how to set propeller depth in the water

I just purchased a used Johnson 1990 115 Hp V4. My old V4 was a short shaft, while the engine I just bought has a long shaft (about 3 inches longer). The guys at the shop mounted the engine about 3" higher on my transom so the trim and tilt bracket hooks are not hanging on the transom, but are about 3 inches above it (the fin at the bottom of the lower unit is still about 1" lower than my old unit). The engine is mounted to the transom by 4 bolts. Is this a problem or not??? In the water, if I don't trim my engine down all the way, a lot water will splash into the aft part of my boat and actually fill the area at the back on my boat! It also seems like there is a lot more drag than my old motor. Also, my wake isn't as clean as my old short engine. Should I lower the engine on my boat? If I do it seems like there will be more drag with the lower unit of my engine.

Re: how to set propeller depth in the water

Anti-ventalation plate (flat plate just above the prop) should be even with the bottom of the keel.
A bit lower or higher is ok to.

Re: how to set propeller depth in the water

Three inches higher than the transom is a bit high. The reason all the water is splashing in is because the lower unit is too deep. It causes drag, and the cavitation plate (large fin over prop) is also too deep, making the set of fins above it too low. The second set prevents splash. The general setup is for the cavitation plate to be even with the bottom of the keel, when trimmed so it is also in the same plane. Get a long straight piece of metal or wood and hold it against the bottom of the keel so it sticks out to the cavitation plate. Then trim the motor so the cavitation plate is parallel to the wood. The cavitation plate should be touching the wood. The motor needs to be raised or lowered accordingly. This is considered the starting point for setting motor height. It works well for most boats with out boards. The more "high performance" and faster the boat can go, the higher the cavitation plate can be over the keel. I just installed a 90 hp V4 short shaft and for my setup, ended up about 1" above the keel.

The solution is to either get a jack plate to properly mount and adjust the height of the long shaft motor, or get a short shaft midsection and lower unit to mount it in the original location.

Do a search on "jack plate" in the parts section too see what they are.

Also search the forums for "prop height" because there are lots of posts about this topic already here. Many have links to websites that talk specifically about all the parameters that go into setting prop height. Boat speed, how far back the motor is from the transom, hull configuration (planing hull vs deep V), etc.

Re: how to set propeller depth in the water

I disagree w/ bifflefan ... the AV plate should run just out of the water (top dry) at WOT. Where that falls relative to the hull bottom is irrelevant. If the hull bottom to the transom top where the motor mounts is ~ 15", you should have a short shaft motor.

Re: how to set propeller depth in the water

Your old motor was probably a long (20") shaft and the new motor is probably a X Long (25") motor. No problem. Just mount the new motor so the AV palte is no lower than the keel or pad. You'll probably end up with a jackplate so you can get it high enough to get your set up right. Your best engine height (X dimension) is a mix of prop design, hull design and intended use.

Re: how to set propeller depth in the water

wilde1j said:

I disagree w/ bifflefan ... the AV plate should run just out of the water (top dry) at WOT. Where that falls relative to the hull bottom is irrelevant. If the hull bottom to the transom top where the motor mounts is ~ 15", you should have a short shaft motor.

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These settings are all specific to the boat design and motor placement. But I agree the cavitation plate should never be below the keel. Often boat manufacturers mount the motors a tad low so that props will not "blow out" on hard cornering at high speeds. This also causes lots of splash. The Force motor on my Bayliner was mounted that way and the splash was horrible. I chose to mount the new motor about 1" higher than the keel as I mentioned earlier.

The real solution is to read about setting prop height as it applies to that specific boat.

Even with the keel is a "generic" starting place that should work for most small boats.

Re: how to set propeller depth in the water

Sounds to me like somebody sold you a mtr that your boat wasn't designed for. Happens all the time.