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Author Topic:   Adding Weight to Affect Trim
mfclapp posted 04-30-2007 11:27 PM ET (US)   Profile for mfclapp   Send Email to mfclapp  
I am running a 1999 Outrage 20 with a 200-HP Mercury OptiMax with a 19-inch-pitch three blade factory MIRAGE propeller [or perhaps a MIRAGEplus?]. It runs around 4,600-RPM at 48-MPH with three people (640-lbs) and a half tank of fuel. It also has a tee top and a Honda 8-HP four-stroke kicker mounted on the starboard side. It always had poor out of the hole performance so I added a set of trim tabs and this seemed to have solved this.

When the boat is off plane the motor well drain plugs sometime at low speeds alone with the side scuppers are right at or below the water line. Will adding weight in the front floor compartment be ok? We had one of the guys who weighs around 170lbs sit in the bow and everything was fine.

Secondly what would happen if I change to a 17-inch pitch propeller? Or is the [engine speed] and [boat] speed about right?


jimh posted 05-01-2007 02:15 PM ET (US)     Profile for jimh  Send Email to jimh     
Adding weight to a boat to cause a change in its trim is often done, but it is not the optimum way to change the trim. Any weight that is added to a boat reduces the boat's performance. This is a basic law of physics. Therefore, as a general rule we never want to add weight to a boat purely to function as weight. An exception might be in the case of a sailboat where we want to ballast the hull to gain stability and a righting moment, and there is nothing else in the vessel's machinery which could be located there and still be useful except for lead. A dense material like lead, or perhaps depleted uranium, is best because it takes up the least space, and, as we know too well, space is always scarce on a boat.

It would be better to move some existing gear that has a useful purpose. This would be a double benefit, as moving weight already on the boat will remove that weight from the stern.

Regarding the effect of a change in propeller pitch on the engine speed:

According to Dave Gerr (in his PROPELLER HANDBOOK), there is a long standing rule of thumb with propellers that states:

Every two-inch increase in propeller pitch will decrease engine speed by 450-RPM, and vice versa.

As a general rule in propeller selection you want the engine's maximum power output to be reached, and this is usually achieved by allowing the engine to spin up to its maxium rated speed for the rated horsepower. The load from a particular propeller will also be greatest at this this point, and thus you will have matched the propeller load to the engine output. You can then be sure that at all lower engine speeds the engine power output will exceed the load from the propeller, and it is this quality which gives the boat the feel of having plenty of power and being easy to accelerate. If the propeller load is too great, the engine is chronically overloaded or bogged down, and the boat has a sluggish response to changes in the throttle.

sosmerc posted 05-05-2007 09:54 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
That engine should reach 5500-5800 rpm in order to develop proper torque and power. I would think that a 19 pitch Mirage would be close to the correct prop...however, there are a number of factors that may be in play.
Your hull could be water-logged and therefore heavier than normal.
Your boat bottom could be dirty.
Your engine could be down on power due to a mechanical problem.
Your propeller pitch could be other than what it is supposed to be due to damage, or mis-repair or defect.
I think I would have the engine checked out by a competent mechanic first.
Tom W Clark posted 05-06-2007 01:05 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Your tachometer is wrong. There is no way a 200 HP OptiMax turning a 19" Mirage (or Mirage Plus) could hit 48 MPH at 4600 RPM. I would expect the motor to be turning 5800 RPM at that speed. I would say your performance is excellent.

However, if you are not satisfied with your acceleration, then a different propeller may be in order. You may not necessarily drop down to a 17" pitch prop if you use a different model of propeller. it will depend on what prop(s) you try.

You may also want to simply open some of your vent holes or use PVS plugs with a partial hole in them to help your motor spool up faster. This is a game of experimentation but an easy one.

Tom W Clark posted 05-06-2007 01:12 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

While Gerr's book may be a good one, I'm not sure that rule of thumb is entirely applicable to outboard powered Boston Whalers.

Tom Clark's rule of thumb states that for large outboard motors, every two inches of propeller pitch change there will be an inverse change in engine speed of 300-400 RPM. This is based on personal and anecdotal experience with many, many Boston Whalers and other outboard powered boats.

sosmerc posted 05-06-2007 12:24 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
That's a good catch Tom. The '99 Optimax is an analog motor so the boat is most likely equipped with an ajustable tach they may possibly be in the wrong setting.
Should be set in the 6 pulse position for this motor.
Using the supplied numbers his current prop would have to be slipping LESS than 5 percent to get 48mph...and we know that isn't possible with this setup.
And yes, 48mph seems pretty decent for a 20ft. Whaler with that prop/motor combination. Bet he'd be even happier with a 19 Enertia...I sure like my 17p on my Ventura.
Brian7son posted 05-08-2007 02:46 PM ET (US)     Profile for Brian7son  Send Email to Brian7son     

