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Weekend Project Fail - (Oday 28 Mast base sinking?)

May 24, 2011
Oday 28 Windmill Harbor Hilton Head, SC
Well this weekend's project didn't turn out like I had planned. I bought the spartite mast wedge/seal kit (http://www.westmarine.com/buy/spartite--spartite-mast-wedge-replacement-system--P002_065_007_501) and planned to remove the mast deck plate and clean and re-bed it and then apply the spartite in an effort to stop leaks at the mast hole.
Well for reasons I am not 100% sure of, there was a SS pin installed through my mast resting on the collar of the mast plate. (See pic). In order to re-bed the plate I have to remove this pin...? What is this pin for? I have a keel step mast..... there was some pressure on the pin but by standing on the deck close to the mast plate I could push down (rather than loosen all the rigging) using the boom as leverage and have a friend push the pin out ....ok, pin out...nothing happend...a good sign..lift/pry plate up, very little sealant found, ..well maybe I can stop the leaking...pretty obvious this is where water is coming from....but now that the plate is up, the area where the plate was was really indented downward...as feeling with your had across the deck...as if there was a lot of pressure in the plate....hmmmm....well I called a marine mechanic who I knew was already in the marina and he came over and eyeballed the area and said " I think your sinking.." ..I looked at him not sure what he meant....he then said,..your mast, ...not the boat.

We then went below and sure enough, he showed me where the shelf that the mast is supported by was delaminted and sagging,...what the hell? So now I am wondering how all this happened and what to do to fix this.
He said, loosen the rigging and we install wedge blocks under the shelf to support the mast...I think on it overnight and think,... in order to do this right, the mast will need to be removed, the sagging shelf lifted up, then supported by fiberglassed blocks and the shelf re-glassed....geez,,,now I think,...maybe this has been this way for a long time and that pin had been installed by one of the PO in an effort to reduce pressure on the failing mast base (that was sagging) and move some of the pressure to the deck? ..that explains why the area under the deck plate is so depressed...not good...HELP!!!!!


May 17, 2004
Beneteau Oceanis 37 LE Havre de Grace
The pin is original equipment. I believe the manual says not to sail in more than light winds without that pin installed. My guess is that it helps hold the deck down against the force of the shrouds (at least the forward lowers) pulling it up.


Jun 19, 2013
Oday 28 Traverse City
i performed that repair 2 years ago. pulled the mast. cut out the top layer, use an oscillating tool to remove the wet core. replace with g4 fiberglass, and use epoxy and fiberglas strips to tie it all together.

we do not have the pin. a po cut the ears off the collar. instead we have a partner to hold the cabin down, counteracting the squeezing forces of the rigging
Jan 27, 2008
ODay 35 Beaufort, NC
I think you'll find that your shrouds are attached to rods inside the boat that take the loads down to the pan/hull in the boat versus pulling up on the deck. However if you have halyards and boom vang lines lead back to the cockpit those forces exert an upward force on the deck. I also have that pin in my mast and confirm it is original equipment.
May 17, 2004
Beneteau Oceanis 37 LE Havre de Grace
I think you'll find that your shrouds are attached to rods inside the boat that take the loads down to the pan/hull in the boat versus pulling up on the deck. However if you have halyards and boom vang lines lead back to the cockpit those forces exert an upward force on the deck. I also have that pin in my mast and confirm it is original equipment.
On the 28 the upper shrouds and aft lowers are carried by rod to the pan, but the forward lowers are anchored to aluminum plates in the deck. Good example with the boom vang; that pulls up on that exact area.
Aug 17, 2010
Oday 35 Barrington
Sorry that I am so late to this. I was looking for info on Spartite for my O'day 35 and stumbled across this thread. The pin is supposed to be installed at the initial commissioning by the dealer. The O'day instructions for this are here: http://forums.sailboatowners.com/in...pping-the-mast-from-oday-30-manual-pdf.41390/

Regarding the fact that the mast STEP (not "shelf") is deformed, you need to straighten that out. If it were my boat, I would pull the mast, and epoxy a 3/4" G10 backing plate - ideally with a strip of G10 running laterally to make it an I-beam.
Aug 28, 2015
Oday 28 St Joseph, MI
The only pin through the mast on my 28 is through the fitting inside the cabin in the attached photo. I seem to remember another post somewhere and the mast sinking problem was the same. That person I believe removed the mast, used wedges to raise the sunken plate and glassed in blocks. In any case I wonder if your shrouds were, or have been in the past, too tight. Seems to me the more concerning problem would be the depressed deck around the mast hole. Could it spring back into shape? I wonder what that lump is, below the mast base plate. I have the same thing.


