I have read some of the posts with indication of ave. speeds reached during various races.

It is obviously impossible to directly compare speeds from one venue on one day to another venue on another day due to effects of wind, tide, current etc.

We've found that depth of water does effect speed - but how much? I see race reports where averages in excess of 10km/h for 14ft boards over 5km distances or longer are quoted - we race timetrials at a number of venues and cannot get anywhere near those speeds, even going flat out on much shorter courses.

Some of our venues are canals which are current free and relatively wind protected but the water level does change depending on how much rain and pump action there has been going on. We definitely see better times when we have more water but it rarely gets deeper than 1-1.5m, if that. Windy days also seem to shave off 0.4 - 0.5km/h....

Does anyone know how much waterdepth affects boat-speed - cause we just feel really bad that we're so far behind you guys.......

Short answer, yes. Here's some good reading on the topic.

http://www.surfski.info/getting-started/tips-other/item/1279-the-science-of-paddling-part-iv.html

yes - very good article indeed! Could someone translate it into what effect a waterdepth of 1.5m has on the speed of a 14ft board. My estimate from reading article is that it could be as much as 1km/h? Which is around 10% - so quite substantial over distance.

We'll have to go and find some deeper waters....or just be content with our resistance training....

Piros said...

Here is a simple translation , I learnt this from paddling & steering a 44ft 6 man outrigger canoe , the bow pushes forward and down a bow pressure wave if it' too shallow that pressure wave bounces off the bottom and rebounds off the boat creating drag pulling on the stern which makes your hull put out a larger pressure wave and creating even more drag. It's like car racing the shortest line is not always the fastest , so it may be faster to take the long way round and avoid the shallow water in a race.

The shorter the hull the shallower you can go. A 12-6 Sup will paddle faster in some cases in shallow water than a 14' or 17'.

Next time you are are out paddle over the edge of a sandbank from 6ft to 1 foot and you will feel the board die , keep doing this until you work out what depth you can cut the corner of the sand bank without loosing glide.

uncanny sir, i was thinking the same think- water bouncing back up at the board/craft

It's all about hydrodynamics Lacey and what's called the Bernoulli effect.

See below:

Warning don't go to this link if you have been drinking This is what it looks like:
H the water depth. In units of knots and feet, FNH = 0.297 VKT / (HFT)0.5. In knots and meters, FNH = 0.164 VKT / (HM)0.5.

://www.hydrocompinc.com/knowledge/whitepapers/HC124-ShallowWater.pdf