stbd to keep the boat going down the wave straight. Perhaps someone with more knowledge can bring some clarity here.
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Old 25-01-2010, 14:49   #11
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I have thought about that but it does not explain to me why the boat arcs towards the wind while surfing down the wave. Mby I am too novice but the effect is the arc is towards the hull that is highest on the wave. The leading edge of that hull is perhaps deeper in the water I suspect but who knows. The solution to the problem was certainly to manual steer the rudder to port against the arc to stbd to keep the boat going down the wave straight. Perhaps someone with more knowledge can bring some clarity here.
I don't profess to have more knowledge but perhaps flying to much mainsail as opposed to jib would contribute to rounding up.
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Old 25-01-2010, 15:39   #12
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The boat was simply sliding/skidding sideways. Without a chunk of something (keel, centerboard, daggerboard) hanging below a hull boats will slide. There is not a alot of surface to stop this with minikeels, your certainly not going to trip on them.

Sliding/skidding is one of the tricks you learn when dinghy racing. Find a hole when sailing OCS prior to the start and slide down into it by raising the board. ex I flag.



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I have thought about that but it does not explain to me why the boat arcs towards the wind while surfing down the wave. Mby I am too novice but the effect is the arc is towards the hull that is highest on the wave. The leading edge of that hull is perhaps deeper in the water I suspect but who knows. The solution to the problem was certainly to manual steer the rudder to port against the arc to stbd to keep the boat going down the wave straight. Perhaps someone with more knowledge can bring some clarity here.
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Old 27-01-2010, 19:45   #13
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I am too chicken to go out to reef the mail I will let the sheet out on it a lot and dump wind that way.
Ken... I have a Priv 435 and have just had the main changed to single-line reefing. I had the reefing lines, the main halyard and topping lift brought back to the cockpit and added a downhaul as well. I can't tell you how much happier I am now that I can reef - or drop the main entirely - on any point of sail from the cockpit!

This work was done in preparation for a two-handed round the world trip.
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