O'day 20 fractional location

[Sastevenson]

Hello again,


As I've stated earlier my oday has a 26.5' hobie 16 mast that has been retrofitted with spreaders etc. I recently discovered that my fractional point is much lower than the original, making my hobie jib not much more than a storm jib. I have just purchased a new mainsail and want toget set up for a new jib. Does anyone know how high up the mast the fractional tang should be? I know that the luff on the original jib is 21.75. I have basically just been dropping the luff measurement down by a foot (example is my new main luff is only 23.5 as opposed to 24.5). Any info will help.

Scott

 

[TLW]

I haven't been following this, so not sure what you are trying to accomplish. The O'day 20 had two different rigs and I'm assuming the mast was in two different locations as the J measurements are different. Regardless, the forestay attachment point on the mast (frac rig) is the same as the I measurement (22' from the main deck - not from cabin top). That doesn't mean the sail leech will automatically be that length. The clew may have to be cut higher to lead properly to your sheeting point, be it a single block or a track.

There really is more to this because of the different mast you are using. The center of effort of the two combined sails could end up way off and result in more helm than should be acceptable. The mast you are using can't be too far off from the original masthead rig, but a little short for the fractional rig. Do you have these dimensions? Do you know what you are trying to end up with? Does anyone know why O'day had two rigs? They must have felt one was more desirable. Have you talked with a sailmaker?

 

[TLW]

By the way, I just looked at the O'day 20 sail plan and noticed the headsail on the masthead rig was not full hoist. Not sure if that says anything. Maybe the fractional rig came first and when they switched to masthead rig, they used the same headsail. All very strange.

 

[TLW]

NOPE! different sail areas. Think I'm going to bow out of this. I obviously don't know enough about the O'day mind-set on this. Sorry. Maybe a sailmaker is your best bet.

 

[Sastevenson]

TLW,

Here's the quick story. About a year ago I purchased my first boat, Harper, a 1977 o'day 20. This boat was originally the fractional sloop with a 27' mast and I believe 46 lb mast weight. The boat hull and interior is superb, the only downside was that the boat had a hobie 16 mast and sails which have far less total sail area than the oday sails. I spoke with many sailmakers, riggers, and sail shops about my dilemma and made many good friends along the way, one being a guy in Florida who completely restored the exact same boat. I began the process of using my mast because it is 26.5 or 26' (I don't recall at the moment) and weighed in at around 45 lbs ( again can't recall exact numbers just know it was really close). I have retro-fit my mast with spreaders per specs that a rigger specified and re-rigged upper and lower shrouds. The boat currently has two shrouds that attach to the fractional point and run aft and those act as the backstay.

With the new sail that I am getting I will be able to eliminate the two aft shrouds and use a true adjustable backstay. The new sail has a total sail area of 109 sq ft, with the original o'day 20 one being 113, so not much difference there. At this point I am trying to figure out what size jib I need and how high I need to relocate my forestay tang. Sorry that wasn't very brief but I hope it helps.

 

[Sastevenson]

Any info you have is helpful. I'm not sure what you meant about this "The center of effort of the two combined sails could end up way off and result in more helm than should be acceptable." Can you explain what the center of effort is?

-Scott

 

[Sunbird22358]

The original rig for the 20 was the masthead rig (1973-75), the Fractional rig replaced the masthead rig for 1976-78.

The original masthead rig used a mast that was 22'4" long (might have been 22'9"? same as original masthead rig on the 22) and a boom 8'4" long. (masthead would be approx. 26'4" above designed waterline)

The later, Fractional rig was 27'0" long and a 9'6" boom. (masthead would be approx. 31'0" above designed waterline)

I think the text from the 1976 O'Day brochure may give some answers to questions posted. Spar dimension drawing came from "WIRELESS" a 1973 model, provided by her owner (wish I could remember his name!)

 

[TLW]

Kind of a long discussion, but basically the center of effort (CE) has a fore-and-aft relationship with the center of lateral resistance (CLR) that determines sailing characteristics, particularly weather helm, neutral helm, or lee helm.

