Sea Anchor versus drogue for multihull

[Dave_S]

When is the right time to set either.

[Sail Houston]

Sorry for the delayed response I have been moving boats.

For any of those still interested. The point of the drogue in combination with the kite is to provide a platform to sail at high speed and stability downwind, remove the need to use the main entirely, push the center of effort very far forward, and to increase the effective waterline length of the vessel.

The last one being the most important for stability control, steerage and speed. My invoiced cost to move a cruising cat that is new from France to South Florida is approximately $30-35k or so. The goal is to quickly and safely get your brand new boat to its destination. Deliveries don't typically have the luxury of weather windows or choice on time of year. When the company says go you go unless it is a hurricane. Hence it is important to adopt safe and effective techniques using sail physics and adaptive methods to safely carry out such a task.

In the first post, I mentioned that I use home-made drogues. Because I have not been able to find an effective commercially viable drogue that actually works for anything other than slowing the boat down in following seas. The way that I use drogues is to speed the boat up by creating a longer waveform for the vessel, not by creating just drag to slow the boat down. The longer the waveform of the wave the faster the waveform travels. This is not a new concept all those beautiful old race boats from a hundred years ago had long narrow swept ends that when heeled way over under sail created a longer waveform and increased the speed and stability of the boat through the water.

What I am doing is essentially the same but on a hull form that doesn't really heel over so the towed drogues under heavy weather and wind conditions going downwind provide for the added waveform length desired. By adjusting the length of the drogue you control the speed of the waveform that you are creating to match the conditions and the target speed of the boat for greatest stability. The kite is just to move the center of effort forward of the boat and to make sail trim virtually automatic.

For anyone interested I charge $450 per half day on your boat or one of mine and I will gladly show you how to set this up or to prep for heavy weather catamaran sailing. A package of ten lessons is $3500.

And for any armchair sailors that like to call everyone a fake or troll. I'm leaving Ft Lauderdale on Tuesday for an eight-day delivery. Come join!!!

[Sail Houston]

Originally Posted by Dave_S

When is the right time to set either.

Hi Dave. If you are referring to when to set a Sea Anchor or drogue under "Standard" normal use. I would suggest not using a sea anchor in most cases as the boat will tend to pitch a lot while using one. Going Hove-to would be much more desirable and save you money and the safety of having to not need to go on deck to deploy a sea anchor in poor conditions.

As for when to deploy a drogue in the standard "slow down" method of use. Continue to reef your sails down until you have them to the smallest viable size. Deploy the drogue setup anytime you are in following seas aft of 130 or so in conditions that call for double reef or more in the mainsail where you need to apply greater than 5 or 6 degrees of rudder angle to maintain your desired heading.

As mentioned previously even if your only goal is to slow down in big seas with a drogue, the important benefit of the drogue is not the slowing down part. The important part is to gain lateral stability and steerage control.

[danielamartindm]

Originally Posted by Sail Houston

Hi Dave. If you are referring to when to set a Sea Anchor or drogue under "Standard" normal use. I would suggest not using a sea anchor in most cases as the boat will tend to pitch a lot while using one. Going Hove-to would be much more desirable and save you money and the safety of having to not need to go on deck to deploy a sea anchor in poor conditions.

As for when to deploy a drogue in the standard "slow down" method of use. Continue to reef your sails down until you have them to the smallest viable size. Deploy the drogue setup anytime you are in following seas aft of 130 or so in conditions that call for double reef or more in the mainsail where you need to apply greater than 5 or 6 degrees of rudder angle to maintain your desired heading.

As mentioned previously even if your only goal is to slow down in big seas with a drogue, the important benefit of the drogue is not the slowing down part. The important part is to gain lateral stability and steerage control.

Lateral stability and steerage control results from the increasing of the waterline with a drogue that Sail Houston mentioned. With an effectively longer waterline, smaller control movements become more effective, and the effect of waves on your track is diminished. If the danger of pitchpoling is ameliorated by slowing down as well as increasing the waterline, cats do well off the wind. The problem is that this requires a rested crew member to steer through the bad weather. Heaving to can afford a short-hander the chance to rest with one eye open, but it loses its utility in the deadliest eventuality, breaking seas. A sea anchor may be the only solution there, especially with an exhausted crew.

[StuM]

The way that I use drogues is to speed the boat up by creating a longer waveform for the vessel, not by creating just drag to slow the boat down. The longer the waveform of the wave the faster the waveform travels. This is not a new concept all those beautiful old race boats from a hundred years ago had long narrow swept ends that when heeled way over under sail created a longer waveform and increased the speed and stability of the boat through the water.

