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Author Topic:   2000 Conquest 21, 225 Opti performance
afishinado21 posted 06-18-2004 10:13 AM ET (US)   Profile for afishinado21  
I have a 2000 CONQUEST 21 w/hardtop, 225 Optimax and 17-inch Mirage Plus SS prop. At WOT I'm getting around 5100 RPM and GPS speed of 34 knots with 2 on board, 3/4 fuel. This has been the general result over the last three years.

Here's the thing, I could swear that during the first season with the boat I got engine speeds closer to 5500-RPM. It is possible I'm wrong. I never run the boat at WOT, but I'm wondering, would my mid-range performance (3500-4000 RPM) would be better if my WOT was closer to 5500?

Can anyone think of a reason why I'd lose engine speed at WOT between seasons? No noticable changes to the boat other than fishing gear and such. I remove the prop every winter to prevent theft. Is it possible to put it back incorrectly every spring? Prop is immaculate, never touched anything but water with it.

Motor has been checked out by certified mercury mechanic and DDT computer a month back. The Digital Diagnostic Tool computer (DDT) shows 100% available power when cruising at 4000-RPM. (The Optimax engine control module stores lots of information on performance and faults. The DDT is used to get to that information. Although many mechanics do so, you cannot properly service an Optimax unless you have one.) My mechanic is not sure why the lose in RPM if boat hasn't changed.

Anyone experience similar issues. Anyone with a similar setup have performace numbers? Thanks...

Peter posted 06-18-2004 11:27 AM ET (US)     Profile for Peter  Send Email to Peter     
If the motor checks out fine, then one answer to a loss of WOT RPM could be a restriction in the fuel line which only becomes noticed at WOT. Likely suspects in that case are the water separating fuel filter and primer bulb.
dauntlass 18 posted 06-19-2004 07:03 AM ET (US)     Profile for dauntlass 18  Send Email to dauntlass 18     
My 21 ft. Conquest w/ hardtop w/ 200hp Optimax and a Mercury Mirage Plus prop # 48-18278 17P two people on board and 3/4 full fuel tank at wot turns 4900 to 5100 rpm it varies from day to day and per gps will show speeds of 39 to the highest I ever got of 44mph{Gps set in mph setting}.Haveing owned the boat three seasond plus I can say the best operation range is 3200 to 4000 rpm.Cruise at 3200 to 3600 rpm gets a approximate seven gallon per hour fuel comsumption.
linhans posted 06-19-2004 07:30 AM ET (US)     Profile for linhans  Send Email to linhans     
I agree with Peter, check fuel line and filters. My sugestion, how clean is your hull. It makes a big difference if the hull is not clean. The difference in speed at WOT can be more than 5-MPH with the difference in RPM.
afishinado21 posted 06-21-2004 10:07 AM ET (US)     Profile for afishinado21    
Thanks Dauntlass for the performace numbers . It seems I'm getting the same performance as you, although I have 25 more horsepower. I agree the most efficient operating range is around 3500-4000 RPM. At this speed I believe I get about 25 knots.

I change the fuel filter every season, so, unless each one has the same problem, I guess this is most likely not the problem.

During the first month of operation, back in 2000, I had a problem where the motor would stall at slow speed. The primer bulb would lose prime. It turns out there was a recall on the primer bulb which was changed by the dealer. The new bulb has a yellow mark on it. After that I never expierieced a problem with the new bulb losing prime at any speed.

A primer bulb problem may make some sense... It was probably during my initial water test back in 2000 that I remember the dealer showing me WOT at 5500-RPM. I guess its possible that after the bulb was changed this problem occured. Since I rarely go WOT I never put 2+2 together.

If there was some sort of fuel flow problem wouldn't I see hesitation, misfiring or something as the fuel, oil air mixutre changes at the higher speed? Motor sounds like its operating fine. When my mechanic originally checked out this problem in 2001, he agreed motor sounded perfect.

