Masthead vs Fractional

[Bradonvashon]

I've been scouting for a mast for my 25' boat which was dismasted--and found one. orginally it had a mast head rig, single spreader--nothing fancy.

The mast I found is 2 feet taller, but is a fractional rig. The calcultations i've done thus far have involved a a 7/8 factional rig. I think the section is plenty strong, my question is whats the differnece? what doI need to think/worry about in hanginf from a masthead to a 7/8 rig? setting aside obvious questions, what does it change in terms of loads, performance Etc For example, do the lowers need to be moved aft or do I need double lowers? :confused:

thanks,

Brad Shinn

[Guest]

The main reason to use a fractional rig is to be able to easily depower the main with mast bend for stronger winds. This is accomplished by adjusting the backstay which pulls on the masthead. Being anchored firmy by the forestay at the hounds, this forces the mast to bend and draw mainsail area foreward, thus flattening the sail and taking power out of it.

Because the mast must bend to make this possible, straight-fixed spreaders and fore and aft lowers as usually found on masthead rigs will not do since they will not allow this bending movement.

On a single spreader rig, the normal thing to find is swept spreaders and single lowers anchored at the same point as the uppers. The tightness of the lowers controls the amount of bend allowed while the windward spreader forces the middle of the mast forward. Working against each other they also hold the mast straight laterally.

Your upper/lower chainplates will need to be in the same position relative to the mast bast as on the boat that the rig was intended for. Unless you are very lucky, there will not be enough hull strength at that point and some kind of linkage to a strong point will need to be built. Without the readily adjustable backstay, the potential of the rig will not be realized.

For a more complete description than I can give, get a copy of "Sailpower" by Wally Ross from your library.

[gonzo]

Will it still be a 7/8 rig if you cut the extra 2' off?

[Bradonvashon]

Yes, but cutting the top two feet would be the same at moving the forestay up to the mast head--but of course then you'd have to move the spreaders and lowers up to 55% of the masthead height. It seems to me that, at least with this particular mast, the best thing to do is leave is be.

The question for me is does it cause rigging issues (i.e. location of the lower chainplate connection) and what kind of changes in sail or sailing qualities does it result in.

I spoke with Brion Toss, and his opinion is that overall a 7/8 is preferable and rigging issues are minimal due to my upper an lower chainplate connections being very nearly on the same lateral plane.

[gonzo]

I think that it depends on the use you forsee for your boat. When they are reefed down, fractional rigs make the mast take wierd curves. They are great for racing in moderate conditions.

[Bradonvashon]

Ok, there's a thought. For you designers-- when do you think of putting a fractional rig on a boat? What questions do you ask yorself to make the decision of one over the other? Under what circumstances would you opt for fractional?

I'd venture that there are more masthead sloops than fractional--why? It takes a stronger, hence beefier, stick to deal with the additinal compression loads, which equals more lbs aloft, but you can hang more canvas. I've heard that headsails are what drive your boat, but in a fractional you live by the main, on the other you have great ability to control the main with a more flexible upper mast section?

Do designers and NA's have a kind of mental check list or flow chart that depending on the answers to a number of questions it would lead them to one choice or t'other?

Brad

[gonzo]

It often depends on racing formulas. A fractional rig is efficient in gusty winds because it depowers the main. However, it doesn't work reefed down. It is the best for club racing or moderate conditions. A masthead rig can have a small section by increasing the amount of spreaders and stays.

[SailDesign]

Personally - I just put on what the owner wants - other than that, I put on what looks prettiest ;-P
Seriously, though, the owners' desires are number one unless there is a clear reason not to use what he wants. Since he usually has a good reason for wanting it, and he is the one that has to sail it, he gets what he wants...

[Bradonvashon]

Jeez, like blood from a turnip...but I ain't giving up yet. So if you were making a dream boat or if the owner said hey whatever...why would you choose one rig type over another?

I am beginning to understand some of the differnces in sail qualities, but I keep belaboring this point because I don't understand the range. I beggining to get the picture that it really doesn't matter. No designer/engineer/Architect I've ever met has had NO opinion whatsoever on a particular system.

If you're boat was dismasted (as mine was) which rig configuration would you choose and (other than theconvenience of not changing the current rig) why?

[Guest]

One of the main limitations of the internet is that people sometimes think that they can get complete and unambiguous answers from forum contributers. Aint going to happen.

You have gotten pretty good guidance from some, some not so clear advice from others and a few bits of wrong information. This is normal for a forum.

Learing is harder than this and there is no real shortcut. Get a book like the one mentioned and study it. After a while you will know enough to make a reasonable choice.

[Bradonvashon]

That's no fun! I actually have several books ( Rigger's apprentice, sailmakers apprentice, boat data book, skenes' elements and a dozen others)--not the one mentioned above, though it is on my list. None of my books are very cut and dried.

I'm certain there is not a "right" answer here, but one of the really good things about the internet and forums is that you do get along of opinions and insights.

I heard another fellow in another forum ask for folks advice on converting a catalina 22 to a gaff rig. oh the howls of derision and protest. I admit admit to thinking why would you go and do that to a PERFECTLY GOOD BOAT!

