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Author Topic: cavitation  (Read 5378 times)

Glen Howard

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cavitation
« on: January 13, 2008, 11:24:07 am »

Hi all - i built a small broads cruiser from a Model Boats free plan. I placed rudder and drive gear exactly where the plan said, but I find when I turn the boat to starboard at any speed above an idle throtte she makes a terrible thrashing in the water, but produces no turn and no go-forward. As best I can tell, the problem is cavitation - that is, there is so much air and bubbles being directed onto the rudder that there is no water for the prop and rudder to 'bite into'.

Has anyone had similar experiences to this one? If so, were you able to find a solution (hopefully one that does not involve re-installing the drive line...)?
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chingdevil

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Re: cavitation
« Reply #1 on: January 13, 2008, 11:31:55 am »

Hi Glen
Three questions
Does she move forward ok?
Is the prop rotating the correct way?
Is she set at the correct height in the water?

Brian
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tigertiger

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Re: cavitation
« Reply #2 on: January 13, 2008, 11:33:18 am »

Hi Glen

I have really no idea what I am talking about, but a few questions spring to mind.

Do you have the correct type of prop?
Is the motor spining too fast, do you have a reduction gear box?
Is she low enough in the water, i.e. down to the water line?

TT
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bogstandard

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Re: cavitation
« Reply #3 on: January 13, 2008, 11:42:53 am »

I think it will be ventilation you are getting rather than cavitation, unless of course you are grossly overpowering it.
It sounds like you are sucking air from the surface.
If it was me I would try putting some temporary weight into the stern to lower it and then still see if it still occurs, or if you are not down to the plan indicated waterline, ballast it to the correct level.
Also check your rudder deflection either way, 35 degrees is really the maximum amount you want to have, any more than that and the rudder starts to act like a brake and throw water back at the prop, this could cause the problems you mention. It might not happen on both deflections because of the direction of prop rotation.

I hope this helps

John
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Glen Howard

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Re: cavitation
« Reply #4 on: January 13, 2008, 11:43:51 am »

My goodness - that was quick, thanks chaps!

Right:

- yes, she moves forward very well, and even gets up on a plane (more or less)

- Yes, she is ballasted to the waterline on the plan - could always put more weight in, would that help?

- yes, the prop is the right one, as far as I know.

- no the motor is not reduced through a gearbox, so she is technically overpowered. However she turns to port with no trouble, only starboard seems to be an issue.

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chingdevil

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Re: cavitation
« Reply #5 on: January 13, 2008, 04:35:48 pm »

Hi Glen
Check that the rudder throw is the sam for both sides, as bogstandard says there is a maximum the rudder should go otherwise it acts as a brake. The other effect of this is the prop just churns up the water instead of moving it.

Brian
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Colin Bishop

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Re: cavitation
« Reply #6 on: January 13, 2008, 04:50:26 pm »

Bear in mind that, depending on which way the prop rotates, in one direction it will be throwing the water down and away and in the other up and away. If the bottom of the boat is flat, as it proabably would be on a Broads Cruiser, then it's possible that as the boat accelerates air is being drawn under the hull with the effect described, especially if the prop is not very deep. Could you post a picture of the boat for us to look at?
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Glen Howard

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Re: cavitation
« Reply #7 on: January 14, 2008, 01:16:21 am »

It is possible that the starboard rudder throw is slightly more than the portside - but it's only very slightly so, if at all. With that in mind I have tried turning at various speeds on a much reduced rate of turn, but the result is maddeningly the same. I am going to try adding more ballast to bring her a bit more 'by the stern', see if that helps.

Anyway, as requested, som pics of the offending vessel ('Clementine') are attached here (fingers crossed).

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bogstandard

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Re: cavitation
« Reply #8 on: January 14, 2008, 06:33:00 am »

Glen,
If the picture is anything to go by, it looks like you already have enough depth at the stern if you are ballasted down to waterline.

The only other thing I could suggest will cost money, though not a lot.
If it was me I would buy a couple of plastic two blade x series racing props of about the same size as the prop you have on now, one LH turn and an opposite RH turn.
These props are designed to have occasionally a bit of air around them and still function satisfactorily. They might also load your motor a bit more (shorter run times) but again they are designed for faster running, if you have in fact over motored the boat. Not pretty to look at, but they are not in view when the boat is in the water.
The reason for the second prop, is to try the boat with the prop in the opposite direction, and see if the problem occurs on the opposite side, if it does then this will show as Colin suggests, air is being drawn under the hull and it might be that the hull form is not designed for the power of motor you have installed. Also it was most probably designed for cruising rather than planing.

You don't say what year the plan comes from, but it just might have been copied from a much earlier design, in which case the required motor for the original design was most probably something that ran on 6volts, fairly slow revving, and had about the same power output as a standard 385 (if you were lucky).

