cavitation problem

Omar Hernandez · 31 August 2001, 05:18

Hi to all, and thanks to batfalcon for inviting me into this forum.

I have a 12.6 inflatable (not a RIB) powered by a 15hp mercury, used mainly in freshwater reservoirs for bass fishing. The problem I have is that more often than not, the motor will start cavitating. I think it may be due to the rounded tubes it has (instead of the usual cone shaped ones) and that at the manfucaturer´s suggestion I got a short shaft OB. Cavitation will usually start after a couple of seconds while gaining speed, but is seldom (or never) a problem once the boat is running at the top speed allowed by the outboard.

I was wondering if it would help to bolt to the anticavitation plate one of those plastic extensions. I would really hate to drill the plate if this will not help or prove more damaging at the end.

Thank you all for your input on this matter.

P.S. This is my first time really using this forum, so please be patient with me as I´m not familiar with the site´s format.

Keith Hart · 31 August 2001, 06:55

Hi

This sounds like a simmilar boat to mine. Excuse the ignorance of a newcommer, but, what is cavitating?

Keith Hart

batfalcon · 31 August 2001, 07:28

Due to extreme water pressure on the front surface of the prop's blades, water tends to boil, thus creating millions of bubbles. In other words a film of air between the prop's metal and the water. The prop looses the contact with the water thus thrust is beiing reduced and the rpm increased to the limit. Of all the above can be fasten up by the wrong installation height of the engine on the transom. That's all in a few words.

Michael

John Kennett · 31 August 2001, 08:52

My guss is that it is more likely to be ventilation rather than cavitation. There's a selection of articles about it here

Is there any damage to the "hull" fabric, especially around the transom? This can cause this problem with inflatables.

Unless the boat is a weird design, then a short shaft outboard is likely to be correct. The bolt on hydrofoils may help, but there's no guarantee! They do seem to to be more effective on smaller, low performance boats like yours though.

John

batfalcon · 31 August 2001, 09:13

Thanks for the correction AND for the list of articles. Whatever is called (cavitation or ventillation) it can't do any damage to the soft hull's fabric, because all these things happen behind the transom , away from it and UNDER the water's surface.

Michael

John Kennett · 31 August 2001, 09:40

Sorry, I don't think I made myself very clear!

What I meant was damage to the bottom of the boat, especially at the transom, can cause ventilation at the prop as the water flow is disrupted.

I had a similar problem with a 4 metre inflatable a few years ago after a previous owner had run the boat onto the beach and then done a dodgy repair. Thinking about it though, this was more of a problem at high speeds once the boat was on the plane . . .

John

batfalcon · 31 August 2001, 12:07

My fault, sorry

Michael

Keith Hart · 31 August 2001, 16:32

My little inflatable has a short shaft engine and it was the one recommended by the manufacturer (in fact I think they are both from the same company Quicksilver and Mariner), I don't recall having any problems like this so short shaft must be the answer.

Keith Hart
(1 more post towards my Senior Membership)

Charles · 31 August 2001, 19:36

What prop are you using?

Keith Hart · 01 September 2001, 07:12

Quote:

Originally posted by Stealth
What prop are you using?

Good morning. Who's prop are you asking about?

Keith Hart

Charles · 01 September 2001, 11:42

I asked Omar what prop he was using, everything begins with the prop!

batfalcon · 04 September 2001, 19:12

Stealth asked you what prop are you using.

Michael

Omar Hernandez · 05 September 2001, 01:44

Well, thanks to all for the valuable information. It does seem like my problem would be more correctly identified as ventilation rather than cavitation. And it seems that the extension should help, at least in theory.

I´m not sure whether I should consider my inflatable of "weird design", you can look at the pictures of such boats on the manufacturer´s website, www.seaeagle.com

I'll let you know if I perform this change, the outcome.

Regards,

Omar

Omar Hernandez · 05 September 2001, 01:47

Oh, about the prop, I am not sure I can give a detailed answer to that one. All I know is that the prop the outboard has is the one that comes as standard equipment with the outboard. I've never given any consideration into replacing it...Should I be?

Omar

batfalcon · 05 September 2001, 09:23

Take a look at the center of the prop. Maybe you are lucky enough to identify the prop's characteristics from the front. In any other way you must uninstall the prop and look at the back.
If that's of any help.

Michael

Charles · 11 September 2001, 14:47

These days props are an art and left the world of Science behind ages ago! I would be so bold as to say that most problems are caused with the WRONG prop. Contact a company in England called Steel Developments and give them all the info they require to sort you out with the correct prop for your boat and application.