Problems with 18ft Jon Boat with Tunnel Hull. Won't get on a plane.

I'd say Eric is spot on. That tunnel looks too small to me, along with the motor being a bit too high. You can probably lower the motor with a minimum of fuss and a bit of fabrication, this would probably do the best with the least amount of cash involved. I don't know if even a different plate would do the job... I would have to see the boat floating. Can you see the tips of the prop at rest? Are they within an inch or so of the surface? I bet if you dropped the motor down 2" or 3" most if not all your problems would disappear. The tips of props on most boats are about 4" below the surface of the water to give you an idea.

 

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First off, the boat pictured is not the OPs boat, but a picture from the manufacturers website.

You can see part of the writing on the mount, and it's equipped with one of these $180 manual jack plates...


http://www.starmarinedepot.com/th-m...zz15541.html?gclid=CL23ve3ygL0CFRQV7AodcCoAsA

Instead of $9,000+ for a new, long shaft motor where you still have a fixed position prop/cav plate, why not a much more useful hydraulic jack plate for $700 for his current motor?


http://www.discountmarinesupplies.c...k_Plate.html?gclid=CP622c7ygL0CFTBk7AodoQIAFA

I'm wondering also if it would help to extend the tunnel by bolting an extended housing onto the lip of the tunnel..?


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If I had bought this boat new, I would be consulting with a lawyer right about now to get my money back. This is a defective design. I suspect Redneck Engineering of the "git 'er done" variety...... The jack plate shown on the company website is a giveaway: they know the boat has problems and the jack plate is their band-aid.
If you bought it used, you are stuck with spending more money to make this butchered up mess a workable boat. Good Luck!

 

Jack plates have a legitimate use and don't mean the boat is bad.

 

Agreed, but I sure think it does in this case..... If a jack plate is necessary to get this "design" to work, then it should be installed on every hull. The fact that the builder has one installed indictates that they are aware of the problem.

 

Oh come on, there is nothing wrong with the boat. Maybe it works perfectly with a fifty. The point is to get the draft as shallow as possible, and that happens to be a lot easier to do with a little motor than with a big one.

If the air is coming from behind the transom, get a bigger plate. Easy to check on. Tie the motor leg to the dock and run the throttle up and look for the whirlpool. If the air is coming through the tunnel, trim the motor down a bit to try to take off a bit flatter. I have this image of the prop ventilated and the nose stuck in the air. In the end, you probably want to lower the motor a skosh if you can accept the draft increase. I would want to be able to roll the throttle open all the way within 5 or 6 seconds.

 

Despite what has been said, I'd first try a stainless prop, I have had some experience with engines mounted behind tunnels, and it is chalk and cheese, alloy props are nowhere near as tolerant of 'starvation' of water to the prop as stainless. Of course if in fact the current prop is a stainless, you just gradually go deeper till it bites reliably. Of course that goes against the whole raison d'etre, which is ability to run in shallow water, but if it means the boat is going to be restricted in places it can access by having the engine an inch lower, you need an airboat !

 

Most of the contributors have pretty much suggested to lower the motor which is of course the easiest solution to the problem.
The water coming out of the tunnel is below the VENTILATION PLATE so when you are accelerating much of the prop is going to see some air. The water will rise a bit between the spot where it exists the tunnel but obviously not enough to cover the prop on acceleration or perhaps at speed. When I first saw this tunnel, it appeared to be the type of tunnel designed for an outboard jet which has a narrow inlet and these outboard jet inlets normally fit up very close to the exit area of the tunnel and slightly below the water flow. I went to the website and saw that the G3 manufacturer also shows a transom picture with the Ventilation plate above the tunnel.

You did not say but I would expect that you may have a problem turning tight when on plane and then the prop ventilates. You may find that when the boat turns, the lowest edge of the tunnel leading forward to the bow may ventilate causing the prop to ventilate as well.

I would not expect that Phils idea of tying the leg to a pier and looking for air getting entrained in the prop will work as you need some boat speed to provide a green water supply to the prop. But leaning over the back of the boat and watching the water flow from the tunnel with respect to the ventilation plate as you have someone else drive the boat will prove to you that the prop is much too high.

