Deck Cracks

[Tom Coons]

I'm new to your site (Cape Dory 31 hull # 15) and have read the comments about deck cracks. I may have missed it but most comments seem to concern how to fix them rather than what causes them. A lot of cheaply built boats step the mast on deck with a compression post or bulkhead below. Stepping on the keel (much better way) as ours do, does present one problem.

When the mast is loaded under sail the compression force is like a bow and arrow. with the boat hull being the bow, the mast being the arrow and the shrouds being the bowstring. When the rig is loaded the mast pushes down and tries to lift the hull via the chainplates. This motion squeezes up and in -- just like the bow with an arrow. The deck and cabin have to be strong enough to resist this force or they will be flexed and crack. There is also some force from the side of the mast at the deck against what on a wooden boat would be the mast partners. On wooden boats this area is the strongest part of the deck with massive beams to carry the load to the hull at the chainplates rather than through the deck and cabin.

Cape Dory tried to accomplish this with a 3/4 inch plywood bulkhead below and the inherent strength of the balsa core deck and cabin. Obviously not strong enough. What we need is a hat shaped piece of metal or more masive piece of wood to anchor the chainplates against one another so that the deck can't be flexed under load.

I haven't done this yet, but I've tried everything else. Ground out the deck core where the cracks were and filled solid with epoxy, mat, roving, and cloth. One season the cracks were back again. I don't know about Capt. Tolley's but I have tried every flexible compound and crack filler that I could find to no avail, before going to the rigid epoxy fix that didn't hold.

If you've covered this previously, I'm sorry to get it going again, but does anybody else have a theory about the cause of the cracks?



TacCambria@thegrid.net

[Newton Page]

I sail a 1980 CD27. For the first 14 years she lived in the water except for a one or two week peroid every second year when she was hauled for maintenance. After one or two weeks on the hard she was put right back in the water. During this period very few cracks developed in the deck, maybe a dozen or so. Four years ago, after
a very nasty winter season with heavy ice problems, I decided start hauling her out in winter. Now, after four years of wintering on the hard she has many random hairline cracks in the deck.

Could it be that sitting on her bottom with no support from the water has resulted in stress/flexing of the deck that has caused an increase in the number of cracks?

Your theory sounds more likely, but It would be interesting to know if there is anyone else that has noticed an increase in deck cracks after a similar change in practice. Or, if those boats that live in the water all year have fewer deck cracks that those that are hauled for the winter.

[Bob Loewenstein]

I have a 1981 27' that is hauled out every season (Wisconsin winters). There are very very few deck cracks on her. The only place on the boat that really has cracks is in the cockpit, which seems to be prevalent in other Cape Dory's. I don't know whether this counters the speculation that leaving the boat in the water helps prevent deck cracks, but I have at least one example where hauling and dry storing doesn't seem to hurt.



rfl@yerkes.uchicago.edu

[Gregg Carlson]

I'm new to your site (Cape Dory 31 hull # 15) and have read the comments about deck cracks. I may have missed it but most comments seem to concern how to fix them rather than what causes them. A lot of cheaply built boats step the mast on deck with a compression post or bulkhead below. Stepping on the keel (much better way) as ours do, does present one problem.

When the mast is loaded under sail the compression force is like a bow and arrow. with the boat hull being the bow, the mast being the arrow and the shrouds being the bowstring. When the rig is loaded the mast pushes down and tries to lift the hull via the chainplates. This motion squeezes up and in -- just like the bow with an arrow. The deck and cabin have to be strong enough to resist this force or they will be flexed and crack. There is also some force from the side of the mast at the deck against what on a wooden boat would be the mast partners. On wooden boats this area is the strongest part of the deck with massive beams to carry the load to the hull at the chainplates rather than through the deck and cabin.

Cape Dory tried to accomplish this with a 3/4 inch plywood bulkhead below and the inherent strength of the balsa core deck and cabin. Obviously not strong enough. What we need is a hat shaped piece of metal or more masive piece of wood to anchor the chainplates against one another so that the deck can't be flexed under load.

A common solution is a tension rod next to the mast running between the deck and the keel, keeping the deck from rising when the mast pushes down the keel.



ghartc@aol.com

[C.C.]

Deck cracks near the chainplates are common on older typhoons. Look at the "Typhoon Slip," there are pictures.



HOOPS3350@aol.com

[Tom Coons]

I'm new to your site (Cape Dory 31 hull # 15) and have read the comments about deck cracks. I may have missed it but most comments seem to concern how to fix them rather than what causes them. A lot of cheaply built boats step the mast on deck with a compression post or bulkhead below. Stepping on the keel (much better way) as ours do, does present one problem.

