JEM Watercraft

David & all,
my reasons for being interested in trying Mick's venturi/gravity drain idea are:

1) Avoid the drain being load bearing by moving it to the side instead of being under the cockpit.

2) I think installation should be easier being on the side of the hull.

However, these are secondary to the issue of will the drains actually drain! Do you think they will drain adequately? I don't know for sure whether the exits will be above or below the waterline on the Tandem SOT Matt has designed for me (it's a brand new design).

Do you think that angling the tubes aft and downward will eliminate the need for a check valve? Would building a little semi-hydrodynamic cover like shown in the photos above decrease drag while still allowing drainage?

Scott,

Even if the exits are below the water line, they will still drain, just like a scupper (with the exit at the bottom of the kayak). It will drain as long as the water can flow down. The rate at which it can drain will depend on the size, length, etc. of the drain. Fluids will flow from a higher pressure to a lower pressure.

Unless you are using this as a racing Kayak, I don't think that you will be able to detect the drag difference whether it is above or below the water level. If you give Matt your loading distribution, he or the software can calculate the waterlevel at the drain location.

If Matt has designed a scupper at this location, you shouldn't need a check valve. Otherwise, it would remain flooded to the sea level with a scupper. My Tarpon 140 had a little amount of water in the footwell whenever I was sitting in it. It doesn't matter as long as you don't mind getting your feet wet, and having a little extra inertia. (harder to get started but more glide)

I think that if you built it the way you like, you wont have any problems. Just make sure that water can flow downhill so that you wont trap any water, and that you don't use too small of a tube and reduce the flow rate.

Ref. the venturi's shown in the picture. I think that they will add a little drag. They are designed to give a low pressure zone at the exit, and therefore pull the water or air through the outlet hole. The velocity will increase going around the hydrodynamic shape. An increase in velocity gives the lower pressure on the back side; however, the increase in velocity will cause a slight amount of drag, which is probably insignificant.

David,
can you elaborate on the relationship between the size and length of the drain and it's drain rate? In other words, does a larger drain/tube size increase drain rate? Same question for drain/tube length? I assume there is a penalty in the form of increased drag for increased drain/tube size. So there must be a tradeoff between drain rate & drag.

Regarding the venturi covers, sounds like they will improve drainage at the cost of some drag. Do you think they are unwarrented for a tandem fishing kayak (we're not talking about a speed demon here)?

Scott,

The larger the diameter and the shorter the length, the greater the flow rate. Most platic SOT kayaks have scuppers that are around 1" diameter, and around 4" - 5" in length. Whether it is horizontal on verticle will not make much difference. Make your drains as per the size that Matt suggested, but not much smaller than 1" and you wont have a problem.

If you are going through the surf, your kayak could be swamped for a few seconds, that is if you dont turn over. The amount of seconds will depend on the supper or drain size. I very rarely get more than a small (less than a cup) splash of water into the cockpit while fishing here in the bays even in choppy water. You shouldn't have any problems.

I think that you would be better off without the venturis. I don't see any benefit in your case. The drain holes look like they are small, and the drain rate could be hampered if you are not moving.

This has been very helpful! I'm going to give Mick's drains a shot.

I have a choice of using round fiberglass tubing that's either 1.25 inches in diameter or 1.75 inches in diameter. I'm leaning towards the 1.25" as I have another possible use in mind for the 1.75" stuff. Suggestions?

The original design called for 4 drains for the 10 foot open cockpit (the tandem is 17'). Two(2) drains up front and two(2) at about the middle at the foot of the aft rowers station. I'm now realizing that the aft rowers seat is a low point. So I'm thinking of adding another set of drains back there. That would bring it up to 6 drains. Thoughts?

The 1.25" size should be fine.

If you put drains in the seat area, you may want to be able to plug them. I am not sure how the side drains will be here, but with bottom scuppers, water will sometimes come up through them and you will end up with a wet seat. Of course you can get water splashed over the side, however, you would usually stay drier without the seat scuppers.

Hi guys,

re the venturis, The discharge will be above water (mostly) so thay are more, just a different type of drain. My reason for going this way is mainly that I want to avoid the posibility of a rooster tail in my face while running down the face of a wave. I won't be relying on the difference of water pressure going past to empty the cockpit.

If they don't work, I will install scuppers a bit later.

All the structural work is done I got the second last coat of resin on the outside hull tonight and sat in her for the first time. (just to check pedal positioning etc. and to also work out measurements for a centre consol etc.

Glad I did. I bought some rudder controls from duckworks and beautiful controls they are too. The way I have mounted them combined with my big feet, leaves insufficient room for a console. Will have to think about re mounting them a bit later. i've been working 14 to 16 hours a day on her this past week and I'm way to tired to even think about it at the moment.

The cockpit is a perfect fit width wise but for me, at 6 feet tall, is about 9 inches longer than it needs to be. Had I realised this, at the start, I could have extended the foredeck and had even more room in the front hatch. Nevermind, It keaves another open space to carry stuff handy or to throw a fish to let it bleed out before going in the cooler

Actually, the more I think about that, the better I like it. .

Here is where I'm at at the moment


Large rear hatch with extra storage under tankwell


Fallback rudder

Mick,

You're a boat building animal!

Mick,
she looks amazing! Tell me about all those round holes in the mid-deck. What are they? I'm sure some are rod holders.

When do you think you'll hit the water for a first trial run?