While we are working our butt of to get our Nacra Inter 18 to a small 20 knots, I look at video's of Infusions and see them going to 30 knots as if its very easy. Of course it has something to do with our sailing, but I was wondering about a few things:

1) Can anybody share the max speed you have done with an Inter 18? We would like to have some comparison, and like to know where we are aiming at.

2) We have difficulties with improving our speed with the spi, compared to without spi. It seems we end up at the same top speed every time. Should't this go faster?

3) Is there really so much difference with the Infusion?

Knowing someone else's top speed does not tell you much since there are many different factors like crew weight, skill, etc.

It is possible that your top speed is not that much faster with spi, but you will sail much lower and therefore towards where you want to go (assuming you are racing on an upwind-downwind course).

Yes, the Infusion is much faster than the Inter 18.
Downwind there is better volume and the read beam isn't slamming into waves.
Upwind there are longer boards and a more modern sail plan.

I'm not buying into 30 knots on an Infusion ....unless it has foiling non-F18 boards and rudders.

don't most modern beach cats (20' and less in hull lenght) top out around 20-23 mph

"While we are working our butt of to get our Nacra Inter 18 to a small 20 knots, I look at video's of Infusions and see them going to 30 knots as if its very easy..."


Where is this video? There's no way any beach cat was doing 30 knots, unless it fell off the trailer going down the highway! From what I've seen and experienced, to get over about 20knots, you've got to get both hulls out of the water, ie. foiling.

I've read tornado guys talk about 30kts but I don't with an F18 is going to get you there. Your probably seeing some exaggeration on those videos. My speed puck would often show 20-21 on downwind legs, however the average top speed over whatever distance the speed puck looks for was never 19kts or more. A better sailor could probably get more out of it though.

Getting much above 22-23kts is going to be really tough without flying. I've seen Mischa touch 25 kts on a Wildcat. Maybe the 30 kt video is from Hydro's foiling C-class?

I was referring to this video: (look at the 2nd part with spi)But start to realize that these might be km's/h?

http://www.youtube.com/watch?v=JKHgygcfuGU&feature=youtube_gdata_player

Another way of looking at this is, is doing math. When you look at the record round of the Round of Texel race (aprox 100 km), it is done by Herbert Dercksen en Mark Bulkeley in 2 hours, 7 minutes and 7 seconds. That means an average of around 27 knots! Or I need to go back to school, and learn to do math again :-)But this is probebly done by an F20

This years winners were:

1
186
formule 18 Spi
Zeekant, Oscar
NL
Begemann, Karel
NL
J
2:46:50

This is still an average of almost 20 knots, but done with an F18 in bad circumstances.

So the conclusion is that an Infusion tops around 22-23 knots? (and the Inter probably peaks at 19-20 knots?)

these numbers seem unreal.

perhaps a strong current was part of this that aided in the overall speed, but lessened the effects of the speed on the hulls?

I think you could get over 20 in ideal conditions for a little while, but I agree, no one around here is doing 30 kn on a "standard" beach cat.

most consumer GPS units are incredibly inaccurate, and will give you spiked readings based on their averaging between (quite inaccurate) fixes.

+1 I would think conditions would include pretty much flat water, or else you're launching/stuffing the boat which can't be fast.

Karl,
Do you have your GPS tracks of Hiram's 2011? ....you and Mark had to be doing mid 20s on the northbound leg.

To date, the fastest I've recorded on a beach at was on a 1978 solcat 18 at 24.4 mph in +30 mph of wind.
Since then I've sailed a N.5.0, N5.7, N6.0, NI20- all but the 5.0 running chutes. The fastest speed I've recorded is 23.6. So I'm more than a little pissed!
A buddy of mine and his crew collectively weighed +450 and ran a N5.8 with a hooter, double trapped, at 28.6 for an extended period, ie 10 seconds, and pitchpoled. I watched them from my 6.0 running parallel, at +_20 and they left me standing still. I'm not sure of their real speed, but it believe it was close to the GPS.
I'm still gunning for the 24.4 clocked on the SolCat dammit! Correct GPS or not!
Did get caught in a squall in Mexico, and had to be running DDW AT OVER 60MPH, with crew trying to pull daggers so I could beach, but I could be way off!!

