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Prop Question 18HPX


Headhunter
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New to us 18HPX with 150SHO, twin power poles and 4 inch setback atlas plate. The boat came with a Tempest Plus 14 5/8 x 24 pitch.  I moved all of the batteries into the forward compartment so far but have 2 issues.  Running home yesterday the engine seemed to lose power for a second at 3200 rpm a couple of times and picked right back up. I am talking a split second but you could feel it in the boat. It happened once last week as well once and I thought I had hit something but in all cases I was running the ICW. I am thinking something with the prop but open to other thoughts. I also get a shudder the first time I hit reverse in the morning. I just had 100 hour service done and everything checked out ok oil and fuel wise. 
Second is proposing. I have to run about 1/2tabs above 2600 rpm to keep the boat steady. I know these don’t run well with plates but I do have one and will use it where I live and fish so I would like to figure it out.  I already moved all the weight I could forward. Any other thoughts besides calling Marcus?

thanks all!

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The 4" Atlas plate doesn't have enough setback to compensate for the additional 2" of built-in motor height that comes with it.  Your prop is probably losing bite and slipping.  You could add a couple of 2.5" CMC Static Setback plates (pics below) to your jack plate setup which would allow you to mount the jack plate lower and provide the additional setback you need.  It also remedies the hydraulic steering clearance problem when you tilt your motor.  However, you still need a prop with extra grip to run at elevated motor height.  Marcus is The Man for the job.  Powertech has some new prop designs with extra cup and extra lift added that minimize ventilation and stay hooked up - similar but better than the Mercury Trophy Plus.

 

THmarine_CMC_65213-static-setback-jack-plate.jpg

THmarine_CMC_65213-static-setback-jack-plate2.jpg

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34 minutes ago, Headhunter said:

Thanks, I had that feeling and can see even when the plate is bottomed out it is about an inch plus above where the motor would usually mount. Are you saying the prop is not getting enough water flow even with the jp down?

Yes, probably so.  I had the same 4" Atlas on my Redfisher 16 and had the same problems - ventilation and porpoising.  I'm assuming your motor is mounted lowest (at the top bolt hole).  If not, you could try lowering the motor first.  Then maybe a grippy-er prop next -or- add more cup to the Tempest if you have some extra WOT RPMs to spare.  It's a game of quarter-inches to get it right.  Good luck!

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4 hours ago, geeviam said:

I think this is/was Joe R's (Dabear) favorite prop to run on his HPX-18 when he had the Yamaha 150 on it.  It's a 23 pitch which might give you some wiggle room to add more cup if needed.  Heck of a good price on eBay:

New Yamaha stainless steel propeller 14 1/2 x 23 Pro Series

After years of test and tune buying and trying way to many props the last 2 or 3 years I had the F150 on my 18 HPX I had two go to props both Yamaha Pro Series one was a 23 pitch that had a little work and the other was a untouched stock 25 pro series . 95% of the time I ran 1/2 tank of gas fished with me and my wife , 3 batteries and a trolling motor . I could not run the 25 with 3 or more people and a full load of fuel . 

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  • 2 months later...

Based on input from Marcus and Ken ordered a OFX 4 blade, 21 pitch. I finally got it this week and tried it yesterday. Much better grip than the 3 blade on turns but still need 3 bars of tab and 1 bar of trim to make the boat stop porpoise. Still looking for a sweet spot. 

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  • 3 weeks later...

My friend had the same setup & the porpoising was unreal. This is what I learned/observed…. 

The 18HPX hull was designed to eliminate the need for a jackplate so your sweet spot will change each time you touch the jackplate control. Porpoising will likely be a constant issue based on water conditions & weight distribution; it will always change. He fought it thru 6 different props until he finally surrendered & removed the jackplate. There was very little difference in usable draft clearance overall. 

Dabear & Gee are the prop masters! 

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