Cruising RPMs from 7.4L

[Doc_Vegas]

Greetings from Lake Mead...

I am trying to get the hang of the Maxum, and we took our first trip around the Lake. It seemed I could not get the Maxum to plane properly without having the RPMs around 4000-4200. Is that too much? The speedo was reading 30-35 MPH and it seemed to ride nice. I am just worried that running the motor at that many RPMs could be too much. I tried trimming the motor and tabs, but it did not seem to help much, but then again, I am a novice.

The motor is the 7.4L Mercury with a Bravo 3 dual-prop set up. 27' Maxum with Generator and we had six passengers at the time.

Any thoughts that will help me keep this motor healthy would be greatly appreciated.

Thanks!

 

[torcano]

Re: Cruising RPMs from 7.4L

6 people on the boat is definetly a factor. What is your WOT RPM? I suggest you start out with the trim tabs in the full bow down position to start. This will get you on plane faster. I usually go to 4000RPM to get on plane but maintain plane at between 3200-3500 RPM. However, I have twin screws. Given the size of the boat, the number of people you had on board and the single engine, I am not surprised that you needed 4000RPM to get on plane. Try the boat by yourself and see how it handles.
RK

 

[QC]

Re: Cruising RPMs from 7.4L

Yeah, hard to tell what you mean as many people report that their boat isn't planing "properly", but like you post 30 - 35 MPH. She should plane around 20 MPH or less, so I am sure you are on an efficient plane much below the speeds you indicate. Need the speed verified by GPS, and we also need to know the RPM at full speed, as fast as you can make her go as Torcano has noted (WOT RPM). This is a VERY important number and will help you make decisions regarding setup etc. You need to do everything you can to get he RPM as high as possible at full throttle. Trim, tabs, etc. All need a little fooling with while running this way until you find a combo that gives the most RPM and speed. When you find that we need to know that RPM. That will help us hone in on a few things for you.

Personally I always thought that she would only run around 35 MPH anyway. 25 MPH would be a very good cruise speed if that's the case.

 

[BigB9000]

Re: Cruising RPMs from 7.4L

What year and condition of the boat?

 

[QC]

Re: Cruising RPMs from 7.4L

Check his sig BigB. It has been in dry storage. Clean bottom today.

 

[BigB9000]

Re: Cruising RPMs from 7.4L

Check his sig BigB. It has been in dry storage. Clean bottom today.

...show offs!


but seriously, I see "Slipped at Lake Mead"

....?

 

[QC]

Re: Cruising RPMs from 7.4L

He just bought it used and it had been out of the water until last week. Bottom is clean at least for another couple of tests . . .

 

[Doc_Vegas]

Re: Cruising RPMs from 7.4L

Thanks guys, I was out for T-giving. Sitting on the Maxum now relaxing... very peaceful, but too late to go roaring off into Lake Mead.

I will check out the max speed this weekend with a GPS. I am not sure if I am getting the motor trim correct. I bring it up until I hear it sound different then go no higher. On the trim indicator, I am only bringing the outdrive to about 1/4 up the scale, maybe 1/3rd. Any higher and it sounded different and I was worried.

Any thoughts? The trim tabs are very odd to me. I was never sure if they were really doing anything, or if was just coincidental to the motor RPMs... Again, I need some more play time to get the feel for the rig.

Thanks again.

 

[180shabah]

Re: Cruising RPMs from 7.4L

Ignor the trim gauge, it will not help you get things dialed in.

Don't think the tabs are helping? Take off with them fully retracted, then do it again with them down. You WILL see the difference.(unless they are not functional?).

 

[QC]

Re: Cruising RPMs from 7.4L

With a boat that size don't expect too much from trim. You need to play some, but it is not going to lift the bow and make her fly. Once you find a comfortable cruise, small bumps on both the tabs and the trim are all you want, then wait and see what effect they have on speed and RPM. If you get RPM rise, but no speed increase (probably the noise) then your drive is probably too high . . .

Oh, oh . . . if your tabs are set up right, hitting the rocker switch up pushes the opposite tab down. This also assumes you have rocker switches, but the switch to the right, moves the port (left) tab, and the switch to the left moves the starboard tab. Think of the switches as your bow. The left switch pushed on the top pushes the starboard tab down which pushes the port (left) bow down. Pushing on the bottom of the switch lifts the tab, which lifts the bow. So to follow Shabah's advice you need to hold the bottom side of the two switches down. Count to 15, the tabs should be full up (they move very slow). Then try to get her to plane and note how long it takes. Stop, and hold the top sides of the switches down for another count to 15, then try again. If she doesn't plane off faster I would suspect that your tabs are not working. Have you seen them move?

