Outboard propeller ventilation

[Mikedefieslife]

My Marinier 2.5 (I guess 3.3 now the carb is modded) will propel my 35kg dinghy with only me in it at around 7-8knts in good conditions with my weight in a certain spot. With any weight that speed drops to around 5 knts.

I seem to find though, that at higher rpms there a loud gurgling noise and the prop slips in the water and RPMs increase while forward momentum slows. I presume this is ventilation.

The problem is that I can't seem to tune it out. My dinghy has an angled transom which doesn't help, so in order to get the outboard level when at any speed, I have to have it trimmed all the way out which helps a little but doesn't solve it.

Other than potter about at around 3 knts, is there anything else that can be done to sort this?

 

[Heckler]

My Marinier 2.5 (I guess 3.3 now the carb is modded) will propel my 35kg dinghy with only me in it at around 7-8knts in good conditions with my weight in a certain spot. With any weight that speed drops to around 5 knts.

I seem to find though, that at higher rpms there a loud gurgling noise and the prop slips in the water and RPMs increase while forward momentum slows. I presume this is ventilation.

The problem is that I can't seem to tune it out. My dinghy has an angled transom which doesn't help, so in order to get the outboard level when at any speed, I have to have it trimmed all the way out which helps a little but doesn't solve it.

Other than potter about at around 3 knts, is there anything else that can be done to sort this?

Cavitation? Cut a slot in the transom so you can get the engine leg lower in the water?

 

[VicS]

My Marinier 2.5 (I guess 3.3 now the carb is modded) will propel my 35kg dinghy with only me in it at around 7-8knts in good conditions with my weight in a certain spot. With any weight that speed drops to around 5 knts.

I seem to find though, that at higher rpms there a loud gurgling noise and the prop slips in the water and RPMs increase while forward momentum slows. I presume this is ventilation.

The problem is that I can't seem to tune it out. My dinghy has an angled transom which doesn't help, so in order to get the outboard level when at any speed, I have to have it trimmed all the way out which helps a little but doesn't solve it.

Other than potter about at around 3 knts, is there anything else that can be done to sort this?

Are you sure this does not occur at all when you get your weight in the correct spot? I am thinking if it did then it could be a spun prop hub.

The real solution I think is a mounting bracket that allows the engine to be correctly trimmed !

However is your engine the correct shaft length for the dinghy transom height. If it is a standard/ short shaft then the top of the transom should be only 15" above the water level

 

[VicS]

Cavitation? Cut a slot in the transom so you can get the engine leg lower in the water?

No cavitation is somethinng quite different. It can occur at any depth, even with submarines!

Good video that explains cavitation:

 

[Graham376]

What's the dinghy? It's unusual for an outboard to be tilted so far. My money (as VicS mentions) is on a knackered prop bush, quite a common fault, at a certain torque, revs increase but speed doesn't. Trying to turn prop by hand it would still appear to be bonded OK so the nut and prop need marking to see what happens under load.

 

[Mikedefieslife]

The dinghy is an Excel SD260 (http://www.excel-inflatables.co.uk/inflatable-boats/inflatable-boat-sd260.php)

The prop is lower than the dinghy bottom, and the cavitation plate lines up with the bottom of the transom.

I don't think it's a damaged prop. I've got two here can check (and spare pinion and split pins). It's seems like air being sucked down into the water it's a quite a loud audible sound. Like a big guy gargling and hacking.

 

[TQA]

Make the cavitation plate larger. I used 3/32 ali plate but I have seen epoxy soaked ply used successfully. about 4 inches to both sides and back worked but I have no idea if this is optimum.

It worked for me.

 

[Heckler]

No cavitation is somethinng quite different. It can occur at any depth, even with submarines!

Good video that explains cavitation:

Ahem! I am aware of cavitation as is described but the use of the word cavitation is also used in this situation as is evidenced by the other posters.

 

[VicS]

Ahem! I am aware of cavitation as is described but the use of the word cavitation is also used in this situation as is evidenced by the other posters.

But incorrectly. Its ventilation, as in the thread title.

 

[Heckler]

But incorrectly. Its ventilation, as in the thread title.

Ah, but that is how the English language has evolved!
PS good job I put a question mark in my first post!

 

[Davy_S]

Try raising the engine by 1 inch, a simple piece of wood under the clamps will do, it will cost you nothing to try it, it certainly works with bigger engines, if no joy you have lost nothing in trying.

 

[Mikedefieslife]

Higher is worse. Tried with it with a piece of pipe under the bracket.

According to the manual the ventilation plate should be 1-2" bellow the bottom of the boat. It is lower, but barely. Though is is probably due to the deep v the dinghy has.

I don't think cutting up the transom (on a dinghy that is still in warrant) for what is essentially a small under powered motor is worthwhile, so I might just have to live it until I get something more powerful