If the prop is also wider than the width of the tunnel at the top, there will be air entrained into the it as well.

The builder if he wants to build a tunnel for a prop should really build the tunnel wider than the prop to keep the prop covered.

Cavitation is caused by low pressure areas created on surfaces of a prop/foil/ If the pressure is low enough, bubbles form which can migrate to another spot on the foil/prop where the pressure is higher at which time they can collapse and cause cavitation corrosion.


Ventilation is the introduction of air from atmosphere or from a hull design which provides air into the prop area.

 

I don't normally frequent this forum, but this post happened to catch my eye. I've been running various tunnel boats for the last twenty years.

First, you need to determine if you have a jet tunnel or a prop tunnel. If it's 2-3" deep and less than 2' in length, it is most likely a jet tunnel and that would be the source of your problems. However, if it is 4-5" deep and about 3'+ long, then you most likely have a prop tunnel. G3 has made these for years, I've driven them, and they are not a horribly bad design. They could use improvements, yes, but they aren't horrible.

What you need is a "tunnel" prop. This is typically a stainless steel prop with a large blade area, designed to grab the water. A good prop will deal with the aerated water and still give you plenty of thrust. It can be either three blades or four blades, but each blade needs to be large.

Contact Matt or Louie at baumannprops.com, give them your boat specs and they will hook you up with a good SS tunnel prop. You test it out and report back to them, they will swap it out until it performs correctly, but they usually get it right on the first try. They are over 1000 miles away from me and I deal with them all the time, props via UPS. If you don't feel like dealing with them, PowerTech props ptprops.com is not a bad option, either.

A good three-blade SS will run close to $400, a good four-blade will run close to $500 (still cheaper and better than good oem props). On my last boat, there was no performance difference between the three-blade and the four-blade, other than the four-blade held the boat better when cornering.

How good are these props? Let me put it this way - on my boat, the water intakes will suck air before the prop will lose bite!

 

Village
What is the height of your ventilation plate with respect to the top of the tunnel and how far is the start of the ventilation plate back from the exit of the tunnel

 

The leading edge of the plate is 5" above the top of the tunnel, and about 8" back from the tunnel. In this configuration, it will still get up on plane when lightly loaded, although it takes a second or two for the water to start really flowing through the tunnel. Once the boat is moving, it is planing within a boat length. It does have pods on the transom, so there is no squat during the holeshot. When the boat has a load on it, I have to drop the jackplate an inch or two to get a decent holeshot.

I should also mention that when the jackplate is at full running height, the intakes do tend to suck a little air. I installed some Mercury Racing Shrouds to alleviate that problem, although it still takes a bit to prime at idle (this is a Merc 115 4s).

 

From what i can see you probably need a bigger pitch on your prop. My race tunnels would cavitate and blow air if the prop was too small in pitch.You could drop the motor but then it wont get the speed it should or the shallow draft.They have made it surface piecing for a reason.The website looks very professional, they are not boats that don't work. Contact the manufacturer and tell them which boat you have, the Outboard you have, the prop size you are useing and the problem you are having.
I am sure they will be able to give you advice to fix it. The photo looks like an alloy prop, so they may pitch it higher than a stainless to compensate for the flex.

Good luck
Don

 

I have worked on a few boats with this design and problem. A good friend builds a very special one. It will get on plane in the length of the boat run in 4" or less of water will jump low levees or run across 50 yard wide mud flats. The only thing you need to remember is to have 6” or better of water to set down in to keep the motor cool and get up again. With that said you need to find someone to double cup your prop and add a vent at the front of the tunnel. The vent is very important to stop blow out. My buddy had tried several of the name brand boats which all had the blow out problem. He found the vent reduced or stopped the problem. Several friends brought their name brand tunnel hulls to him to modify. The tunnels size looks about like the one in your picture. The first one I saw My commit was the tunnel was to small for the 200HP, but was told to big of a tunnel will not direct enough water to the prop and lower unit.

 

You are unlikely to find any tunnel design that does not interfere in some way with the operation of the propeller and in particular when turning if the prop is expected to work in disturbed water flow and the prop is set up higher than would be normal (without a tunnel) on the transom.
A surface piercing prop may be your only chance.