I need to adjust the knotmeter on my Catalina 27 {paddlewheel type}. Hull speed reads 7.4 when motoring at full thottle and the hull speed is 6.3 so I am thinking it's off a bit. There is a screw on the back of the meter for adjustments so I was thinking of running the boat under power at full thottle and adjusting the screw back a little to read 6.3 knots {hull speed}. I will take currents and tides into consideration while doing this. My question is, is there a more accurate way of doing this with out a gps? As always, thank you for your help and ideas, Jack
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instruments
You don't say what type of instrument it is but I have used the internet to contact my instrument servicing folks (Data Marine, DMI.com) with good results. Search on Yahoo or some other search engine for your make of instrument and contact the folks there for advice. The guys at DMI were very, very helpful when I had problems with my depth sounder a couple of seasons ago. Good luck, Mike
You don't say what type of instrument it is but I have used the internet to contact my instrument servicing folks (Data Marine, DMI.com) with good results. Search on Yahoo or some other search engine for your make of instrument and contact the folks there for advice. The guys at DMI were very, very helpful when I had problems with my depth sounder a couple of seasons ago. Good luck, Mike
measured mile
Is there a measured Nautical Mile in your area? Look over your charts or maybe the coast guard or even better your local Power Squadran in your area should know. Sometimes you can use land references. speed * time = distance.....
Is there a measured Nautical Mile in your area? Look over your charts or maybe the coast guard or even better your local Power Squadran in your area should know. Sometimes you can use land references. speed * time = distance.....
There's a measured mile in Tampa Bay
Used to be four of them actually, set up in a square off Pinellas Point. I don't if all of them are still there but I know at least two are. I'm not at the boat right now so I can't give you coordinates. If your unit is a Datamarine (like mine) you'll basically be calibrating your log (not speed) against a known distance. My manual has me go out a mile with the switches on the back set to 'record' this, then put it on 'hold' as you turn around and then 'record' again for return to compensate for wind and current. When finished, you'll then turn the screw to adjust the displayed distance. You can also do it with a GPS by setting an arbitrary mark to go out and back from. Most GPS's read in hundredths of a mile which is a little more than a boatlength over a 2 mile run. It's not perfectly accurate but it's pretty darn close. If you're sure you don't have any wind or current you can just do a one way trip although it's not as accurate.
Used to be four of them actually, set up in a square off Pinellas Point. I don't if all of them are still there but I know at least two are. I'm not at the boat right now so I can't give you coordinates. If your unit is a Datamarine (like mine) you'll basically be calibrating your log (not speed) against a known distance. My manual has me go out a mile with the switches on the back set to 'record' this, then put it on 'hold' as you turn around and then 'record' again for return to compensate for wind and current. When finished, you'll then turn the screw to adjust the displayed distance. You can also do it with a GPS by setting an arbitrary mark to go out and back from. Most GPS's read in hundredths of a mile which is a little more than a boatlength over a 2 mile run. It's not perfectly accurate but it's pretty darn close. If you're sure you don't have any wind or current you can just do a one way trip although it's not as accurate.
Hull Speed
Your knot meter may be correct...it's not difficult to exceed hull speed, that's what you are probably doing. If you are going to actually try to measure your boat speed, tides and currents are irrelevant as you want speed through the water. Don't use the gps, that will give you speed over ground...no help in this case. Have fun!
Your knot meter may be correct...it's not difficult to exceed hull speed, that's what you are probably doing. If you are going to actually try to measure your boat speed, tides and currents are irrelevant as you want speed through the water. Don't use the gps, that will give you speed over ground...no help in this case. Have fun!
exceding hull speed?
Bert, I always thought you couldn't push a sailboat faster then it's hull speed with a motor. I thought the only way to excede hull speed was surfing down a wave. I mean, we are only talking about 1 knot or so. Thanks for the mile marker idea, my knotmeter doesn't calibrate distance, just knots. Maybe it is working correctly,getting the exact speed with a paddlewheel can't be a perfect science. Anyone else want to back up Bert's idea? Can you excede hull speed with the stock motor? thanks for your help, Jack
Bert, I always thought you couldn't push a sailboat faster then it's hull speed with a motor. I thought the only way to excede hull speed was surfing down a wave. I mean, we are only talking about 1 knot or so. Thanks for the mile marker idea, my knotmeter doesn't calibrate distance, just knots. Maybe it is working correctly,getting the exact speed with a paddlewheel can't be a perfect science. Anyone else want to back up Bert's idea? Can you excede hull speed with the stock motor? thanks for your help, Jack
Jack
It depends.
Sorry. Hull speed is a mathematical/theoretical point that guesses when the bow wave meets the stern wave. At that point most of the energy driving a displacement hull pushes it down, not forward and faster. Hope that's close enough for the purists here. I haven't put that one to paper before. Anyway, there are a lot of variables such as horsepower, weight and hull form. Well, actually, that's about it.
It depends.
