How High Can My Boat Be for the Great Loop?

great loop boat height

Written by The Crew

Rev writes all the posts as "The Crew" at What Yacht To Do. It's just part of the First Mate duties. That list just keeps growing...

February 26, 2021

One of our most frequently asked questions via crew email is about the fixed bridge of 19.1′ or so south of Chicago on the Cal-Sag.  Remember that this 19.1′ height is at the high water datum, so you might see other heights (19.6, 19.7 etc.) on the internet or in discussions.

We get the questions mostly from folks who are getting ready to buy a boat and are concerned about air draft.

Manufacturers vary on what they use for air draft of a boat. So it can be confusing to say the least if you are trying to calculate your air draft to see if you can get under that bridge.

We passed under it on our Carver 504 with no problem. So let’s tackle air draft first, then discuss that pesky limiting bridge.

Let’s Talk About Air Draft

The Carver supplied data for air draft on our boat was listed as 18’ 9” as I remember, but we reconfigured our hard top when we did an upgrade to our electronics.

It is real important that you know the height of everything on your boat.

So here is ours as it stands now. This was measured with half fuel and water.

  • The top of our hardtop, specifically the uppermost portion of the upper portholes is 15’ 10” (our lowest attainable “get down to” air draft) .
  • There is another 1’ 3” for the new Garmin Radar slim mount and dome, that puts us at 17’ 1” (our lowest practical “get down to” air draft).
  • We have two VHF antennas (flexible) that can fold easily from the gunwales that are 19’ when up.

If we need to get down to the 17’ 1”, Rev goes up one of the portholes and she folds down three things:

  1. the anchor light which is normally at 17’ 4.5”
  2. the windvane which is normally at 18’ 2”
  3. the FM antennae (somewhat flexible) which is normally at 18’ 11.5”

Check Your Waterline

The Here’s to Us!! carries 688 Gallons of fuel and 350 Gallons of water.

So take a look at your waterline before and after you fuel up and see how much it varies. It really is not that much and I don’t think ours varies more than 2”.

And, there is a difference on how the boat sits between salt and fresh water, so there is that.

Overall, if you are down to calculating squeaking under by an inch or two, you are too close. A wake from a nearby boat could affect you more than that.

Now About That Bridge So Many Seem to Fret Over

It really is a non-event for 95% of loop boats and the other 5% fold down the anchor light or wind vane like we do.

I know someone who had to tie up and wait for the pool to go down and it was less than a day. He had a special custom built steel hull boat, so that was the issue.

I use Navionics a lot, so just last week I went there to see what they have posted on the bridge.

To be certain we are on the same page, the limiting bridge for the loop is identified on Navionics as the “Atchison Topeka and Santa Fe” bridge in text on the map. See picture below. It is at MM 300.6.

However, when I hover over the blue bridge icon and select the data, it is named “Burlington Northern Santa Fe Railroad Bridge.”

To locate this on the map, it is near the town of Lemont and about 450 feet north east of the Lemont HWY/Road Bridge which has a height of about 45 feet or so.

Here is the textual data cut and pasted from Navionics:  

Last Updated: Thu, 29 Dec 2016 12:14:18 GMT  Location: 41° 40.744′ N 87° 59.993′ W

Summary:  Bridge does not open, this is the low clearance point. Height is at normal pool, add or subtract if high or low water. It is reported that the easiest way to find out the clearance at this bridge is to call Lockport Lock at 815-838-0536.

To calculate the clearance go to: http://rivergages.mvr.usace.army.mil/WaterControl/stationinfo2.cfm?sid=IL02P&fid=&dt=S  Add 1.3 to the latest stage to convert to NGVD. Subtract the result from 597.2 low steel elevation (per Illinois waterway 2013 chart 101) and that will give you your bridge clearance. Bridge closed height (at high water): 19.1 Feet.

Bring Your Sense of Humor

So, you can go to the rivergages website (above link) and do the math exercise, which is interesting to do, but confusing because of the datum reference used (if you do this, have a sense of humor or an adult beverage), or just give the Lock a call and they can tell you the delta from normal pool.

From my own experience while trying to calculate whether or not to attempt downtown Chicago (limiting height there about 17.3 or so), I watched those gages fluctuate as much as 2 feet in a day. That is why we did not attempt downtown Chicago.

There is only about two miles of the downtown Chicago River worth seeing – so take a tour boat and you will see a lot more and relax while you are doing so.

You can read about our experience here or watch us explain what we did.

Feel free to pass this on to your boat broker, because they need to know as well. If you don’t have a broker in mind, send us note and we can recommend a good one for your needs and area.

Now stop worrying – this is pleasure boating after all!

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7 Comments

  1. Thanks for the sage advice on “How High??”. As is your custom you have a knack for explaining things in layman’s terms. Bravo Zulu!

    Reply
    • Yes, Sam has a gift. 🙂

      Reply
  2. Thank you!

    Reply
    • You’re welcome! Happy to help.

      Reply
  3. Hi Rev and Sam. Thanks for the good info. Do you have a good way to calculate your exact air draft? We have a pretty good idea, but would feel better if we actually knew for sure. We understand it can fluxuate depending upon fuel and water levels.

    Reply
    • Hi Susan! Measure midship near the tallest immovable object. Take a 2×4 and drill a hole at one end. Tape a level on the 2×4. Thread a plumb bob through the hole. Hold the 2×4 on that tallest immovable object so it is parallel to the waters surface by looking at the level. Adjust the length of the plumb bob until it touches the surface of the water. The distance from the bottom surface of the 2×4 to the tip of the plumb bob is the air-draft. Try to not use a plumb bob that is too heavy or a line that stretches too much. Of course, this is a team activity, so we would expect to see a YouTube video of you two having fun. And of course, with your resourcefulness, I am sure you can improve on the process! Some people have used laser distance measuring equipment, but I think it is more difficult. And nothing like using gravity and simplicity to solve the problem!

      Reply
  4. Thanks, Sam! We’ll try to make that YouTube video happen!

    Reply

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