The hull bottom seems fine except for some slime. No growth that I can see, although I haven't gotten in the water yet to check it out. The boat was bottom painted in 2000 and 2001 with a 2-year ablative paint. In 2003 my marina was pushing the Dolphinite Go Fast Bottom paint which turned out to be garbage. I believe I had a tree growing on the hull before the Marina re-painted. This season I re-painted again with 2-year ablative. I've never stripped the hull but instead scored the previous paint before applying. I guess it is possible the numerous layers of bottom paint are degrading performace.

bsmotril posted 06-22-2004 09:57 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Two more easy things to check are:
1) Is the throttle butterfly opening fully with the throttle wide open? If the cables were ever disconnected for service, then put back, they may have been set differently and your throttle butterflye is not now opening all the way

2) Check spark plugs. Worn plugs will cause a loss of top end performance.


Bigshot posted 06-22-2004 02:31 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Slime will kill performance and the bottom paint is the reason you are slower. Not because of the paint, because of the condition. You also have to check how she runs with 1/4 tank of fuel. You might remember running it when empty. My Hydra-sports is set up for speed and with a full tank (85gals) I get about 5600-RPM and about 47-MPH. With 1/4 tank I get about 5900 and 50+ MPH.
afishinado21 posted 06-23-2004 10:46 AM ET (US)     Profile for afishinado21    
My plugs were changed last month and are changed every year. I run the $15 NGK plugs. Ironically, during the first year of operation, I was running the $2 factory installed Champion Plugs. That's when I recall getting a higher WOT.

Never removed throttle cables. I will check to see if butterfly opens fully.

Thanks for all your suggestions.

jimh posted 06-23-2004 11:29 AM ET (US)     Profile for jimh  Send Email to jimh     
With a hard top, the wind direction could affect your speed. Upwind versus downwind might make a difference of several miles per hour.

To regain those lost 400-RPM, if they were really lost as you conjecture, you might have to duplicate the boat weight, wind, and sea conditions.

northwhaler posted 06-24-2004 09:22 AM ET (US)     Profile for northwhaler  Send Email to northwhaler     
Your performance numbers seem reasonable to me. I run a 20 Ventura with a 200 Opti, turning a 17P Vengeance prop. (A little smaller diameter than the Mirage) WOT is 5600 RPM.

Keep in mind that my prop is less in diamaeter, and my boat weighs in at about 1000 lbs less than yours, with your hardtop. Seems reasonable to me that you would be turning at about 5100 WOT. Have you considered going to a 15P Mirage? With your user name, I assume that you fish with the boat, and the smaller pitch prop would offer two improvements; slower trolling speeds, and a WOT number closer to factory specs. Because of the higher WOT, you probably wouldn't lose (if any loss) much in top MPH.

I don't do much mechanical work to my Opti, other than change plugs and lower unit grease, (regular maintenence stuff) and am curious as to where one would look for the throttle butterfly on these engines. Since they are direct injected, I didn't realize they even had one, or several, throttle butterflies.


Peter posted 06-24-2004 09:25 AM ET (US)     Profile for Peter  Send Email to Peter     
The throttle butterflies should be where the carburetors would be. These butterflies control the air intake. They are connected via linkage to your throttle cable.
afishinado21 posted 06-24-2004 10:33 AM ET (US)     Profile for afishinado21    
When I go WOT the conditions are generally optimal, flat seas with little or no wind. Hardtop is factory installed and came new with the boat in 2000.

Regarding the butterfly, I don't know if that is visible on the opti. I was planning on advancing the throttle fully forward then check on the engine to see if the throttle arm was actually fully extended or had additional play. Not sure this would have solved anything.

I was thinking of trying a 15p prop just to see how performace changes. I guess I'm not sure if its possible to try a prop before buying. If I end up buying can I trade in the 17p? When i read about others testing different props I would guess they have a relationship with a dealer to try and return. I can see if my mechanic has one lying around.

It seems most who have responded feel I'm getting adequate performance from my setup, which works for me. I guess I shouldn't fix what ain't broke.

I'll be monitoring for additional suggestions or input. Thanks again all.

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