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Checkmate 2400 BRX Porpoising Issue

lleopold1

New member
I have a 2009 2400 BRX that I recently re-powered with a Mercury 300 R and can't get it to stop porpoising without using trim tabs. I have a CMC manual jack plate with 5.5 inches of travel that came with the boat when I purchased in 2011. I have moved it in all positions but it doesn't help. Previously I had an Evinrude 225 HO Etec on the boat with the same jack plate and zero porpoising issues. I also did not have trim tabs. The boat starts porpoising at 35 mph and doesn't stop until I run the trim tabs down 4 positions. I also changed props from a 3 blade to a Bravo FS 4 blade but it did not help.

Just curios if anyone has had this issue?

Thanks,

Lance
 

Glenn Kornett

Active member
I have a 2009 2400 BRX that I recently re-powered with a Mercury 300 R and can't get it to stop porpoising without using trim tabs. I have a CMC manual jack plate with 5.5 inches of travel that came with the boat when I purchased in 2011. I have moved it in all positions but it doesn't help. Previously I had an Evinrude 225 HO Etec on the boat with the same jack plate and zero porpoising issues. I also did not have trim tabs. The boat starts porpoising at 35 mph and doesn't stop until I run the trim tabs down 4 positions. I also changed props from a 3 blade to a Bravo FS 4 blade but it did not help.

Just curios if anyone has had this issue?

Thanks,

Lance
I have a 2020 closed deck 2400 with a 300R. I have a hydraulic Atlas plate. How much set back do you have? How far below the hull is your prop shaft centerline? I assume this still happens at full negative engine trim? Do you have a torquemaster or Sportsmaster lower unit? Do you have 1 or 2 batteries in the back? How heavy are they? Does it porpoise with passengers in the bow? I think the 300R is about 50 - 60 lbs heavier than the HO. Is the jack plate mounted against the transom without any wedges? I have no trim tabs on mine but I have to run full negative trim at less than 35 mph to ensure no porpoising. If I have passengers in the back seat, it is worse and I have to run full negative trim and run a bit lower on the jack plate. Have you tried adding about 50 lbs in the bow? Do you have an anchor in the bow anchor locker?
 
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lleopold1

New member
I have a 2020 closed deck 2400 with a 300R. I have a hydraulic Atlas plate. How much set back do you have? How far below the hull is your prop shaft centerline? I assume this still happens at full negative engine trim? Do you have a torquemaster or Sportsmaster lower unit? Do you have 1 or 2 batteries in the back? How heavy are they? Does it porpoise with passengers in the bow? I think the 300R is about 50 - 60 lbs heavier than the HO. Is the jack plate mounted against the transom without any wedges? I have no trim tabs on mine but I have to run full negative trim at less than 35 mph to ensure no porpoising. If I have passengers in the back seat, it is worse and I have to run full negative trim and run a bit lower on the jack plate. Have you tried adding about 50 lbs in the bow? Do you have an anchor in the bow anchor locker?
Thanks Glenn for the quick response.

Currently I have 17.5 inches of setback (Measuring the center of the prop shaft with the motor & boat level to the back of the boat/pad below drain plug) and the center of the prop shaft is 6 inches below the bottom of the boat. I failed to mention that the boat does not porpoise when at negative trim but as soon as I begin trimming it starts lunging forward (unless the trim tabs are used). I have a torquemaster lower unit but it did it worse when I originally had the Sportmaster. I have 1 battery in the boat and yes it porpoises when I have multiple people in the boat (It does it with 6 adults and 2 children) and weight doesn't seem to affect it. The jack plate is mounted without wedges and yes I have an anchor in the bow.
 

Glenn Kornett

Active member
My prop shaft is 3.25 - 4.0 inches below the V, and since yours is 6 inches, which I think should help your porpoising a bit more than mine. My jackplate engine mounting surface is 10" from the transom. If yours is 17.5" from the transom that would theoretically make it worse than mine for porpoising due to moving the center of gravity further back (plus having a heavier motor than HO). I do run negative trim up to at least 35 mph to prevent porpoising. After 35 mph I can slowly start adding positive trim, the faster I go the more trim I have to add to keep the bow up. By the time I reach about 68 mph, I have it trimmed almost to the stops as I continue to WOT. If your jack plate mounting surface is really 17.5" behind the transom, I would seriously consider moving it to 10" - 12" behind the transom and testing again.
 

jjg

Well-known member
Thanks Glenn for the quick response.