Damn you're slow. 24.4? We almost touched that surfing off a wave while the GPS was flung airborne on a Swan 39 in the stream. Ok I think it said 17 kts as we fell down the wave. Next year it'll be "yeah we were doing 22 kts on that run no question".

You know what, I've set my GPS on the dashboard of my truck to check its accuracy, and it did pretty good. Considering that I was going up to 75 mph in the truck, and considering the max speed recorded by Manny and Julianne of well over 28mph, if you consider that their gps was a max of 6mph off, putting their speed at +22, I should have observed in excess of 12 mph differences in GPS speed vs speedo. Nope.
Also, I have oversized tires on the truck uncompensated by the odo, GPS shows about 6% faster than the odo and is consistent. Confirmed by police radar.
I think the inaccuracy of GPS may be overstated. Probably decimals in most cases. OMHO

A gps will be accurate at a fairly constant speed in a straight line, such as a freeway. In fact, the GPS will be more accurate than the speedometer in your truck. They don't do as well on boats constantly changing speed and direction. I believe its an apples to oranges comparison.

and the faster they go, the more accurate they are (measuring a greater change in position over the same amount of time).

Oh, see what I know

Oh if only GPS accuracy were as good as when you're driving in your truck, how that would make my life easy. Well, it's kind of a good thing that isn't the case...otherwise I might be out of a job.

Part of the problem is the best consumer GPS has a 10Hz update rate, and most are down in the 5Hz range. Turn that GPS mode to battery saver like I do and it's probably closer to 1Hz. That means at best you are probing the GPS receiver once every 10 seconds for data. Hey that's pretty good right? Nah, it's ****. At 25 kts you're covering 43 ft/s or about 4.3 ft every GPS parse. That's not terrible and within the realm of P1 GPS accuracy, but just about. What happens in the simple case of "my mast blocked the GPS signal", so you're loosing a packet of data ever other hit. That means now your sampling at 5 times per second, and traveling 9 ft between samples. That's right on the published 95% accuracy spec of 3 meters. And really you only get that accuracy with high PDOP values and when you're stationary. In short, if you're off by 9 feet at every reading for a 5Hz GPS, you could be off by as much as 18 feet between readings, 18 ft/0.4 seconds between readings = 45 ft/s or 26.6 kts!! That's the delta on your actual speed. Ouch right? Well, luckily the filtering within your receiver is usually much better, as is the accuracy (WAAS). But you can see how a small error between readings can result in a max speed burst several knots higher than your average. This is ignoring the effects of accelerating hands, masts, changing headings etc. that can all mess with the receiver itself (well, namely in degrading positional accuracy).

All of this can be eliminated by adding additional sensing equipment. For instance, 6-DOF IMU's do a really nice job at short term position prediction and hence velocity, can be sampled at high rates, well known filters etc. can be applied and you'll get good data. In conjunction with GPS information you are now looking at some really nice data. This is how UAV's manage speed so nicely (well, most also have an air data source, or speedo in our boat example). It's also how your phone knows it's orientation, the AC boats know their heel, and how those sick on screen VMG readings are produced with a high degree of confidence. The beauty is all this hardware is less than $300.00, if you just care about logging the data and getting within 1/10th of a knot.

Besides all this, average speeds matter much more. I bet you had much nicer VMG's on your I20 than the Solcat 18. To add a little more to that fray, most of the boats you owned aren't faster on a reach than your old solcat, more stable maybe so you can keep a higher average but faster? Probably not, the sail plans aren't optimized. I'll say with some confidence the new F18 hull shapes + rigs are faster on a reach than the I20 or pretty much any straight boarded 2-person production cat.