Please explain what you mean by "longer waveform". And what does it matter how fast the "waveform travels". Once the hull has created a wave, how can the speed at which that wave propagates away from the hull change the speed (or stability) of the hull?

I am familiar with the concept of hull design increasing LWL when heeled and consequently increasing potential "hull speed" but I can't see how that can possibly apply to a drogue towed behind the vessel?

Originally Posted by Sail Houston

And for any armchair sailors that like to call everyone a fake or troll. I'm leaving Ft Lauderdale on Tuesday for an eight-day delivery. Come join!!!

Please take photos of these magic drogues that make you go faster and a kite flying in strong winds with no mainsail.

[Sail Houston]

Originally Posted by StuM

Please explain what you mean by "longer waveform". And what does it matter how fast the "waveform travels". Once the hull has created a wave, how can the speed at which that wave propagates away from the hull change the speed (or stability) of the hull?

I am familiar with the concept of hull design increasing LWL when heeled and consequently increasing potential "hull speed" but I can't see how that can possibly apply to a drogue towed behind the vessel?





Please take photos of these magic drogues that make you go faster and a kite flying in strong winds with no mainsail.

Ok, let's start with some basic wave theory. Please excuse this if this is already understood knowledge as it might help others understand.

Take a piece of paper and a pen and travel across the paper once using a wave line with a high frequency than one with a very low frequency. Count how long it takes you out loud to draw the line. This is a very easy illustration to show that the further apart the top of waves are the faster that wave travels.

Most of the sailing vessels we will come in contact with for the purpose of this discussion are what they call displacement hulls. They are designed to push through the water or displace it to the sides as the boat moves through the water. As we move the boat through the water we effectively trap the boat inside a wave at its maximum speed with a wave that is the length of the boat. This wave that we are creating by moving through the water determines are maximum speed through the water before we overtake that wave we are creating and start to surf the wave/or plane across the top of the water.

The simplified calculation of this wave speed is the square root of your waterline length times 1.34. There are other factors and it is more complex than that but that will get you pretty close in all cases. So that is the speed of the maximum wave you can form with your boat before surfing.

So why is surfing bad? surfing in itself is not inherently bad. However typically when a cruising vessel starts to plane it cannot maintain that surfing so it quickly falls off the back of a wave after speeding up and falls into the trough of the wave that goes past it. When doing this the water going past the rudders which give you all of your steerage is diminished greatly and makes it difficult and tiresome to steer properly without the danger of swinging up into the wind.

A drogue in a "standard slow me down" method aids by preventing you from surfing down the wave in the first place and this inherently makes your control and steerage better. Using a drogue in this way is a perfectly fine way of maintaining control. But like so many things in life, there are always other ways to do things.

The method I am describing allows the use of a drogue to effectively increase the length of that wave being displaced. So the wave I am creating by dragging stuff behind the boat makes the wave that is creating longer then the actual boat can produce on its own. As described above the longer that wave the faster you can go at maximum speed.

So as an example if we take a standard 45-foot cruising cat and use the formula above we get around 9 knots of max speed. And before you guys jump and say my 45-foot cruising cat goes upwind all the time at 9.7 I know let's just stick to the simple formula for now to explain the theory. If by using a drogue I extend my 'waveform' being created by the boat as it pushed through the water to say 100 feet using the formula above we would get a calculated speed of 13.4 knots. This is 13.4 knots without surfing. I know someone is going how is that possible. It is possible because the wave you created now to push out of the way is longer. So you no longer need to surf to keep and maintain that speed. So you gain speed, stability, and steerage in the process. By adjusting the drogues length, what depth and shape you pull you can change the shape of the wave is created. This theory is not much different then say drafting in car racing. Or if riding a motorcycle and trying to find the smooth air behind a big truck going down the highway.

In closing, there is no one way to sail. And if you ask 100 different guys how to do something you will probably get close to 100 different answers. Some time with physics and theory can help most anyone understand sailing better when considering longer passages or offshore trips.

I will work on putting some detailed theory together on this to post in a few weeks after this next trip. Until then everyone is safe on the water.

[StuM]

For some reason, you are conflating ocean waves coming from astern with bow waves and stern waves generated by the movement of a vessel through the water. They are two toally separate issues. A displacement hull will not "plane" or "surf" on its own bow wave. The issue is one of surfing down the face of a following ocean wave which is travelling faster than the boat.

And the old sqrt(1.34x LWL) is well known to be of limited relevance to catamaran hulls with LWL/BW ratios greater than 8:1.

