How much weight for mooring?

[Scott]

Our boat is a 27' Starwind, displacement is 5100 pounds and ballast is 2000 pounds. We may have the opportunity to keep our boat on a mooring at a county park near our home, which will be more convenient than our current slip. The park officials told us they think they have space but may not have enough blocks for the weight they need to hold our boat. I offer to make the blocks but am wondering about the appropriate weight. Any "rules of thumb"?Oddly enough, it seems that they pull the weights out every fall and put them back every spring (they only have a handful of small sailboats on moorings). It makes more sense to me if they simply wired all chains together each fall and fished them out in the spring, but it's their "system".

 

[Gord May]

Deadweight Moorings

Deadweight anchors/moorings (concrete, granite, etc) rely almost entirely on their submerged weight to provide holding power. A deadweight anchor will set into the bottom, if left to soak, and thereby increase it’s holding power through “suction”. A 3,000 dry weight of concrete will have a submerged equivalent weight of about 1,650 Lbs. Ie: Dry Weight x 0.55 ≃ Submerged weight - accounting for material buoyancy.A 27' sailboat should have a Permanent Mooring capable of withstanding about 1,700 to 1850 Lbs. of force. Most Harbours provide specifications for moorings, which have been designed to reflect local conditions (bottom type, depth & scope allowances, wind & wave protection, etc.)I know that many Harbours / Yacht Clubs specify mooring weights that reflect MUCH LOWER capabilities than I indicate. I can only assume that they have either (a) very limited exposure, (b) very good holding bottom, or BOTH.Helix Anchors (embedded) are now much preferred over deadweights for permanent moorings.Some comparative breakout strength tables (approximations): http://www.helixmooringsystems.com/benefits.htmlhttp://www.boatmoorings.com/hm.phpFWIW,Gord

 

[Scott]

Thanks, Gord

I feel like I'm armed with a little more information. Yesterday, the park told me they think they have the space and will make the anchor for me. My boat is heavier than the boats they normally have moored, but last year they did have another that looks to be about the same as mine, so I hope they know what they need. It looks like 3/4 yard of concrete is about right. I think they bring a barge in to place the anchors each spring.The helix system is interesting. I just hired a contractor to install helical bearing piles for a boardwalk that my company is building along one of our communities on the Jersey shore. They auger into the sand and are less disruptive than driving wood piles (but more expensive).