Cavitation Issues

[gouldy]

Well im having cavitation issues with my 4.5m stacer with a 60hp yammie 2 stroke on the back. I went the the dam yesterday to do some ski tubing and on tight turns the prop cavitates badly causing me to loose speed until it grips again. There is the standard yammie 13 1/4 x 17 K series prop on there. Should i get a solas prop cause I have heard the K series props are a crap prop or should i drop the motor down another hole and see how that goes.

At the moment the cav plate is inline with the bottom of the hull and just sits above the water when at cruising speed.

Any input to help me resolve this issue would be great. Cheers

[Noelm]

lower it down a hole, will cost nothing, take next to no time and just may solve your problem if the cav plate is above the water now.

[FNQCairns]

The k series is perfectly fine for yours and mine, you have 2 choices lower the engine for the tubing etc or increase the diameter or blades all with a pitch change even the cheapest prop will perform well enough for tubing duty if it is the right prop to suit the application.

Your boat is setup for efficiency now as per your engine height...you cannot have it perfectly both ways...you can throw money at it and see an improvement but certainly not in keeping with the cost involved.
.
Tubing and run of the mill ski boats have engine heights way low in relative term so they can perform this duty.

cheers fnq

[wiz]

Why not just trim in for the turn ?

[Chimo]

From what you say and as confirmed by FNQ your boat set up is set up for efficient running and is probably fine most of the time.

If you went tubing in the sea you cavitation would probably be even worse due to the difference in bouyancy between salt and fresh water.

So either trim the motor in closer to the transom, as opposed to out trim that you do when running in efficent cruise mode, or drop the motor a hole when you are going tubing if its a big worry.

My old boat used to cavitate in hard turns in the sea but was fine in the fresh; all due to the difference in bouyancy.

Cheers
Chimo

[Jabba_]

Why not just trim in for the turn ?

You would be suprised how many boat owners are out there who don't know how to use the trim tilt button to optimize there boats performance....

[FNQCairns]

Just a little caution on the trim in at fun tubing speeds if you might be whipping your tow especially when towing an adult.

Your boat is light and short, when you trim in to cover a problem like this you can unsettle the boat. Anyway it's not that uncommon for light boats to end up on their side then upright and pointing in a direction near 60 deg different than where the where originally heading all in the blink of an eye.

If it where a 5m glass the equation becomes somewhat different.

just a thought.

cheers fnq

[gouldy]

Thanks for the info guys i will drop the motor a hole next time and see how it goes. I also did try trimming down the motor as well but didnt make much of a difference.

[Hornet Rider]

gouldy, have a read of this when you get a tick.. http://www.boatinglife.com/article/Maintenance/Ventilation-vs-Cavitation-The-Facts

all good advice here from FNQ, Chimo & Noelm

Jabba - what's this trim tilt button thingo & where on the boat is it?

[oldboot]

The young bro'inlaws 4.2 savage, is propped right and realy planes efficienty, especilay on a flat day......but a tight turn at speed will guarantee cavitation with 1 aboard.

Just a thaught... would a foil improve the situation?

cheers

[Hornet Rider]

The young bro'inlaws 4.2 savage, is propped right and realy planes efficienty, especilay on a flat day......but a tight turn at speed will guarantee cavitation with 1 aboard.

Just a thaught... would a foil improve the situation?

cheers

Short answer - most likely not, but I only know that because more experienced foilers gave me some good advice.

Hey Chimo, here's an opening for you. Get in here & do your foil thing, I love it;D

[Chimo]

Thanks HR but I thought perhaps Gouldy could do one thing at a a time and if no worthwhile results that would be the time;) for "the magic of foils":P

How have you been going?
Thought you may have disappeared up some jet exhaust or similar, been quiet for a while::)

Cheers
Chimo

[Crocodile]

Hello Gouldy
I vbmenu_register("postmenu_1091547", true);

[Crocodile]

Hello Gouldy
I have the same motor and prop on a Webster Twinfisher catamaran.
The motor has a foil and it works in the disturbed water between the hulls.
There is a vee shaped splitter in the tunnel to reduce drag from the motor leg.
I have it mounted in the top hole to lift the powerhead when at rest in steep swells.
It never cavitates in sharp turns so it is safe to say that your problem is not engine/prop.
I do get ventilation when trying to lift/trim the bow when there are heavy and un-cooperative passengers (which is all passengers!) on the front seat. This is because the prop is lifted high enough to suck air.

On you tinny is there a substantial keel in front of the motor?
If so this might cause disturbed water to hit the prop.
I had an old DeHaviland that had a keel it it cavitated in turns.

[Mindi]

What you have is ventilation not cavitation...ventilation is when the prop gets high enough to suck air or aerated water at least...either in a turn or white water or whatever....but the water is not solid enough for it to work to it's design "grip"........loses solid grab and spins out...Cavitation is when the opposite happens and tiny low pressure areas on or near the prop surface collapse in mini pressure explosions against the trailing side of the blade often showing as small paint spots and metal removed... ( the name "cavitation" comes from the collapse of this cavity agaist the surface)....it will damage the propeller, is noisy, and lowers performance. Different problem completely....you cant usually feel cavitation (inspect your prop) which is a major issue in fluid dynamics generally not just boats. And no....it isnt really called the cavitation plate it is the anti ventilation plate.
Hope this helps if anyone interested in the difference..? Otherwise boring as...!!!

Lower the motor or turn less sharply...?

[Rowdy_2002]

Gday All
I have the issue of ventilation with my allycraft 475 bar raider. The boat is fitted with a 60HP motor that has a foil fitted. When the boat is run through calm water I don't have a problem but when the conditions get rough I have found that unless I trim the nose down and slow down the speed the prop hits the disturbed water and starts to over rev. The cure then is to back off the throttle until the prop grips again.

