jonboat: spray coming off motor shaft and over the transom ???

[Jobu88]

I have a Grizzly 1648, had been running a 9.9 and just upgraded to a 15 4-stroke, so about 25 pounds heavier motor. Usual load is trolling motor on the bow, 2x batteries just behind the raised casting deck, gear distributed evenly on the floor, me + gas tank + battery back in the stern area. When running at half-throttle during break-in I noticed spray coming off both sides of the motor shaft, spray directed up and out (not back away from the boat, but outwards so the spray is more or less parallel to the transom) and rising about as high as the top of the transom. When running 3/4 throttle the spray actually comes into the boat over the top of the transom and it's enough water coming in that it's not trivial. 20-inch transom and 20-inch shaft. I had the outboard set in the second of the six trim positions, with one being the farthest down position and six being farthest up. Reading the comments in a previous similar thread it sounds like I need to trim the motor all the way down closest to the transom? And just continue past 3/4 until the boat gets on plane? I never had the same problem with my 9.9 no matter how fast I was running.

 

[lckstckn2smknbrls]

Just because the transom and motor are factory spec'd at 20" doesn't mean they actually are. Where is the anti-cavitation plate in relation to the bottom of the hull?

 

[fhhuber]

is the drain plug inside or outside the boat?:nod:

 

[Jobu88]

Plate is within an inch or so of being even with the bottom of the transom. Drain plug is inside the boat.

 

[ondarvr]

The correct trim position may or may not produce the least amount of spray, the two really have little to do with each other.

Until you actually go full speed there isn't much you can do to help alleviate the spray.

20" doesn't always mean 20" when it comes to transoms and motors, it can vary a few inches and usually does. Once you can go full throttle you may find you need to raise the motor on the transom an inch or so.

 

[jbcurt00]

Jobu: an inch or so ABOVE or BELOW the bottom of the hull?

 

[airshot]

In theory the bottom of the hull and the antivent plate should be in line. Use a straight edge coming off the bottom of your hull toward the motor and where does the anti vent plate on the motor align? Optimum is level or slightly above the bottom of the hull..but some people find there prop grabbing air when turning so they end up dropping the antivent plate just a smidge below the hull bottom. If you motor is tilted at a great angle this can also cause excess spray. Also take a look at the bottom of your hull and make sure nothing is hanging down that could create this spray. Reminds me of a story many years back where a gent had a similar problem only to find his drain plug handle was dragging in the water causing his to spray up and inside. Just some thoughts here.......

 

[Jobu88]

Sorry for the delay, been on travel. To answer:

- the cavitation plate is an inch below the the hull, which according to the Mercury manual is acceptable. The manual recommends being even or up to an inch below the bottom of the hull.

- there is nothing hanging down, and my transducer is not the problem, the small wake it throws is well off to the side and not the problem.


Also I put the engine in the 4th highest trim setting today and the problem was reduced somewhat, previously I had tried it in the lowest setting, then the second lowest. I also noticed that at 3/4 throttle the problem was the worst, but upon going up to full throttle it wasn't as bad. I will shoot some amateur video next time so I can show exactly what I'm talking about and how it looks at different trim and throttle combinations. Thanks for all your input.

 

[GA_Boater]

Moving the trim out to the 4th position gives less spray - I think that's saying the motor is too deep. I would try raising the motor if this was my set up.

 

[Jobu88]

Yes, after talking to a bunch of my local friends I have already decided to look into a jack plate to raise the motor an inch or two.