I remember when the first 'high performance' motors were starting to be imported into the country, and they cost a fortune in those days, and all they were, were badly made and noisy 540's that sucked your, then wet celled, lead acid batteries dry in minutes. 'Orbit' and 'Jumbo' spring to mind.

Hope this helps (or maybe confuse you even further)

John
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grasshopper

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Re: cavitation
« Reply #9 on: January 14, 2008, 11:28:18 am »

I've had this similar problem!  Going along with what Colin has said - try attaching an extension  to the transom level with the bottom of the hull and as wide as the hull - this will stop the air being sucked back into the prop.
just use some plastic extrusion to make an extension that will go beyond the back edge of the rudder and double sided tape it on to experiment.

When you've sorted that out and got the right size - disguise it by making it look like a swim platform.
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Peter Fitness

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Re: cavitation
« Reply #10 on: January 14, 2008, 10:17:46 pm »

Glen, I built one of these from the same plan, but enlarged it to give a hull length of about 76cm, and have had none of the problems you describe. It throws some water up when turning to starboard, but that is due to the prop wash being deflected by the rudder, and cavitation does not occur. Mine is driven by a 540 type motor with a direct drive to the prop, and the power is from 2 4.2ah 6v gel cell batteries wired in parallel, one in the bow and the other in the stern.
Some photos attached.
Peter.
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Glen Howard

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Re: cavitation
« Reply #11 on: January 14, 2008, 10:42:07 pm »

Thanks all - I will have a think about my next move, and post results in due course.

Grasshopper - do you mean extend the transom at the waterline? Have you a photo of this - though I think I know what you mean.

Peter - nice looking boat. It's always a bit sobering to see the work of someone who knows what they are doing and compare it to a beginner / muddler like me!

For my future reference, do you think this problem would have been avoided if the prop was set further forward than it is?

watch this space.

Glen
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Peter Fitness

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Re: cavitation
« Reply #12 on: January 15, 2008, 03:57:28 am »

Glen, it's possible the prop and rudder are a bit too far back, although I could not say with any certainty that this would cause your problem. The prop on yours does appear to be further back than mine, and also seems to angle downwards more. I have attached a photo of the stern of my model, unfortunately not a very good one, but it should give you the general idea.
Peter.
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tigertiger

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Re: cavitation
« Reply #13 on: January 15, 2008, 06:37:17 am »

Just a thought Glen.

Your prop does look to be dirrected down a lot more than Peter's.
Does the stern lift noticably when under power? If it does is could be lifting far enough for air to be drawn in. In which case re-aligning the prop might help.
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grasshopper

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Re: cavitation
« Reply #14 on: January 15, 2008, 08:01:22 pm »

Sorry no pictures available at the moment, but it should be easy to understand -
you're not extending the transom, more the hull bottom   - this will give the same effect as the prop being further forward under the hull , and will reduce air being drawn into the prop. one thing about doing this little 'mod' is that it can be done with a little bit of plastic and some double sided tape - if it makes a difference . then make it more permanent
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Glen Howard

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Re: cavitation
« Reply #15 on: January 16, 2008, 11:45:42 am »

Ok, will check her for a lifting stern. Might give this extending hull idea a go too. When I do get to this I'll let you know how I go, though it will be a couple of weeks.

Thanks all for your thoughts.
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IainM

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Re: cavitation
« Reply #16 on: January 16, 2008, 01:10:14 pm »

Hi Glen, I've been reading this thread with interest!  On the subject od transom flaps/platforms, perhaps these pikkies may help to explain.
I know these are on a Brave class hull and the purpose is to improve the pitching of the hull in rough water at speed, , but the construction principle is exactly the same.





The transom flap was made up from plasticard using liquid solvent adhesive and the adjustment uses two brass bottle screws salvaged from an old yacht rigging.  I have used plasticard tube for the hinge pin (no corrosion) and this is held in place by applying a hot soldering iron to the ends to create a slight belling effect.

You could easily make the flap look like a dive platform instead.
Since it bolts onto the boat with 8BA brass machine screws, it can be removed if you don't like it or its performance.
Just make sure to minimise the gap between the front edge of the flap and the back edge of the boat transom.  If you get this too big, it shovels water straight up through the slot and, whilst looking spectacular does not do the speed any good at all!!

BTW When looking at the pikkies of your boat, it strikes me that the rear end of the keel (just in front of the prop) seems to be rather large section (thick) and could well be masking the water flow into the prop.  I'd be inclined to carefully file away the profile to narrow down the cross section or, better still, file a semi-circular profile on it.  The less obstruction to smooth water flow into the prop the better the prop will perform.
You don't have room to extend the prop shaft without fouling the leading edge of the rudder, which would be the other alternative.