P.S. I have a pdf version of the Bennett trim tab manual. PM me your email addy and I can send it to you if you don't have one.

 

[180shabah]

Re: Cruising RPMs from 7.4L

Trim Tab Owners Manual

It is 4.53Mb so give it a minute to load.

 

[180shabah]

Re: Cruising RPMs from 7.4L

Here are the Troubleshooting Links

 

[Doc_Vegas]

Re: Cruising RPMs from 7.4L

Thanks guys, I can hardly wait until Saturday to try it out again.

Here is a simple question, I was running with a full-canvas top installed. At one point we removed most of the Eisenglass, but we still had the double-bimini up. Could that be changing the equation at all? Seems like a giant parachute....

I will report back on the weekend, cheers!

 

[torcano]

Re: Cruising RPMs from 7.4L

I run my boat that way all the time. Having the bimini up should not significantly effect the performance. In the end, I think the answer is that your boat has reached the size where twin screws would have made a big difference in performance. I think you will find a big difference with just one or two people on board. My previous boat, 240 Searay Sundancer, had a 5.7L Mercruiser with a Bravo III. It got on plane very quickly when I was alone or with my wife. One time, I had 6 guests on board. The boat barely got on plane and the handling charcteristics changed dramatically when I was trying to dock the boat. Let us know how the boat handles with less people. I wish I could take my boat out for a trial this weekend but it's 27 degrees today!

 

[180shabah]

Re: Cruising RPMs from 7.4L

Yes the bimini will make a difference, but it will be a tiny one. You have a lot more drag on the bottom of the boat, and at 30-35mph wind drag is negligible(sp?).

 

[QC]

Re: Cruising RPMs from 7.4L

I agree, Bimini makes a very small difference and could even be an improvement if there is any lift created (unscientific, unsubstantiated, poorly contemplated, dataless, wild-arse speculation). I run 50+ with my bimini up all the time . . . my boat won't run much faster with it down, but it is a lot faster on fumes, no ice chests and just me . . .

 

[Doc_Vegas]

Re: Cruising RPMs from 7.4L

OK, had some time on the Maxum yesterday. Used a Garmin GPS, which really did help. Here is what I learned. I had three people on-board with a combined weight of just over 500 pounds. Also had a full tank of gas, full tank of water, and unknown amount in waste tank.

The fastest I could achieve was 38.7 MPH. There was a bit of chop and a slight head wind. If I had let it wind out a bit more, I think I could have gotten to 40, but that would be about it.

I learned that I was pulling the motor trim up to high. Seemed like about 1/4 of the trim gauge achieved maxum speed with the conditions listed. I pulled the trim tabs to complete bow up for the speed tests.

I played around with the trim and tabs and RPMs, and found a nice cruising speed of around 25 MPH at 3600 RPMs. Was very stable at that speed and I ran across the entire basin with just steering. Below 3600, I had a hard time telling if I was planing or not. Seems the key is to bring the RPMs down SLOWLY, that what I was doing wrong before.

I verified that the tabs were working by running one tab down and one up (briefly) they are working.

I just need more practice. I would like to get the boat to plane at a bit lower RPMs since we are generally not in any hurry and I would like to keep the stress on the motor and fuel consumption to a minimum.

Just to verify, our dealer suggested we keep the tabs to Bow Down and the outboard trim DOWN, then launch with full throttle. At about 4000 RPMs, I begin to slowly back off the throttle and try to maintain around 3600.

So, what to you fellows think?

I will post pictures later today. Heading back to the Lake now, December in Vegas is fabulous

 

[QC]

Re: Cruising RPMs from 7.4L

I think she is doing exactly what she should be doing . . . In fact very good with that much load. As far as a lower cruise, my guess is that you're at the most efficient point right where you found,but I bet she'll stay on plane down to about 18 MPH with the tabs full down. Technically you are on plane when the transom goes dry . . . Basically if you can see the transom, down to the water, then you are planing. With the big step this will be hard to see. Another way is to watch the wake. If you see two lines, as opposed to a big roller behind you, then you are also probably planing. With the tabs down, she might hold down to 15 MPH even, you will know, if you mess around down there . . .

 

[torcano]

Re: Cruising RPMs from 7.4L

I concur with QC. Given the a full tank of gas and 3 people, I think your boat is performing exactly as it should. I find it difficult to keep my boat on plane under 3000RPM. I need to practice more with my trim tabs. Unfortunately, it's going to hard to do on a windy day with the air temperature at 32 degrees, especially with my boat up on blocks. It's only 14 weeks until I re-launch my boat....not that I'm counting.

 

[Doc_Vegas]

Re: Cruising RPMs from 7.4L

Thanks again guys. I will post the pictures in a few minutes. Cheers.