Re: [505-north-america] Mast step position and other tuning considerations

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Alexander Meller

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Mar 27, 2015, 2:32:24 PM3/27/15
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Joel,

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On Fri, Mar 27, 2015 at 1:20 AM, Joel Bedard <bedar...@gmail.com> wrote:
I just got a used 505. I think it is from the early 70's (sail number 3452).

The sail number (on any original sails, not sails bought used) should also be inscribed on the back of the centreboard trunk, aft spine, or inside of the transom.  There may also be a builders plate on the inside of the transom.  That may have a number than is NOT the sail number.
 
The rigging of the boat is basic, but works nicely (main/jib cunningham, boom vang, traveler, main outhaul, adjustable jib leads (fore and aft) and adjustable forestas/shrouds (turnbuckles)). The hull is still relatively stiff and the sails are in pretty good conditions (I also have the original sails, but won't use them as these one are worn out). I went towards the 505 class as we are getting a bit heavy for the 420 class which are used in university/college competitions across Canada. I understand that this boat will never be very competitive, but I'd like to get as much as I can without investing too much. My objective is to become familiar with the 505 by racing it locally.

While trying to set up the boat to standard tuning sheet numbers, I realize the mast gate is short and I cannot put the mast at the most aft position as suggested in most tuning sheets. I understand that it is critical to have the mast supported laterally at the gate/partner.

While older 505s differ in detail, many of them had mast gates that were open at the back.  I agree that you need lateral support for the mast, at the gate/partners. You can probably get away with only part of the mast section fully contained within the mast gate, as long as the widest part of the mast is still in the gate. 
 
However, the boat is a classic and it is probably not worth modifying the mast gate (it would require structural work). I was thinking I could simply move the mast forward in the step and start from there, but I'm not completely sure of the consequences of doing so. As I don't know of any other 505 in my area (Montreal, Canada), it's hard for me to get inputs from locals. This is why I seek for advices here.

Many 505s, particularly older ones are not able to step the mast as far aft as suggested in the tuning sheets (which is essentially as far aft as class rules allow).  Moving the mast butt forward in the step a short distance is fine. 


I analysed the problem with the little experience I have. Here are different solutions I am considering right now (other solutions are the most welcome):

1) Moving the mast forward in the step without changing the recommended rake/tension settings. I think this will mainly change the mast prebend and I will get more jib luff sag, which may not be convenient in very low wind speed. I'm not sure if pulling harder on the jib cunningham can fully compensate for this (any hints someone?!?). The reduced mast prebend may also slightly reduce the ability to depower the boat in high winds because of the fuller main shape and increased jib luff sag. I'm not sure if I will see much difference because I don't go out when there is no wind and also, we can rake back a bit more or use more cunningham/outhole if possible to compensate the effects in high winds.

What does the boat have to control mast for and aft bend?  Some sort of a mast ram (which pushes back on the mast)?  Blocks in the mast gate/partners?  I don't see how moving the mast butt forward say 25mm changes the mast bend unless that movement also changes something in a mast ram or moves the mast so far forward in the mast gate that the fore and aft bend is constrained.

505s built that early generally had fewer controls and many from that era did not have mast bend controls.

The rake settings that we use now, and particularly the large range from the more forward/upright rake for light air to the more aft/raked setting for heavy air, require quite a bit of control range in the forestay or jib halyard (you almost certainly have a jib halyard) and in the shrouds.  Your boat probably does not have enough adjustment range to give you the range in rake from the tuning sheets.  But you point out that you do not expect to sail the boat in heavy air, so you will not need to rake aft as much.

Also the older boats cannot normally carry significant rig tension, so you are going to have a lot of jib luff sag in medium or heavy air, anyway. 

2) Maintain a fair amount of prebend while moving the mast forward in the step. To do so, I would have to increase significantly the shrouds and forestay tension (I hope she won't collapse). Doing so, I will probably loose quite a bit of power as I will have a flatter main and less jib luff sag. On the other hand, it might increase the pointing ability, but I'm not sure if it is worth trying.

Modern 505 sails are cut for less jib luff sag than your boat would normally have (except in light air when there is not enough pressure to sag the jib much).

Generally less luff sag means a flatter "entry" on the jib, which gives you better pointing.  However, I like a lot of jib luff sag in very light air.  A lot of jib luff sag will give you a lot of fullness in the jib, losing point and may give you more power than you want.  Even if your boat will not carry lots of rig tension to keep the jib luff straighter in breeze, you can ease the traveler to leeward and pull on the mainsheet, using leech tension on the main (pulling the mast aft) to reduce jib luff sag.

3) Another solution would be to move all the rig forward trying to keep the prebend with relatively low forestay tension. However, moving the mast away from the chainplate would probably reduce the effect of the spreaders on the mast. Thus, I will probably fail prebending the mast with relatively low forestay tension. I think I would also loose some of the lateral/athwartship mast support from the spreaders which is probably not desirable (unless I modify the spreader length/position).

My comprehension of the problem is not perfect, so I invite you to correct me or clarify certain point that I might have missed. Any hint would be the most welcome. Also, there are a few things that you may want to clarify:

- How does the mast position affect the slot between the main and jib and how does this affect the boat ?

What we have learned over the years since your boat was built is that stepping the mast as far aft as we can, seems to make the boat go better upwind.  This is probably due to opening the slot, but different people have different theories as to why it works.  Raking the mast aft also opens the slot and depowers the boat for heavier wind conditions. 

- What is the impact of the forestay angle (changed through mast position) on the shape of the jib (similar to the fact that the forestay is attached in different location depending if it is a bag or a launcher boat) ??? Is it sufficient to play the jib car position to compensate for that  ?

Some sailmakers cut jibs slightly differently for a forward tack boat and an aft tack boat.  The aft tack jib could be slightly shorter on the luff.  A forward tack boat typically has the jib sheeted further forward in the boat, as well. 

I understand that in all cases, I will have to experiment with the boat and fine tune the centerboard position at the same time to adjust the helm. I'm looking for a good starting point here. A few facts you may want to consider while giving me some hints: winds are relatively turbulent in my area as the lakes and rivers are relatively small, so the urban shores have huge effects. The crew weight is on the light side (330-360 pounds) and we almost never sail in winds above 18 knots.

For upwind in light, board as far down as it will go for upwind/close hauled.  Almost full down for reaching, most of the way up for running.
For upwind in medium, leading edge of CB essentially vertical (perpendicular to the bottom of the boat), bring it up a bit if you are overpowered.
The more overpowered you are in breeze, the more you can bring the CB up, but probably not more than 1/4 up and that would be for very windy. 

Bon chance!

Alexander "Ali" Meller


Thank in advance !

Joel

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Alexander "Ali" Meller
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