When the mast is loaded under sail the compression force is like a bow and arrow. with the boat hull being the bow, the mast being the arrow and the shrouds being the bowstring. When the rig is loaded the mast pushes down and tries to lift the hull via the chainplates. This motion squeezes up and in -- just like the bow with an arrow. The deck and cabin have to be strong enough to resist this force or they will be flexed and crack. There is also some force from the side of the mast at the deck against what on a wooden boat would be the mast partners. On wooden boats this area is the strongest part of the deck with massive beams to carry the load to the hull at the chainplates rather than through the deck and cabin.

Cape Dory tried to accomplish this with a 3/4 inch plywood bulkhead below and the inherent strength of the balsa core deck and cabin. Obviously not strong enough. What we need is a hat shaped piece of metal or more masive piece of wood to anchor the chainplates against one another so that the deck can't be flexed under load.

A common solution is a tension rod next to the mast running between the deck and the keel, keeping the deck from rising when the mast pushes down the keel.

A follow up to Gregg's observation. I think that's a good solution. Have pondered how to do it. There's a little well where the masts steps thus no obvious way to attach a plate for the bottom of the tension rod. At the deck a plate on the exterior through bolted to some sort of eye pad on the inside? What kind of forces are we talking about here. Any engineers who can calculate the forces for us? I presume it has to be pretty sturdy and probably cut away the liner to bolt to the deck. Anyone know if the bulkhead is glassed to the hull or floats in the liner? Perhaps the bulkhead doesn't touch the hull which could be the problem since there would then be an inch or so of "flex room" on each side between the liner and the hull. Perhaps just anchoring the bulkhead to the hull would do it. Tom - following up the follow up.

TacCambria@thegrid.net

[Gregg Carlson]

When the mast is loaded under sail the compression force is like a bow and arrow. with the boat hull being the bow, the mast being the arrow and the shrouds being the bowstring. When the rig is loaded the mast pushes down and tries to lift the hull via the chainplates. This motion squeezes up and in -- just like the bow with an arrow.

A common solution is a tension rod next to the mast running between the deck and the keel, keeping the deck from rising when the mast pushes down the keel.

A follow up to Gregg's observation. I think that's a good solution. Have pondered how to do it. There's a little well where the masts steps thus no obvious way to attach a plate for the bottom of the tension rod. At the deck a plate on the exterior through bolted to some sort of eye pad on the inside? What kind of forces are we talking about here. Any engineers who can calculate the forces for us? I presume it has to be pretty sturdy and probably cut away the liner to bolt to the deck. Anyone know if the bulkhead is glassed to the hull or floats in the liner? Perhaps the bulkhead doesn't touch the hull which could be the problem since there would then be an inch or so of "flex room" on each side between the liner and the hull. Perhaps just anchoring the bulkhead to the hull would do it. Tom - following up the follow up.

I figured a 100 lb tug at the Typhoon masthead will steady the rig/boat horizontally, resulting in about 475 lb shroud tension/mast compression. A 1/4" mild steel rod will support this, larger diameter or S.S. even better. In reality, the load is of course shared, and anything you add will help (if the deck *does* move). If I were using all-thread, I might look for 1/4" or 5/16"... Secure the bottom (can you tap into some keel metal?) well. I would drill right through the deck and add a large fender washer ("marinized", of course) on top to distribute the load and snug it well up.



ghartc@aol.com

[Robin Meigel]

In response to Newton Page: The 1979 CD 27 I purchased came with 17 yrs of maintenance history at Hartge's, Galesville, MD. The boat was hauled for painting but wintered in the water rather than "on the hard". The records reflect gelcoat cracking and repair throughout the history of the boat, including shortly after the previous owner purchased her. For that reason, I do not think that water vs. hard storage is a significant factor. I think, for all our boats, the gelcoat can be expected to get more brittle with age. Rebuttal to this expectation would be welcome.

rlmeigel@aol.com

[Joe Impellizeri]

Idon't know anything about deck cracks (yet) but I did use Cap't Tolly's Creaping Crack Cure on a front window leak. One shot, worked great. I worked it in by flexing the area near the window leak by finger pressure to get the stuff in and bubbles out. It dries like a thin rubbery stuff. Goes on like milk dries in a few minutes to an off white color. Of course this isn't going to help you structural problem But it will stop your leak until you do figure it out (Great web site. Were just getting our's off the ground. Hopefully it will be as good as CD's.)

P.S. I call myself a powerboater since I own a 33' Egg Harbor. I just happen to be fixing a 1978 25 CD as a hobby. Never sailed it;
don't think I want to.



Impeljr@aol.com

[Andy!]

I would like to find a crack for the game "Half-Life" and was wondering if you could tell me where i could find one.

Andy!



king@Andyh1.freeserve.co.uk