Would removing the foil cure the problem as it seems to me that the back may be riding too high?
Thanks
Greg

[Chimo]

Greg

Maybe.

What reason did you have to fit the foil?

Dont get me wrong, I have 3 of them fitted 2 on the Vag to aid tail support at slow speeds and 1 on a 10 ft tinny on a 9.9 to increase rear lift and to enhance turning stability ie get rid of the twitchyness which happens with a few horses on a short boat. I dont have one on my 2.3 on a 12 fter tho!

Cheers
Chimo

[Geoff-]

Just a little caution on the trim in at fun tubing speeds if you might be whipping your tow especially when towing an adult.

Your boat is light and short, when you trim in to cover a problem like this you can unsettle the boat. Anyway it's not that uncommon for light boats to end up on their side then upright and pointing in a direction near 60 deg different than where the where originally heading all in the blink of an eye.

can you elaborate on "whipping your tow" and the light boats ending up all angles when trimmed in? reason I ask is our 14' hinton seems to ride best when the motor is trimmed all the way in, it makes the bow sit flatter. any trim out and the bow lifts too much when you accelerate, and the prop loses bite much more easily. the cav (ventilation!) plate is in the water at cruising speeds so the motor's definitely not too high on the transom.

[Razgo-]

What you have is ventilation not cavitation...ventilation is when the prop gets high enough to suck air or aerated water at least...either in a turn or white water or whatever....but the water is not solid enough for it to work to it's design "grip"........loses solid grab and spins out...Cavitation is when the opposite happens and tiny low pressure areas on or near the prop surface collapse in mini pressure explosions against the trailing side of the blade often showing as small paint spots and metal removed... ( the name "cavitation" comes from the collapse of this cavity agaist the surface)....it will damage the propeller, is noisy, and lowers performance. Different problem completely....you cant usually feel cavitation (inspect your prop) which is a major issue in fluid dynamics generally not just boats. And no....it isnt really called the cavitation plate it is the anti ventilation plate.
Hope this helps if anyone interested in the difference..? Otherwise boring as...!!!

Lower the motor or turn less sharply...?

always wondered about that, thanks for the info. any pics of cavitation at its best?

metal removed? what does that exactly mean? pitted?

[Hornet Rider]

Thanks HR but I thought perhaps Gouldy could do one thing at a a time and if no worthwhile results that would be the time;) for "the magic of foils":P

How have you been going?
Thought you may have disappeared up some jet exhaust or similar, been quiet for a while::)

Cheers
Chimo

Hi Chimo, hope all is well with you & family. All good here, been busy that's all & very little time on the water. Catch up soon. HR

[bar raider]

Gday All
I have the issue of ventilation with my allycraft 475 bar raider. The boat is fitted with a 60HP motor that has a foil fitted. When the boat is run through calm water I don't have a problem but when the conditions get rough I have found that unless I trim the nose down and slow down the speed the prop hits the disturbed water and starts to over rev. The cure then is to back off the throttle until the prop grips again.

Would removing the foil cure the problem as it seems to me that the back may be riding too high?
Thanks
Greg

NO FOIL and i get damn ventilation too >:( .

rides real smooth, motor correct height, correct trim and still does it even in a medium turn.

[Mindi]

Pics as requested....It is more likely the faster the surface is moving so tends towards the propeller tip or outer edge...see first photo.

If actually interested then have a look at this short paper ...the intro is good enough for me.

http://www.pumped101.com/cavitation.pdf


Cheers

[Razgo-]

Thanks for the pics Mindi, the pictures say it all :o

[FNQCairns]

can you elaborate on "whipping your tow" and the light boats ending up all angles when trimmed in? reason I ask is our 14' hinton seems to ride best when the motor is trimmed all the way in, it makes the bow sit flatter. any trim out and the bow lifts too much when you accelerate, and the prop loses bite much more easily. the cav (ventilation!) plate is in the water at cruising speeds so the motor's definitely not too high on the transom.

Hi Geoff whipping my not be the right word for it these days but it is simply when the driver uses the boat in a fast turn/s to force the tube back across the boats wake and a fair way up along side still with a near tight rope.

I do not know the Hinton off hand but my comments are based on trim use at speeds befitting fun tubing. When a 70-100kg pendulum is hanging of the end of the light short boat that ventilates on turns and the boat is trimmed in to force bow steer it's a recipe for something to go wrong quickly.

cheers fnq

[Rowdy_2002]

Gday Chimo,
The foil was already on the boat when I purchased it. I suspect it was to assist getting on the plaqne quicker seeing as it is powered by a 60HP when the max HP is 75.

Bar Raider - It is interesting that you also get ventilation without a foil. It sounds a lot like the motor is too close to the water surface.... Or is the bottom shape causing a lot of disturbance???

If anyone else has had the problem and solved it maybe they could elaborate on their solution...

Thanks

[Zooter]

I heard that Prop cupping can also eliminate these issue's ???

Spoke with steve at solas props a while back as mine started to cav after adding the foil on tight turns, etc. As i cant live without the foil on he suggested cupping the prop to counter act what problems the foil was creating, extra lift etc,

Havnt had the cash yet to do it but i know another poly owner with the same problems and the cupping worked a treat for him.

Good luck with it :)

[black runner]

I purchased a spare prop a pitch size down for tubing and heavier loads. Greatly reduced breakout on turns to almost zero.

Cheers

[Jarrah Jack]

Once had a nameless savage boat that needed a new bottom. When they replaced it they added a full length keel which produced ventilation probs that weren't there before. Although I sold the boat I think they fixed it by cutting the rear 500mm off the keel.