Ideally, the CE would be a little forward of the CLR to provide a little weather helm.

The CLR is determined by the under water shape of the boat and, particularly, the keel. In your case, a "trunk" keel with centerboard. Without lateral resistance a boat would travel sideways (slip to leeward). The shape of the keel and centerboard resist that motion and turn it into forward motion. The fact that you have a centerboard gives you the ability to move that CLR forward and back slightly by adjusting how far down (forward) you allow the board to go. You are probably in the habit of putting it all the way down and forgetting about it.

A simple way to think about CE is to consider a single sail. If you drew line from the middle of each side of the sail to the opposite corner, that is, from the clew to middle of luff, etc., the spot where these three lines cross would be that sail's geometric center or CE. If you find this spot on each sail and draw a line from one CE to the other, the COMBINED CE falls along this line; not in the center (unless both sails are the same size) but in a ratio of the area of one sail to the other.

Writing this out sounds terribly complicated, but it really is over-simplified. There is a little more to accurately finding the combined CE, but drawings would help a lot. I think you can find this info readily.

I guess if I were you, I wouldn't worry too much about this as you are coming pretty close to the original rig dimensions.

The shrouds you mentioned: were these aft lower shrouds, or are your upper (maybe, only) shrouds terminated at chainplates somewhat behind the mast? It is common on many boats this size to use "swept" spreaders and anchor shrouds behind the mast to eliminate the need for a backstay. I am going to go look at rig plan for O'day 20 right now and see if a backstay was originally used. Do you have an attachment point for backstay on transom? The only rig plan I am finding is for masthead rig.

 

[TLW]

Yes, O'day 20 masthead has backstay and upper and lower shrouds in line with the mast.

 

[Sastevenson]

I was just about to answer your question. To incorporate the hobie rig, the po installed two stainless steel u bolts aft of the side chainplates and those are what these aft backstay shroud things attach to. Once I get my new sail I will abandon these and go with the original backstay that has a triangle plate about a foot up from the chainplate and that is where the mainsheet attaches. My spreaders do sweep back a touch and I consulted with a rigger as to the height those needed to be. So you think that I should be fine with what I am doing beings that It's only a foot shorter luff? My plan was to have a jib made for me that is about 20.75 luff and 8 foot. The original was 21.75 and 8.17.

As for getting a higher percentage Jib, does that just extend the foot out by percentage or how does that work. Like for a 110% jib does that just mean that the foot is like 9'? That may be a stupid question.

 

[TLW]

RE SUNBIRD POST: the line drawing shows CE for each sail & combined. SAILBOATDATA shows slightly different numbers - 8' boom (E) for masthead boat and 9' for fractional. I wonder if the guy that sent you those numbers measured the entire boom and not just the "black band" length (maximum sail foot)

 

[Sastevenson]

I used the photo that sunbird posted to attempt to find a forestay tang point. I used the given mast height and boom length to determine that the photo is very close to scale. I then used an algebraic calculation to determine that the forestay attaches roughly 20' up from the bottom of the mast (cabin top). Would that concur with a jib that has a luff of about 21.5"

 

[TLW]

The overlap - 110%, 135%, 150%, etc. is actually the LP measurement - a line from clew that meets the luff at a right angle (not the foot) - a slight difference, but other than that, yes you are right in how it is calculated.

 

[TLW]

We're all out of order here. Post flyin' in while others are being answered I've got to stop and catch up. No real calculation is needed to find the height of forestay attachment. it is same as the I measurement. You would have to calculate how high the mast base is above the deck, however. And allow for the position of halyard sheave.

 

[Sastevenson]

HAHA I was thinking the same thing...This seems like a chat messing thing not a forum. I think we are caught up now. So let me understand this...The I measurement, which is the luff of the jib, is the measurement from the deck to the forestay attachment plus a little for the halyard sheave on top? Now, what are they considering the top of the deck? The top of the cabin or the top of the deck lower than the cabin? Because if they are considering it the cabin then it is essentially the bottom of the mast correct?

