Bubble In The Mainsail

[Don Guillette]

I decided to open this as a separate topic. Since I’m just a non-scientific country boy and a common seaman I’m going to keep it very simple so I don’t get confused. In the sail trim scheme of things the bubble is not very important but since a lot of you east coast guys are stuck at home due to the snow storm I thought you might find what I have to say interesting.

To start, if you are a cruiser or middle to back of the pack racer, I would not worry about the bubble. Unless you know exactly what you’re doing don’t mess with it but if you’re fighting for first or second place in your race fleet maybe you’d want to think about getting rid of the bubble. By changing the angle of attack and adding power it might give you the edge you need.

Let’s set the scenario. You have a masthead rig and your sailing closehauled in 10 to 12 knots and the jib is set for 100% efficiency. The main is not exactly set for 100% efficiency because it has a bubble.

As I’ve mentioned, the bubble is caused by backwind from the jib and to get rid of it just move the traveler up a bit. Do not mess with the jib. You don’t have to move the traveler very much. To see what I mean, the next time you’re on the water (might be a bit for you east coast guys!!) set your jib and main for 100% efficiency and the drop the traveler and watch the bubble appear. You’ll see how little movement of the traveler is required to induce the bubble and conversely how little movement is required to eliminate it.

Someone said trimming in the mainsheet or traveler causes the boat to slow down. Actually, the opposite is true and here’s why. When you move the traveler up or trim in the mainsheet you’re changing the angle of attack and applying the accelerator. Easing off slows the boat down. Easing the jib sheet will cause the boat to slow down because you’ve changed the angle of attack. Additionally, someone suggested you switch from the inside track to the outside track. Again, this is counter productive. While closehauled you want the inside track. On all other points of sail you use the outside tracks. As I said, don’t mess with the jib since it is your engine.

On my boat I have 2 mainsail telltales on both sides of the sail (like the jib) which are located about in the middle of the sail or about 2/3 back. They are located far enough back so I can get away from the wind shadow of the mast. When the bubble develops you should see what happens to the telltales. Their movement indicates the wind flow is disrupted. That is the reason I want to get rid of it but that is just me. It is a matter of what “floats your boat”. If it doesn’t bother you then don’t mess with it.

 

[shemandr]

Oh goodie! An discussion about sail trim - just what I need as the snow flies ....

Assuming that your jib is indeed trimmed for best speed at highest sailing angle (Best upwind VMG), and that your main is trimmed in the all around best setting, i.e. upper battens parallel to the boom, and that you have a bubble. If you raise the traveler are you not in danger of stalling the main? If you stall the main you should expect the boat to have a case of the slows, a dead helm and an unhappy crew.

Here's an interesting question: Suppose you are moderately overpowered. The jib is driving the boat and you would reef the main if it were convenient or if you were sure the condition of being overpowered would last. Would it work to raise the traveler to stall the main to reduce power?

(I think I know the answer but I'd like to stimulate some discussion, hear others opinions and help pass the winter evening)

 

[All U Get]

Hi Don,
Little to no snow in NH. My question is if the traveler or mainsheet pulls the boom beyond the midline of the boat, is that an indicator of a tired main? My boat will slow down if I do this.
All U Get

 

[Todd Smith]

I did say that sometimes, trimming in the main causes the boat to slow down.

If, as shemandr proposes, you are slightly overpowered and thus if you trim the main to it's theoretical maximum, the boat heels over past the optimum and creates too much weather helm and drag. This can be fixed by reefing or changing to a smaller headsail, if the leg is long enough to make the effort worthwhile.

Also, the cut of the main can be such that even when not overpowered there can be a bubble in the main as the best possible trim. I know because I have done just as Don suggests. We get the jib trimmed correctly. Pull the main in until the bubble goes away. Then we ease the main and the speed goes up. I would rather go fast and look at the bubble.

 

[Don Guillette]

General wisdom is that you never move the boom beyond the center line of the boat but I don't buy into that. During the Catalina30 National events I frequently went further than the center line as I was trying to pull every bit of power I could get. Your telltales and the mainsail itself will tell you when you've gone to far and then you just back off from that point.

I raced on a Newport30 in Long Beach, Ca and on that boat i went past the centerline everytime simply because the sail let me. This particular boat was owned by a live aboard and its weight probably exceeded a fero cement boat. We never won any races and were always at the back of the pack but we had a great old time. If we were first across the line, which we mostly always were, we considered we won the race.

In one race we almost sunk the Shoreline YC race boat!! We never saw them until it was too late because the deck sweeper blocked our view. They came screaming across our bow and almost made it but we caught them just about at the rear quarter. The Newport 30 bow punched right thru the side and into the cabin. What a mess.

As far as moving the traveler up when overpowered. You're going the wrong way. You're improving the angle of attack and the situation just gets worse. The higher up you go the faster you go. The correct adjustment is to drop the traveler which will bring the boat back on its feet.