Outboard Bracket Help

[JOHN P.]

I have an outboard mounted on an Armstorng Bracket (new boat). On my first run I noticed that I'm getting quite a bit of verticle spray on either side of the motor shooting upward toward the motor cover (top cowling). I also noticed that I was shooting quite a "rooster tail" behind the prop. What does this mean? How do I resole it?

[gonzo]

Is the bracket bottom hitting the water?

[nevd]

Rooster tail is not a problem if the motor mount height is high, the spray up the motor cowl is however potentially a problem. You need to provide much more info - eg.

What is the trim angle of the motor compared to water surface?
Is there damage to the prop or leg which is producing the spray?
Does this happen at all speeds or just at low speed?
Is there something else near the motor producing the spray eg sounder transducer?
How far is the motor mounted back due to the bracket?

Regards,

nevd

[JOHN P.]

Originally Posted by gonzo

Is the bracket bottom hitting the water?

I'm not sure but I don't believe it is. The spray is more defined at lower RPM's when the boat planes out ther is less spray but the rooster tail is still there.

[JOHN P.]

Originally Posted by nevd

Rooster tail is not a problem if the motor mount height is high, the spray up the motor cowl is however potentially a problem. You need to provide much more info - eg.

What is the trim angle of the motor compared to water surface?
Is there damage to the prop or leg which is producing the spray?
Does this happen at all speeds or just at low speed?
Is there something else near the motor producing the spray eg sounder transducer?
How far is the motor mounted back due to the bracket?

Regards,

nevd

The trim angle is between 2-3 on the Yamaha Tach. This is a brand new motor and prop. The problem is more defined at lower RPM's. I do have a transducer mounted in the aft starboard corner but the spray is equal on both sides of the motor. The motor is mounted 30" back on a 10 degree aft deadrise hull. I was told that the motor should be raised up one hole. They think the water spray is coming off the cavitation plate. I don't have a situation where I'm cavitating. The tach RPM's are holding pretty constant.

[gonzo]

The cavitation plate may be submerged

[JOHN P.]

Originally Posted by gonzo

The cavitation plate may be submerged

Would this cause spray?

[gonzo]

Yes, because the plate is lower forward. It would catch the water and make a rooster tail.

[nevd]

Originally Posted by JOHN P.

The trim angle is between 2-3 on the Yamaha Tach. This is a brand new motor and prop. The problem is more defined at lower RPM's. I do have a transducer mounted in the aft starboard corner but the spray is equal on both sides of the motor. The motor is mounted 30" back on a 10 degree aft deadrise hull. I was told that the motor should be raised up one hole. They think the water spray is coming off the cavitation plate. I don't have a situation where I'm cavitating. The tach RPM's are holding pretty constant.

If you never see any cavitation or ventilation even in tight turns, then the motor is mounted too low regardless of the spray problem. As a general comment, the higher you can mount the motor the better and the limit will be either
cavitation
ventilation
prop walking
or lack of cooling water flow

You need to aim to do most of your running with the cavitation plate horizontal (not parallel to the keel) as that produces highest efficiency and best steering. Trimming the motor out will give a rough approximation of lifting the motor and if you can trim out so the cavitation plate is lower at the front than the rear without the problems mentioned above, then that is another indication the motor is too low.

The motor trim angle takes some checking as the comment above on the cavitation plate slope refers to the actual operating slope compared with the water surface. Take a spirit level out on the water test and determine the running trim angle of the boat at the normal operating speeds (likely to be 2 to 8 degrees positive). Simulate this angle on the trailer (or in the water) and check the cavitation plate slope with the same motor trim angle from your trim gauge.

With a good production boat and a good OEM propellor, you should be able to run the cavitation plate above the water level at speeds above 35 mph on a shallow vee boat.

You may also want to check the cavitation plate height compared with the boat keel at the motor trim angle described above. The front of the cavitation plate should be at least 40 mm up due to your transom bracket offset unless your running trim angle is less than 2 deg.

Let us know how you go.

nevd

[JOHN P.]

Originally Posted by nevd

If you never see any cavitation or ventilation even in tight turns, then the motor is mounted too low regardless of the spray problem. As a general comment, the higher you can mount the motor the better and the limit will be either
cavitation
ventilation
prop walking
or lack of cooling water flow

You need to aim to do most of your running with the cavitation plate horizontal (not parallel to the keel) as that produces highest efficiency and best steering. Trimming the motor out will give a rough approximation of lifting the motor and if you can trim out so the cavitation plate is lower at the front than the rear without the problems mentioned above, then that is another indication the motor is too low.

Thank you for the help. I'll let you know.
Thank you,
John P.

The motor trim angle takes some checking as the comment above on the cavitation plate slope refers to the actual operating slope compared with the water surface. Take a spirit level out on the water test and determine the running trim angle of the boat at the normal operating speeds (likely to be 2 to 8 degrees positive). Simulate this angle on the trailer (or in the water) and check the cavitation plate slope with the same motor trim angle from your trim gauge.

With a good production boat and a good OEM propellor, you should be able to run the cavitation plate above the water level at speeds above 35 mph on a shallow vee boat.

You may also want to check the cavitation plate height compared with the boat keel at the motor trim angle described above. The front of the cavitation plate should be at least 40 mm up due to your transom bracket offset unless your running trim angle is less than 2 deg.

Let us know how you go.

nevd

Thank you for the help. I'll let you know.
John P.