Port Townsend, WA Bridge Clearance

[Marksman]

I don't know if this is the right place for this thread but.....

Has anyone with a 50ft. stick (or more) used the Port Townsend canal? The chart gives a clearance of 58ft. and I assume that means MHHW so my mast (50ft. off the water) should clear at any tide but I would feel much better having heard from someone who has actually been there.

Thanks,

[delmarrey]

My stick is 58' off the water and I've been under it several times. I just have to wait for a negative tide like a -1. I just stay in the center of the channel and go real slow. At -1 tide the channel is only 15' deep down the center.

With a 50' mast you should have no problem even at a +5' tide..........._/)

[Marksman]

Thanks Del, Looks like 7ft. for the low tide on fri. when I can be there, should be ok, math wise anyway. I just get nervous because it's imposible to judge distance looking straight up your mast. Bridges alsways look much closer than they realy are when your rigging is on the line.

[cal40john]

Originally Posted by delmarrey

My stick is 58' off the water and I've been under it several times. I just have to wait for a negative tide like a -1. I just stay in the center of the channel and go real slow. At -1 tide the channel is only 15' deep down the center.

With a 50' mast you should have no problem even at a +5' tide..........._/)

Why do you have to wait for a minus tide? Bridge heights are listed based on mean high tide, (OP was off by little, not MHHW) which looks to be about 7.7 feet. (Don't have my chart here, found a number on a NOAA website.) So at a zero tide the clearance is less than 66.

I'm 53 with the junk on top, I don't remember what the highest tide I've been through on.

Chart no. 1 bridge clearance reference:
http://www.nauticalcharts.noaa.gov/mcd/chart1/H.pdf

NOAA MHW Pt. Townsend (near bottom):

Published Bench Mark Sheet for 9444900 PORT TOWNSEND, ADMIRALTY INLET WASHINGTON

John

[delmarrey]

Originally Posted by cal40john

Why do you have to wait for a minus tide? Bridge heights are listed based on mean high tide, (OP was off by little, not MHHW) which looks to be about 7.7 feet. (Don't have my chart here, found a number on a NOAA website.) So at a zero tide the clearance is less than 66.

I have a 1-1/2 ft of antenna + an $800 wind instrument sticking up and just like to play it safe. When ever I go under it sure seems like I'm going to hit.

[ixtlan22]

On August 20th...we caught it on a rising tide...meaning that it was almost at peak on the inside of Marrowstone Island. The question we had was, "is the bridge clearance calculated at MHT or not? or...is there a built in variance to allow for antenna's and stuff? (I know what it says). So, we are 50ft from the waterline...plus antenna's, masthead light, and wind indicating cluster.

We figured we were probably 53ft 3in to 55ft something and the clearance was posted as 57 ft 7in at MHT (or so I assumed) but mean high tide was 8ft 6in by the book when we went through...so...do we add in the caculations and take a chance or take the extra couple of three extra hours going around the outside? I should note at this point we had been on the water over 10 hours coming up from Poulsbo and we had 35 knot headwinds out in the Staight with small craft advisories...and the still-incomming tide) and it was getting darker...anyway...We went through at 4:00pm on Thursday August 20th...and the tide chart shows an 8'.6" high tide at 5:55 PM. We shot through it (doing 6.9 knots by the way) and held our breath...and then cheered when we cleared it (but our relief was short lived as we got clobbered by 40-45 knot winds in the bay as we came out of the hole).... the hard part, other than the risky calculations, was that the channel is only 13ft deep in the very narrow middle...and less than 4 ft on each side of the banks just a few feet away from the boat with rocks and debris from the flow.

Just to let you know... that "theoretically" we didn't make the clearance. (We're 53' 3" with antenna's...the clearance was 57ft 7in...which would be okay until you add in the 8ft 6in tide...which should make us 61' 9" and the bridge clearance was as stated 57' 7" on our new Navionics Electonics chart as well as our paper chart dated 1998). BTW we cleared the bridge easily (well, looking up it didn't look all that clear!).

Hindsight...we probably shouldn't have taken the bridge and roughed it on the outside. When we left on the return trip to Poulsbo we elected to go around...we didn't need the stress. When we shot through we we're doing 6.8 kts under bare poles with the throttle pulled all the way off...after we entered the channel there's really no "re-deciding not to go" 'cause once your in the channel your committed (no room to turn around) and I don't know about other's... but our boat don't back worth &^%$#!...(it's S-1 rotate for you flying types).

Our new motto...when it doubt...stay out...we're cruisers...not racers...and that goes for wind and weather too!