Hope this all helps,
IainM
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John W E

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Re: cavitation
« Reply #17 on: January 16, 2008, 02:29:56 pm »

hi there Glen

By your photograph of your boat, it looks like you have a fairly small brass propeller on there - in the region of between 20 - 30 mm diameter.   This may be the cause of your problem.

Also, as IanM has suggested, the deadwood/keel area just in front of your prop - looks to be rather on the thick side.   This will not help along with the size of the prop.  It will in fact cause more problems I think.

If possible try and go for a three-blade plastic propeller - as large as diameter as you can manage to fit, and, also try and reduce the area on the deadwood.

Hope this helps.

Aye
John e
Bluebird
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Martin13

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Re: cavitation
« Reply #18 on: January 18, 2008, 11:15:23 pm »

Hi Glen, I've been reading this thread with interest!  On the subject od transom flaps/platforms, perhaps these pikkies may help to explain.
I know these are on a Brave class hull and the purpose is to improve the pitching of the hull in rough water at speed, , but the construction principle is exactly the same.

The transom flap was made up from plasticard using liquid solvent adhesive and the adjustment uses two brass bottle screws salvaged from an old yacht rigging.  I have used plasticard tube for the hinge pin (no corrosion) and this is held in place by applying a hot soldering iron to the ends to create a slight belling effect.

You could easily make the flap look like a dive platform instead.
Since it bolts onto the boat with 8BA brass machine screws, it can be removed if you don't like it or its performance.
Just make sure to minimise the gap between the front edge of the flap and the back edge of the boat transom.  If you get this too big, it shovels water straight up through the slot and, whilst looking spectacular does not do the speed any good at all!!

Hope this all helps,
IainM

IainM,

Nice pic's. Any chance of more pics or info on Brave as I am attempting to build Brave Borderer.

Martin Down Under
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Glen Howard

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Re: cavitation
« Reply #19 on: March 09, 2008, 11:17:19 am »

Hi all, well since I last dropped in I have had a chance to do some experimentation with this problem. After very careful observation and testing, I have to conclude that the wash from the prop is indeed deflecting off the rudder. I'm also very concerned about the angle of the drive shaft, the amount of deadwood behind the prop and the propp being so far back on the hull. I am going to make some drastic adjustments, namely:
-  pull the drive shaft out
- move the whole drive train and motor forward in the hull
- re-drill for the shaft so that it doesn't angle down so much

Lots of work! When I eventually get this done I'll post an update. If this doesn't work, I'll be posting some pictures of a viking funeral for the boat!

thanks again,

Glen
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malcolmfrary

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Re: cavitation
« Reply #20 on: March 09, 2008, 02:06:34 pm »

Hi Glenn, while you are doing drastic surgery in that area, slimming the skeg might help as well, by improving the water flow before it gets to the prop.  See the pictures of Peter's boat.  Alternatively, turning the trailing edge into a wedge rather than an almost square would help.
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Glen Howard

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Re: cavitation
« Reply #21 on: March 09, 2008, 11:27:39 pm »

Yes, quite right Malcolm. In fact I had every intention of thinning the skeg, just forgot to note it down! Thanks.
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Willit

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Re: cavitation
« Reply #22 on: March 13, 2008, 10:10:03 pm »

I have this problem with the Springer tug, but only in astern.

Its no biggy as theres no steering in reverse at all anyway and paddle wheel effect does ruin astern running a lot (I'm going to fit stabalising fins to counter this) but it does mean the props not especially efficient in astern, resulting in a slightly long stopping distance.


but HEY!!!

It all adds to the fun!    ;D
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hopeitfloats

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Re: cavitation
« Reply #23 on: March 22, 2008, 11:45:18 am »

CAVITATION.... at last a subject i know a bit about. i would have to agree with Bluebird and IanM about the keel being too thick. this would most likely be the cause of cavitation. the area behind the prop is a perfect design as it is to create air pockets and especially so if the boat is over reving. tapering the square edges off  at about 45 degrees may help. could at least be worth trying before a major rebuild.
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Glen Howard

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Re: cavitation
« Reply #24 on: June 29, 2008, 03:17:29 am »

Hi again all, well it's taken me a long time, but I have now finally been able to comprehensively address this problem. I am delighted to be able to tell you all that Clementine now sails beautifully, and turns both port AND st'bd satisfactorily.

Here's what I did:
- ripped out the entire driveline, including motor, and re-set them further for'ard than they originally were;
- replaced the thick wooden skeg with a brass plate one;
- changed power to one battery, rather than running two batteries in parrallel (to reduce power output);

These modifications have had the desired effect, which leads me to conclude that she was overpowered, that the prop wash was being blocked by the rudder when it was in the st'bd position, and that the skeg was probably too thick to allow good water flow into the prop.

Thanks to all contributors on this august forum, your advice has been very valuable, and I look forward to many hours happy sailing Clementine from now on. 

Glen
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