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Old 08 April 2014, 04:13   #1
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Country: South Africa
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Inflatable with tunnel hull, help please.

Hi guys.

I am new to this forum and would really like some help from experienced inflatable boaters.

I have 3.3 m fully infalable with a 15 hp 2 stroke,

I recently installed small pontoons underneath (high jackers ) to create a tunnel hull, this was done professionally by one of the companies that specialize in tunnel hulls for racing.

I currently am strugling with handling issues,

It is realy difficult to to steer and have directional stability. If i am on the dam , it is realy hard to drive underneath a bridge as the boat flanks left and right and is extremely sensitive to any movement on the throttle in terms of steering.

My highjackers was around 0.8 bar, should it be pumped harder?

Should i avoid sitting on the oversized pontoons?

Should i try lowering the engine deeper in the water?

I am really worried , and would love just to have directional stability,

Can this be caused by running a trim which is too negative !?

It feels as if when i am on the plane, that the tunnel hull suddenly bites on the water trying to then change to direction of the boat.

Please help ,

Thank you.
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Old 08 April 2014, 08:34   #2
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Anyone on here with a tunnel hull?
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Old 08 April 2014, 08:43   #3
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Interesting. Any chance of a pic of the boat inflated, especially from underneath?
My old cat needed 14psi in the highjackers so 0.8 bar is in the right ballpark.

Tunnel hulls tend to need their engines run pretty shallow, at least on the zapcats so I generally wouldn't be lowering the engine, use a prop with some cup if you are ventilating.

We had to get the weight distributed carefully to get best performance - my mate at the front was effectively just mobile ballast to keep things balanced. If it's just you and you are sat on one side perhaps that is unbalancing the boat a little?

Can't help but wonder if your highjackers aren't quite straight or are moving about on the plane- hence the interest in seeing how they are attached and how straight they are!
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Old 08 April 2014, 09:10   #4
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The front parts of the high jackers looks as if it bends slightly inward, like in the direction of the bow.

Please see pics, so the motor should be way higher than it is now hey?

At the moment i am chasing stability more than speed,



I tried correcting the negative trim on the motor, see timber wood in place for demonstration.

I was geting alot of kicked up water on the bow side but i guess that is just to much weight in front.



Thank you for the reply
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Old 08 April 2014, 10:35   #5
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I have no experience of add on jackers, but even to my eye the engine looks far too deep. Is it a long shaft engine on a short shaft transom ? Even without the jackers this could give problems, once you put the additional tubes in there who knows what will happen to the handling characteristics of the hull.

The nearest we have to your set up in Europe is the Zodiac Futura, but the speed tubes are much smaller. The Futura runs fast a true with no handling problems till you are right on the performance edge.

I'm not suprised its got handling problems, suggest you need a short shaft motor and a trip back to the firm that fitted the jackers.
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Old 08 April 2014, 11:14   #6
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My problem is my water inlet in underneath the anti ventilation plate, (the horisontal plate above the prop) .

So i am really scared for overheating of the placement if too high, or am i missing something guys?
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Old 08 April 2014, 11:17   #7
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Does this look any better?
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Old 08 April 2014, 11:25   #8
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From a non expert but keen enthusiast

I would say that the engine looks about right. if you can get the sib to ride on the tubes (enough speed) I do not think it should be any higher than it is.

Someone will be along that has a futura or the like and let you know 100%

This is in reply to your earlier engine height not the 1 above.
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Old 08 April 2014, 11:42   #9
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These speed tubes look pretty big. Since they were professionally done, what did the installer say about a new engine height? Surely they might be able to offer some insights into their proper use.

I don't have any speed tubes but I suspect you need to:
1) sit in the absolute center of the boat
2) raise the engine up about 8 or 10cm
3) try not to go slow
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Old 08 April 2014, 14:09   #10
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Hi

A few thoughts for consideration beyond the engine position, but am I right in thinking the engine needs to be lower because the boat will be much higher in the water when planning?

I had a similar factory built set-up on a Lodestar Trimax 3D-V. The two main difference are:
1: The sponsons on the Lodestar are more inboard, so the main tubes still contact the water to provide lateral stability.
2: An inflatable keel runs the length of the hull to the transom, giving a much deeper V. This effectively produces a trimaran, offering better directional stability as the V cuts through the water.

As a comparison, your sponsons are nearer the bottom of the tubes, and your boat is much flatter. Therefore, when planing, the sponsons are the only part of your boat that can have contact with the water. There is no centre V to help directional stability, and there is no lateral stability from the main tubes, if the boat keels over beyond the sponsons.

This is only an idea, but perhaps the sponsons need to be moved further inboard, so the main tubes can help stabilise the lateral movement?

Here are some images of the Lodestar's design, including one showing engine height and tube position.
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