Is 19" Tempest still over rev

[Knottymate]

Helping a very inexperience boating neighbor. He has 20' Tahoe Fish/ski (555T ??) with 150 Merc 4/S and 19" Tempest. Doubt he knew on transom dealer put screw type jack plate that appears all the way up and motor is one notch up on jack plate. He was not familiar with the trim and for the years the boat porpoised.

A short burst showed 7000 RPM at 35 mph and generally 28 mph around 5500/6000 with moderate trim down with no porpoising. Pulled the boat and obviously cavitation plate is about 2 inched above center keel

Have the boat at a shop to get the motor properly aligned. Do ya all think with the (current prop) max 6000 Rpm at full throttle is realistic?? What is the max RPM this motor should turn --- around 5800 to 5500 I suspect is the target.

If revs are to high what prop is recommended. For his purposes, a basic aluminum should be fine. He may pull some tubers but for him a level riding rig at moderate speed with quick planning is best.

 

[GA_Boater]

What is the setback of the jackplate? A motor is raised higher on a jackplate and the normal anti-ventilation even with the keel setup goes out the window.

Is he trimming the motor properly? Saying "moderate trim down" sounds like it isn't trimmed correctly.

 

[Scott Danforth]

^ +1....what he said

proper use of the trim button should wear it out about once every 7-8 years. trim all the way down for the holeshot and as the boat comes out of the hole, trim up until it porpoises then a few blips down

a screw type jack plate should be rigged for the task at hand. (fishing with one, two or 3 people, etc.) everyone I know uses a powered jack plate because they always need adjustment depending on load and they want to get into really skinny water.

you can pull tubes with a 20hp motor

 

[Chris1956]

7000RPM at 35MPH for that motor/boat combo indicates ventilation or some other cause for prop slip, IMO, provided the tach and speed are accurate. The obvious choice is to lower the motor on the jack plate, until she holds water better. A Tempest is a reasonable prop, I would think it will give reasonable performance, and 19P sounds about right.

I would not expect much more speed out of her, say 40MPH, at redline. I would target that performance.

 

[porscheguy]

Sounds like the motor is way too high. The 150 4stroke has a range of 5000-5800rpm. Obviously the closer you get to 5800, the better.

To give you an idea just how bad his setup is, let’s do the math.
His gear ratio is 1.92
At roughly 5750RPM he’s running at 28mph.
That’s a prop slip of 48%.

7k rpm at 35mph comes out to 46% slip.

Even if the data is off by 15-20% you’ve still got major slip issues. These issues are clearly a direct result of the current motor height.

The tempest prop is a good prop IMO. It’s the 3 blade variant of the rev4. People buy this prop when they want to go fast on their larger bass and/or walleye boats. It’s application should be appropriate on the boat we’re discussing. Even the pitch should be about right. The prop at this point is the least of your worries.

If you do the math with sensible (albeit theoretical) data you’ll see that at some point, the correct prop was installed.

At 5800rpm (which is the factory set maximum rpm on this engine) a 19P prop should be moving the boat along at 47mph factoring in 12% slip. And mind you, I’m being conservative here.

 

[QBhoy]

All the above should be listened to for sure. Something way wrong. Even without a jack plate and engine sitting lower...it isn’t going to loose 1000rpm or anything like it. It must be bouncing off the limiter in seconds. Sounds like it either needs a much bigger pitch and/or the tacho/speedo verified.

 

[Knottymate]

As mentioned in the beginning the owner was not familiar with the trim meaning I doubt he even knew it what it did. After my sea trial driving the boat I knew immediately the motor needed lowered and had him next morning get it to a reputable shop and NOT to the dealer he bought it from.

When I mentioned moderate trim I took the trim down from porpoise to level smooth ride. My inquiry is concern at non porpoise trim the excessive 7K RPM at 35. Is it reasonable to think with motor down and cavitation plate even with keel bottom do we expect at wide open the 19 tempest should only tach 5800 +/- 200. I agree the boat should run at least 45 at 6K with this prop

 

[porscheguy]

Yes it’s reasonable to expect the RPM to come down That much by lowering the engine to a proper level. At 46% slip, something is way off, in this case, he’s surfacing a prop that likes to run high, but not necessarily on the surface.

remember, a prop is fine tuning, motor height is basic setup. You’ve got to get your basic setup right before you can fine tune with the prop.

 

[Knottymate]

Another opinion please, The motor is one notch up on the jack plate. IF getting the jack plate all the way down still has the cavitation plate just above the keel bottom why is the shop reluctant to drop the engine down the One notch available.

The shop seemed reluctant because of labor (which they would get paid for anyway). Possibly some liability damaging the crank/internals/ due to the stress lifting the motor etc etc

 

[porscheguy]

Any decent repair shop should have the ability to raise or lower a outboard motor with minimal difficulty.

Their reluctance could be justified or simple laziness.

How much higher than the keel is the bottom of the cavitation plate? Ideally it should be 1 inch higher for every 12 inches back from the transom because the water comes up immediately after it passes the keel. He may be reluctant to drop the motor itself because it may put the powerhead too close to the water.

with the jack plate all the way down, if the cab plate is slightly above the keel, I’d test it and see what happens. If you still have sky high slip and poor performance, then it may be time to lower the motor a notch.

 

[GA_Boater]

Another opinion please, The motor is one notch up on the jack plate. IF getting the jack plate all the way down still has the cavitation plate just above the keel bottom why is the shop reluctant to drop the engine down the One notch available.

The shop seemed reluctant because of labor (which they would get paid for anyway). Possibly some liability damaging the crank/internals/ due to the stress lifting the motor etc etc

Has the shop taken the boat on the water?

 

[Knottymate]

The shop will water test next week with the ''Slidemaster'' jack plate all the way down. Finally found the plate that is on the
boat attached is photo. Mfg says it is an 8" plate. Your analysis about set back should resolve with proper performance

 

[Knottymate]

Sorry that link did not upload correct plate. This is the plate. His is set exactly like shown in the photo;