why 2 similarly pitched props have such drastic differences

kitecop917

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I have a 2003 proline 22 sport with a 2003 Mercury 200 XL SWB (carbed) and I have been to the moon and back with this issue. When I got the boat the guy had a tiny 4 blade 17P prop on it. It would turn in excess of 7000rpms so I changed it right away. I picked up a stock SS merc prop in a 19p with a larger overall diameter. With that prop im still able to rev over the max RPMs and im only getting 26mph at 5000rpms. I picked up a 21p mirage plus and it is giving me the efficiency im looking for but one major issue. Once I hit 5000rpms with that prop on, the engine immediately drops the rpms down to around 3000 and then continues to go down from there. If I get up on plane and stay at 4600rpms or under then I get no drop in rpms. im running 34mph at 4300rpms with the 21p. So one, why would the 21p prop cause the engine to drop at 5k and two, why such a huge performance difference between the 2 props. I should only see a difference of about 400rpms from a 19p to a 21p right? not over 1000rmp difference? Also, I just all the fuel lines and inline filters. However I don't suspect a fuel problem because there are no problems with the 19p on there its just not efficient. The motor idles fine and runs great aside from the rpm drop with the 21p prop.
 

steelespike

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Speaking generically it sounds like the motor thinks something is amiss.No doubt the 21" pitch requires a little more throttle to reach the same rpm.
Perhaps it thinks the oil ratio is low or it thinks it is running hot.
As far as the props my guess is you replaced a generic ss prop with a more sophisticated prop.
There is more to prop design than pitch, there is cup and rake for instance.
I'm a seat of the pants guy so can't tell you the details on the Mirage plus but I bet its more sophisticated than the 19" prop.
While the boat is a bit of a load 2750 lbs. dry; your slip seems high at about 29% Could be real but also the tach and/or the speedometer may be off.
Is the bottom clean and smooth?
 

kitecop917

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yes the bottom is perfectly clean. I don't think the tach is off, i looked into that already. I was getting an alarm with the problem until i replaced the thermostats. Now no alarm but still dropping rpms. I am starting to think a false overheat may be causing it. It is def. not overheating, it has a new impeller and i can hold my hand on both heads for a second or so right after running it at speed and the water out of the pisser is tolerable too. I understand the mirage is much more high tech than the old 19p but the difference between them is still too much to chalk up to design.
 

Tail_Gunner

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You are having some kind of powerhead issue.

I picked up a 21p mirage plus and it is giving me the efficiency im looking for but one major issue. ....Once I hit 5000rpms with that prop on........, the engine immediately drops the rpms down to around 3000 and then continues to go down from there.

That say's it all for some reason the engine can push the boat out or the hole read heavy demand. Once the load actually lightens it lose's rpm...fuel supply come's into mind. Time for a trip up to outboards. Good luck.
 

kitecop917

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You are having some kind of powerhead issue.

I picked up a 21p mirage plus and it is giving me the efficiency im looking for but one major issue. ....Once I hit 5000rpms with that prop on........, the engine immediately drops the rpms down to around 3000 and then continues to go down from there.

That say's it all for some reason the engine can push the boat out or the hole read heavy demand. Once the load actually lightens it lose's rpm...fuel supply come's into mind. Time for a trip up to outboards. Good luck.

this does not explain why I can run over 5k with the smaller prop though. if it were powerhead then it would drop no matter what prop was on it. I have 120 in all 6 cylinders.
 

Tail_Gunner

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this does not explain why I can run over 5k with the smaller prop though. if it were powerhead then it would drop no matter what prop was on it. I have 120 in all 6 cylinders.
I am not dismissing your point's and welcome to iboats my bad...Now unless i am missing something you have a spark..timing or fuel issue The Mirage plus is a good app for your boat engine combo and should be used as a starting point. Now there is one possiblity do not take offense you do know how to trim your engine for launch and trim it for plane both seetting's are remarkably different..again do not be offened.
 