[Marksman]

Thanks again everyone, it's funny how even when all published indications show everything is cool, there is much relief in hearing others confirm from experience that it is indeed OK. I guess I have seen too many statistics that proved to be inacurate when put into practice. When I was young, foolish and indestructible I used the canal often, and never even thought about how tall my mast was (it was well under 50ft. back then all though I never messured it) or the state of the tide. Most things seem much more important now and I don't relish the idea of dodging falling masts and flying wires. These things are best left in stories from other sailors. Well the Admiral gets her way, we'll go shopping in PT!

[cal40john]

I believe that you did your math wrong. You cleared by your numbers. Your mast is 55' off of the water. Bridge is 57' at MHW. MHW at Pt. Townsend is 8.5' (All your data) If you went through at a + 8.5' tide, the same as MHW, the bridge clearance is 57' off of the water, you cleared by two feet. Now if the tide were at +10.5' then the clearance of the bridge at that water level is 55', and you're theoretically at the same height of the bridge.

What you did wrong was compute your mast height from chart datum, then used bridge clearance from MHW.

By the way, I think the 8.5 number is MHHW or some other datum, and MHW is actually 7.7', but I haven't been to the boat to check the chart. So that puts you most of a foot closer to hitting the bridge.

Also Garmin Bluecharts does not have MHW in their charts they had some other high water datum. When I asked them about it they said we're just an aid, you should always refer to the original charts. If you're refering to software charts make sure you know exactly what number you're looking for.

John

Originally Posted by ixtlan22

On August 20th...we caught it on a rising tide...meaning that it was almost at peak on the inside of Marrowstone Island. The question we had was, "is the bridge clearance calculated at MHT or not? or...is there a built in variance to allow for antenna's and stuff? (I know what it says). So, we are 50ft from the waterline...plus antenna's, masthead light, and wind indicating cluster.

We figured we were probably 53ft 3in to 55ft something and the clearance was posted as 57 ft 7in at MHT (or so I assumed) but mean high tide was 8ft 6in by the book when we went through...so...do we add in the caculations and take a chance or take the extra couple of three extra hours going around the outside? I should note at this point we had been on the water over 10 hours coming up from Poulsbo and we had 35 knot headwinds out in the Staight with small craft advisories...and the still-incomming tide) and it was getting darker...anyway...We went through at 4:00pm on Thursday August 20th...and the tide chart shows an 8'.6" high tide at 5:55 PM. We shot through it (doing 6.9 knots by the way) and held our breath...and then cheered when we cleared it (but our relief was short lived as we got clobbered by 40-45 knot winds in the bay as we came out of the hole).... the hard part, other than the risky calculations, was that the channel is only 13ft deep in the very narrow middle...and less than 4 ft on each side of the banks just a few feet away from the boat with rocks and debris from the flow.

Just to let you know... that "theoretically" we didn't make the clearance. (We're 53' 3" with antenna's...the clearance was 57ft 7in...which would be okay until you add in the 8ft 6in tide...which should make us 61' 9" and the bridge clearance was as stated 57' 7" on our new Navionics Electonics chart as well as our paper chart dated 1998). BTW we cleared the bridge easily (well, looking up it didn't look all that clear!).

Hindsight...we probably shouldn't have taken the bridge and roughed it on the outside. When we left on the return trip to Poulsbo we elected to go around...we didn't need the stress. When we shot through we we're doing 6.8 kts under bare poles with the throttle pulled all the way off...after we entered the channel there's really no "re-deciding not to go" 'cause once your in the channel your committed (no room to turn around) and I don't know about other's... but our boat don't back worth &^%$#!...(it's S-1 rotate for you flying types).

Our new motto...when it doubt...stay out...we're cruisers...not racers...and that goes for wind and weather too!

[JEFFREY20]

Bridge heights are now based on HAT (Highest astronomical tide).

[cal40john]

Originally Posted by JEFFREY20

Bridge heights are now based on HAT (Highest astronomical tide).

I think it depends on who's making the chart.

For U.S. it's MHW: Mean High Water
http://www.nauticalcharts.noaa.gov/mcd/chart1/H.pdf


For Canada it's HHWLT: Higher High Water Large Tide
see figure 2 on the below link
CHS-Vertical Datums



Found this non-government link for U.K.
Sailtrain: Navigation and Chart work, Chart information.
Appears to be mostly MHWS: Mean High Water Springs

But:
Heights above H.A.T.

In some parts of the World the height of objects such as bridges or cables that cross waterways are given above Highest Astronomical Tide (HAT). These are normally area with small tidal ranges such as the Baltic or Mediterranean.
On chart 3 and 4, below the title there is a reference to this. It states, "Vertical clearance heights are above Highest Astronomical Tide." Whilst this is unusual in UK waters, you will encounter this in other parts of the World, so the rule is, always read the text below the titles of charts when you move to an unfamiliar area!