Currently I have 17.5 inches of setback (Measuring the center of the prop shaft with the motor & boat level to the back of the boat/pad below drain plug) and the center of the prop shaft is 6 inches below the bottom of the boat. I failed to mention that the boat does not porpoise when at negative trim but as soon as I begin trimming it starts lunging forward (unless the trim tabs are used). I have a torquemaster lower unit but it did it worse when I originally had the Sportmaster. I have 1 battery in the boat and yes it porpoises when I have multiple people in the boat (It does it with 6 adults and 2 children) and weight doesn't seem to affect it. The jack plate is mounted without wedges and yes I have an anchor in the bow.
Reduce your setback to a max of 8” and your issue will go away. These boats do not do well with that much setback. I have a 6” atlas JP and have zero issues with porpoise at any speed, and also do not have tabs. Im running a much heavier 400r with a sport-master and the boat handles like a dream.
 

Hayabusa

Well-known member
I agree with “jjg“. On my BRX when I had my 300 Pro it was mounted on a 10”CMC and like you i was getting lots of porpoising, the Verado sits almost 6” further back compared to its cousin the 300XS, so like your present set up my 300 and its drive shaft where almost 16” back. I switch engines and installed a 400R and although it’s heavier than the 300 Pro the porpoising was less and more controlable probably due to the SportMaster, so I started to experimenting with more setback for more speed, my porpoising got worse at mid speeds but my top end benefited, I went setback almost 19”, no bueno. Since 95% of the time we’re cruising I took it all apart and installed a Bob’s 6” and set my prop shaft to be in its lowest point at almost 2” below the pad and any porpoising I might get it’s totally controlable, 6” setback for the BRX is perfect.
 
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lleopold1

New member
Reduce your setback to a max of 8” and your issue will go away. These boats do not do well with that much setback. I have a 6” atlas JP and have zero issues with porpoise at any speed, and also do not have tabs. Im running a much heavier 400r with a sport-master and the boat handles like a dream.
Reduce your setback to a max of 8” and your issue will go away. These boats do not do well with that much setback. I have a 6” atlas JP and have zero issues with porpoise at any speed, and also do not have tabs. Im running a much heavier 400r with a sport-master and the boat handles like a dream.
Thank you very much! Sounds like I need to get a new jack plate.
 

lleopold1

New member
I agree with “jjg“. On my BRX when I had my 300 Pro it was mounted on a 10”CMC and like you i was getting lots of porpoising, the Verado sits almost 6” further back compared to its cousin the 300XS, so like your present set up my 300 and its drive shaft where almost 16” back. I switch engines and installed a 400R and although it’s heavier than the 300 Pro the porpoising was less and more controlable probably due to the SportMaster, so I started to experimenting with more setback for more speed, my porpoising got worse at mid speeds but my top end benefited, I went setback almost 19”, no bueno. Since 95% of the time we’re cruising I took it all apart and installed a Bob’s 6” and set my prop shaft to be in its lowest point at almost 2” below the pad and any porpoising I might get it’s totally controlable, 6” setback for the BRX is perfect.
Thank you! Sounds like I need to get a new jack plate.
 

JUPITER PULSARE

Well-known member
The 300R has the AMS (advanced mid section) which has roughly 4+ inches of setback built into the design. Whatever jack plate you are using, you would have to add the 4+ inches to get your total setback. When you wind up with too much set back and a heavy motor, the boat will porpoise at mid speeds due to the CG not being correct for the speed and will only level out when enough speed is met to overcome the offset CG. Sometimes, the porpoise can be controlled (to a degree) with a prop change...
 

Glenn Kornett

Active member
The 300R has the AMS (advanced mid section) which has roughly 4+ inches of setback built into the design. Whatever jack plate you are using, you would have to add the 4+ inches to get your total setback. When you wind up with too much set back and a heavy motor, the boat will porpoise at mid speeds due to the CG not being correct for the speed and will only level out when enough speed is met to overcome the offset CG. Sometimes, the porpoise can be controlled (to a degree) with a prop change...
My 300R does not have the AMS, but I believe they started offering it as an option about 1 year ago on the 300R. I have 10" of setback.
 
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