MID-BOOM SHEETING ON o'dAY 23

[dicknatta]

Want to add bimini to O'Day 23. Want to position it at aft end of cockpit for ease of dock/cockpit access, but main sheet complicates things. Was thinking of installing either a short track for main sheet block at companionway threshold, or a triangular mainsheet block system with blocks attached on cabin top at either side of companionway.

Please share mid-boom sheeting system experience, thoughts, or plans.

Popeye -- O'Day 23 "Free Ride"

 

[LANCE ON CLOUD NINE]

you may find that placing the bimini just in front of the current mainsheet and backstay works well in the long run. I say this because this leaves just enough room to stand up in the very back of the cockpit. This comes in handy for using the boarding ladder, fishing, handing a line to the dock, or leaning over the rail to tinker/remove engine. Mine is this way, and it has worked well for me. Best of luck.

 

[Daveinet]

Be careful about sheeting towards the middle. It will mess with the sail twist, as it changes the leverage the sail has on the boom compared to the sheet. I would never go less than 2/3rds back on the boom. My previous boat had the sheet in the middle of the boom. It sailed terribly as a result. I switched to a different boom that had the sheet attached at 2/3rds, and it acted much better. Incidentally, I did not move the location of the track, so sheet was on an angle up to the boom. That worked perfectly fine.

 

[Brian S]

Not sure what problems Daveinet had... I would say, if you are going to go to the trouble of moving from end boom sheeting, then you should figured out how to rig a traveller. O'Days, and many other boat like Precisions, are intended to be a cruising boat. As such, they were designed with end boom sheeting to maximize cockpit space and comfort. However, one obtains much more sail control and performance from having a traveller. With the mainsheet on a traveller, for close hauled sailing, leech tension, and thus twist, are controlled by mainsheet tension. After that is set properly, the traveller can be used to adjust angle of attack. For lighter winds, if the weight of the boom does not pull the leech closed too much, the traveller can be brought to windward so the boom is on the centerline. As puffs come, the traveller can be let down. As one bears off to a close reach, the traveller can be let down further, all without changing the mainsheet tension. It's true that a boat without a traveller relies on mainsheet to set angle of attack and leech tension. This can be controlled better using an adjustable topping lift in conjunction with the vang to keep the boom in position for main twist, and then the mainsheet is used to control angle of attack (so called vang sheeting.) Unfortunately, the boom still cannot be brought to the centerline using vang sheeting. If one were to move the end boom sheeting to mid-boom, without a traveller, just a fixed point on the bridge deck or cockpit floor, one would also want to arrange for an easily adjustable topping lift (or Boomkicker, or even rigid vang) along with the vang, to better control leech tension and twist. If one were concerned about maximum performance...

I have thought about this at length on my end boom sheeted boat. I pretty much have two choices: 1) build a raised traveller over the tiller on the rear of the coaming, or 2) build a raised traveller across the nicely sculpted seats in the cockpit and move to mid-boom sheeting. 1) is undesirable, because the further aft you go, the longer a traveller you need to make meaningful adjustments, and also it would interfere with climbing in up the boarding ladder and 2) is undesirable, because now you've got tracks right across the seats, making moving fore and aft in the cockpit more difficult, and making is so you can't lay full length on the seats. So I continue to use vang sheeting to get the best performance I can, and keep reminding myself that she's not a racing boat, or even a racer/cruiser. I actually would have a third traveller option, to put a short track across the flat bridge deck, between the seats, up by the companionway, but even though this moves the mid-boom sheeting point forward, I still don't thing the short width of the bridge deck would give me enough traveller movement to make the change worthwhile.

 

[Daveinet]

Its really an issue of physics. Sail twist is determined by the load of the wind on the sail, the weight of the boom, and the downward pull of the main sheet. Moving the traveler to windward results in less downward pull, compared to holding the boom in the correct position athwartship. This allows the boom to rise slightly, changing (increasing) the twist. If the main sheet attaches too far forward, it looses downward leverage on the boom compared to the wind load. Moving the traveler to windward lets the boom rise too easily, which tends to negate the use of the traveler. On my first boat, with the main sheet attachment point in the middle of the boom, you had to run the main sheet about as tight as you could pull it, otherwise the boom would just raise up. It was very finicky to adjust, as there was no range to the adjustment.

