Inflatable Questions

[Richard]

I just bought a used (2003) 9'4" West Marine/ Avon inflatble with a 5 hp Mercury 2 cycle OB today. When I took it out for a shakedown, the engine started to rev way up as I accelerated. It sounded just like a slipping clutch in as car but I suspect it is cvitatition. The prop seems tight on the hub and it does not look like it has ever hit anything.

I dropped the engine all the way down and sat as far forward as possible (I was by myself) and that helped a little bit. It finally got up on a plane but it would rev up if I hit a wave or turned. It just does not seem right but this is the first inflatable I have owned and I have not been able to try it with two people onboard.

The shaft length looks long enough and the boat and motor were purchased from West as a package by the original owner so I would ssume that they were a properly matched pair.

Is this just the "nature of the beast" and will it get better when I get a passenger up front?? (my wife is away for a week and I have not been able to find a passenger yet).

Anybody else experience this with their inflatables when running solo? or do I have a problem? I am trying to decide whether or not to stop payment on my check first thing on Monday morning.

[jcollins]

Richard,
I'm not an expert but I have owned a couple of inflatables. My first was Zodiak. Loved it but lost it to Isabel.
Next was a Boat US brand. I had the same problem when running alone. You tuck the drive down to get up on plane but when your there it will cavitate when you hit a little chop. I chalked it up to being a light boat. Try to trim the motor up one notch. It will be harder to get on plane but may help with the cavitation. I keep my fuel tank in the bow as well. (maybe move it back when alone?) With 2 people in the boat it should be fine. Mine is a 10'3" w/ 9.9 but the principle is the same. Unfortunately, the plywood floor has rotted out and a very large friend jumped in and finished it off. (CRACK) I am pondering replacing the floor or just bite the bullet and buy a new one. I was worred about coming to the rendezvous without one but since you have a new one, we'll hitch a ride with you.

[Mariner]

That's definitely cavitation/ventilation. Now to determine what is causing it. Cavitation can occur for two reasons: aerated water reaching the propeller (techincally called ventilation), or an improperly pitched propeller being put under to great a load.

The first is usually the most likely. Does the anti-ventilation plate stick below the bottom of the hull? Is the propeller always in the water? Ventilation is generally more likely to occur when the engine is trimmed too far OUT, not in. Trimming it in will usually reduce it.

True caviation occurs when the vaccum on the lee side of the propeller blades is so great that it actually sucks suspended nitrogen and oxygen out of the water and the propeller loses it's "grip". The end result is the same: essentially like a peeling out in a car. Cavitation can be caused by the use of a propeller that has too much pitch for the engine/boat combination. Lightweight but bulky boats like inflatables are particularly prone to this. Their heavy drag when loaded and accelleration requires a very low pitch, but once they get on a plane, they have almost no resistance and can accomodate a very high pitch. People tend to size propellers for the latter condition, because they do not want to give up the high end speed. Unfortunately this may mean that getting up on a plane in certain loading conditions will be nearly impossible. When an engine encounters extreme resistance, as when accellerating this condition, the result is caviation.

It sounds to me like you may be experiencing both caviation (on acceleration), and ventilation (in chop once on a plane). First ensure your engine is mounted properly, then consider reducing the pitch of the propeller. It may also help to relocate weight forward, or even just to lean forward when accellerating.

[jcollins]

After thinking about it, I agree. Triming up is not the answer. Moving the tank forward will help with getting up on plane. I never experienced it with the Zodiac but I do with the Boat US brand.

[Richard]

The large plate on the lower unit just above the prop is well below the bottom of the boat but a smaller, upper plate is above the bottom.

There is a lot of turbulence in the prop area between the tube extensions when I power up and this may be introducing air.

[Merry Kate]

Richard -

I have the same rig as you and with just me and no gear, it will barely plane and then only if you get your weight way up to the bow when coming on plane. (I weigh 200 lbs and it is an Avon 280 air floor which is very light.) I cut a 2ft long piece of 1.5" PVC drain pipe and jam it onto the motor grip for a tiller extension - works great to get your but up on the seat. Also you better make sure you are in control in this mode cause it is REAL squirrely.

I would'nt expect this boat/motor combination to get you around fast. We load it up with the family & just putt along. The 5 hp is plenty to get through any current and works great with it.