I'm sorry, but the idea of a drogue towed on a flexible line with a separation of fifty feet from the stern acting as part of the hull and increasing the effective LWL of the vessel is nonsensical.

[Sail Houston]

Originally Posted by StuM

For some reason, you are conflating ocean waves coming from astern with bow waves and stern waves generated by the movement of a vessel through the water. They are two totally separate issues. A displacement hull will not "plane" or "surf" on its own bow wave. The issue is one of surfing down the face of the following ocean wave which is traveling faster than the boat.

And the old sqrt (1.34x LWL) is well known to be of limited relevance to catamaran hulls with LWL/BW ratios greater than 8:1.

I'm sorry, but the idea of a drogue towed on a flexible line with a separation of fifty feet from the stern acting as part of the hull and increasing the effective LWL of the vessel is nonsensical.

I'm not conflating anything. Nor did I ever say you are surfing down the wave that you are displacing ever. I simply stated that you are changing the displacement wave that you are creating to prevent surfing down wind-driven waves in heavy air conditions by physically displacing those waves out of the way.

I prefaced the use of the formula stated above as already not 100 percent relevant to the catamaran hull form but easier to understand for the purpose of explanation then calculating the displacement formula for the catamaran hull form.

Who on earth said I'm towing a flexible line drogue fifty feet behind my boat creating a waveform?

[StuM]

This is getting really bizarre.

Originally Posted by Sail Houston

I'm not conflating anything. Nor did I ever say you are surfing down the wave that you are displacing ever.

"This wave that we are creating by moving through the water determines are maximum speed through the water before we overtake that wave we are creating and start to surf the wave"

Who on earth said I'm towing a flexible line drogue fifty feet behind my boat creating a waveform?

"if we take a standard 45-foot cruising cat and use the formula above we get around 9 knots of max speed. .... If by using a drogue I extend my 'waveform' being created by the boat as it pushed through the water to say 100 feet"

Boat = 45 ft. Boat + drogue = 100 ft. Ergo, drogue is 55ft behind the stern.
Your supposed explanations just make no sense at all.

[Sail Houston]

Originally Posted by StuM

This is getting really bizarre.



"This wave that we are creating by moving through the water determines are maximum speed through the water before we overtake that wave we are creating and start to surf the wave"



"if we take a standard 45-foot cruising cat and use the formula above we get around 9 knots of max speed. .... If by using a drogue I extend my 'waveform' being created by the boat as it pushed through the water to say 100 feet"

Boat = 45 ft. Boat + drogue = 100 ft. Ergo, drogue is 55ft behind the stern.
Your supposed explanations just make no sense at all.

Stu, I apologize in advance that I can not teach you full sail physics and waveform theory in a few forum posts. Every vessel when moving slowly starts in displacement mode. When they overtake the wave they create they are planing. Doesn't matter what you call the vessel or its size that is physics.

If you take a small powerboat and you press down on the throttle it forms a bow wave, the bow lifts, and the power that you have allows you to overtake the wave you are displacing. Cruising sailboats can do the same it just takes a bunch more power to push them past that big wave they are forming.

You may want to google sail physics and waveform theory if you are really interested in how this works. Or you can pm me and I will recommend a few books for you to start with if you like.

[StuM]

Originally Posted by Sail Houston

Stu, I apologize in advance that I can not teach you full sail physics and waveform theory in a few forum posts.
...
You may want to google sail physics and waveform theory if you are really interested in how this works. Or you can pm me and I will recommend a few books for you to start with if you like.

Than you for your kind offer, but I must decline.

Your concepts of "sail physics" and "waveform theory" are very different to mine and quite frankly I find your poorly explained concepts and inconsistent statements to be totally illogical and absolutely bizarre.

[swampbush]

Anybody checked the date lately?

[Cadence]

Yep! It's all to complex for me on April fools day. I can only see a drogue keeping someone from taking green water when diving into the next wave. I doubt anyone will calculate their waterline length and the length of the line to the drogue. It's fly by the seat of your ass depending on the situation.

[Mike Banks]

The danger in towing anything that floats on the surface closely behind a vessel is having it thrown aboard by a wave breaking on the stern. Any drogue or tyres etc need to be towed well astern, and preferably weighted, and having a float on the surface to stop them sinking too deeply..

[StuM]

Originally Posted by swampbush

Anybody checked the date lately?

I would have thought the same if we hadn't had the Bizarro universe, where trailing a drogue makes a boat go faster, introduces on 26 March.

Of course, he could well be a SA troll but if so, I'm having fun feeding this one.