Islander Jack

Mast not directly over compression post

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1972 Islander 30 MkII (3/4" plywood core deck, deck-stepped mast). I noticed my mast -- not the original but still a rigid cruising mast -- has a very slight bend, maybe 1 inch, with the center of bow aft rather than forward. Measurements show, to my surprise, that the center of the mast is 2.5 inches aft of the shrouds and, similarly, 2.5 inches aft of the center of the compression post. Yet the J measurement is exactly as reported on sailboatdate.com.  I've asked around for measurements on any other Islander 30 but haven't got any response.

I'm guessing the boat was designed with this 2.5" offset between mast and post/shrouds because I have no other reasonable explanation. And I 'm guessing my deck core has crushed or sagged a little  around the mast heel so the mast is now supported mostly at the forward end of its section, causing the mast to bow slightly.

So, what to do?  Certainly unstep the mast and replace the core with solid, scarfed-in laminate.  I'm thinking of polyester resin rather than epoxy for the core replacement because it only needs to be incompressible,  and the modulus, strength, etc. would match those values of the rest of the deck better than epoxy would, so the scarf joint may be more secure.

But there would still be an offset between mast and post. I have space on the sole for a 1.25" wider (fore and aft)  compression post, so should I fab and install a new one? Any other options?  Move the mast heel forward, even though it's where it belongs according to sailboatdata?  If so, leave the masthead where it is or move it forward also?


I gladly welcome any opinions.

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No need for the mast foot to be directly over the post. The deck needs to be sound. Rebuild the deck with whatever was there in the first place.

Chainplates forward of the mast foot is common. Doubt it makes much difference.   Mast bases are often trimmed a bit out of square to induce some bend. Depends on deck, shoe and rake. Preload on the forward lowers should be all you need. Aft lowers are usually tuned without preload. They just control overbend in a blow.p

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You don't mention where the top of the mast is relative to (anything) everything else.  I think you need to start from zero.  On a calm day, loosen all the shrouds, and the head stay and back stay.  Get a long metal tape measure, and attach to the main halyard.  Then center the top of the mast side to side by adjusting the upper shrouds till hand tight.  Then using the tape measure, center the top of the mast fore and aft by adjusting the head stay and back stay.  At this point the mast should be mostly straight and in column.  Measure from a common point at the middle of the stern to both the center of the compression post and the center of the mast.  Compare those measurements.  They should be the same (or awfully close), those as Daddle says, there is no need for the mast foot to be directly over the compression post based on the design and soundness of deck.  That said, I suspect on the Islander, they should be.  Once you have the top of the mast over the base of the mast, and centered side to side and fore and aft, tighten the forward lowers first, then the aft lowers, again to hand tight.  You should be able to have a dead straight mast at this point.  If not you have some other issue - like a headstay that's too tight, a crushed deck as you speculated, a backstay that's too long, etc.  Right now the biggest issue seems to be the aft lowers are too tight, loosen the aft lowers a couple turns and tighten the forward lowers and see what happens.

As far as repairing the deck, I'd go with epoxy.  Its secondary bonding ability is much better than poly resin, and you want it to bond well to the inner and outer laminate.  The price difference on this size repair is really kinda negligible...

Just my 2 cents, which isn't really worth much...

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7 hours ago, Crash said:

.... Then using the tape measure, center the top of the mast fore and aft by adjusting the head stay and back stay.  ....

Crash must have left out some important detail here. The headsail hoist can be compared to the datasheet. Or the boat can be trimmed to her lines and the mast eyeballed. Or level the floors then plumb the mast. Or just put the headstay turnbuckle at 1/2 and assume you are close. Or some blend of all those.

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LeoV, yes, the mast certainly looks aft of the post in that drawing, and that settles the question of original design. Thanks for pointing that out. I have that drawing in the original brochure (yep, still got it) but never thought to look at it! 

Replying to others about measurements and adjustments: I already loosened all stays and checked mast bend (no change), checked shroud lengths, plumbed the hull as best I could to the boot stripe and the cockpit (my cabin sole has a liner with a slight upward bow) and then plumbed the mast.  I measured mast-to-post with two measurements: distance from an aft deck screw to the mast minus distance from that screw to post, and distance from forward hatch edge to the mast minus distance from that edge to post.  For shrouds I measured fore-and-aft post to chainplates  and I measured shrouds to mast as shown in the attached sketch, which requires only a ruler measurement between shroud and the eye that's sighting along the mast to opposite shroud.

One more question: would an epoxy repair preclude refinishing with gelcoat?  Put another way, will gelcoat adhere to epoxy/FG?

 

 

mastToShroudsOffset.png

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Chainplate position fore/aft makes no difference in mast bend. They had to bolt the chainplates to the bulkhead which for whatever reason is slightly ahead of the mast center. Remove all tension from the aft lowers. Tighten the forward lowers until they feel like the cap shrouds: really tight. Report back.

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11 hours ago, Islander Jack said:

LeoV, yes, the mast certainly looks aft of the post in that drawing, and that settles the question of original design. Thanks for pointing that out. I have that drawing in the original brochure (yep, still got it) but never thought to look at it! 

Replying to others about measurements and adjustments: I already loosened all stays and checked mast bend (no change), checked shroud lengths, plumbed the hull as best I could to the boot stripe and the cockpit (my cabin sole has a liner with a slight upward bow) and then plumbed the mast.  I measured mast-to-post with two measurements: distance from an aft deck screw to the mast minus distance from that screw to post, and distance from forward hatch edge to the mast minus distance from that edge to post.  For shrouds I measured fore-and-aft post to chainplates  and I measured shrouds to mast as shown in the attached sketch, which requires only a ruler measurement between shroud and the eye that's sighting along the mast to opposite shroud.

