Optimal venturi and bullet placement.

I worked on the venturi concept with Dave Hartwanger (a flow design engineer based in East London South Africa - and represented SA in rowing!) - commissioned by Red7. We looked at various designs but the bullet was the most practical, hardy, easy to install and very effective! Well done to Oskar on developing the one piece. Add the non return valve concept used by Carbonology and then you have a real winner!

For the technically minded I measured the speed at which the venturi starts sucking air.

On an Elite with 80kg paddler in the ocean it's 7kph.

That's very good! Maybe too good

Hi

Just mounted one of Oskars venturies on my Robberg Express surfski from Knysna Racing.
At only arround 7.4 km/h the ventuti starts sucking air. So no more wet feet at low speeds . With the original venturi the speed was arround 9.2 km/h. I have only mounted one and closed the other hole (two is standard on the Robberg). And it seems that is´s sufficient with only one of Oskars venturies to drain the foot well.

This is what the latest version of all-in-one venturis looks like. Now you can slide your ski in and out of racks without taking off the front lip.

Performance is the same as the previous version, it sucks the footwell empty between 7 and 7.5kph. For competent paddlers paddling a Fenn Elite or Glide one venturi is perfect for nearly all conditions.

I have been told that there are other brands of skis that have problems with excess water getting over the sides of the ski, and that for these skis one venturi is not enough. So we ended up making a larger venturi for them, it's 20% larger in all dimensions. The large venturi sucks the ski empty at the same speed (7-7.5 kph) but has a higher water extraction rate. From preliminary tests it seems as if one of the larger venturis sucks about the same volume of water as two of the normal size ones.

Size of the larger venturi is:
33mm wide, 156mm long, 19mm high.

The factory Fenn venturi by comparison is 40mm wide and 17.5mm high, so roughly the same cross-sectional area as the larger all-in-one.

Oskar,
Have you compared it to the Epic or Anderson Bailer? I'm building a ski and have been thinking that a bailer would be the way to go but maybe your Venturi would be a simpler solution.
Thanks.
Ken

Ken,

I only paddle Fenns so did not know about the problem of water coming over the sides until people started telling me about it. Apparently Epics are somewhat prone to it, but there is another brand (won't mention any names) that is even worse.

What size venturi you will need depends on how much water comes over the sides of the ski. That in turn depends on the ski, and to a certain extent also on the paddler. If you seldom get water over the sides (like in a Fenn Elite or Glide) one normal size venturi is more than adequate. For a Fenn double I would recommend two normal venturis or one large venturi front and back.

Oskar, I've been thinking of reducing my venturi's to one this season (got to take all advantages to beat Willy this year), how did you remove the stock venturis? Is it a case of grinding and sanding down to the hull? I might get one of your new venturi's at the Tuesday races in a few weeks.

Oskar,
Thanks for the response. What I am curious about is the rate at which your venturi drains and the drag, as compared to a bailer. THANKS.
Ken

Always drag on a venturi, 100% of the time! even when the foot-well is empty.

My experience with Oskar's bailers might help answer some questions. I added one to a Stellar SR to try and make it drain quicker as the huge bucket can take a long time to drain and in some conditions can be a real problem. Before I replaced the bailer I tested the draining speed to get a baseline. I filled the bucket to the brim with water and pulled the plug when going 10 kph. The Stellar bailer took 1:30 min to bail the full bucket. With Oskar's bailer it took 2:30 min to bail. I feel the ishue is there isn't enough flow threw the small 'mouth' of the venturi to make it bail fast. It's size acts as a restrictor to the potential volume that can come threw the 2cm hole in the ski. It did bail at very low speeds, much lower than the Stellar bailer. I also think it's more hydrodynamic in the water than the Stellar bailer and likely faster.
If your looking to drain small amounts of water from your ski and maybe get a tiny bump in speed then it's an excellent choice, but if your wanting to drain a flooded bucket as fast as possible look elsewhere. For comparison, my Epic V10 with the new bailer type bails a full bucket in 15 seconds at 10 kph when open all the way.

I just want to be clear, I think Oskar's bailer is a really good bailer, but maybe not for all applications.

Bill

Ranga wrote: Always drag on a venturi, 100% of the time! even when the foot-well is empty.

That's the crux of it. The only reason we need venturis or anderson bailers is because water comes over the side of the ski. We wouldn't be bothering with these silly devices otherwise.

The idea is to put on the smallest venturi you can get away with. I've played around with a medium size and large size (small size is next on the list) and know that for me a single medium is the optimum solution. I've paddled with a large one and the drag feels the same as a medium one, the difference in drag is most likely negligible but the psychological advantage of only having a single medium is massive

Greg - I notice that you suggest 5-6mm gap between scupper and bullet. Can you tell me if that distance is between the tip of the bullet and the end of the scupper, lengthways along the boat. Or are you talking 5-6mm opening gap?

Thanks heaps

When I researched the optimal bullet placement for my SES I came up with the following, the bullet should poke inside the venturi hood, if you get a vernier the gap around the bullet measured to the venturi hood should be 5-6mm (depends on the size of your hood and bullet) the whole way around, this gap does not restrict the flow but streamlines the flow, my SEL came with this dimension as well, I did the same dimension on my Swordfish, all of them drain under 8kph. Been awhile now so can't find the reference for this placement but it works for me, if you think about the water tumbling past the hood you want the bullet to prevent it tumbling at the rear of the hood, so it makes sense for the bullet to be slightly inside the hood to keep the flow moving past smoothly.

Hi Oskar,
I reply to an very old thread. I'm new in this forum and a surfski rookie. I'm flat water padddler and last days I puchased an old Custom Kayaks Synergy.  I feel good with paddling it, but speed could be a little better. The surface of the bottom of the boat is quite rough, there are some decent fibere patches an a little bit of colour and the two venturis are very big and coarse. Beginning with 8 kph you can hear load sucking noise under your feet
I like this ski and I want to make it pretty again and optimize it. Looking for venturi, I found Your all-in-one venturi and it looks great to me. It looks like a 3D print. If so, could You please share the 3D model with me?
Or do You know, where I can buy two of them here in Germany?

Best regards
Richard