Sorry. Hull speed is a mathematical/theoretical point that guesses when the bow wave meets the stern wave. At that point most of the energy driving a displacement hull pushes it down, not forward and faster. Hope that's close enough for the purists here. I haven't put that one to paper before. Anyway, there are a lot of variables such as horsepower, weight and hull form. Well, actually, that's about it.Hull Speed
Jack...this is not gospel, just telling you what my boat does. In a good blow, it will exceed hull speed by over two knots, when motoring, if I run at full throttle, it will also exceed hull speed. As Fred mentioned, I learned the term as "theoretical" hull speed. Who knows, maybe my knot meter's incorrect!
Jack...this is not gospel, just telling you what my boat does. In a good blow, it will exceed hull speed by over two knots, when motoring, if I run at full throttle, it will also exceed hull speed. As Fred mentioned, I learned the term as "theoretical" hull speed. Who knows, maybe my knot meter's incorrect!
dropping stuff overboard
With a stopwatch and something environmentally friendly to drop overboard you can calculate your speed. Convert feet per second into knots. Roughly (as my calculator isn't handy) a knot is 6000' per hour this would then be 100' per minute. or 1.66 ' second. So drop a piece of bread overboard at the bow and time it to the stern. Say 30' in 15 seconds for 4'/second divided by 1.666 to get roughly 2.5 knots. Use a calculator to be more precise. Tom
With a stopwatch and something environmentally friendly to drop overboard you can calculate your speed. Convert feet per second into knots. Roughly (as my calculator isn't handy) a knot is 6000' per hour this would then be 100' per minute. or 1.66 ' second. So drop a piece of bread overboard at the bow and time it to the stern. Say 30' in 15 seconds for 4'/second divided by 1.666 to get roughly 2.5 knots. Use a calculator to be more precise. Tom
One more
Chris, just curious, how do you measure the current if it's your knot meter you're trying to calibrate? I suppose you could use Tom's bread trick. Bert, covering a reciprocal course using either mile markers or a GPS negates current and/or wind effects. If there's wind or current it's the only way to calibrate, assuming you're out of bread Here's my take on hull speed. Waves with a certain distance between crests travel at a calculable speed (please don't ask me to calculate it.) As a hull moves through the water at a certain speed it moves (displaces) the water around it at that speed creating waves with crests a certain distance apart. As the speed increases the crests move farther apart. At some point, the speed of the hull produces waves that are as far apart as the waterline is long. The hull is cradled in the trough between the bow and stern crests. The hull has no means of pushing the waves farther apart and is then 'trapped' between the crests. This is it's hull speed. If the hull is subjected to enough driving force it will then begin to climb up the bow wave and at some point will completely escape the bow and stern waves (planing.) Exceeding hull speed requires enormous power relative to boat displacement but given the push of a following wave (surfing) and/or sail power off the wind a displacement sailboat can be pushed into the bow wave enough to exceed 'theoretical' hull speed. As many on this board have, we've surfed our H40 into double digits with the spinnaker up in 'brisk' conditions but it's a rare ride. If you've done this you've probably noticed the abnormally large (and spectacular)spray coming off the bow as it is being shoved into that 'immoveable' bow wave. Bob Perry has discussed (on, ahem, another board) that although many use the quick and dirty formula of 1.34 x square root of LWL for hull speed calculation, it's actually a little more fluid that that (sorry) and that there are more factors at play, many of which I don't pretend to understand. It's highly unlikely anyone with a typical diesel powered cruising sailboat generates the power needed to exceed their hull speed in flat water by motoring alone. How'd I do?
Chris, just curious, how do you measure the current if it's your knot meter you're trying to calibrate? I suppose you could use Tom's bread trick. Bert, covering a reciprocal course using either mile markers or a GPS negates current and/or wind effects. If there's wind or current it's the only way to calibrate, assuming you're out of bread
Here's my take on hull speed. Waves with a certain distance between crests travel at a calculable speed (please don't ask me to calculate it.) As a hull moves through the water at a certain speed it moves (displaces) the water around it at that speed creating waves with crests a certain distance apart. As the speed increases the crests move farther apart. At some point, the speed of the hull produces waves that are as far apart as the waterline is long. The hull is cradled in the trough between the bow and stern crests. The hull has no means of pushing the waves farther apart and is then 'trapped' between the crests. This is it's hull speed. If the hull is subjected to enough driving force it will then begin to climb up the bow wave and at some point will completely escape the bow and stern waves (planing.) Exceeding hull speed requires enormous power relative to boat displacement but given the push of a following wave (surfing) and/or sail power off the wind a displacement sailboat can be pushed into the bow wave enough to exceed 'theoretical' hull speed. As many on this board have, we've surfed our H40 into double digits with the spinnaker up in 'brisk' conditions but it's a rare ride. If you've done this you've probably noticed the abnormally large (and spectacular)spray coming off the bow as it is being shoved into that 'immoveable' bow wave. Bob Perry has discussed (on, ahem, another board) that although many use the quick and dirty formula of 1.34 x square root of LWL for hull speed calculation, it's actually a little more fluid that that (sorry) and that there are more factors at play, many of which I don't pretend to understand. It's highly unlikely anyone with a typical diesel powered cruising sailboat generates the power needed to exceed their hull speed in flat water by motoring alone. How'd I do?Find flat water and ...
