Mast step position and other tuning considerations 1 post by 1 author

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bedar...@gmail.com

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Mar 27, 2015, 11:30:15 AM3/27/15
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I just got a used 505. I think it is from the early 70's (sail number 3452). 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. 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.

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.

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.

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 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  ?

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.

Thank in advance !

Joel
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