Cavitation or Ventilation ?

[Bleeding Green]

HI Raiders,

I have always had the issue of loss of power/slipage in sharp turns with my boat. I am not sure if it's cavitation or ventilation, but from what I've read it sounds like it a ventilation problem.

Basically when heading out of a sharp turn at speed the prop will slip and the speed will drop. It's likle a loss of traction in a car. It seems to be worse the higher trimmed the outboard is.

Question is, how can I fix or eliminate it? Is it possible to stop it altogether? Do the permatrim attachments fix this problem? Is it a prop issue?

FWIW, my boat is a 4.8 quinnie with a 70hp yammie 2 stroker.

Thanks.

[oz_brett]

as you go in to turn trim your motor in (down) so that the nose of the boat comes down this will reduce / prevent catation. perma trim are good & changing to a different prop can make a great difference, for extra traction (less cavataion) You could try a 4 blade stainless prop at about the $800 but all the above will not convert your fishing boat to a jet boat!

[Geoff]

HI Raiders,

Question is, how can I fix or eliminate it? Is it possible to stop it altogether? Do the permatrim attachments fix this problem? Is it a prop issue?

FWIW, my boat is a 4.8 quinnie with a 70hp yammie 2 stroker.

Thanks.

Trim is one aspect , height of the motor is another. The cavitation plate should be leval with or perhaps a few mm below the bottom of the boat.

I have a 70 Yammie & when the boat was delivered it was supplied with a 13" prop that cavitated when turning.

The dealer replaced it with a 15" prop & problem basically fixed. Later I did add a Permatrim & now 100% OK

Does your prop look like this or is it the one with the rounded ends rather than the "pointey" end

Geoff

[Bleeding Green]

Looks very similar to the one in the photo, b ut I'll run down and take a look. I'll also look to see where the cavitation plate is in relation to the bottom of the boat.

When you changed your prop to the larger diameter, how much clearance did you have remaining from the end of the prop blades to the cavitation plate? I currently have about 30mm, but I'll measure it to be sure. That wouldn't leave much clearance with a 15 inch prop.

[Bleeding Green]

Here is a photo of the current prop:

and another showing how close the current blade tips are to the underside of the cavitation plate. I measured it at 20mm:

Also, another showing that the cavitation plate is approximately in line with the bottom of the boat. Probably slightly above it I'd say. The motor is trimmed down all the way in this photo:

[Geoff]

Here is a photo of the current prop:

and another showing how close the current blade tips are to the underside of the cavitation plate. I measured it at 20mm:

Also, another showing that the cavitation plate is approximately in line with the bottom of the boat. Probably slightly above it I'd say. The motor is trimmed down all the way in this photo:

Alfa-Rosa Yours is a 15" prop. Mine also measurers 20mm from the tip to the cav plate.

The height looks OK but to check , trim the motor out so the cav plate is horizontal

to the bottom of the boat as shown here.

Disregard that on mine the cav plate is 50mm lower than the bottom of the boat.

This is necessary so the prop is in "clean" water when coming through the tunnel of a twin hull boat.

Of the 3 options , trim , height & prop I'm now leaning towards trim. When on the plane how many bars are showing on the trim gauge,

Whilst mine is a different boat the trim gauge reads 2 bars when the cav plate is leval with the bottom of the boat. When on the plane 4-5 bars is is best position.

Any thing more , even with a Permatrim & cavitation begins to occur.

Geoff

Geoff

[BFB]

You can not get a normal boat to turn like a race boat. Caviation becomes worse in a boat if you are trying to pull skiers or tube in a tight curve. It is actually the drag of the skier or tube causing it. Another reason why a lot of ski and race boats are inboards with the prop coming out under the boat in the water.

[Bleeding Green]

My boat if fitted with an analogue trim gauage so I can't compare it directly with yours. When I'm not towing (I've only tubed a tube so far) and on the move I usually keep the motor trimmed up so my gauge points to a level boat. It sits nicely and I can then drop the throttle back to lower the revs and maintain the same speed.

The issue I was having, and as BFB said above, is worse when towing. When doing this I had the motor trimmed down all the way, mostly because it's a pain to keep lowering and lifting again with the amount of times the riders feel off (all added to the fun btw).

When I tried trimming up a bit to save on some fuel the cavitation was even worse.

BFB, I realise that the boat is never going to corner like a speed boat, but if I can reduce the problem a bit it will be more fun when towing.

Does everyone agree that a permatrim will reduce the cavitation and improve the planning time (for pulling skiers)??

[Bleeding Green]

Alfa-Rosa Yours is a 15" prop. Mine also measurers 20mm from the tip to the cav plate.

My prop is stamped with 13 1/4 X 17K so maybe the motor model is differant to yours??

[Geoff]

My prop is stamped with 13 1/4 X 17K so maybe the motor model is differant to yours??

I think there the same but just to check , the model No. on mine is a 70BETO.