I had a similar problem getting on plane and no issues with my top end or cruisng speed performance. However, I dont have the optimax. I have twin 225 Merc. 4-strokes which are known to have a slow hole shot. Mine boat is a BW 285 Conquest. JimH had suggested switching the PVS vent plugs on my props. Since I had bought the boat used, I didnt know the brand props that I had and I didnt know that they had vents. If you have a Mercury Mirage prop, it may have the PVS plugs. Look on the tube section between the blades for a black plug (about the size of a nickel). Mine had 4 plugs on each prop and they were solid "closed plugs". I replaced them all with large vent hole plugs and it increased my RPM's and gave me better hole shot. If you go on Mercury's website you can read all about the "PVS system". They only cost $1.49 each. It may fix your problem. If it doesnt work, it's the cheapest mistake you'll ever make.

Good luck,


20dauntless posted 05-08-2007 08:09 PM ET (US)     Profile for 20dauntless  Send Email to 20dauntless     
The performance you are seeing is excellent. My 1996 20 Dauntless uses the same hull as your boat does and I typically see top end around 55 mph at 5400 rpm with my carbed Yamaha 200 when lightly loaded. Forgive me, but I do not know what prop I have on the boat. Is your tach showing 4,600 RPM at WOT? If it is then your tach is definately off by several hundred RPM. I would reccomend having your tach adjusted correctly and then testing to see how many RPM your engine is turning at WOT. If it is not within the factory RPM range for WOT it is time to get a new prop. However, it sounds like your getting pretty decent performance from the setup as is. Remember, the T-top will cost you a few MPH at top end due to the increased windage.

Now, to the issue of adding weight to the bow. My boat is a bowrider so it is easy to experiment with changing weight distribution. I have found that with more weight up front the boat planes quicker and rides smoother, but the scuppers do not rise significantly from the waterline. I have a Yamaha T9.9 kicker and find that when loaded with a full tank of fuel and six people (plus gear) the scuppers are at or sometimes just below waterline. I bought some cheap plugs to put in the scuppers when the boat is resting so that water will not flow in. These plugs can be removed when on plane to let the scuppers do their work. I would second the reccomendation to put as much gear forward as possible but do not add extra weight simply for the sake of adding weight.

jimh posted 05-08-2007 09:04 PM ET (US)     Profile for jimh  Send Email to jimh     
Tom--Your thumb (as in your rule of thumb) appears to be slightly narrower than Mr. Crouch's (400-RPM versus 450-RPM). This surprises me, given your background as a professional master wood craftsman. I would have thought that you might have hit your thumb a few times with a hammer, broadening it.

Among fellow Boston Whaler owners, 50-RPM is generally consistent with the difference in the size of our thumbs.

mfclapp posted 05-09-2007 12:31 PM ET (US)     Profile for mfclapp  Send Email to mfclapp     

I am running a Mirage Plus 19 and it doesn't have any plugs to remove.


mfclapp posted 05-09-2007 12:35 PM ET (US)     Profile for mfclapp  Send Email to mfclapp     
Also the motor was just tuned and all fuel pumps, filters, plugs, injectors, oil pump, etc, etc were replaced. Engine temp running continuously at WOT runs about 1/4 on the gauge.


mfclapp posted 05-10-2007 10:31 PM ET (US)     Profile for mfclapp  Send Email to mfclapp     

Checked the back of the tach and it is set in the 6 p position.


Plotman posted 05-11-2007 03:55 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Was your 48mph by GPS in still water, or via some kind of in the water speedo? You don't say - so it could be that the tach is right, but the speedo is wrong.
mfclapp posted 05-13-2007 09:27 PM ET (US)     Profile for mfclapp  Send Email to mfclapp     
The speed was what was showing on the two GPS units on the dash.


Tom W Clark posted 05-15-2007 08:52 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I still say no way you can run 48 MPH at 4600 RPM with a 19" Mercury Mirage Plus. I use the same propellers on my own boat and know what the typical slip figures are. Mine are very low, about 4 percent but doing the math on the data you have supplied means you have a negative slip of about 8-9 percent. That is not possible.

Let's talk about speed. It must be measured with a GPS. Paddle wheel sensors or pitot tube speedometers are notoriously inaccurate. Many GPS units also use a paddle wheel sensor and will display that data.

Speed must always be measured traveling in two opposite directions and then averaged. This will cancel out the effects and advantages of wind and current.

Tom W Clark posted 05-15-2007 08:56 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

The part nuber of your Mirage Plus is 48-13700 A45 19P. This is a slightly older Mirage Plus design without the PVS holes.

You could try a 48-13700 A46 19P, the A46 suffix indicating the hub is a Flo-Torq design WITH the PVS vent holes.

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