Hull & Keel Question

[kevintacher]

SOS HELP!!! Expert opinion requested... so the 1996 Hunter Legend sailboat has a 4.9+ foot draft with a wing keel. The dock we are looking at has a 5.5 foot depth at low tide out 5 feet from the dock.

So is this good enough, or is this measured with an empty boat and when you add people, fuel and storage it becomes that you need more depth? Or is this calculated fully loaded per manufactures specs? We've been told we need 6.5 feet minimum but the draft is only 4.9 feet per the manufacture specs...

Question One: Is the 4.9 draft fully loaded or empty or part in between. Also at fully loaded capacity what would the actual draft be?

Question two: Does anyone know the measurement from the top of the hull to the bottom of the keel so we can measure while in the water looking at the hull line. Maybe that is the 4.9 marker... yes I should have measured before it went in the water.

Any help is greatly appreciated!

Captain Kevin!

 

[Roland5048]

Welcome aboard, Kevin. I'm no expert, but from what you described, it looks like you should be OK. A fully loaded boat may sit a little lower than an empty one. How much depends on the boat. I don't know your tidal range but here on the Fl. west coast, we have a negative tide about twice a month on the new and full moons. Range can be anywhere from 1 to 6 inches below mean low. Like me, you might be sitting in the mud for a couple of hours. Really no big deal. I wouldn't back in, however. Rudders don't like to sit in the mud

 

[pateco]

Hey Kevin,
Great to see you here. I don't think you will have any problems, but If you like, I can stop by and look at the dock with you.

My dock too is a little shallow at low tide. I made a simple air lift with some PVC pipe and a compressor, and was able to clear an area under my keel, so that even at the lowest tide I don't hit bottom. It only took a couple of hours diving under the boat. I can't leave or return at low tide, but there is usually only about a 2 hour window I have to avoid.

 

[FastOlson]

Even with all of your friends and some extra beer on board, doubt that you'd lower the waterline by any more than an inch, if even that much. Check your boat's displacement #, and see what % of that the poundage of crew and supplies might be.....

Loren

 

[Benny17441]

Like some have suggested the important factor is the composition of the bottom. In Ft. Lauderdale I would surmise you are looking at a sandy bottom. Like Roland indicates there is no problem for the boat sitting on the bottom for a couple of hours. The warning about not backing the boat in is most valid as you do not want the rudder touching bottom. Probably the most annoying problem that you might experience at that dock might be the inability to depart or arrive during low tide hours. I have played the tides game in the past and it is a pain to have to schedule your sailing around them. How is the rest of the approach? As far as calculating the draft I have calibrated the depth meter to provide a 6 inches margin of safety under the keel. By intentionally soft grounding the boat I was able to calibrate the meter to 0 and then added 6 inches to allow for load variations. The specs for my keel are 4'6" which I can round to 5' when comparing to chart depths. We have a low spot in our canal and over time I have developed that I need a reading of 4.2' of water under the keel at our dock to be able to clear by inches that low spot. We have gotten used to watch the tides and South winds that might empty the Bay but is not as bad as it once was at a South Florida location.

 

[jibes138]

How about some math? Back of the napkin style. 40 foot boat, maybe a 32 foot waterline? so 32 feet times 12 foot beam times 1/2 = 192 square feet times .0833 displacement = 16 cubic feet of water displaced for a 1 inch change in draft. 16 times 62 pounds per cubic foot = 992 pounds. So you can put 992 pounds on your boat to make it sink 1 inch in the water. This is a very rough calculation but it will give you some idea. I assumed 1/2 the area of a rectangle of your length by beam. In fact your area might be greater if your beam is wider or your waterline length is longer or the fraction is greater than 1/2. This would mean a lot more weight to sink 1 inch. Fuel and water and water in your bilge can add a lot of weight. 50 gallons of water is about 400 pounds for instance.

 

[CloudDiver]

Judging from the picture posted, you aren't even sitting in the water all the way up to the designated WL to begin with. Look at the scum line... So I echo the statements above, it takes a LOT of additional weight to make a hull sit another inch deeper at the DWL. You should have few or no issues with the depth as it sits, especially if you have soft sandy bottom.
This got me thinking, because I can resist the urge to over engineer the crap out of stuff... If I had a similar issue and it was determined I needed to clear material from the bottom to ensure draft clearance; I'd go to a gov't auction and get an over-stock or retired Navy portable fire fighting pump or rig something similar. Place the pump engine on rigid fiberglass dinghy (Paint Punt) and dive the bottom using 2 inch hose to suction away bottom material like the gold miners do in Alaska waters. Once I proved the concept for the safety of my own slip draft, I'd get the Marina to pay me to clear out the rest of the place. people hate parting with ca$h so maybe free slip for life?

 

[Gunni]

Keep shopping, that isn't enough water. You will be bumping, sticking, and generally rubbing the barrier coat off your keel. And wings don't slide, they plow. I have the same draft and won't take a slip with less than a guaranteed 6 ft of water. Assume a draft of 5 ft.

 

[pateco]

If I had a similar issue and it was determined I needed to clear material from the bottom to ensure draft clearance; I'd go to a gov't auction and get an over-stock or retired Navy portable fire fighting pump or rig something similar. Place the pump engine on rigid fiberglass dinghy (Paint Punt) and dive the bottom using 2 inch hose to suction away bottom material like the gold miners do in Alaska waters. Once I proved the concept for the safety of my own slip draft, I'd get the Marina to pay me to clear out the rest of the place. people hate parting with ca$h so maybe free slip for life?

I made a simple air lift with some PVC pipe and a compressor, and was able to clear an area under my keel, so that even at the lowest tide I don't hit bottom. Below is a rough idea of how they work.

 

[Finally Free]

Kevin, don't know if this helps or muddied the water but here goes. We have a 1995 Hunter 42 Passage. Design draft is 4'11". But with nothing but fuel and water aboard she draws 5'5". Since our boats are basically the same year, take design draft with a grain of salt and figure out a way to measure it (we measured before launch).