make it read the same as your GPS. Make sure there is no tidal current and after the first check turn 180 degrees and check again. Borrow a portable unit if you don't have one. Tweek the screw until they are close I've seen local charter boats calibrated was too high so the charter fools can tell their buddies they sail 8-9 knows on 30 footers
make it read the same as your GPS. Make sure there is no tidal current and after the first check turn 180 degrees and check again. Borrow a portable unit if you don't have one. Tweek the screw until they are close I've seen local charter boats calibrated was too high so the charter fools can tell their buddies they sail 8-9 knows on 30 footers
Your knot meter tells you your
speed through the water. for navigation purposes you have to know your speed made good. or your speed over the ground. For the Tampa area find a tide current chart, they are usually made to cover a complete tide cycle and show the speed and direction of the current for each hour of flood and ebb. They also show the periods of slack water. If you choose a time of slack water and run a measured course you should be able to get an accurate readout. The charts will also show you the areas with the smallest currents.
speed through the water. for navigation purposes you have to know your speed made good. or your speed over the ground. For the Tampa area find a tide current chart, they are usually made to cover a complete tide cycle and show the speed and direction of the current for each hour of flood and ebb. They also show the periods of slack water. If you choose a time of slack water and run a measured course you should be able to get an accurate readout. The charts will also show you the areas with the smallest currents.
The manual says
My knotmeter manual says to make two timed runs (one out and one back) over a known course and average the speeds. This factors out the current and wind and gives you a true speed through the water. To do this get out your nav charts and find a measured mile nearby. If one is not handy then use two fixed lights. You can get the distance from the chart and the time from your watch. The math looks like this: Average time = (T1+T2)/2 Speed through the water = Distance one way/average speed. What the manual does not say is to note the reading on the knotmeter while you are doing the runs so you can adjust the darn thing. I found that by doing the runs under power I could duplicate the speed by duplicating the RPM. Then it is just a mater of starting the run again and adjusting the knotmeter to the right setting. If the wind changes speed or direction accuracy will suffer.
My knotmeter manual says to make two timed runs (one out and one back) over a known course and average the speeds. This factors out the current and wind and gives you a true speed through the water. To do this get out your nav charts and find a measured mile nearby. If one is not handy then use two fixed lights. You can get the distance from the chart and the time from your watch. The math looks like this: Average time = (T1+T2)/2 Speed through the water = Distance one way/average speed. What the manual does not say is to note the reading on the knotmeter while you are doing the runs so you can adjust the darn thing. I found that by doing the runs under power I could duplicate the speed by duplicating the RPM. Then it is just a mater of starting the run again and adjusting the knotmeter to the right setting. If the wind changes speed or direction accuracy will suffer.
Calibrate with a GPS
Why not just take out a GPS and callibrate the knot meter to what the GPS is proviving as a read out?
Why not just take out a GPS and callibrate the knot meter to what the GPS is proviving as a read out?
GPS reads MPH
My GPS reads Miles Per Hour. Knot meter reads Knots per hour. Does your GPS have a KPH/MPH select? I don't think that mine does.
My GPS reads Miles Per Hour. Knot meter reads Knots per hour. Does your GPS have a KPH/MPH select? I don't think that mine does.
Liam, There should be a settings screen
on your GPS that allows you to specify units and formats. Also for others, GPS gives you SOG (speed over ground) knotmeter is STW (Speed Thru Water) any current and they are not the same.
on your GPS that allows you to specify units and formats. Also for others, GPS gives you SOG (speed over ground) knotmeter is STW (Speed Thru Water) any current and they are not the same.
Can't do that
You can't calibrate a knotmeter with a GPS because they measure two different things. A GPS measures speed over ground, while a knotmeter measures speed through the water. So, for example, if you are sailing with a 3 knot current and your knotmeter read 4 knots, you'd actually be going 7 knots an hour over the ground. If you were going against the current, your knotmeter would read 7 knots an hour, but you'd only be going 1 knot an hour over the ground. (I'm sure I messed the numbers up and someone will correct me.) Any marine GPS should be able to toggle between mph and kph.
You can't calibrate a knotmeter with a GPS because they measure two different things. A GPS measures speed over ground, while a knotmeter measures speed through the water. So, for example, if you are sailing with a 3 knot current and your knotmeter read 4 knots, you'd actually be going 7 knots an hour over the ground. If you were going against the current, your knotmeter would read 7 knots an hour, but you'd only be going 1 knot an hour over the ground. (I'm sure I messed the numbers up and someone will correct me.) Any marine GPS should be able to toggle between mph and kph.
Liam: GPS units are selectable
and MOST (at least this is true for my Garmin GPS II, Garmin GPS Map 162 and Garmin E-Trex) will allow you to select either Statute (MPH), Nautical (Knots), or Metric (KM / H) units. Steve Alchemist C-320
and MOST (at least this is true for my Garmin GPS II, Garmin GPS Map 162 and Garmin E-Trex) will allow you to select either Statute (MPH), Nautical (Knots), or Metric (KM / H) units. Steve Alchemist C-320
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