Long-term anchoring in tidal river

[Nate M]

I'm building a custom 28' x 13' houseboat, and looking to be able to leave it unattended for days at a time. I'd be anchoring on the Potomac, which is a tidal river, and was wondering what kind of set up you'd recommend. I'm thinking of using two 16lb danforth anchors with 10' of 1/2" galvanized chain and 5/16 chain connecting the rest of the way from a swivel. Your thoughts would be much appreciated!

[pjShap]

If you plan to be anchored in the same place most of the time, a mooring provides far more security. If that's an option in your location, I can recommend Inland Marine. They gave me excellent advice and a well-designed system. Your anchoring plan sounds substantially too light to me. I'd be uncomfortable overnighting with that setup.

[Montanan]

Excellent question you ask. Wise to seek guidance for achieving proper ground tackle.

Danforth:

Weight: 16 lbs
Estimated holding power: 1300 lbs
Designed for boats up to 36 feet (boat size based on 20 knot winds)
Recommended chain size: 4' x 1/4"
Dimensions:
Height (A): 31-3/4"
Stock Length (B): 24"
Fluke Length (C): 16-1/2"

Basically in my opinion that is what is called a lunch hook, not an anchor of any substance to handle even moderate winds or currents, let alone a combination of winds or currents. The power of the wind is a cubic function of velocity and you are dealing with a substantial House Boat which can impose very large windage loading.

Swift current can of course induce strong pulls and can cause erosion of river bottoms exposing even deeply set anchors to loss of holding power.

Of keen issue in a river or tidal current zone is that the rode and a hull [particularly pontoon style or catamaran style] can readily collect flotsam and debris by entanglement and then the load on the rode become massive, pulling the boat downward submerging into the current increasing load dramatically, and regularly dragging even massive moorings along the bottom, and for certain can drag temporary / dropped anchors, including oversized temporary dropped anchors. Debris can be extensive during high water flows and after storms, think complete trees and branches floating down stream. If your rode gathers debris the vessel can easily be lost. Many youtube videos of such in rivers and tidal basins after a storm which are painful to watch. It requires urgent and prompt action, cutting the rode to allow the vessel to break free and then the boat towed to a safe mooring.

Removal of the vessel is appropriate before the river is subject to ice flow.

Ice makes it challenging to keeping a float and rode on the permanent mooring. Typically the rode is removed and reset to the mooring in the Spring when icing is no longer an issue. Ditto as to if debris flows are expected.

If you know that a river is being subject to storm flow, it is imperative that you keep watch, and have emergency boat tow assistance available for the duration of the debris laden river or tidal flow. This is very hard to accomplish during a storm or at night.

Since the current is tidal the boat will swing on a single drop style anchor causing the anchor to upset and swivel about increasing the chance that it will drag and / or the rode will become entangled. If the boat is to be held fore and aft so as to not allow swinging on anchor it will have two rodes which can entangle flotsam and debris, and of course two large ground tackles as only one is holding the vessel at anyone time.

Definitely recommend discussion with a competent permanent mooring placement company, not a typical boater's scope of capability.

A deeply set screw anchor [requires a dive team to position] or a heavy Pyramid [Dormor] type mooring will provide the best permanent mooring holding power. A block or mushroom mooring are not as secure as the deep set screw anchors or the Pyramid style which tends to continue to deep set due to its pointed end loading.

A substantial swivel and a heavy duty chain rode with routine inspection [e.g., six months by a professional diver, typically the same as the person that set the permanent mooring] and replacement of rode components are called for, e.g., every two years.

Reference: https://www.jamestowndistributors.co....do?docId=1107

[Nate M]

thanks for the feedback. The setup seemed light to me, so I appreciate your opinions. I really don't want to do a permanent mooring, as there are a number of spots I'd like to move back and forth from. I live on the water, so going and checking on it isn't really a big deal. Any suggestions on a better system given that i'd like to be able to relocate the rig? Thanks again!

[JPA Cate]

Hi, Nate,

I'm assuming (yeah, I know that's risky in itself) that occasionally they are strong winds on the Potomac. For your anchoring setup, if you both want to move around and have security, it's going to be important to understand how much windage your houseboat has. In case you don't know, the force of the wind on the boat goes up as a geometric progression (imagine the sharp curve rise typical of corona virus infections). What this means for you is that your ground tackle is going to have to cope with all your windage, plus your displacement in whatever the highest winds are that you can reasonably expect to encounter there. (Info available from NOAA, probably.)

Our sailboat displaces roughly 12 tonnes. Her primary anchor is an approximately 60 lb. Manson Supreme (27 kilos). Our chain is 10 mm, and we use all chain rode. We have perhaps one half the windage of your boat. We do anchor in rivers, but tend to up anchor every third day, because of the tidal cycles occasionally causing chain to wrap around anchors. We find this preferable, because of the ease of getting up only one anchor. In sustained winds over 50 knots, we have occasionally had to put out two anchors to keep from dragging. That depends mostly on the nature of the bottom in the actual spot where the anchor is buried.

I predict that your plan will be an interesting thing for you to do. Rivers are notorious for biological and other trash getting swept over the bottom, fouling it for anchors. One of your challenges will be how to deal with your set up so that it is easy to deploy and retreive. I think the Danforths are good for the job, but are probably too light. You really need to find a chart that will give you what their holding power is.

