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Author Topic:   Raise Those Engines
dr john posted 04-23-2007 12:51 PM ET (US)   Profile for dr john   Send Email to dr john  
Just moved up my Mercury OptiMax 225-HP motors to second to last hole on a 2002 OUTRAGE 290. What a difference in performance with significant reduction in fuel burn. I would think this would apply to Conquests as well.
poker13 posted 04-24-2007 02:55 PM ET (US)     Profile for poker13    
So where were the cavitation plates in relation to the bottom of the hull before and where are they now? Higher, lower or at the same level as the bottom of the hull?
Brian7son posted 04-24-2007 09:50 PM ET (US)     Profile for Brian7son  Send Email to Brian7son     
Dr. John,

What was the reason that you raised the motors? Did you have [problems with accelerating onto hydroplane] before you did this? I have a 1998 285 Conquest with 2004 four-stroke 225-HP Mercury motors. It's slow to get on plane and has big problems doing so when full of 295 gallons of fuel. The engine is mounted as low as it can be on my set up. With my step hull, I was actually concerned that the props still aren't low enough.

bkloss posted 04-24-2007 10:34 PM ET (US)     Profile for bkloss  Send Email to bkloss     
This brings up an interesting question; why doesn't the factory mount the Mercury motors so that the cavitation plate is at the correct level? I raised my Optimax 135-HP two holes on my 2004 Dauntless 180, and it, too, [made a] night and day difference. The motor came from the factory mounted flush with the transom top.

The problem is that the bigger and heavier the motor is, the more difficult it is to raise it, and, since they are mounting the motors anyways, wouldn't they want to mount it at the "right" level?

Maybe the "right" level is ultra-conservative with a lot of drag. Maybe I am missing something. Would anybody know why they wouldn't mount it in relation to the cavitation plate and the bottom of the hull?

Rough ocean seas? Mine doesn't have any problems raised two holes with large swells or 4-foot high waves in slop.


dr john posted 04-25-2007 08:26 AM ET (US)     Profile for dr john  Send Email to dr john     
This resulted in close to 3-inch rise of the cavitation plates in relation to the hull. I initially had the Mercury MIRAGEplus propellers which were poor out of the hole and to keep it on plane required more than 4,000-RPM, and in rough seas that was tough. I switched to the Mercury REVOLUTION4 which are, and still are, great out of the hole. I was now able to stay in plane around 3,300-RPM. The downside was poor fuel burn and loss of top end speed at WOT. Raising the engines decreases the drag and now compensates for the latter. Also increases RPM at WOT by about 200-RPM.
dr john posted 04-25-2007 08:33 AM ET (US)     Profile for dr john  Send Email to dr john     
There is a black, hard, synthetic, 90-degree angle [molding or trim piece] covering the top of the transom, and, when the motors are mounted low, this results in a flush mount that looks good and is easy to install for Whaler. Raising the engines mean you have to make cut outs [in the transom trim molding piece], but that was easy.
Brian7son posted 04-25-2007 09:11 AM ET (US)     Profile for Brian7son  Send Email to Brian7son     
Dr. John, bkloss,

This is VERY interesting. The funny thing is that if this resolves my problem, it's the exact opposite solution that I would have expected. I would really love to resolve my planing issue with a load of fuel.

I, too, have Mercury REVOLUTION4 17-inch-pitch props. My boat performs fine when up on plane, it just doesnt like getting there. With one passenger and less than a 1/4-tank of gas, I've had it up to 43 mph at WOT (around 5700 RPM's). My problem is that I really cant put in more than 3/4' of a tank of fuel without big planing problems (that is ridiculous!!!). You should be able to fill up on gas and "get up and go". I had also tried switching to 3 blade props and had the same problem that you had with your Mirage props. My boat came OFF plane at much higher RPM's with the 3 blades than it does with the 4 blades. So, if I take it to a shop and request them to raise the motors, how do I know how much to go? Did you guys both raise yours 3 or 4-inches? Do you guys think this will solve my slow planing and [problems accelerating onto hydroplane]? I really dont have any [problems] at crusing speed or top end or any of that. It's just getting up on plane. Even without a lot of fuel, it takes much longer that it should. With topped off tanks, I actually have to drive in a zig-zag pattern to get the props to "bite" so I can get on plane. That is just crazy.

