Prop Ventilation Help

[Confused]

Have a 5 metre Rib powered by 50 HP Johnson ( power trim & tilt 2000 model ), when I first used the rib I noticed prop slipped when going into any turn or bouncy water. As the prop was not in excellent shape replaced it with a stainless one (15 " secondhand but in very good nick). There appears to be no difference.

I am beginning to think that this is not cavitation but perhaps hull induced ventilation which could be harder to cure. I am unable to lower the outboard any further to see if this helps the problem as it is on its lowest setting.

Now cavitation / ventilation during turning I have heard is quite common, how many have experienced this and how have you been able to cure this very frustrating problem.

The cavitation plate would appear to be about 1" below bottom of hull vee.

All sensible advice gratefully received. I guess what I would like to hear is that I can fit some type of better prop which will cure the problem, but after changing to the stainless steel prop, I am now not convinced and very concerned that this engine is simply not the right combination for my hull.

 

[BarryH]

Still need a bit more info. What length leg does the outboard have and what do the manufacturers of the hull recommend. Sounds as if its about right if the cav plate is just below the keel. Is it underpropped and can you hit the rev limit easily. Is it a power trim model or a non trimable model. If its not, is the trim pin set to high.

When you say you changed the prop for a stainless one, do you know what pitch and diameter it is. From what you have said it does sound like its cavitating. It will in turns if its trimmed out or up too far. Some hulls do cavitate when pushed hard in the turns. Mine does when I'm being a bit too heavy handed on the throttles.

A good place to start would be the people who built the boat and see what they recommend re the prop size.

 

[BILLABONG]

Had similar problem on a 5 mtr Gemini I use to have with a 75 bhp Mariner and 19" aluminium prop. The cavitation/ventilation got better on tight turns (prop slip was less) when I went down a pitch to 17". I do not know whether this little bit of info helps at all.


Playing on the beach again

 

[Confused]

Engine is a Johnson 2000 model power trim & tilt.

Yes I can reach maximum revs easily ( I have to be careful not to over rev when it slips ), I have thought of trying a 17" to reduce the revs.

The diameter of props are fairly standard according to pitch and is therefore not a variable I can consider. The prop size is 15 ".

I appreciate many boats will cavitate in tight turns and if you trim the engine up too much, but this happens in light turns and choppy seas and trim gauge says that it is trimmed quite well down.

Keep ideas coming please

 

[jleaworthy]

I suspect that the only change to your present setup which would help would be a propeller with a higher blade area ratio ie wider blades. You could talk to your nearest Johnson Evinrude centre. Probably the only real answer is to get your propeller deeper into the water either through a longer leg or by taking a chain saw (I've actually seen someone doing this) and reducing the height of your transom. But watch out for the overtaking stern wave as you come off the plane!!
However you did ask for sensible advice so please disregard the above.

 

[byron]

<font color=blue> Indeed! From recollection of such matters, shouldn't the cavitation plate be located 1" <font color=red>above</font color=red> the bottom of the Vee? Sounds to me that on your tight turns you are sucking air repositioning the lie of the cavplate will rectify this.

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[BarryH]

Surely, if the cav plate is above the keel whn the boat is on the plane it will be riding clear of the water and the prop will be able to suck air from the surface. The one thing the cav plate is there for. When you look over the stern at mine the cav plat is riding just below the surface of the water.

Maybe those Doel Fin things may help sort iur the cavitation probs. Cheaper than replacing a stainless prop. I used to use them on a small fletcher that I had. Improved the effect of the power trim and popped straight up onto the plane and pstayed on the plane at lower revs. Just a thought.

 

[byron]

<font color=blue> I am reasonably confident that for optimum performance the cavplate should be where I said. I know I am getting old, frail and somewhat doolally but I can still just about recall my race days of the 1960s.

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[Chris_d]

Ahh.. well your both wrong, I think !, general practice is to position the cavitation plate level with the bottom of the vee, so it will become an extension of the planing surface.

If changing the pitch of the prop is having no effect, and I don't see how it could, unless its a completely different shape, could it be you are trimmed too far in or out
both would cause prop slip.

 

[gonfishing]

this is a tricky problem that a lot of boats suffer usually do to engines /boats/props
being sold as average /std set up to accomodate the general market ,as a users experiance increases and you start to look for a particular type of performance then things change and you will need to choose the right prop the only person that can really sort this type of problem out is a naval architecht, propeller design is a science and i mean a science i have had a look at the math involved OUCH you can only really hope to get something close to what you want unless you want to spend a small fortune in getting a prop designed for your boat and again as the boats load.,number of people on board etc, all have a serious impact on performance, as Byron will tell you if he has been involved on the racing circuit just like tuning an F1 car to suit the conditions there is no perfect prop on the day. so spend a fortune or put up with it
sorry not to be of much help
here endeth the sermon

 

[BarryH]

Yes true, but they were the good old days of cleaver,s and surface piercing props, out and out speed machines. This is a rid with a low tune 50hp on the stern.

Have you any pics of your racing boats, would sure be interested in having a look. What class did you run in, type of hull, engines, that type of stuff...oopps sorry getting into anorak mode!

 

[Confused]

Ok here are some pics so you can view set up and guage height of cavitation plate etc

http://homepage.ntlworld.com/p.lyon/rib2.jpg

 

[BarryH]

last bit of the link is wrong it should be [/url].

 

[longjohnsilver]

Byron, did you ever come across a guy called Micky Mantle when you were racing, used to live next door to him when I was a youngster.

 

[byron]

<font color=blue>Didn't actually race myself, you saw all the Plaques on the wall at my place ? I was a Patrol Boat (Steward) however the name does ring a bell. The only racing I did was off-shore water ski racing which never really caught on over here.

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[byron]

<font color=blue>When I raced (water ski racing) Shakespeare supplied my boats and Carniti my outboards. Shakespeare would set up the boat as I described.

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[longjohnsilver]

Were you the driver or the skier? Think I can probably guess /forums/images/icons/smile.gif

 

[byron]

<font color=blue> In those days, both. Depended on rules of race, long distance i.e. cross channel & back we were allowed to swap skiers. I wasn't always old and fat John.

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[longjohnsilver]

Oops...................

We all know that you were/are a suave debonaire man about town. Still that waterskiing sounds like hard cold work. Didn't realise they were over such long distances.

 

[byron]

<font color=blue>I am not familiar with the effects of those wings you have fitted, they didn't exist in my day. I would however reiterate that the main cavplate should be 1" above for maximum speed. Or level for day to day operation. Having said that I observe that I don't know what those wings are for.

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