Interesting about the prop ,they look the same with the same distance from the tip to the cavitation plate but mine is stamped , 13 1/2 X 15K

Geoff

[BFB]

Geoff you will not pick up with the naked eye, unless you are very good, the difference on the distance to the caviation is only 1/4". The difference between the 2 props is the 15 and 17. In theory a 15" prop will move 15 inches through the water for every one rotation where the 17" prop moves 17 inches. Therefore a 15" will supply more low down torque and the 17" will provide ahigher top end speed. Just like changing gears in a car.

Hope this helps explain the difference

[Bleeding Green]

BFB your explaination clears it up.

Geoff, your prop is 13 1/2 inches diameter, mine is 13 1/4. The thrust on yours is 15inch and mine is 17.

I wonder how much top end speed I would lose, and bottom end torque I would gain with your prop??

[Us13feb05]

Hi mate

This is not the case with all alloy boats but in some the keel can be large and come all the way to the back causing air bubbles when u turn

Witch causes ventilation around the prop this could also be caused on some boats by a transom mounted speedo pitot or transducer or even a bait tank pick up

Any of these things mounted close the keel on the transom could cause ventilation/cavitation

U could try a larger prop or have a prop specialist put a little bit of cup in the end of prop blades this will help the prop hold on better depending on ur engine rpm at full throttle it should be between 5000&5500 rpm putting a bigger prop on could make it less than that causing to much load on the engine

Hope this can be some help

Clemo

[Bleeding Green]

HI mate, thanks for your input.

The motor revs out to about 6000rpm, if the gauage is acurate, and is within the specs shown in the owners manual.

From what I read on the net ventilation is caused by differant issues than cavitation, and it seems that ventilation is more common.

[Us13feb05]

Hi mate

Yeh I agree ventilation is more common especially in alloy boats because of large keels,chines ect causing air bubbles not like fiberglass where the bottom of the boat is more smooth

U could go up a prop size or have cup put in ur prop it would help and bring ur rpm down a bit saving a bit of fuel and engine wear but it wouldn't pull a ski/ tube out of the water as well.

A permatrim will help get on the plain but I don't think it will fix ur ventilation

I had cup put in my prop it stopped the ventilation and lowered my rpm by 350 rpm it just allows the prop to hold the water better.

It's well worth checking those transom mounted things u will be surprised how something small can effect the problem

[BWV]

I have always had my engine height adjusted so the anti ventilation plate is higher than the bottom of the boat. My Boston Whaler was about 5-10mm higher and the Skeeter is 60-70mm higher from memory. You'll find 90% of people have their engine height wrong and there for not propped correctly. Lift the engine height and you will gain more RPM’s then you can run a larger pitch prop which will bring the RPM’s down to the engine manufactures limits. Running a larger pitch prop will not over load the engine as long as the height is right and it within the rev range. You will gain much better all-round performance and longevity out of your engine. As Clemo80 has said a "cup" in the prop will work but that isn't your first concern.

Geoff and Alfa-rosa, your engine height is too low from your pictures! Running engines that low causes excess drag and poor water flow around the vent plate and prop!

Have a look at this web page it’s a very good explanation (much better thanh I can give) on engine height and once you have that sorted start with trial and error with different props. http://www.veradoclub.com/smf/index.php?topic=137.0

Just to clarify race boats want as much efficiency out of their props and I think you’ll find they run surface piercing props that are designed to run with varying degrees of cavitation/ventilation as the blades pierce the surface.

Edited January 18, 2012 by Whaler 255

[Geoff]

Geoff , your engine height is too low from your pictures! Running engines that low causes excess drag and poor water flow around the vent plate and prop!

BFB Tks for clarifing the difference between the two props.

Whaler. For a mono hull boat your correct about the height but as I mentioned earlier ,

Disregard that on mine the cav plate is 50mm lower than the bottom of the boat.

This is necessary so the prop is in "clean" water when coming through the tunnel of a twin hull boat.

It does create additional drag which effects performance but this is the trade off with only requiring one motor on a twin hull boat.

Geoff

[Bleeding Green]

Thanks for your input whaler. I will have a look at the link you provided shortly. In your opinion where whould the cav plate sit in relation to the bottom of the boat?

This might be a completely unrelated issue, but when I did my practical course to get my licence 2.5 years ago, we practiced beaching the boat. It was easy to reverse the boat off the beach. With my boat when I beach it I struggle to get it off the sand using the outboard alone (I usually just push it off first now). Would this also be effected by the height of the motor? I can see it two ways:

1) raising the motor would help as I could then trim the motor down to a better angle while still keeping it clear of the bottom.

2) Raising it would make it worse as the the prop would be closer to the water surface and therefore in lower water pressure.

3) Completely unrelated.

I don't want to drag this thread away from the original discussion, just trying to draw a link between other issues I experience and what you are saying above.