One more question: would an epoxy repair preclude refinishing with gelcoat?  Put another way, will gelcoat adhere to epoxy/FG?

 

 

mastToShroudsOffset.png

Jack,

Its possible that the headstay and backstay are both too short, resulting in the mast being compressed in a static position.  But the problem with that is how do you get the turnbuckle to engage on which ever stay you hook up second?  If you've really loosened all the stays and the shrouds, and you till have an 'inverted bend", then either you have a mast with a permanent bend in it, or your are not measuring accurately (which I highly doubt.).  As Daddle says, what happens when you loosen the aft lowers such that there is only a couple thread showing below the turnbuckle, and tighten the forward lowers until they are really tight?  I can't believe you wouldn't be able to induce the middle of the mast to move forward and induce a "normal" bend where the middle of the mast is a couple inches forward of plum...

If I wasn't leaving Socal for the next three weeks, I'd offer to drive down one day and help you troubleshoot, etc.  I'm back the second week in August if you want a second set of eyes on things...

Crash

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Daddle, I'll ease  -- even more -- aft lowers and tighten forward lowers, though I'm not excited about putting a lot of tension on the forward lowers because the geometry is, well, less than optimal. (See photo.) I don't have turnbuckles, by the way.  I don't know what you call them, but they're very nice. A captive nut (not shown) at the top of the hollow cylinder (shown) screws the wire stud into the cylinder.

Crash, thanks for the offer!  But I think I got this one figured out: mast is designed to be aft of post, plywood core is 46 years old and weak, and I may be able to be straighten mast with forward lowers, but core really should be replaced.  The only questions are when and with what material and finish.

I'm planning a trip to Baja Naval in Ensenada in August for other things (bottom, a through-hull, topsides, re-seal deck-to-hull joint).  I wanted to get this core replaced at the same time but they're pretty busy and don't want to schedule it without seeing it first. I'll have to make a second trip.

But thanks to everyone here I'm no longer worried about mast position. 

fwdLowerSm.jpg

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54 minutes ago, Islander Jack said:

Daddle, I'll ease  -- even more -- aft lowers and tighten forward lowers, though I'm not excited about putting a lot of tension on the forward lowers because the geometry is, well, less than optimal. (See photo.)

That geometry is common on boats of that era. The bending load on that chainplate goes into the deck. My Cal was the same way. Works fine except for the leaking issue.

The forward lower shroud carries a huge load while sailing. Far more than you will ever adjust into it while tuning in a bit of mast pre-bend.

On the Cal 36 the aft lowers did not even have a chainplate. Just a deck padeye.

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I have horizontal stainless plates under the deck, roughly 8 x 15 inches, with welded-on eyes that protrude through the deck.  Maybe those can be called "chainplates."

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On 7/5/2018 at 4:28 PM, Islander Jack said:

1972 Islander 30 MkII (3/4" plywood core deck, deck-stepped mast). I noticed my mast -- not the original but still a rigid cruising mast -- has a very slight bend, maybe 1 inch, with the center of bow aft rather than forward. Measurements show, to my surprise, that the center of the mast is 2.5 inches aft of the shrouds and, similarly, 2.5 inches aft of the center of the compression post. Yet the J measurement is exactly as reported on sailboatdate.com.  I've asked around for measurements on any other Islander 30 but haven't got any response.

I'm guessing the boat was designed with this 2.5" offset between mast and post/shrouds because I have no other reasonable explanation. And I 'm guessing my deck core has crushed or sagged a little  around the mast heel so the mast is now supported mostly at the forward end of its section, causing the mast to bow slightly.

So, what to do?  Certainly unstep the mast and replace the core with solid, scarfed-in laminate.  I'm thinking of polyester resin rather than epoxy for the core replacement because it only needs to be incompressible,  and the modulus, strength, etc. would match those values of the rest of the deck better than epoxy would, so the scarf joint may be more secure.

But there would still be an offset between mast and post. I have space on the sole for a 1.25" wider (fore and aft)  compression post, so should I fab and install a new one? Any other options?  Move the mast heel forward, even though it's where it belongs according to sailboatdata?  If so, leave the masthead where it is or move it forward also?


I gladly welcome any opinions.

that's not right..  mast should never bow aft... something is not tensioned correctly..  you have a masthead rig..

to do it properly,  take the mainsail off, remove the boom..   ease the tension on the shrouds until the aft bend is gone..  use the main halyard to hoist a measuring tape and center the mast from side to side.. etc..

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Mainsail and boom are supported at the masthead via the topping lift, not a rigid vang or kicker, so they can't bend the mast aft (though possibly forward).

Everything else has been checked.  The mast still bows aft by about an inch, just a little, but I won't ignore it and hope it goes away.  If I squint I can kinda see a slight depression in the deck, aft and  starboard of the mast foot.  I can't get the depression to show up well  in photos.

mastFootSm.jpg

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16 hours ago, Islander Jack said:

Mainsail and boom are supported at the masthead via the topping lift, not a rigid vang or kicker, so they can't bend the mast aft (though possibly forward).

Everything else has been checked.  The mast still bows aft by about an inch, just a little, but I won't ignore it and hope it goes away.  If I squint I can kinda see a slight depression in the deck, aft and  starboard of the mast foot.  I can't get the depression to show up well  in photos.

 

 

IJ

 correct,   but when you go to center the mast by measuring side to side etc.. tensioning the rig,  you don't have the boom on the mast was what I was saying...

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