Tunnel drive performance issue

It does not show in the picture, at least not as good as I intended, but there is no sharp edge anymore. And I forgot to mention in the last post, that there are no vibrations anymore, so that is also part of the good news.
But the improvement in flow increases the load on the props and the engines cannot cope with that.

 

Do you have an updated sketch of the tunnel, as constructed? Looking at the photos it really looks like a sharp edge down there, and a very steep, nearly vertical diffuser wall in front of the prop.

Don't misunderstand me or consider this as jinx-invoking, but I would really wait to arrive at the hump speed and full throttle before saying that vibrations are gone. That's the moment of maximum prop and engine load.
Only after you have successfully gone over the hump and entered the planing regime, you will know that the boat has passed the test.

Good luck, I think you have really deserved it after all the effort you've put into that stuff!

 

Standard procedure on planing tunnel hulls is to inject air at the leading edge of the tunnel, using a PVC pipe epoxied through the hull. The pipe can be run up to the helm where an inexpensive ball valve is used to modulate airflow.

 

In reality it looks like an overstretched Italic "S". The leading edge is horizontal, then it curves up gradually and is horizontal again where it touches the ceiling of the tunnel.
Of course there could be vibration surprises at higher speed, but for now I can only compare it with the previous construction, which had a sharp 75 degrees angle the water could not follow. When I opened the throttles then, the whole boat shuddered and the wake was white with irregular hills and valleys that moved continuously; now it looks like that of an overpowered displacement hull. Optically you don't see that there are two props.
At full throttle the bow rises considerably, like a half-glider, but there is not enough power to go any further.

Village_Idiot, can you elaborate on that? Should the air be distributed or just coming out of a plastic pipe of say 2" diameter? The pipe should of course be positioned somewhere where the pressure is low enough to pull air in.
I expect that once planing speed is reached and the props are very near the surface, air will be drawn in from the sides of the tunnels, so the air injection can be closed or reduced. That should of course be done without intervention from the helmsman, otherwise nobody but myself would be able to control the boat.

 

Often a simple approach helps .....when we fit a pod to the transom to hold an outboard the rule is that you can raise the outboard 2 inches for every 6 inches of length of pod. the pod does not extend down to the bottom of the transom ... ie 18in behind the boat the water is 6 inches higher than the bottom of the transom..naturally we are talking a v hull with one motor. ( 6 metre Stabicraft 15 deg and 2.4 m wide

So is not your solution to move the props back ...fit maybe a diving platform at the rear and support them from that .....

 

The concept behind the vent is to allow air into the tunnel to "break" the vacuum that develops inside the tunnel on a typical planing tunnel hull. Not sure how the application would work in your case, but if you think air introduction is the proper fix, then it may be worth a try.

However, before poking any holes in your hull, you might "test drive" the application by epoxying a pipe or hose in each tunnel, running it up the roof of the tunnel, then up the outside of the transom of the boat where the free end is above the waterline. This way, you can see if adding air into the tunnel will improve performance without drilling any holes in the hull. It will also allow you to experiment with various pipe diameters to see what works best for you.

For practical examples, boaters using 40hp outboards on small tunnel boats typically run 1/2-inch diameter vents, whereas those running 115hp outboards are running 3/4-inch. The ball valve allows you to somewhat modulate the airflow if you end up with too big of a pipe or need to shut off the airflow (which you want in your particular case). Many users report a noticeable vacuum at the upstream intake of the vent.

I believe you will want the vent located in the tunnel at the furthest point forward that you think air would be introduced or present in the tunnel even if no vent was in place. You might want to check out the following link for more info and possible pictures: (boats, blinds, & gadgets forum on refugeforums.com website) http://refugeforums.com/refuge/forumdisplay.php?s=&daysprune=1&f=10

 

There is/was a company called OPS Jetpac that marketed a similar rig, basically a floating pod that housed engine and jet drive, which basically attached to the transom of a boat. They no longer seem to have a web presence, so am not sure if they are still a functioning entity. However, here is a link to give you insight on their product http://www.boatdesign.net/gallery/showphoto.php/photo/1266/size/big/cat/

 

The attachment shows the current situation. Between the new deflector and the transom the cavity is filled with polyurethane foam and also the box on top of the tunnels. I spent lots of time making the construction completely watertight, so I am very reluctant to drill new holes for another experiment.

