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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Hull Speed
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Thread: Hull Speed Reply to Thread
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Topic Review (Newest First)
07-24-2007 10:08 PM
MeanzBeanz
Quote:
Originally Posted by sailingdog View Post
tim-

It is far more likely that the knotmeter is less accurate than the GPS.
Yes but we are talking about speed through the water (log), not over the ground (gps). Calibrating your knot meter with a two way run (average of the two allows for tide current etc.) using the GPS is going to give you the best shot at accurate boat speed measurements.

One caveat is if you are sure you are in completely slack water. Then the gps speed is good for hull speed... but then if your log is calibrated they should match.
07-24-2007 10:00 PM
MeanzBeanz You REALLY notice it at low speeds on certain sailboards as well
07-07-2007 01:49 PM
tomaz_423 One think is sure about fresh and salt water - (This if from my Laser sailing times): You need more speed to start planning in fresh water.
07-07-2007 12:04 AM
sailaway21 But the hull speed is a theoretical maximum based on hull length. Back in the real world; for a given hull, the more she displaces, the slower she is going to be. So, what little hull length gained by your fresh water allowance is mostly consumed by your greater friction losses from a greater surface area. And since the difference in potential speed is negligible to begin with, vis-a-vis fresh/salt, we've probably rung about all we can out of this topic.(g)



but I could be wrong. (g)
07-06-2007 09:30 AM
Sailormann
Quote:
Sure 'Canadian Sailcrafts made (make?) great boats' and all that but never mind the CS BS for now. Yes, there is more wetted surface in fw that slows you down. Yes, there is more waterline in fw that makes you faster. So, my question is; would a given monohull sailboat be faster in fresh water than salt water?
CS is making sailboats again and the world is a better place as a result As far as the fresh vs salt speed differential goes, there may be a difference, but it's going to be pretty negligible. If a displacement hull is sitting an inch or two lower in the water, it may go 1/10th of a knot faster, IF you are moving through perfectly flat water. I don't think that you could find that flat water anywhere though...
07-06-2007 03:45 AM
CapnHand Sure 'Canadian Sailcrafts made (make?) great boats' and all that but never mind the CS BS for now. Yes, there is more wetted surface in fw that slows you down. Yes, there is more waterline in fw that makes you faster. So, my question is; would a given monohull sailboat be faster in fresh water than salt water?
07-06-2007 01:48 AM
Sailormann When you sail fast boats (like a CS) in salt water you have to be careful because the boat moves through the water so fast that the friction makes the water boil and vaporise leaving salt crystals on your hull that slow it down....
07-06-2007 01:22 AM
pigslo But if they sink further in the fresh water, they will have a lot more wetted surface and slow down.
pigslo
07-05-2007 11:50 PM
RichH Not due to density but freshwater has a lower absolute viscosity (and lower surface tension) than seawater ... therefore faster in fresh water.
07-05-2007 11:44 PM
CapnHand So with fresh water being lower density than salt water, boats will float lower in fresh water. This gives them more water line. Will hull speed be greater in fresh water than in salt water?
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