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kitecop917

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I am not dismissing your point's and welcome to iboats my bad...Now unless i am missing something you have a spark..timing or fuel issue The Mirage plus is a good app for your boat engine combo and should be used as a starting point. Now there is one possiblity do not take offense you do know how to trim your engine for launch and trim it for plane both seetting's are remarkably different..again do not be offened.

yes, I trim down to get out of the hole then trim up once on plane. I have messed with the trim a lot and it has no affect other than the typical decrease in speed when running with motor trimmed all the way down, forcing the bow down. I have trimmed up to cavitation then back down in increments.
 

steelespike

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The Mirage has knocked 1000 rpm off the wot. To me thats like adding a full boat load and skiing in relation to throttle position for a a given rpm.
Have you tried loading the 19 up with a full boat load and giving it a good work out?
The 19 only produces 26 at 5000 rpm thats a lot of slip and a heck of a lot easier to turn.
I'm assuming the motor has a safe mode as I understand it safe mode kick in when when temperature or other key condition"appears" wrong.
I'm not saying it is actually in distress, something may be in error.
 

kitecop917

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The Mirage has knocked 1000 rpm off the wot. To me thats like adding a full boat load and skiing in relation to throttle position for a a given rpm.
Have you tried loading the 19 up with a full boat load and giving it a good work out?
The 19 only produces 26 at 5000 rpm thats a lot of slip and a heck of a lot easier to turn.
I'm assuming the motor has a safe mode as I understand it safe mode kick in when when temperature or other key condition"appears" wrong.
I'm not saying it is actually in distress, something may be in error.

I have only recently ran the 19 with 2 ppl onboard and less than 1/4 tank of fuel. Last time i ran the 21 i had 2 ppl and about 1/2tank/50gal of fuel. I believe there is a false reading somewhere causing this, i just don't buy the fuel issue, its way too precise and consistent of a problem to be fuel.
 

Tail_Gunner

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OK bear with me here is the below what actually happened you launched......the motor reached 5000 rpm...then went to 3000 rpm and held there?

I picked up a 21p mirage plus and it is giving me the efficiency im looking for but one major issue. ....Once I hit 5000rpms with that prop on........, the engine immediately drops the rpms down to around 3000 and then continues to go down from there.
 

kitecop917

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OK bear with me here is the below what actually happened you launched......the motor reached 5000 rpm...then went to 3000 rpm and held there?

I picked up a 21p mirage plus and it is giving me the efficiency im looking for but one major issue. ....Once I hit 5000rpms with that prop on........, the engine immediately drops the rpms down to around 3000 and then continues to go down from there.

It drops very quickly to about 3k then slowly continues to drop to idle until i back off the throttle then, i can immediately throttle back up and repeat the process or stay under 4800ish and run normal.
 

Tail_Gunner

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Take the question to the O/B section just for the sake of well maybe i do have a engine issue.
 

kitecop917

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Did some diagnosis today. here is a list of what i did.

-observed carbs with air box off- even and normal fuel flow at WOT.
-ran with gas cap off- no noticeable changes
-ran on an aux tank- no difference
- replaced plug one at a time and did a test run between each- no changes
- cleaned grounds that are attached to the power pack plate- no change
-ran both 19p and 21p props again- same issue with 21p but 19p was also bogging once it hit 5800-6100rpm, only 32mph at 6000rpm b4 bog.
- did compression check- all cylinders are 120psi or just over
-observed ride plate while at speed. its completely submerged so i will have to raise it a good 2-3"

Im thinking the motor height is what is creating so much slip and lack of speed but the bogging has got to be an electrical issue. I think my next step is testing power packs... something is failing under load.
 

Tail_Gunner

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Your cav plate should be riding 1" above the keel for a starting point, outboards can be played with a lot and can go very high on the transom column that could be a real adventure. There is a gearing ratio on your leg or drive could you post that....i'm quite sure you will find a fuel or some kind of issue with the engine.
 

kitecop917

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Your cav plate should be riding 1" above the keel for a starting point, outboards can be played with a lot and can go very high on the transom column that could be a real adventure. There is a gearing ratio on your leg or drive could you post that....i'm quite sure you will find a fuel or some kind of issue with the engine.

fuel is pretty much ruled out in my book, I've done everything to solve a fuel issue and never even verified one but did verify a heavy and even spray of fuel into the motor while running at WOT. I will be running it with a spark tester to see if there is a cylinder or multiple breaking down under load. If just 1 then i will go with a new CDM and if multiple then i will try a control module. checked the timing today, idle timing was a bit off but high end was spot on.
 

kitecop917

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Anyone in jacksonville fl have a merc 135-200hp that is a 2000-2005 that would let me borrow their control module to test b4 i shell out $250 for a new one?
 
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