[BWV]

Geoff, I don't think that to 40mm+/- difference in height is going to improve in the water quality between the hulls as there is going to be lots of positive water pressure between the hulls. Running the engine that low is going to reduce your speed and efficiency of you vessel and place undue stresses on the transom. If you have a look on the link on my previous post there are some very good photos on the correct engine mounting height and how it should look when youre underway. I dont believe that your position is completely wrong mate I just think it can be done better but it can big a big game of trial and error to get it right.

Alfa-Rosa, to answer your question. I cannot give you an answer in the way of a set distance e.g. 10mm or 20mm. It is trial and error. Too get the position right you can only do that by running the vessel at planning speeds and sticking your head over the stern and having a look. It could be in the correct position now just propped wrong but engine height is the first place to start looking. Measuring it in your drive way will get you somewhere in the ball park but it wont be perfect Have a look at the link and take note of the photos on that page when the vessels are running the vent plate is clear of the water when on the plane and trimmed correctly. Every boat is different, they act different when underway, and some ride with the bow high some ride pretty flat. You should be able to get enough info off the other page to work it out or at least point you in the right direction.

To the other question; reversing off the beach. To be honest I dont know it may improve it may not. When you are trying to drive off the beach turn your engine from hard to port the hard to starboard and pause when hard over to get the vessels to screw around then go back the other way.

Edited January 19, 2012 by Whaler 255

[Bleeding Green]

So had a read of the info on the link provided above. Outlines things very clearly. I will check out the height of my outboard the next time I am on the water and also take some hard notes of the rpms.

It has been recommended to me also that I can possibly reduce the slipping but increasing the prop pitch. I am not that keen on doing this at it would reduce the torque and as I'd like to try sking reducing the torque is not ideal.

So if the outboard height seems right and the rpms are within range (which I am sure they) I can either try a hydrofoil or just put up with the cavitation/ventilation problem.

Thanks everyone for your help.

[Bleeding Green]

So finally got to take the boat out today. Got up on the plane, conditions were good so it was an easy cruise. Went straight up to 6k rpm, stuck my head out the back and couln't see the plate. Dropped the revs a tad and took some photos.

Did another check later with the boat a bit flatter and could just barely see the plate, but I don't think it's enough. I'll post some pics tomorrow for your opinions to be added.

[Bleeding Green]

Photos attached.

First photo is at 5500rpm, nose up a bit too high according to trim gauge, and 62k/h according to finder.

Second shot of outboard is 5000rpm, boat a bit flatter and 55km/h.

Minimum load in boat with only about 25lts of fuel, one adult and 2 kids on board.

Is the current height affecting the cavitation problem? Should I raise the outboard and if so how much? I need your opinions please.

[BWV]

Lift it up one hole and see how it goes. I wouldnt really care what the trim gauge shows it really counts for nothing at this stage. Its just a reference point for the future. Remember bow lift isnt a bad thing, it reduces drag and increase speed and range of the vessel with reason. I have always set my vessels up with as much bow lift as possible without making the vessel feel loose at higher speeds.

[Bleeding Green]

Lift it up one hole and see how it goes. I wouldnt really care what the trim gauge shows it really counts for nothing at this stage. Its just a reference point for the future. Remember bow lift isnt a bad thing, it reduces drag and increase speed and range of the vessel with reason. I have always set my vessels up with as much bow lift as possible without making the vessel feel loose at higher speeds.

Thanks whaler.

The outboard has a lot of silicon around where it's bolted onto the transom. I expect that once I undo the bolts the silicon will cause it to stick to the transom. What's the best way to get it off, and to clean the silicon off once I get the outboard away from it? Also, is it paramount that I reapply new silicon when I bolt it back on?

Also, the outboard has one hook at the top under the cowl. Is this sufficient for taking the weight of the outboard? I intend to hang it from here then lift the front of the boat so the the back drops then re bolt in the new position.

[BWV]

Thanks whaler.

The outboard has a lot of silicon around where it's bolted onto the transom. I expect that once I undo the bolts the silicon will cause it to stick to the transom. What's the best way to get it off, and to clean the silicon off once I get the outboard away from it? Also, is it paramount that I reapply new silicon when I bolt it back on?

Also, the outboard has one hook at the top under the cowl. Is this sufficient for taking the weight of the outboard? I intend to hang it from here then lift the front of the boat so the the back drops then re bolt in the new position.

Yes the one hook will be strong enough to hold the weight of the engine, thats whats it designed for. Although it might not hold the outboard level its small enough to manhandle. I think it would be easier to just move the engine around on a lift or on a block and tackle than moving the boat around but its your choice.

To remove the silicone I would have a razor blade or stanley knife handy to cut it free to ensure it does pull and paint off. Clean it rest of it up with acetone or metho, what ever works the best. I would apply new silicone but not until I have the height sorted. Then I'd apply new silicone or sikaflex, but with an alloy boat its not as important as GRP one but its fairly cheap so just do it.