 

[TLW]

O'day 20 Fractional Rig: (P=24.5' E=9.0" - mainsail luff & foot)

(I =22.0' J=7.3' fore-triangle or 100% jib) notice I is not the luff, but height of forestay attachment above deck (not cabin top - deck) -

 

[Sastevenson]

TLW Thank you for all of your help. So just one more question...Since the I measurement is 22' I am going to make mine 21' to compensate for the shorter mast. Does that mean I measure from the deck perpendicular to the mast about where my chainplates are or is this a measurement from the deck at the bow chain plate. I am just questioning because my deck slopes up toward the bow as you can see in the photo uploaded by sunbird.

Thanks again for all of your help. There are some great people on this forum.

Scott

 

[Sunbird22358]

See diagram below for info on how the various dimensions are measured.

 

[TLW]

Good morning. I woke up early this morning thinking about this. Data I see shows fractional rig main area as 110.25 sq..ft. You have 109. Don't know where the 113 info you gave comes from. Regardless, all very close.

I'm not sure if this discussion matters much since you are not trying to arrive at an "original" rig dimension anyway. I assume you are planning a new forestay. The question is: where to attach it to the mast. I am not sure why you would make your I measurement a foot shorter. Your main is almost "designed" size. Why shouldn't the fore-triangle also be "designed" size. You don't have a mast the exact height of original, but it doesn't matter if the rig is 3/4 fractional (as I think the original is) or 4/5 fractional, or 7/8, or anything else. Since your main is about right, I would try to arrive at a fore-triangle that is also "about" right.

THe J measurement forms a right angle with the mast and extends to the tack, or forestay attachment position on the bow. To get that right angle you would have to measure from a point BELOW the cabin-top. Hard to do, but you CAN measure from the base of the mast - level to keep the right angle - to a point above the tack. Then measure down - perpendicular - to the tack (a few inches.) Subtract those few inches from the I measurement and use the remainder to measure from the base of the mast to the new forestay attachment point on the mast.

Think of it any way you want, but you are just trying to arrive at a fore-triangle that is a RIGHT TRIANGLE with a base (J measurement) of 7.30' - a height (I measurement) of 22.0 ft - and a hypotenuse (forestay length) of 23.18 ' {23' 2.25"} The difficulty lies in the fact that that right angle is below the cabin top somewhere and you have to figure out how far. Easy enough to do with method above.

I think you are over-thinking this as you don't care if the new rig is EXACTLY as the original. I would just create the new fore-triangle and tell your sailmaker what you have and where your sheeting points are. It doesn't matter "how fractional" the new rig is.

On the subject of your shrouds: the original rig had lowers, of course, attached to the same chainplates as the uppers. When you add the new back-stay, I wouldn't eliminate the lowers altogether. You might even leave them where the are to limit the amount of mast bent you are able to induce with the back-stay adjuster. After saying that, I'm thinking maybe an adjuster wasn't in the plan anyway. Are the shrouds so far back now that they interfere with downwind sailing?

 

[Sastevenson]

TLW,

I can't tell you enough how much you have not only helped me with this topic, you have also given me a ton of valuable information. I can deffinately say I have learned a lot through this one forum. I believe in give and take and for the last year I have been taking and taking when it comes to information about sailing. I do have to say however that someone just emailed me through this forum saying that he has the exact same boat as me with a hobie rig so it's time to give a little.

You have a very good point as to why I should just take the forestay up to original height and then get original o'day 20 foresails. Like you said I am not trying to get it perfect because I don't have a few grand to dump on it right now for the original mast. This one is very close and I am no racer...just a weekend cruiser. Thank you very much for ALL of your help. Oh and I was going to put an adjustable backstay on it so that I can control the rake of my mast. I have a friend who will be helping me with that however any input is always appreciated.

Thank you again very much,

Scott