Wake vs. hull speed

OK armchair boat designers...latched on to an Alerion 38 as I was paddling back into the Newport Beach Harbour. (Alerion 38 is a nice performance oriented sailboat ). Real nice bump and the skipper was amused to see me drafting. Looked up and could see his boat speed indicator , so...the harbour water surface was relatively smooth and the sailboat was reaching close hauled maintaing a nice straight course right up the middle of the channel , at boat speed of 7.5 kts I could sit on the wake with minimal paddle strokes to maintain sweet spot , when sailboat speed increased to 8 kts. I had to paddle hard and found little respite in terms of sweet spot to draft . We played this game all the way in so not just a flash in the pants , My question ( and I am sure the answer is a combination of factors) , was the increased effort in the speed increase more a factor of sailboat wake "waveform" or surf ski speed beyond optimal efficiency for hull design ( a huki S1-XL ) . I was surprised how much difference there was and assumed, which was not the case , that as sailboat speed increased the bump would deepen and be easier to sit on .

Not sure if this answers your question but I have easily maintained just over 9knots (17km/hr) behind a fishing boat, just depends on the wake size and angle. I dont think its likely you will notice a great deal of hull speed resistantance until you move from an elite/intermediate type ski to a beginner type skis where there is noticebly more drag. I have surfed unbroken waves with guys on various stable to advanced skis at over 20km/hr and no-one was working noticebly harder to stay on the wave.

As you say, often people on boats get quite a surprise to see you easily sitting behind them and tourist boats are the best with them smiling and taking photos of these water sportsman following them, makes you feel like some sort of superhero even though we know it's not that hard to do

It's an interesting question. In my Stellar SR, I've noticed an exponential increase in drag when I hit speeds of around 7 mph on flat water. It seems to require massively more effort to go from 7 to 7.5 mph than it does to go from 6.5 to 7 mph. In air, drag varies according to the square of the velocity, but in water it seems more like the cube of the velocity.

My goal is to be able to average 8 mph in the quarter mile sprint, but I don't know whether I'll ever be able to manage it. At 7.5 mph, I'm already putting so much force into each stroke that it feels as if my paddle blade will break off!

I live near a lake in Germany with a lot of boat traffic in the summertime. Due to the lack of real waves I try to surf boat wakes as much as I can. It´s a lot of fun ! My experience is that the wave always gets bigger when the speed of the boat increases but only to a certain point. If the boat goes faster than 20 kmh (11 knots ?) it comes out of the water and produces less drag so that the wave gets smaller. I get the best wakes from small cruise ships or ferrys
that go along with 15 kmh. Sometimes I get lucky and catch a wave from a big yacht and I will reach speeds close to 20 kmh according to my GPS but this is quite demanding because these waves are sometimes steep and short and difficult to ride with a 6 m + Surfski !

Michael

Most likely, the sailboat's speed is changing at that particular speed such that its wake is becoming less steep and therefore harder to sit on while on a ski, i.e. total ski's hull resistance equalizing the pull down the wave from gravity . In your ski, that should occur moments before it might on an elite ski of similar fore/aft weight distribution and length.

Aurelius > Wiki: "Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed."

however, see example:

drag diagram
www.thomassondesign.com/Images/panthera-calculated-drag.jpg
of
www.thomassondesign.com/en/catalog/my-kayaks/panthera

Great discussion - agree with the general direction of everyone's thinking. They key to it is likely the considerably increased drag experienced by your Huki just from that 0.5knot extra speed v's the proportionally less extra wave energy offered by the sailboat wake.

Looking at Uffilation's attached chart, that kayak generates about 40% more total drag from 7 knots to 8 knots, and the charts published by www.oceankayaks.com suggest around 25% increase in drag just going from 7.5 knots to 8 knots for a low drag 17' kayak. Given you were riding a racing sailboat wake, probably just below its theoretical hull speed (~8knts), the wake energy is not comparable to a cruiser or commercial boat, so while the wavelength is opening out, the extra power in the wave (height) is less than the ski needs at the higher speed.

Even being quite slender, the ski is working at a higher + steeper point on its wave resistance curve than the yacht, just based on waterline length. And to Aurelius' point, gaining that last extra 0.5 mph (+7%) might well take 15% more muscle power, which is a painful lot!