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

Hey guys, I'm having problems with my 18ft JonBoat. I bought an 18ft G3 JonBoat with tunnel hull and 60HP Yamaha outboard motor last year and I have always had problems when getting onto a plane. In order to get on a plane, I need to slowly increase throttle so that the motor doesn't kick out. When I give it too much throttle too fast, the motor stops pushing water and just spins out. I've read on different forums about people having the same issue that water just isn't getting to the prop and eventually just spins out, but I'm not interested in putting bulky float boxes on the back or anything drastic that could get in the way. Help!

 

It sounds like air is getting in, or as you say, water flow to the prop is interrupted. Is the boat a "brand" name, or home made ? There could be a difference when she squats before planing where the flow of water, or air, into the prop, is unsatisfactory. Bear in mind too, that this is when the prop is most likely to "let go", the prop is spinning faster per unit of forward distance gained than when cruising along. Might be an idea to check your prop isn't slipping on the rubber hub, too.

 

Try experimenting with a couple different props.

Or add a power lift jack plate. Lower your outboard down a couple clicks to plane and then raise it up for full speed.

 

A what height is the engine installed? With the engine trimmed level, how far above (or below) the bottom of the boat is the cavitation plate?

 

Mr Efficiency, it is a name brand name G3 JonBoat. Matt, the transom height is 27" and the motor is mounted on the lowest height possible. The cavitation plate is an inch higher than the tunnel hull. IMP-ish, if I were to put a power lift jack plate on it would I be able to lower the motor lower into the water? I have been looking into those cupped props, but haven't really talked to anyone about if they work or not. Thanks fellas!

 

These pictures were taken from the G3 website, but they pretty much sum up what my boat looks like. However, I do not have the buoyancy pods since they will more than likely get in the way.

 

Is that an alloy prop ? I suspect you might need to either lower the engine, or change to a stainless propellor, they are less prone to cavitation problems, mainly because of the cupping and thinner blades.

 

It looks like it's got a jack plate on it to use as IMP-ish says. How would the pods get in the way?

 

Many to choose from with different mounting holes.
The jack plate has to be mounted so its low position is lower than where your outboard sits now.
Worst case --- four new bolts thru the transom to mount the jack plate lower.
Then you have a button attached to the wheel or throttle that you hit to raise the outboard once you're up on plane.

 

That second photo, I suspect shows the problem - the fence (on the motor) is way above the bottom of the transom even with the tunnel. Air will be getting down the leading edge and giving you the problems you describe. Simply lowering the motor or using a long shaft variant of the motor should sort it out.

It looks like the maunfacturer has tried to get a very shallow draft for the boat and not allowed the prop to bite in solid water. I bet if you can get someone to watch the leading edge as it is popping onto the plane, air will be seen intermittently being drawn downwards and swept onto the prop. You might be able to film it, or take shots with a GoPro or similar.

If air is not causing this problem and it would surprise me if it was not, subtle angling of the engine should cure it.

 

He might find a stainless prop solves it, but unless he can borrow one to test, it is an expensive experiment, if not successful.

 

Any chance you can try a 4-blade prop?

 

I agree. That motor is too high in that picture and prop ventilation would occur pretty easily.

Those boxes on the stern most likely significantly improve the take-off characteristics as you attempt to get the boat on full plane. There is a lot of additional planning lift in those...

 

That tunnel is pretty small compared to others I have seen. It appears to be smaller than the projected cylinder that the prop moves through. It leaves the cavitation plate some 4-5" above the true bottom of the boat, but doesn't give a large volume of water for the prop to move.

Is there a "G3" user forum somehwere on the web, where you can see the setup of other similar boats? If you bought new, I would be consulting with the dealer.

Other ideas...some manual jackplates can be mounted upside-down to change the range the motor goes through. I assume you have the engine trimmed all the way down when attempting to plane? Have you moved weight around (fwd)? Those floatation boxes would also serve to keep the boat more flat and reduce the load to get over the hump, like a pair of trim tabs.

A 60hp should have no problem planing an 18' jon boat.

 

JonBoater, I checked the G3 website and other on-line photos of the G3 jonboat, and I suspect that there are two things wrong. First, I the design of the boat, with the tunnel, is to allow the boat to operate in shallower water than it otherwise would do if it did not have the tunnel. That allows the motor to be set a little higher with less draft to the bottom of its skeg. As a result, the propeller is too close to the waterline, and the propeller is sucking air from directly above, causing full blade cavitation, and so the prop just spins in place. The fact that you can get the boat to plane if accelerating very slowly confirms that this is happening. Pushing hard and quickly on the throttle causes the propeller to spin very fast right away with a lot of sucking power, enough so that it sucks air down from directly above.

The second problem may be that the shape of the tunnel is not the best. It could be too abrupt in shape at the front end, thereby allowing a lot of air to travel with the water into the prop. This could be aggravating the problem of allowing more air into the prop. The tunnel also does not look very wide compared to the diameter of the prop, so that could be part of the problem. Unfortunately, if the tunnel were made wider, that would adversely affect the planing performance--it would be harder for the boat to get on plane, which is what you are trying to achieve.

Changing to a stainless steel prop or going to a 4-blade prop are not going to help. Also, I don't think the float boxes are going to help either. They look to be fitted flush with the bottom surface of the hull, and while they might add bottom surface area that will help with planing, they will also add buoyancy which will tend to raise the hull out of the water even more, and bringing the propeller closer to the surface, and therefore likely to keep sucking air.

The solution to your problem is to at least prevent air from being sucked down from just above the propeller. You can try adding a larger anti-cavitation plate onto the outboard motor's existing anti-cavitation plate which is not big enough in this application. What an anti-cavitation plate does is put a wall, a barrier, between the propeller and the water surface, thereby killing the suction directly above the propeller. There are any number of add-on anti-cavitation plates on the market.

Another solution, suggested above, is to replace your outboard with a long-shaft model of outboard. This does basically the same thing as anti-cavitation plate--makes it harder for the propeller to suck air from the water surface above simply by being deeper. The drawback is that it makes your running draft deeper, but maybe you can live with that.

I hope that helps.

Eric