 

[Brian S]

I'm sorry, I have to disagree, Daveinet. When you have the mainsheet tightened how you like, because it is rope and not a rigid arm, it will deflect off to the side. This is why you can't sheet the boom to the centerline with a mainsheet fixed on the centerline. The longer the distance between the boom and the attachment point, the greater this effect. We try to sheet the main in harder to get the boom in closer for better pointing performance, but at a certain point, all it does is close off the leech. A traveller allows you to account for the mainsheet deflecting off to leeward. In fact, because we use a straight traveller, and the boom describes an arc, one could say that pulling the traveller up to windward actually slightly increases the tension on the mainsheet, because the further ends of the traveller will be a longer distance from the gooseneck pivot point than the boom attachment point (which is the radius of the arc.) Is this increase in tension on the mainsheet significant? Probably not all that much. If one were to use a traveller that was bent to the same arc radius as the boom describes, the mainsheet would always be pulling down on the boom with the same amount of force on all points of the arc. (The vang on the Star is on a curved track, for example.)

Vang sheeting with an adjustable topping lift tries to get around this. With the topping lift holding up the boom, increasing mainsheet tension will pull against the topping lift and (hopefully, without too much stretch) will not pull the boom down anymore, maintaining the desired twist. Pulling harder on the mainsheet will then start to eliminate sideways deflection, bringing the boom closer to the centerline, but never quite getting there (in my experience.) Of course, you still need the vang to prevent the boom rising. So the vang and topping lift hold the boom in position horizontally, so that leech tension remains constant, where one wants it set, while the mainsheet adjusts angle of attack.

I may speak in absolute terms on this, but I'm open to your thoughts.

 

[Daveinet]

I'm sorry, I have to disagree, Daveinet. When you have the mainsheet tightened how you like, because it is rope and not a rigid arm, it will deflect off to the side. This is why you can't sheet the boom to the centerline with a mainsheet fixed on the centerline.

yes, of course

The longer the distance between the boom and the attachment point, the greater this effect.

Yes

We try to sheet the main in harder to get the boom in closer for better pointing performance, but at a certain point, all it does is close off the leech.

Yes, and I think by closing off the leach, we could also call that reducing twist. In reality, the twist moves higher and higher until it disappears. I think so far we are still on the same page.
Pretty much agree with what you said after that, but it has no impact on my original point, although I should also state that my first boat did NOT have a boom vang. My original point onlyhas to do with the location of the main sheet on the boom. In your original discussion, you point out that how the length of the main sheet affects the rise of the boom. Or you state it from the opposite perspective, that pulling down on the boom closes off the leach. Move the traveler to windward, lengthen the mainsheet to put the boom back to near center. This means the downward pull on the boom is reduced, so the sail is allowed to twist. Are you with me so far? Do we agree?
OK, so if we agree, then to my original point. If the sheet is attached to the end of the boom, there is a lot of leverage to pull the boom down. As the traveler is moved windward, when the wind picks up some, the sheet is pulled tight, which limits the side to side position of the boom. HOWEVER, the boom can move up (of course that does move the boom slightly side to side, but negligible.) If the sheet attachment point is at the end of the boom, the downward vector of the sheet has a lot of affect on the boom angle. If you want to allow the boom to rise a bit, you have to move the traveler farther to windward. Are we still in agreement?
OK, now picture attaching the sheet in the middle of the boom. The side to side movement is still limited by the main sheet becoming tight, however (and this is the big point) the boom can much more easily move upward, because the attachment point is in the middle. You've lost some of the leverage on the boom to pull it down. So, the wind picks up, and the boom rises because nothing is pulling it down. Of course a boom vang would prevent that, but not sure if the OP's boat has one.

 

[Brian S]

Or you state it from the opposite perspective, that pulling down on the boom closes off the leach.

Yup, increase leech tension=close off leech=reduce twist.

Move the traveler to windward, lengthen the mainsheet to put the boom back to near center. This means the downward pull on the boom is reduced, so the sail is allowed to twist. Are you with me so far? Do we agree?

