Hull speed Vs GPS sailing app

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MillingManDan

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Has anyone tested the accuracy of speed (sog) indicated by sailing apps Vs dedicated devices, and how this compares with hull speeds? I ask because the app I used this weekend on Rutland water (lake) indicated 5.9 kts but the hull speed of my little Hunter 490 is 4.8kts with a water line length of 13ft 11. I'm curious to know what the margin of error is likely to be with each?
 
Hull speed is grossly misunderstood.
With a typical boat, it's not a brick wall, it's an area on the speed/drag graph where drag is rising fast.
Some friends of mine had GPS 'speedpucks' at the weekend, clocking 6 or 7 knots upwind in 14ft boats. Not twin trapeze international 14s, conventional sitting out dinghies.
GPS will sometimes tell you some nonsense, but equally some people have a concept of 'hull speed' which isn't very useful.
 
Theoretical considerations have no place in sailing. Your boat is either doing 7 point whatever knots or it isn't. GPS can give a pretty good indication of the ground speed (providing your boat isn't going up or downhill?!) but has to be interpreted within its possible errors. I once managed to get a 26' boat with a V section hull to get the electronic log hard against the 10 knot stop for a short while, which should not be possible.

In steady conditions, GPS ought to be correct but in open water or even on canals I have found it impossible to know that one is free from currents and so 'correct to GPS' is not a good way of calibrating a water log.
 
lw395 said:
GPS will sometimes tell you some nonsense, but equally some people have a concept of 'hull speed' which isn't very useful.
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I don't really have any established concepts of Hull speed, only what I've read and watched on YouTube. I'm interested to know what margins of error there are for each, or how literally I should take them. For example, my old 250 motorbike had a top speed of 66mph, but with a good wind and my head down I could push this above 70. I've read that above the rated hull speed you'd begin climbing your own 'wave' (I forget the terminology), so the gains in speed rapidly become limited. I wonder if this is accounted for in the hull speed ratings? Or perhaps the max hull speed is maximum efficiency? If anyone has a link or article recommendation that can explain it succinctly I'd be very interested to read it.
 
A displacement hull has a max speed. As an example, my own boat is fitted with a powerful engine, which is capable of pushing the boat in excess of it's max hull speed, in calm conditions. If i operate the engine at full speed it pushes the boat through the water increasingly fast, until hull sped is reached. At this point, the hull can't go any faster, but the prop is still spinning faster than it needs to be for the current speed. So, as it cannot push the hull any faster, it pushes the water at the stern in the opposite direction, digging a hole in the water, into which the stern of the boat descends. Keeping a careful watch on GPS speed, i reduce the engine speed, the stern will rise as the hole disappears, the engine sounds much happier and the GPS stays the same. As soon as GPS speed drops, add a few RPM back. That's flat out for the boat.

A planing hull will have some lift and will surf on it's own bow wave, which is what you're thinking of above, i think.
 
GPS signals have an inbuilt inaccuracy of around 5 m. This means that it is detecting you are anywhere in a 5m circle. Whilst you are in this circle your speed changes as you move around. So when looking at a GPS speed ie SOG watch for a few seconds and take an average. Strangely the apps don’t average this value consequently your speed is constantly changing.
Whilst at a consistent speed watch for max and min and guess a midway point
 
The best way to understand GPS errors (for open sky use) is to monitor speed (and direction) when you are dead still. But then plot your average position.
You will often see speeds of 2mph or so for a static receiver, but average position (over 10's of seconds) is accurate to better than a few feet.
So, if you want sensible SoG from GPS then use end-point time and position (or aggregate speed if your device has that) for as long a stretch as possible.
 
Lon nan Gruagach said:
The best way to understand GPS errors (for open sky use) is to monitor speed (and direction) when you are dead still. But then plot your average position.
You will often see speeds of 2mph or so for a static receiver, but average position (over 10's of seconds) is accurate to better than a few feet.
So, if you want sensible SoG from GPS then use end-point time and position (or aggregate speed if your device has that) for as long a stretch as possible.
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Most GPS receivers, as in the packaged consumer units do quite a bit of averaging to give reasonable indications of speed, so long as your speed and direcion are steady.
If you take a raw GPS chip set like the one on my desk here, it give a position every second with a random error, often moving around at a few mph.
If you look at a Garmin handheld GPS, it susses that the last 'n' fixes are all within the random error of each other and averages them to give a good position. If you walk in a straight line with it, it susses that there isa general trend and averages the speed and direction over a short period to give a more useful ouput. If you suddenly stop or change direction it will show a bit of a lag.
 
