Dock construction for high winds and tide

[diver dave]

So, there is a new (to us) house in our near future. It has 180' of saltwater seawall; the concrete wall cap itself is just 2' above normal high tide.
It's in NE FL, so I consider it to be in a hurricane zone. Matthew caused 6' of surge. There is about 100 yards of fetch to the wall. Its a meeting of three canals. Normal tide range is 3'. With that set up; here is my question.

Lets say I want to keep a 35' x 15' 13,000 lb catamaran there.

I'm half considering a 4 pile lift, but I'm not sure about hull support issues.
The more basic option is to bang in a couple of dolphin piles, wood or 10x10 concrete some distance off the seawall. These would be tall enough to manage a 6' surge, but the seawall would be underwater, along with those two cleats. How far from the seawall should those dolphins be? Note sure how much wave action would occur in a CAT 3 with a 100 yard fetch. My last waterfront home was in a narrow (70') saltwater canal with decent isolation from any real fetch. I won't have a real option to tie the boat in the middle of this new canal. If I do, it will take a lot of line!

The no routine bottom job is a real draw, but on 4 tall posts, not sure how survivable that will be either.

 

[DavidM]

Seems like we recently discussed this issue, but maybe with another boater. IMO anywhere on the lower east coast can see up to 10' of hurricane surge. But with 100 yards of fetch you will also get 1-2' of waves. We got that much in Irene, a Cat 1+, with 200 yards of fetch. To survive that on a lift you need piles about 14' tall.


In my area, SW Florida, piles are 8-9' above the water. So yours would have to be 5' taller. For that to work I would definitely use two piles on each corner to deal with the side forces.


Another way is to install two piles adjacent to the seawall about 12' out of the water and another pair or maybe three about 20' away. Use one of the pile riding systems (bead rollers or whatever) to let the side ties ride up and down. That would probably survive the side forces and surge ok.

It isn't easy to design a system to deal with 10' of surge plus 1-2' of waves. Near Oriental, NC a floating dock system did ok in 9+' of surge but with little waves. Others in New Bern got washed away because their piles were too short.


David

 

[bayview]

Multiple piles at the corners, dolphins, set 10' or more from the boat to allow long lines to accommodate the high and low water.
I put the lines inside washer machine hose and looped them around the three piles. The hose protected against chafe and sun and the lines were healthy after ten years and multiple storms

 

[FF]

I would install a boat hoist for the 95% of the time only thunderstorms or passing sport fish arethe problem..

And I would install a proper mooring (3) 60H or 90H Danforths or a screw in set anchor for the "BIG " named storms.

Should you change boats or homes the mooring can always be recovered

 

[DavidM]

I would install a boat hoist for the 95% of the time only thunderstorms or passing sport fish arethe problem..

And I would install a proper mooring (3) 60H or 90H Danforths or a screw in set anchor for the "BIG " named storms.

Should you change boats or homes the mooring can always be recovered

I doubt if he would be allowed to preinstall a screw type mooring anchor in the canal outside his home. The one time I saw a triple Danforth rig installed as a hurricane mooring, it failed and the boat washed up on shore whereas mushroom moorings survived. So....


But I think your underlying premise is true: It is very difficult to install a docking system that will survive Class 3 winds.


David

 

[PMF1984]

Please do not listen to anyone on this site about how to design a dock at your site that will withstand a serious storm. Get a Civil Engineer who specializes in marine construction. I’ve seen a wide range of construction efforts on docks, lifts, and sea walls in JAX and half of them are scarier than a hurricane

 

[DavidM]

Please do not listen to anyone on this site about how to design a dock at your site that will withstand a serious storm. Get a Civil Engineer who specializes in marine construction. I’ve seen a wide range of construction efforts on docks, lifts, and sea walls in JAX and half of them are scarier than a hurricane

+1 Including me. But that sage advice applies to almost anything discussed on this site .



David

 

[PMF1984]

Dave,

Actually no, there is a lot of good technical info on this site.

It's just that the particular question about how to design a dock for a storm relies on some solid engineering and knowledge of the site, and the boat. And no one who is qualified to give an answer would without that info.