There is also an accessibility problem. From above there is no room for a tool unless I drill through the top of the box as well, but that would require a 2 1/2 ft long drill. From the engine bay I could achieve something, but only very near the top of the tunnel, where the injected air may not mix with the water and probably will escape through the gap between the prop tips and the tunnels. Near the prop tubes I will encounter a thick stainless steel flange that will resist any drilling efforts with a hand tool.
Drilling through the bottom just in front of the tunnels is another option, but there I will need forced air injection because there is no vacuum. Maybe I could connect the exhaust hoses at that point with a T or a diverter valve.

As we've entered the summer season now and want to make some use of the boat instead of only working on it, there is time until the autumn to sketch, contemplate and discuss.

 

Of course I know nothing..... but if you removed the tunnels you would have the props 18 inches behind the sharp edge of the bottom of the transom so water should rise up to hit the props just fine ( like a pod on a power boat ..2 in lift per 6 in of length ) so are the tunnels in some way causing bad flow over the props which is sapping power and flow ....of course you need to get it up on the plane ...what is the water doing between the tunnels and outside the tunnels p and s at planing speed ....

Having searched back to the photos since I wrote the above you have done far too much heavy engineering to go back but examination of the water on each side of the jets where it leave the original transom would give a good guide to what should be happening inside the tunnels and of course between the jets ..

 

If I would remove the tunnels a number of things would happen, like the rudders falling off, the prop tubes loosing there support and of course the boat would sink.
But you already guessed that....

It is not possible to determine exactly what is going on behind the transom, except at low speed when there is little wake and the boat behaves almost like any other small displacement cruiser. I use 'almost' because there are no other 26' boats around here with 2 such large props and left/right rotation. The course stability is better than anything I've even been on: even movement of passengers hardly influences the set course.

At 10 knots it is already way over the hull speed, so it starts to climb. I estimate the bow to rise 20-25 degrees. In that situation it is not possible to see that there are 2 tunnels and props, it looks as if there is only one.
The tunnels, which are normally 6" submerged are near the surface. The water is relatively flat and starts to rise approx. 3 ft. behind the transom. At the flanks of the hull, the water curves inward because it was displaced by the hull.
On the next trip I will take some pictures...

 

well I said I know nothing but I do have a working tunnel drive and it is similar to your set up but only one prop..I find it very salutary that an expert on this forum can spend lots of bucks and it dont work too good...as the yanks say ...**** happens ......keep us informed .......the pictures would be great ....

 

I would never call myself an expert in marine propulsion. My field is electronics and computer hardware. I started by studying mechanical engineering, but was forced to. A long story, not very interesting.
From making mistakes I learned more than from going to college, the process is still continuing.....

 

Didn't know photographing the wake would be that difficult. The autofocus goes berserk and blocks the trigger most of the time. From 30-odd pictures I found two sharp ones that are attached.
Wake1 is at hull speed or slightly above, the wake is not symmetrical, probably because there was quite a lot of crosswind so the rudders were at an angle.
Wake2 is at approx 10 knots when the bow starts to lift. There is already a flow from the bottom on both sides of the tunnels like on a planing hull. This is at about half throttle and almost as fast as the boat will go.

Frankly I think that only less steep props could increase the performance a little bit and even then I'm not sure the boat will plane. And there is also the voice in my head that says "do you really need it?"

 

From what you say I dont get this ,,, surlely the tunnel drives will not work until you are on the plane lifting the water up and ejecting it above "normal water level" also much less power per mph on the plane...as for the photos we need to see the tunnels relative to the water ..are they in or out ? as for focus you half press pointed at an object the same distance away then swing round to the photo you want to take and then fully press the button ...its called pre focusing .....

 

I know about pre-focusing, but do you have any suggestion which object I should have the camera focus on when there's only sea around me? Someone has to take the pictures from another boat, that will also provide a better perspective.
When I look down, I can see part of the tunnels protruding, but that may be partly caused by a dip in the water level behind the stern. And of course at 10 knots the hull will also be lifted several inches.

Is the prop in your tunnel pumping just water or do you consider it a surface drive?