JEM Watercraft

Mick,
can you elaborate on your venturi drain idea? I'm really interested.

Hi guys,

Got the venturis' tacked in tonight. also got the fall back ruder completed.
I reckon I'll put it on anyway. Just to get on the water. If I need a more central skeg, I'll retro-fit one. I put a big hatch in the back partly for that purpose.

Gonna have a check valve on those hummers

Nah Jack, I reckon she'll be right.

Are they 2 pieces with a tube connecting?

I guess I'm kinda lost on how you'll do it.

Hi Matt,

I just worked out the angles, shot some measurements. Once the holes were cut, I fitted a short piece of fibre-glass tube through the holes, creating a drain from the cockpit to the outside of the boat. tacked em in with woodflour. will trim em back tomorrow and finish the fillets.

Paddling for Sharks,

Mate, that is just priceless.

Hi Javier,

You could always seal yourself into the kayak with one of those rubber gaskets, then have a large plate of beans. The resulting positive pressure will keep the boat totally dry

Mate, I paddle warm water. A dry ride is not necessary. i just want something to drain the slop and blood out.

Hi Scott,

Best I can do is post some piccies. i am afraid I am not much of a photographer but i will try to explain things as we go along. The reason i am trying the venturis' is that i thing a big hole inthe bottom of a boat will creat unnecessary drag. Also, when i was young and just for kicks, some mates and I drilled a 1/2" hole right through the back of our surfboards. Riding solid waves the roostertail thrown up behind was amazing. I don't want a rooster tail in my face while running waves in my yak. I don't know if a scupper in a SOT would do this but I don't see why it wouldn't.

Most of the SOT fisherman in Australia end up putting plugs in their scuppers anyway.

First Pic shows running a straight edge across the top edge front of the cockpit and measuring down to the lowest point. This is the where the venturi hole will be cut. The discharge port will be a little lower and slightly behind this point.



next, the discharge hole is measured from the same straight edge.
The angled thing sticking through the hole is a welding electrode to illustrate thttp://i24.photobucket.com/albums/c32/hairymick/Kadaitchi%20Build/Kadaitchi165Medium.jpghe angle marked and drilled.


Once these pilot holes are drilled and checked as being satisfactory, they are cut bigger to fit the venturie pipe. ( in this case 3/4" nominal bore fibreglass tube.)

Don't know what just happened there. will try again. Some more piccies.

Electrode poking through hull & cockpit to show angle of discharge.


Venturi pipes cut


Pipes in ( this is one of the pipes from the tankwell) Pic taken facing aft.))

Makes sense! You said you angled them back about 10 degrees. What do you think the downward angle is?

Have you used this design before or know of others that have done it? What's your/their experience with them? What drawbacks might there be to this approach vs. the under the cockpit design? I can see that it would cut down on drag and also have a side benefit of not carrying any load like the under cockpit design would have.

Just checking this option out before I decide which way to go.

Hi Scott,

I have never seen the idea used in a kayak before. some high performance sailing skiffs use something similar I believe.

I think the vertical scuppers were a development for the plastic SOTs and they are an integral part of that type of hulls strength. None of the plastic SOTS that I have seen have internal bulkheads or any other kind of deck support and so, IMHO are necessary to keep the deck and hull apart. the fact that they can also drain some water out of the cockpit/seat/footwell recesses is just a bonus.

If they were stuck out the side like on my kataitchi, they would provide no support for the weight carrying area.

Venturis are fairly common in industrial applications. ie aluminium smelters use them to extract molten metal from the reduction cells. A simple carbuaretta uses one to suck petrol out of the float bowl.

My biggest concern with a conventional scupper was the potential for a rooster tail in my face while trying to negotiate a run down a steep wave.

Oh, yeah

Downward angle is a simple plumbing drainage angle about 10 degrees again. not very important, as long as it is down a little.

will try to get a better piccie today.

Mick,
for the venturi exits, would something like this (see photo below) be useful at all on the hull exterior? Any and all photos you can post of your venturi drains would be great!

Any one else have any thoughts on the cockpit/hull-side venturi drain option vs. the standard under cockpit/hull design?

Here's a larger view of it:

I'd have some concerns about how well it would drain vs. scuppers. But I have nothing to really compare it with. So I'm waiting to hear Mick's report on them too.

But if it works, I certainly would look at designing the next SOT, whatever that may be, to make installing them easier.

I can see how this would work well as a gravity drain; however, if the outlet is above the water line as Mick suggests, you would need a much higher air velocity (well above kayak speeds) for the venturi drain to work. If venturi is below the water level, if designed properly, it can increase the removal rate of water out of the kayak while underway. I don't see a need for a venturi drain on a kayak, unless the floor level is below sea level. Then your venturi should have a check valve to prevent incoming water while the kayak is not moving.

I think that this is a great idea for a gravity drain if the oulet is above sea level; otherwise, drag will be nearly identical as to a drain to the bottom of the kayak. The choice would be what ever would be easiest for the builder. If the size and length of the drain, either horizontal of vertical, is similar, then the flow rate will probably also be similar.