Motor, props, and cavitation

[Jaysen]

Had “good testing” conditions today. Basically high wind aft port quarter and tide from dead to stern. I added 15’ to the dock lines to give me room to play. I had pretty much the same results. Boat moves slowly and veers to starboard.

Because I wasn’t endangering any other vessels I had some time to try things. First thing I noticed; wicked cavitation. The prop looked like it was doing nothing but aerating the water. I also noticed that in reverse the prop lifted a bit. If I pushed down a bit on motor the cavitation lessened and the boat moved a lot faster and straight. The motor never bogged or showed any signs of loading.

I’m wondering if a larger prop would get me more motion at lower rpm. I’m also thinking this would reduce the prop cavitation.

I’m not sure of is the whole tilting thing. The motor is clearly locked but it seems “sloppy”. I don’t need tilt at all as the transom mount lifts the motor clear of the water. Any one think of a reason not to just throw a bolt through some holes to prevent motion?

[Fuzz]

How about posting a picture of the motor and how it sits compared to the waterline. Props are made to push so they never work as well in reverse. Only problem with locking the motor down is how will you get it out of the water for sailing. Does the outboard swivel the way it is mounted?

[fallguy1000]

Many motors lock the tilt up in reverse. But keep the tilt up forward for hitting bottom.

I would say something is not working or the engine doesn't have the needed feature, but most of them do lock in reverse.

You definitely need to get the engine to lock, but only in reverse. A larger prop is not the main issue. If you prop bigger, the larger prop is going to lift the engine even more as it has more contact with the water.

Also, based on what you are saying, the engine start position MAY be too high.

Can you attempt, for testing only, to put the engine on the other side?

Picture of the engine and cav plate in the water would also be good.

[fallguy1000]

Also, don't you have a pin setting?

Make sure that is all the way down.

[fallguy1000]

https://www.boatdesign.net/threads/merc ... rse.32848/

[TomW1]

Jayson a little more detail please. Boat, motor HP, prop size. where is the prop set per the keel line of the boat, distance between keel and water line? Thanks. Cavitation is normally caused by setting the prop to high and it grabbing air as it does not have enough water above it.

Tom

[fallguy1000]

2004 Merc 9.9 2 stroke long shaft

he said on the other thread

[Jaysen]

2004 Merc 9.9 2 stroke long shaft

he said on the other thread

Helms 24 (20.9’WL)
4.9’ draft
4200lb displacement
Engine is setback 16” when motor dropped.

Prop is 18” below hull but that’s deceiving as the posterior of the boat is light due to missing atomic. As you can see, without my fat posterior in port aft position (piloting spot) the cav plate is out of the water. When I’m in the drivers seat it is 1.5-4” below.





Fg1k, moving motor to other side requires holes. Lots of holes. I’d prefer to not do that. It also seems that most out birds are mounted port side? I think it has to do with avoiding throttle (tiller motor) and tiller (rudder) collision.


Fuzz, this is the normal outboard lifting system on a sailboat. Basically a moving transom that lifts the unit 12”. Note that this is “no fat man on board” so motor is higher than underway.

[Jaysen]

Many motors lock the tilt up in reverse. But keep the tilt up forward for hitting bottom.

I would say something is not working or the engine doesn't have the needed feature, but most of them do lock in reverse.

You definitely need to get the engine to lock, but only in reverse. A larger prop is not the main issue. If you prop bigger, the larger prop is going to lift the engine even more as it has more contact with the water.

The motor locks but is sloppy. With motor off I can get the proper to move about 12”. Im thinking a bushing or something is worn.

I didn’t look hard for the low pin setting. I’ll check that.

[cape man]

Can you lower the engine on the bracket? Looks like the clamps that would go over a transom are butted up to the top of the bracket. You may need to do some surgery to get them out of the way. I think you are too high, even when you are sitting your fatness on that corner (hey, you said it first). A bigger prop may give you more oomph, but it won't stop the cavitation.

But I'm usually totally wrong...