why does my prop make this trail?

[flyguy90]

I have a little 13ft fiberglass jon boat micro skiff ive put together with a 76 mercury 9.8 outboard. made a homemade jackplate to move the motor up and down to test out heights. No matter what height I run it makes a V spray behind me. Ive tried cavitation plate even with keel, 2"above and 1" above. Pics are of 1" above but it does the same no matter. Really dont want to lower cativation plate below the keel. What would be causing this weird spray? Is it because I do not trim up the motor and let it rest on the jackplate? Because if I do trim it to the first hole, it seems to loose performance and nose up too much. and still makes this spray. Right now it has the 2 blade prop. Im buying a new 9x9 3 blade and going to run that here in the next week or so.

 

[QBhoy]

I'm thinking probably because it doesn't really have the design profile on the lower leg to be ran like that, but mainly it looks like the trim angle is too far in. The cav plate it likely angled down and ploughing in efficiently through the water and throwing the spray ? Just initial thoughts.
All the best.

 

[QBhoy]

Saying that. It looks the opposite, looking at the close up pictures when out the water. Haha. Is that a long shaft ?

 

[flyguy90]

not a long shaft. I always thought it was until I measured it. you may be right, the cativation plate as the motor sits like that is angle away from the boat a few degrees. however I have always found that if i put the trim up a bit i loose power

 

[Sea Rider]

With OB trimmed to 90? and deck weight well distributed, have someone, preferably light to pull head over transom at plane and visually check at which lower leg height water flow passes at speed. Check if under uppar plate and how much even or avove lower plate. Will know what to do if in need to rise/lower OB a bit.

If pic is not playing tricks on me, see 3 horizontal plates, which one of upper 2 has a protruding lip if any next to plate ?

Wild ear guessing or checking water wake to assume if OB sits ant the Tip Top Transom Spot is a waste of precious boting time, a visual inspection will tell so spot on.

Happy Boating

 

[flyguy90]

in the pics you can see the upper plate which is always out of the water. the middle plate is actually just the crease where the lower unit goes together and the lower plate is the ventilation plate.

 

[jbcurt00]

not a long shaft. I always thought it was until I measured it. you may be right, the cativation plate as the motor sits like that is angle away from the boat a few degrees. however I have always found that if i put the trim up a bit i loose power

Not a long shaft? Your skiff has a less then 12in transom? Looks like you've got it lifted up at least 6in.

I'm surprised its not a 20in long shaft

 

[Texasmark]

In picture #2 the plate protruding out front of the lower unit adjacent to the red strap on the far side of the boat, is there to stop water from coming up the front of the LU and into the boat like you are experiencing. It has to be above the top of the trailing water coming from the stern of the boat to function. From your picture it appears to me that it is several inches above a straight line extension of the bottom which should put it up and out of your way and should function as designed.

What I think is wrong is that you have the engine in too low a hole in your tilt bracket.....prop is too close to the transom (rear of the boat) I don't even see a pin in your tilt bracket for your engine's mid section to rest against. No wonder it's tilting in too far.

Get a piece of "all thread" that will just fit through the holes and a couple of locking nuts and make one. Then set it to the mid hole with your engine tilted all the way up and when installed, let the engine come down against it. That should put your engine about at a 90 degree angle to your hull.

This will probably cause porposing where the bow (front of the boat) wants to move up and down in a repetitive fashion. To correct this condition:

Move some weight forward and/or speed up, and/or move your rod you installed in the clamp bracket to the next hole closer to the transom which will put the prop closer to the transom, bringing the bow down and reducing the tendency to porpoise. If you boat with a friend, their weight up front will/should prevent any problem with porpoising when using the center hole for the tilt position.

Do it and get back and tell us how it went.