I have asked in several posts and have had no replies regarding heavy Conquests and planing performance with a full load of fuel. One gentlemen with a 305 Conquest was replying to someone else in a post about performance with 225 four-strokes and he said "every ounce [is] critical". That really should not be the case. Boats are designed to fill up with fuel and have a heavy cooler or two on the deck and still be able to operate. I really hope the raising the motor works, I want to try it. Do you guys know anyone that tried that with a Conquest and 225 four-stroke motors? Mine's a 1998, it has the 10-foot 4-inch beam, 296 gallons of gas and 8,500 lbs dry, with the step hull.



dr john posted 04-25-2007 09:27 AM ET (US)     Profile for dr john  Send Email to dr john     
I'm not sure why you are having such problems out of the hole as my hull design is similar to yours. The four-strokes do have less punch than an OptiMax, and I would wonder whether this could be an engine [problem]. I will run this by a friend who is a marine surveyor and get his take. I don't think raising the engines would solve this.
bkloss posted 04-25-2007 09:43 AM ET (US)     Profile for bkloss  Send Email to bkloss     
First, I would examine where the cavitation plate is in relation to the bottom of the hull. You don't want the plate lower than the hull otherwise the plate will never ride on top of the water when on plane.

It is more a trial to determine where the best position is for the motor (and the boat). I raised mine two holes and it was perfect. Personally, I would not raise it in greater increments. If you raise it too much it will be self defeating as you could get prop blow out when making sharp turns or in rough seas. I have the original MIRAGEplus 17P prop and it has worked well for me. Each hull design has different characteristics that relate to its overall ride so you really need to test what's best for your boat.

I have emailed Boston Whaler to find out what determines their placement of the motor height on the transom. I will share the answer when they respond.


Brian7son posted 04-25-2007 11:37 AM ET (US)     Profile for Brian7son  Send Email to Brian7son     
Thanks for the input. My boat had the hull replaced under factory recall. It was in the factory in November of 2003 where they installed a complete new hull. There was a recall on the 1998 285 hulls and the prior owner had this done at Whaler (which I confirmed before I bought it). I don't know if in the installation of the new hull there was any additional weight involved. I was kind of enthused that I bought a 1998 BW CONQUEST with a 2004 hull from the factory and 2004 four-stroke motors. However, it's always had this [problem accelerating to plane] when full of fuel.
jimh posted 04-29-2007 01:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Let me make a few comments:

--my experience with Boston Whaler's newer boats is that they receive plenty of on-water testing to determine what the best propeller and engine mounting height should be. However, they may not have been as diligent back in 1998 as they are now. But in any case, the engine mounting height would tend to be on the low side of the range, as this is a more conservative approach.

--the mounting holes are spaced at 0.75-inch increments. On larger motors there are usually five sets of holes, and this means there is a range of 3-inches in the mounting height choice. You cannot raise a motor 4-inches just by changing from the lowest hole set to the highest hole set.

--the 225-HP four-stroke motors sold by Mercury were actually re-painted Yamaha 225-HP motors. These motors were some of the initial high-horsepower four-stroke motors available. They have a reputation for being somewhat light in horsepower. I believe the EPA test data shows they are closer to 215-HP motors. They also have a reputation for lacking low-speed torque. The problem with low-speed torque in a naturally aspired four-stroke motor was the basis for Mercury moving to a supercharged four-stroke. This problem has also been the target of a great deal of advertising by two-stroke engine makers, viz., the Evinrude E-TEC.

--boat performance is always related to the ratio of weight to horsepower. When more weight is added it always extracts a penalty on performance. If you do not have enough horsepower in reserve to handle the added weight, the performance will be limited. The distribution of the weight is also important. If the boat becomes stern-heavy, this aggravates the problem of getting on plane.

--trim tabs are often used on larger boats to help affect the boat trim during transition onto plane. If your boat has trim tabs, they may help getting on plane.

--the engine mounting bracket has a relief which allows the engine to clear the transom trim piece if the engine is mounted using the lowest possible position, but, as mentioned, the trim piece may have to be cut away if the engine is mounted in a raised position. Whether or not this affected the decision by Boston Whaler on what mounting height to use is hard to say. If you decide to raise the motors, it probably is a good idea to remove the trim in the section where the engine mounting bracket will bear against it so that the mounting bracket can be truly flush with the transom.

--raising the engine mounting height is not a magic potion to cure all problems.

--the best solution for improving the performance of a boat which is struggling to get on plane is to reduce the propeller pitch.

bkloss posted 04-29-2007 03:21 PM ET (US)     Profile for bkloss  Send Email to bkloss     
As promised, the response from BW regarding the determination of motor mounting height was -

"basically overall performance and handling. especially cavitation in a tight corner. Our engineering dept tests and recomends the setup."


Jefecinco posted 04-30-2007 09:40 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
A possible consequense of raising the engine will be that the boat cover may no longer fit.

Obviously this is a very minor issue when seeking best performance from your boat. But worth considering under some circumstances. For example, if you're in the market for a new cover and also considering raising your engine, raise the engine first.


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