He replied his reasoning and rationale and it made some sense. Got me thinking outside the Sloop Box-- and why we do certain things. So why wouldn't I put a 7/8 rig on my little 25' cruzer-- here's my thinking-not necessarily neg/pos

less compression on a not-overly-stout deck
Less weight aloft
better down wind control
smaller or at least better sized standing rigging
less headsail--less overall sail
could increase weather helm (more reliance on main)
others???

[SailDesign]

Brandonvashon says:
"Any others?"

Fashion? ;-P

Seriously - there was a time when most, if not all, BOC-type boats were masthead rigged, because it was perceived that this was safer, and that the guys would not have time to really "race" the boats. Things change.....
The fractional rig is cetainly more controllable under normal racing conditions, but from a cruising standpoint it is more finicky, and most just opt for a masthead they can set and forget.
When reefed, there is little to choose between them.
Steve

[gonzo]

One of the downsides of fractional rigs is the need for running backstays. Downwind, the spinnaker pulls from lower than the back stay.

[Guest]

Only on some boats Gonzo. My S2 Grand Slam is 7/8 rigged and needs no running backs. I'd not have it if it required them.

[Guest]

The first thing is to be sure that changing to the fractional rig doesn't cost you too much sail area. Most boats need all the sail they can get, and then some.

If I was to design an easily handled sailplan from a clean sheet of paper it would be about 85% fractional rig with masthead assymetrical spinnakers.

The fractional rig is better aerodynamically. The ability to twist the top of the main above the headsail is very important. The fact that the top of the rig is higher off the water helps as well.

The fractional rig is also better when reefed. In a MH rig, when reefed, the headboard is mid panel in the rig. That's unsupported, and the main leech loads are pulling aft below the headstay as it pulls forward. That's not good, inversion wise. The fractional rig, when reefed, has the leech loads closer to support from the upper shrouds and opposing the headstay. All very good.

The ability to bend a fractional rig is not quite so important. Modern MH rigs bend to depower the main just as well. In fact, most modern fractional rigs are being built stiffer and stiffer for less bend, since the newer sailcloths stretch less. Less stretch means more stable shapes and less need to flatten the sail.

Fractional rigs do not require runners unless you are looking for ultimate control. There are a couple of options here:

You can sweep the spreaders like a J24. That will provide a very stable rig that is easy to handle. However, since your original rig probably had in-line spreaders you would have to add a new bulkhead to take the chainplate loads.

If you keep the in-line spreaders you can add jumpers to transfer the headstay loads to the backstay. If your rig is stout enough at 7/8 you may not even need these. In-line spreaders do not hinder mast bend at all.

You could add the jumpers so you can run masthead spinnakers. This will really help your offwind performance.

What are the dimensions (I, J, P, E) of your old rig, and what are the dimensions of your planned 7/8 rig?

[Paul B]

The above post is mine. For some reason the forum shows it as Guest. Strange, since I was logged in when I wrote it.

[Bradonvashon]

Wow that is very helpful feedback. I don't know the measurements off the top for the old rig--but I can get them. I haven't done a drawing of the new rig,so haven't done the calcualtions.

The twist on this is that this is an existing mast that met the basic dimension and strenght I needed--so I've kinf od back in to this whole thing. But the cost and the availability were too tempting to walk away from this as a replacement t the mast I lost.

This thread has been very helpful to me. One questions I have is about mast bend depowering the sail. If the mast bends it should make the sail fuller not flatter, right? As the wind rises don't you want is flatter to shed lift?

[SailDesign]

If the mast bends, Bradon, then the distance from the leech to the luff increases, which perforce (sorry - big word for the day) flattens the sail. I know it FEELS like it should make it fuller, but it donesn't.
Imagine the sail lying on the ground. If you want to flatten it the most, you have to pull on the luff.....

[Bradonvashon]

Sorry I'm having trouble with the geometry of this:)

If the mast bends aft, Doesn't the distance from the head to the clew decrease? You are closing the angle.

Using an anology if I stand up straight I have no wrinkles in my shirt (normal luff), if I bend backwards (fore) my shirt is really taught (tight luff), if I bend toward my knees (Aft) my shirt is all loose and poochy (loose luff). A rather odd example, but does't that work?


Will the book Sail Power explain/illustrate this all a little better for me??

Brad

[Paul B]

Yes, the head does get closer to the clew, if the clew doesn't move. Simply pulling on backstay opens the leech. But the boom is not locked at a 90 degree angle from the mast. Pull on a bit of mainsheet as you bend the mast and the distance between the head and clew remains the same, so it doesn't add fullness or twist.

Anytime you make a change to a sail control you will probably need to make other changes to get the result you want.

Bending the mast does increase the distance from the leech of the main to the luff, so this removes camber and flattens the sail.

I often suggest that people should tie their boats to the dock on various points of sail (in light breeze) and pull all the control lines one at a time to watch the effects from all angles. This will teach you a good deal about what strings you want to pull to get the shape you want.