Aug 17, 2010
Oday 35 Barrington
Because some people seem reticent to click the link that I provided above, here are the instructions from the PDF that I linked to above, which was provided by O'DAY YACHTS that address the REQUIREMENT for the mast pin if you have an O'day 28, 30, 34 (therefore 35) or 39 with an Isomat mast. I have added that Butyl sealant is an acceptable substitute for silicone.

Mast Wedge and Pin Instructions

These instructions are for installation of the mast boot, wedges, and deck pin on O‘Day Yachts equipped with Isomat spars.

l. Before stepping mast, slip neoprene collar over the mast and up to the gooseneck. The smaller end goes on first.
2. After stepping mast, and before final tuning of the shrouds, install the rubber wedges between the collar and mast.
3. After final tuning, and a short sail in light-to-moderate winds, drill a l/2" hole through the mast, just above the collar. Install the l/2" stainless steel pin through the spar and fix in position with cotter pin. This pin will hold the deck from flexing. It is important that it be installed. The pin should be fust forward (approximately l/2") at the extruded niche in the side of the mast.
4. Silicone (or Butyl) sealant must be placed around the mast and mast collar where the clamp will tighten. Be sure to fill the spinnaker track and mainsail track grooves well with silicone (or Butyl) sealant. The rubber boot is clamped around the collar and the mast. For the top clamp, leave enough boot to roll it back over the clamp.

Illustration 1 - see the PDF

Illustration 2 - see the PDF

Before stepping the mast, be sure all running and standing rigging is properly installed, cotter pins are spread, and halyard sheaves are free to rotate. The upper shroud is run through the slot in the outboard end of the spreader. After the shroud is in place, tape over the end of the spreader to protect the sails. Check the spar lights to be sure they are operational.

Open all turnbuckles to their full extension.

Refer to separate sheet for mast-collar instructions.

Step the spar through the deck and table (if applicable), and then onto the mast step. Be careful not to pinch the mast wires during the stepping. The mast step was set at the factory to provide an aft rake. If you wish to adjust this, loosen the mast—step bolts and slide the step fore or aft.

The mast—ground wire should be attached to the mast at this time.

Attach all the shrouds, tighten the headstay, backstay, and upper shrouds to a taut condition. Refer to the mast-collar drawings to install the rubber wedges, mast pin, and boot.

Installation of the pin is essential to prevent the deck from lifting, when full rigging forces are applied. Do not sail until this pin is properly installed. For now, leave the lower shrouds slack. Attach a weight to the main halyard and allow it to hang freely. Adjust the headstay and backstay to achieve a straight spar, when sighting up the trailing edge.
O'Day 37 - 12"; O'Day 34/35 - 8"; O'Day 30 - 9"; O'Day 28 - 7";
aft of the spar, when measured at the gooseneck. Next, with the boat level athwartships, tighten the upper shrouds to get the mast straight. Finally, tighten the lowers no more than hand tight.

Final tuning must be accomplished while sailing. In a light breeze (6 - 8 knots), adjust the shrouds to achieve a straight spar on either tack. In heavier winds, any curvature should be gradual and constant from the deck to the mast head.

The rig will need adjustment after a few sails to compensate for the stretch in the wire. Be sure to install cotter pins in all turnbuckles and clevis pins, and tape over them to prevent injury to crew or damage to sails.

Fine tuning for the best performance will depend upon your local conditions and your sails. Consult your dealer or local sailmaker for their suggestions.


O'DAY 28, 30, 34/35, 39
The chainplate rod system used on the shrouds of your boat is designed to carry rigging loads to the structural floor pan. IT IS IMPORTANT THAT THE INSTALLATION BE CHECKED FOR PROPER ROD TENSION. The system is designed to carry rigging loads to the pan, rather than the deck.

After the boat is launched and the spars installed, but before the rigging is tightened, the rods should be taut. Pulling on the rod, at mid—height from pan to deck, you should notice a very slight deflection. If the rod is loose, it should be tightened. After final rigging and a few early sailings, recheck the rod tension. If the deck is deflecting at the chainplates, the rod needs further adjustment.

