Model Boat Mayhem - Forum

Dry Dock / Shipyard: Builds & Questions => Pleasure boats, Sports, Race, Power and Leisure Boats: => Topic started by: bbdave on April 08, 2012, 12:38:42 PM

Title: cavitation cure?
Post by: bbdave on April 08, 2012, 12:38:42 PM
Hi i have upgraded motor battery etc. in my atlantic 21 which now runs loads better and cooler the only small problem now is occasionaly it looses grip and i get cavitation whats needed? smaller/larger prop,more/less pitch i do need to add a little weight to the bow as it porpoises a little. It does run fine at scale speed it's just when i give it that last bit for some fun it cavitates.

Dave
Title: Re: cavitation cure?
Post by: derekwarner on April 08, 2012, 01:53:55 PM
Dave.....

Irrespective of the type of hull...machinery or propeller installed.....any such issue of ... "occasionally it looses grip and i get cavitation" is that some form of hydro dynamic limitation has been exceeded  >>:-(

There is no simple answer for a fix  :((....but rather go back to the original vessel design if possible & start to rethink the design concept............

You cannot simply add more [kW] power and expect all things to be equal.........something must give  %) ......Derek
Title: Re: cavitation cure?
Post by: Shipmate60 on April 08, 2012, 01:59:17 PM
What prop and how deep is it?
Very few models get cavitation but do suffer with aeration where the prop pulls the water level down to the blades and sucks in air.

Bob
Title: Re: cavitation cure?
Post by: bbdave on April 08, 2012, 02:51:01 PM
Like i said it's fine at scale speed just whe i push the stick  to fun does it happen also as i run in the sea wave do pop it out of the water now and then it has an x35 on at the moment which is probably only around 10-20mm below surface. No dramas really i was just curious.

Dave
Title: Re: cavitation cure?
Post by: Andyn on April 08, 2012, 04:06:10 PM
Drop the props a few mils, and angle them a couple of degrees downwards, that should do the trick :-)
Title: Re: cavitation cure?
Post by: martno1fan on April 08, 2012, 04:09:05 PM
If you upgraded the motor try a bigger prop or same size more pitch.
Mart
Title: Re: cavitation cure?
Post by: Subculture on April 08, 2012, 04:12:49 PM
Is this using outboards like the fullsize versions? If the prop is sitting beyond the bottom of the hull, you're always going to get problems with the prop sucking down air- it's a common problem with submarines with spindle sterns. The solution for a surface runner is either relocate the prop, or fit a plate above the propeller to act as a false hull bottom.
Title: Re: cavitation cure?
Post by: Kim on April 08, 2012, 06:58:54 PM
Hi all,

Given that your boat is 'porpoising' then your problem (assuming the problem is while underway, in a straight line) is probably caused by Ventilation. Easiest cure then (assuming props are correctly mounted in relation to transom)  is as you say a little more weight in bow.

If Props are out of line then maybe they are trying to drive to surface? May need trimmed out slightly (although if porpoising then I'm confident this is not the solution)

If it is good old fashioned cavitation then as per the full size - addition of 'Dole fins' might be the way to go or lower the outboard / prop.
Cavitation is most noticeable on initial start (hole shot) and in turns.

A big subject with a rather brief and general answer to get you started .
Hope it helps.
Regards,
Kim
Title: Re: cavitation cure?
Post by: bbdave on April 08, 2012, 07:02:08 PM
Thanks for the replies i'll get it trimmed up and see how it goes i'm not to worried about it really i'll just lay off the top end of the stick.

Dave
Title: Re: cavitation cure?
Post by: Kim on April 08, 2012, 07:17:46 PM
Hi Dave,
A school boy error on my part Atlatic 21's are twin engined aren't they?

Props should be contra-rotating if not then that is not helping the problem.

Or just go easy on the 'Laughing stick' / throtle... lol

Regards,
Kim
Title: Re: cavitation cure?
Post by: bbdave on April 08, 2012, 07:59:40 PM
The outboards are dummies on this the Lesro kit

(http://i127.photobucket.com/albums/p127/bbdave/Model%20boats/Atlantic21.jpg)

Dave
Title: Re: cavitation cure?
Post by: Kim on April 08, 2012, 08:06:29 PM
Hi Dave a pic tells a thousand words ....

Single prop + Bow sitting high + Porpoising ....


= Ventilation  + more weight over the length, trim down the bow and full throtle is a go go !

= more laughing stick!

Regards,
Kim
Title: Re: cavitation cure?
Post by: bbdave on April 09, 2012, 10:42:50 AM
I 've got a bag of lead shot to experiment with to get the trim sorted next weekends job sorted

Dave
Title: Re: cavitation cure?
Post by: scoop on April 09, 2012, 04:57:10 PM
I 've got a bag of lead shot to experiment with to get the trim sorted next weekends job sorted

Dave
Why not just move the drive battery a centimetre or two further forward, saves adding extra weight  :-))
Scoop
Title: Re: cavitation cure?
Post by: triumphjon on April 09, 2012, 08:41:22 PM
what about fitting a small trim tab to help push the nose back down / lift the stern ? remembering that the full size vessels have a powered trim on the outboards so are able to adjust while underway !
Title: Re: cavitation cure?
Post by: bbdave on April 09, 2012, 08:52:36 PM
I know a little weight will help the ride as i tried a stone on the bow i don't have any room to move the battery fwd it's a lipo so not very heavy anway.

Dave
Title: Re: cavitation cure?
Post by: bbdave on April 15, 2012, 10:37:25 AM
Well i've bunged 100 or so grams of lead in the bow and it rides loads better more scale like in fact it is a little bows down at rest so maybe i'll experiment further, but much happier with it and just a little water ingress which has been narrowed down to the cooled motor mount which needs sorting before the salt does it's worst.

Dave