*********

If you plan to spend weeks in one location before moving, you will still need to solve the ground tackle handling problem. We have drug during flood conditions, too much weed and tree trunks come down the river and go crosswise on your chain. We tied a knife to our boat hook to slice through the weed, and eventually were able to get the log off with the boat hook. I guess what I'm really saying, is you need to carefully think through how you will handle the disadvantageous things river life brings, and be prepared to get your tackle up and out of the way to keep the boat safe. Strong winds can make life very difficult for house boats.

Ann

[GordMay]

Greetings and welcome aboard the CF, Nate.

[skipmac]

While it is only 28' long if shaped like a typical houseboat it will have the windage of a 40' or longer sailboat. 16 lb Danforth is way, way too small and not the anchor I would choose for a river with reversing tide unless you drop one in each direction.

The reason, Danforth anchors are not the best at resetting when the pull reverses, the anchor pulls loose and has to reset to hold in the opposite direction.

For leaving a boat like this unattended in a reversing current area I would go with one 45 lb new gen anchor like Mantus, Rocna, etc.

[Montanan]

Wind power - cubic curve.

Do the math, the force becomes amazingly powerful at moderate speeds.

The Danforth specs were at light 20 knot wind. And every bottom has different holding power capabilities. The specs are calculated under optimal holding conditions not less than optimal.

[Nate M]

Thank you all again for your valuable feedback. Such a pleasant surprise to get such good information from this excellent community! Based on your responses, I've revised the plan to include the following. I would of course be more comfortable on a proper mooring, but given my needs and use I'm proposing the following:

Swivel
https://www.mantusmarine.com/product/s1-mantus-swivel/

Anchor
https://www.mantusmarine.com/product...mantus-anchor/

½” galvanized bottom chain

5/8 rode chain

If recommended, i might also use the 16lb danforth anchor

For reference, this is the boat I'm building:

https://mooreconstructiongroup.com/houseboat

Some additional notes based on responsesThanks again for your insight!

[rhubstuff]

You don't say where on the Potomac you will be located, but be aware that it is subject to serious flooding, which can raise the water level several feet. I agree with the concern others have expressed about logs and other floating debris, and I would definitely NOT moor your boat anywhere in the main river current. A single log or large patch of weeds/sticks/grass/etc. will increase the drag on your mooring system exponentially, so find a place to moor that is protected from the main river current, and make a generous allowance for the increasing depth of water during floods. I would also suggest that vandalism is probably going to be your biggest problem, unless you spend most of your time aboard.

[thinwater]

Originally Posted by Montanan

Wind power - cubic curve.

Do the math, the force becomes amazingly powerful at moderate speeds.

The Danforth specs were at light 20 knot wind. And every bottom has different holding power capabilities. The specs are calculated under optimal holding conditions not less than optimal.

Sorry, but this is not correct. Wind FORCE is a square function. Wind POWER is a cube function (since power = force x distance).


The scale is in energy units, not force.
http://drømstørre.dk/wp-content/wind...s/enrspeed.htm


(I've done a lot of anchor testing--just needed to make sure I was not loosing my marbles)

[Don C L]

With what you are describing I'd sleep fine with two Danforth 20Hs if they are set well, have a scope of around 7 to 1 at high tide, AND importantly, your cleats or posts are really stout too! With 2 35Hs it will be there until the next millenium. You didn't how much will your houseboat weigh and how much scope, (that is the ratio of length of rode to depth of water,) you'll have. And is this on two pontoons? If the boat is a barge and presents a large flat surface to the tidal current, then that's a number that may need to be calculated.

[Nate M]

Thanks again guys, it's near DC on the Potomac, currently at Washington Sailing Marina. There are actually four runs of pontoons (updated on blog below).

Based on your feedback (again, thank you all very much) I realized it would be nice to have a proper mooring to store it at least over the winter season. There are a ton of moorings right where i'd love to keep it part-time (Belle Haven Marina), but they tell me there are over 500 people on the wait list to get one... I went by today and find it strange because half the boats on moorings look like they barely float. Unfortunately, there are no other locations anywhere else.


https://mooreconstructiongroup.com/houseboat

[Montanan]

Originally Posted by thinwater

Sorry, but this is not correct. Wind FORCE is a square function. Wind POWER is a cube function (since power = force x distance).


The scale is in energy units, not force.
http://drømstørre.dk/wp-content/wind...s/enrspeed.htm


(I've done a lot of anchor testing--just needed to make sure I was not loosing my marbles)

Oops, was thinking power from the days when we designed wind turbine generators.

Wind force on the windward surface is indeed more along the square function but with the addition of drag forces the total force is more than just the force on the windward side of the barn.

When moving air - wind - is stopped by a surface - the dynamic energy in the wind is transformed to pressure. The pressure acting the surface transforms to a force

Fw = pd A

= 1/2 ρ v2 A (1)

where

Fw = wind force (N)

A = surface area (m2)

pd = dynamic pressure (Pa)

ρ = density of air (kg/m3)

v = wind speed (m/s)

Note - in practice wind force acting on a object creates more complex forces due to drag and other effects.

[Montanan]

The Lincoln Memorial Reflecting Pond on the Mall looks rather empty of boats and promising as to lacking currents or flooding; great views, well policed neighborhood. Not real deep so easy to let out adequate rode for 7 to 1 scope.