To tighten the rod, remove the cap on the pan at the base of the rod and get a wrench on the nut. Above the pan is a flat spot on the rod. Use another wrench or locking pliers to turn the rod. DO NOT OVERTIGHTEN. If the rods are too tight, with the rigging slack, the deck could be pulled down, creating a depression on deck and damage to the gelcoat surface.

The U-bolt, to which the turnbuckle attaches, is bolted through the deck to an aluminum plate that holds the top of the rod. Annually, the U-bolt should be disassembled and rebedded. When reinstalling, tighten the U-bolt nuts to a snug condition. Again,do not overtighten, or you will damage the deck finish. After reinstalling the U-bolt, be sure to check the chainplate rod tension."
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Aug 28, 2015
Oday 28 St Joseph, MI
Eherlihy, sorry, didn’t mean to cause any anger issues. I simply mentioned the bolt through my mast as an aside to which I dedicated one (1) line as I obviously don’t have an Isomat spar. The bulk of my post, 5 lines, was addressing Euphoria2011’s post about his mast “sinking”. Again, sorry if I caused any anger issues to surface.
Jun 11, 2004
Oday 31 Redondo Beach
Regarding the fact that the mast STEP (not "shelf") is deformed, you need to straighten that out. If it were my boat, I would pull the mast, and epoxy a 3/4" G10 backing plate - ideally with a strip of G10 running laterally to make it an I-beam.
Euphoria, have you done anything about this yet?

I agree with eherlihy that it looks like your mast step is the first problem. His suggested fix sounds effective and doable. MIght be able to do this without pulling the mast all the way out if there is a way to raise it a couple/few inches.

Is the core around the hole in the deck wet or rotten? Can't tell from the pictures but if it's not you might not have a problem there once the mast step is fixed and the mast is raised to its proper height.

Just curious, are the turnbuckles for the shrouds turned in fairly closed, indicating that someone has tightened them down to make up for a sinking mast?
Aug 17, 2010
Oday 35 Barrington
@wing15601, No anger on my part. However, I do want @Euphoria2011 (the OP), and anyone following in his wake to get this question answered with accurate information. I suspect that the tension of the standing rigging is the reason why Euphoria2011's mast step is/was bent. I suspect that the pin was added after the mast began to sink. I can tell from the pix that Euphoria2011 has an Isomat spar and the mast pin is supposed to be there.

Regarding your thread on the mast partners creaking, I suspect that the lack of a pin with your non-Isomat mast is also the reason for this symptom.

There was another thread in which @Panta_Rei pointed out that many boats with keel stepped masts need a tie rod to keep the compressive load on the keel from squeezing the hull, and causing the deck to rise.
The pin is needed to prevent the deck from rising and the boat squeezing together midships (placing greater than designed loads on the bulkheads below). There is a large force from your halyards if run them back to your cockpit thru blocks attached to the deck plate. The boom vang also tries to lift the deck. My buddy who has owned his O'day 39 since it left the factory and sails it across oceans says the pin is a cheap alternative to the proper solution which are tie rods running from the deck plate to the mast step as seen here https://www.google.com/imgres?imgurl=https://theriggingcompany.files.wordpress.com/2012/07/images3.jpg&imgrefurl=https://theriggingco.com/2015/06/17/deck-tie-downs-2/&docid=bpBPMKRnptciEM&tbnid=1Sk4osTbJk8OHM:&vet=10ahUKEwiogrGA8sTeAhUl_4MKHSAIAhEQMwhIKAQwBA..i&w=259&h=194&bih=882&biw=1280&q=mast tie rods&ved=0ahUKEwiogrGA8sTeAhUl_4MKHSAIAhEQMwhIKAQwBA&iact=mrc&uact=8 and here http://www.jordanyachts.com/1840 He has two tie rods on each side of the mast. They run much closer to the mast than the picture I attached and look great down below. Handy hand hold as well.
I'm happy with my O'day solution but don't plan on curcumnavigating any time soon.
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Jun 23, 2021
O'Day 30 NSSC
My first post here! On an old thread, which seems appropriate for an old boat.

We just bought an O'day 30, complete with the rotting & sinking mast step. Pretty much exactly as pictured here.

Has the repair held up over time? This looks much simpler and easier than the full mast step rebuild I am contemplating.