PaulRainbow said:
A displacement hull has a max speed. As an example, my own boat is fitted with a powerful engine, which is capable of pushing the boat in excess of it's max hull speed, in calm conditions. If i operate the engine at full speed it pushes the boat through the water increasingly fast, until hull sped is reached. At this point, the hull can't go any faster, but the prop is still spinning faster than it needs to be for the current speed. So, as it cannot push the hull any faster, it pushes the water at the stern in the opposite direction, digging a hole in the water, into which the stern of the boat descends. Keeping a careful watch on GPS speed, i reduce the engine speed, the stern will rise as the hole disappears, the engine sounds much happier and the GPS stays the same. As soon as GPS speed drops, add a few RPM back. That's flat out for the boat.

A planing hull will have some lift and will surf on it's own bow wave, which is what you're thinking of above, i think.
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If you look at a decent book on yacht design, you will find graphs of drag vs speed for various hull shapes.
There is no 'brick wall' at 'hull speed' for a typical yacht hull.
It's just the drag rises quite steeply at (speed in knots)~=(1.4xsqrt(waterline in feet))
More power and your boat will always go faster.
But every 1% more speed starts to need several % more power.
Much of the last 80 years of yacht design has been about working on that curve, so good sailing boats don't really need all that much wind to significantly exceed 'hull speed'.
The key is that a yacht in 20knots of breeze has a lot more power available than one in 10 knots. Wind power is proportional to speed cubed, so maybe 8x the power to go 20% faster than 'hull speed' isn't so surprising?
 
My understanding, in simple terms, is a displacement boat produces a wave with wavelength proportional to square root of waterline length. As you increase speed you "catch up" on your wave and your stern is in the trough meaning you need to push uphill to go faster. The height of the wave basically depends on width to length, narrow giving smaller wave. This is why a catamaran is quicker and more fuel efficient than a monohull. Designing lift into the hull and shaping the bow to go through rather than up the wave helps.
 
Perhaps I am lazy (after all I am past 72) but I tend to rely more on what the GPS tells me regarding SOG, COG, distance to the next waypoint and the ETA at that point. I am not really interested in my speed through the water, which doesn't really matter because this is affected by currents and leeway.
On a long leg the info that the GPS gives is accurate enough for practical purposes because the values are continually being averaged.
Upon arrival I am usually amused when I call up the 'MAX speed'. I have a shot of the instrument that shows "MAX 11.34kts"; this on a 32-ft boat that weighs 5 tonnes and with a 24' LWL. Yes, the Centurion32 is certainly not a sluggish boat but hardly "Hull Speed" by any stretch of the imagination; all it means is that in that particular second the GPS aerial was probably going over a largish wave, hence the spurious reading.

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PuffTheMagicDragon said:
Perhaps I am lazy (after all I am past 72) but I tend to rely more on what the GPS tells me regarding SOG, COG, distance to the next waypoint and the ETA at that point. I am not really interested in my speed through the water, which doesn't really matter because this is affected by currents and leeway.
On a long leg the info that the GPS gives is accurate enough for practical purposes because the values are continually being averaged.
Upon arrival I am usually amused when I call up the 'MAX speed'. I have a shot of the instrument that shows "MAX 11.34kts"; this on a 32-ft boat that weighs 5 tonnes and with a 24' LWL. Yes, the Centurion32 is certainly not a sluggish boat but hardly "Hull Speed" by any stretch of the imagination; all it means is that in that particular second the GPS aerial was probably going over a largish wave, hence the spurious reading.
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No, young man, I don't think that you have to view the figure as spurious. On a (very few) occasions I have seen my boat speed go up to 11.5kn in semi-surfing conditions. On each occasion the speed has built up steadily over the course of several seconds and the peak speed was maintained for a short while. We certainly got fluke speeds before selective availability was turned off but modern systems seem to be well damped.
 
johnalison said:
No, young man, I don't think that you have to view the figure as spurious. On a (very few) occasions I have seen my boat speed go up to 11.5kn in semi-surfing conditions. On each occasion the speed has built up steadily over the course of several seconds and the peak speed was maintained for a short while. We certainly got fluke speeds before selective availability was turned off but modern systems seem to be well damped.
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You could well be right! ;)

However, you might wish to reconsider your use of "young man" when referring to me! I certainly don't feel young at the moment. I am still managing to shuffle slowly with the aid of a walking frame. Last year I fell off the deck onto the concrete when the boat was at the yard fracturing my pelvis on both sides. This year, practically on the anniversary, I managed to lose my balance while trying to hold a line when the boat lurched in a sudden gust. This time I broke my right Femur, necessitating the insertion of a DHS, hence the need for the walking frame!
Still, I have long learned to take life one day at a time. Maybe this year I'll still be able to fit in some sailing... all in due course. :D
 
This week I broke the bracket on the gooseneck .uncontrolled jibe!!!! Don't ask !!! a 1500 Euro boombrake that was not connected.
So we sailed back to my home base on Genoea alone and was able to get high 8s close hauled and 9.5kts on a reach . not for a few seconds,but, over a couple of hours. Wind speed in the middle to high teens .
 
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