cavitation - how to avoid it?

[Si]

gday. I have Yamaha 50 hp 2 stroke on a 4.55 seajay. I found last weekend the outboard/boat was cavitating in choppy conditions in the bay. I normally drive bigger fibreglass boats and hadn't experienced this before. Is this normal for a tinny in choppy conditions? or is it function of motor height/balance of boat issue?

cheers

[wayno60]

First I thought prop size but if in chop maybe it is motor height...have you tilt/trim?? if so play with that makes a big diff getting in the right place for the conditions....

[Blindfreddy]

Slow down............

[Fed]

What were the symptoms?

[tunaticer]

Is it cavitating under power or aerating from leaving the water?

My prop will aerate when the back of the boat is on a wave and the engine is meeting the trough, lasts all of a second though.

[Si]

Is it cavitating under power or aerating from leaving the water?

My prop will aerate when the back of the boat is on a wave and the engine is meeting the trough, lasts all of a second though.

its pulling in air under power. its not leaving the water but possibly getting close peaks and troughs I guess. if I slow down it does go away but i'm not going fast at all

[gofishin]

Without wanting to start another one of 'those' threads, what we are talking about here is ventilation! However let's just move on ;D


...I normally drive bigger fibreglass boats and hadn't experienced this before. Is this normal for a tinny in choppy conditions? ... For a pressed tinny in comparison to a glass boat, yes! In terms of the likelihood of providing disturbed/aerated water to the prop, it's chalk and cheese between these two types of boat!

Firstly, 99% of this size tinnies will have an external keel - I nice big inverted 'T' running from the keel foot to the transom. In the old days the keels were cut-back and tapered away aft of the last rib, however for many years this means time and money, so it has long gone! On choppy seas the external keel provides an easy path for air bubles/disturbed water straight to the prop. This is exaggerated of course when you are turning, as the keel carves a big air pocket under the hull.


... or is it function of motor height/balance of boat issue?...Motor height plays a big part, also the condition and sometimes the type of propeller. One of the 'compromises' you have to wear with any pressed tinny with an external keel is that the motor height has to be lower than it would be in a similar size glass boat, which in comparison has a nice smooth bottom.

Even in my biggest tinny to date (6.1m 'pressed plate' as they say), I cut back the keel which reduced ventilation markedly. Changing from the stock alloy prop to an SS also helped a lot.
cheers
Brendon

[soulfish]

Try dropping your engine one hole....it could be a number of issues but you have to start somewhere..
Does the yammy have power trim or is it gas assist lift?.makes a huge difference if you can trim it underway..I used to have same boat & motor.
make sure you have the boat balanced the best you can .
Do you have underfloor fuel tank or portable tank aft?So many variables you have to go through...could be as simple as taking weight out of the front of the boat or adding weight to the front of the boat

[Si]

Try dropping your engine one hole....it could be a number of issues but you have to start somewhere..
Does the yammy have power trim or is it gas assist lift?.makes a huge difference if you can trim it underway..I used to have same boat & motor.
make sure you have the boat balanced the best you can .
Do you have underfloor fuel tank or portable tank aft?So many variables you have to go through...could be as simple as taking weight out of the front of the boat or adding weight to the front of the boat

cheers mate. it's a gas assist lift pull start so I cant trim underway. yes I have a fore 40 litre cruise tank plus I also had another 20 litre container sitting on top. I also have a keel protection plate on it. could be a combination of reasons.

[Si]

Without wanting to start another one of 'those' threads, what we are talking about here is ventilation! However let's just move on ;D

For a pressed tinny in comparison to a glass boat, yes! In terms of the likelihood of providing disturbed/aerated water to the prop, it's chalk and cheese between these two types of boat!

Firstly, 99% of this size tinnies will have an external keel - I nice big inverted 'T' running from the keel foot to the transom. In the old days the keels were cut-back and tapered away aft of the last rib, however for many years this means time and money, so it has long gone! On choppy seas the external keel provides an easy path for air bubles/disturbed water straight to the prop. This is exaggerated of course when you are turning, as the keel carves a big air pocket under the hull.

Motor height plays a big part, also the condition and sometimes the type of propeller. One of the 'compromises' you have to wear with any pressed tinny with an external keel is that the motor height has to be lower than it would be in a similar size glass boat, which in comparison has a nice smooth bottom.

Even in my biggest tinny to date (6.1m 'pressed plate' as they say), I cut back the keel which reduced ventilation markedly. Changing from the stock alloy prop to an SS also helped a lot.
cheers
Brendon

cheers mate. that's good info. I never realised my keel protection plate would be making things worse. I have one hole position left in allow the motor to trim right in so to speak. it's the bar on the spring that you can adjust. I might give that a go next time I'm out there. Been thinking about upgrading my prop also.

[gofishin]

... I have one hole position left in allow the motor to trim right in so to speak. it's the bar on the spring that you can adjust. I might give that a go next time I'm out there. ... Yeah mate, that's the go, try the simple/cheap things first. With gas-assist it doesn't matter if the pin is closest to the transom, should not only help reduce ventilation but will also improve the ride into head-on chop. However just remember to trim out a bit going down-sea, especially if she is a bit twitchy under normal trim.
cheers
Brendon