Weeeeell... Here's how I understand it. On a traveller equipped boat, preferably with easily adjusted traveller, not some pin stop mess... (And I'm wearing my racing hat, here.) sail close hauled. Watch the top two leech telltales, and set main tension such that both fly, and maybe the top telltale flicks around every so often. From this point, twist is set for the wind, and the boom "sags" off to leeward with the mainsheet angled from the attachment point as it is wont to do (angle of attack.) For lighter winds, it may be necessary to bring the traveller to windward, to get the boom to centerline. (I am assuming that the weight of the boom is not pulling the leech closed - that either the wind is strong enough to hold the boom up, the Boomkicker is holding the boom up, the topping lift is holding the boom up, or the GNAV-style rigid vang is holding the boom up.) Moving the traveller to windward should maintain the same "angle of sag" of the mainsheet (provided divergence of the tangent to the arc of the boom doesn't significantly effect the mainsheet pull, as from my previous post.) At the same time, a puff allows the traveller to be dropped leeward to momentarily depower, and then bring back to windward to keep the sail driving, all while maintaining the correct twist for the prevailing wind strength. If one were to foot off, possibly as far as a close beam reach (depending on the amount of travel of the traveller) one could drop the traveller, again maintaining the correct amount of twist without touching the mainsheet.

OK, so if we agree, then to my original point. If the sheet is attached to the end of the boom, there is a lot of leverage to pull the boom down. As the traveler is moved windward, when the wind picks up some, the sheet is pulled tight, which limits the side to side position of the boom. HOWEVER, the boom can move up (of course that does move the boom slightly side to side, but negligible.) If the sheet attachment point is at the end of the boom, the downward vector of the sheet has a lot of affect on the boom angle. If you want to allow the boom to rise a bit, you have to move the traveler farther to windward. Are we still in agreement?

OK, now picture attaching the sheet in the middle of the boom. The side to side movement is still limited by the main sheet becoming tight, however (and this is the big point) the boom can much more easily move upward, because the attachment point is in the middle. You've lost some of the leverage on the boom to pull it down. So, the wind picks up, and the boom rises because nothing is pulling it down. Of course a boom vang would prevent that, but not sure if the OP's boat has one.

Ok, so you are saying that if you move the mainsheet attachment from end boom sheeting to mid boom sheeting, the former end boom purchase (what, 3:1?) is no longer strong enough to pull more leech tension and reduce twist, as the wind gets stronger? Well, that may be. So one would want to increase from 3:1 to 4:1. Or use the vang more.

But it's all academic, because if one is moving from end boom to mid boom, with 3:1 mainsheet purchase, and no traveller and no vang, one is not a speed weenie like I am, and wouldn't care anyway!

Ok, so the OP was about moving from end boom sheeting to mid boom sheeting, solely to allow a bimini to be used further aft. In this scenario, one would want that mid boom attachment point as close to the house as possible. And if you're going to do that, you want a traveller so that when not sailing, one could pull the traveller off far to one side to clear space in front of the companionway, and allow for better cockpit lounging and moving in and out of the cabin. But I think this is less desirable, because if there's rain, it may be good to have the bimini on a track, so that it can be move forward to cover the sliding hatch to some degree, allowing the drop boards to be left out for ventilation, while also keeping rain out. I don't know, I could probably keep a bimini up, but slid forward some, with end boom sheeting. I don't ever sit at the back of the cockpit while sailing anyway, I sit in front of the tiller so I can shift sides while tacking. Plus, on my small boat, if I sit aft too far, I don't want the stern to drag and slow me down. See, speed weenie, even in a slow cruising boat!

 

[Brian S]

Also Daveinet, I am not picking on you or trying to be a PITA.

 

[Daveinet]

Also Daveinet, I am not picking on you or trying to be a PITA.

I spend time in the war room, so I don't get bothered by too much. I am super anal, so I really don't mind picking something apart to make sure it is fully understood. There are always things to be learned, so I see value in these kinds of discussions. And I do remember being wrong once about 3 years ago...

 

[Brian S]

I spend time in the war room, so I don't get bothered by too much. I am super anal, so I really don't mind picking something apart to make sure it is fully understood. There are always things to be learned, so I see value in these kinds of discussions. And I do remember being wrong once about 3 years ago...

Yes! Deep discussion leads to greater understanding! (Especially when done respectfully!) I find a lot of good manners here. Not like Sailing Anarchy forums...