I would say the majority of posts in the maintenance section are spot on, and I go there quite often after I've damaged by boat while I am trying to fix something.

John

 

[Northern Spy]

I too would seek professional help. (•_•)

But, the Army Corps of Engineers has their incredible Coastal Engineering Manual available here: https://www.publications.usace.army...74616C20656E67696E656572696E67206D616E75616C/

It is literally (littorally?) the text used to get a master's in Coastal Engineering.

Not bad to peruse before and while you are communicating with an expert.

 

[Nomad Willy]

Shearwater’s floats got blown away about 12 years ago.
They probably know something about floats for high winds.

 

[diver dave]

I appreciate all the replies. Two new questions from me. What is a screw mooring? Second; does a dolphin/pile system need to be higher than surge? Floating docks are not being considered.
Seems to me that i want the piles to act only as line anchors.

 

[O C Diver]

I guess I would start by seeing what other people have. I would also see who is across the canal from you and if they plan to keep a boat in the water during a hurricane.

As far as holding the boat off the bulkhead in a hurricane, I might consider 2 lines coming across from the other side of the canal. They could either be attached to the bulkhead if no boat was there, or 2 large anchors dropped just off the bulkhead. In stead of running rope, I would consider 1/4" or 5/16" chain for the first 200'. This would eliminate a lot of the stretch and certainly handle storm surge on that side of the boat. If I were doing this with anchors and chain, it would be deployed for storms and recovered afterwards to maintain the integrity of the ground tackle and eliminate a possible snag hazzard. Depending on storm surge and normal canal depth, this could give you between a 10:1 and 20:1 scope for secure anchoring.

Ted

 

[diver dave]

I would install a boat hoist for the 95% of the time only thunderstorms or passing sport fish arethe problem..

And I would install a proper mooring (3) 60H or 90H Danforths or a screw in set anchor for the "BIG " named storms.

Should you change boats or homes the mooring can always be recovered

I like this idea:

I can put it in 50' or so off the seawall. 5 feet long; would be an interesting dive for me.

 

[PMF1984]

I appreciate all the replies. Two new questions from me. What is a screw mooring? Second; does a dolphin/pile system need to be higher than surge? Floating docks are not being considered.
Seems to me that i want the piles to act only as line anchors.

helix moorings.

 

[cmgreeniv]

I have put in helix moorings like the one pictured. I have pulled them out too, in only 25 knots of wind. After consulting an engineer it appears the ones you would need would be at least 8-10 feet into solid bottom. With16” helix... Not 5 feet. I use slide moors on my 100 foot wide canal. I have 18 foot tracks bolted to two sets of doubled pilings. My pilings are 9” tipped 35 footers. The main piling is 20 feet in the ground, the secondary piling is bolted to it and it is 10 feet in the ground. The two pilings are bolted in three places with 5/8 stainless. My boat is pointed straight down the canal. Take a look on google earth. 1040 Aquamarine Dr gulf breeze fl. It’s a 46 footer between the two boat houses. Pm me if you want pictures. I have the boat rigged where I have about two inches from the dock to stepping on the boat, no matter the tide. .

 

[ssobol]

If it were me and a class 3 hurricane was coming, I'd be moving my boat somewhere else.

 

[diver dave]

If it were me and a class 3 hurricane was coming, I'd be moving my boat somewhere else.

I agree totally. And forecasting has improved too. But, there are cases when moving becomes less than desirable. For instance, a storm path that rakes the coastline, S to N. And the boat is 6 kts with 50 feet of air draft. I just want as many options as possible.

 

[Redsky49]

I suspect that I know the area you are considering. Two points:


The holding in this area is frequently poor - ranging from a pudding-like mush, to a sandy, tiny shell-filled mud. A good holding sandy bottom can be found, but typically not in the canals.


You are right about the Matthew storm surge. At 6'-0" it was quite damaging, all up and down the coast. However, this was an actual blessing as we were preparing for a 10'-12' surge. If the hurricane track had not veered unexpectedly at the last moment it would have been catastrophic.
As it was, the damage was quite extensive, and salt water over-flooding of roads, golf courses, marinas, homes and hotels is still being repaired.

A fall back position of hauling out might be your best option.