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Discussion Starter #1
My new skiff came with a Mercury Spitfire 4 blade 19 prop. I am only able to turn 4700-4900 rpm's even when I am light on fuel and gear. I have yet to see 5000 rpms. I was hoping there were others out there that have the same set up and were dialed in a prop that will allow for the correct rpm's of up to 6300. Thoughts would be appreciated!
 

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What type of speed are you seeing at WOT? I have a 115 ProXS on my boat (different boat) but am running a 3 blade 20p prop that I've had some work done to. I've had some cup added to it because I was easily hitting max RPMs of 5750 (I have the 2015 ProXS that is a 2 stroke.) Now I can hit the max RPMs and see about 48-49mph tops. Hole shot is acceptable.
 

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What type of speed are you seeing at WOT? I have a 115 ProXS on my boat (different boat) but am running a 3 blade 20p prop that I've had some work done to. I've had some cup added to it because I was easily hitting max RPMs of 5750 (I have the 2015 ProXS that is a 2 stroke.) Now I can hit the max RPMs and see about 48-49mph tops. Hole shot is acceptable.
I am seeing 44-45 at 47-4800 rpms. I should easily see 49-51 with the right prop.
 

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No doubt you need to move up in pitch. If you stick with a 4 blade, I would think you would want to be in the 23-24 pitch range. Maybe 22 in a 3 blade.
 

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For starters you don’t want to go up in pitch if the motor isn’t turning 5000 rpm. I believe the recommended redline on the XS merc 115 fs’s is 6300. Are you able to trim the motor up some at speed or does the prop cavitate ?

If you can trim out to get the bow up and less wetted surface you should pick up some speed compared to the engine being trimmed all the way down. If that rpm is with the engine trimmed for best speed you need to drop down in pitch, not go up.
 

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For starters you don’t want to go up in pitch if the motor isn’t turning 5000 rpm. I believe the recommended redline on the XS merc 115 fs’s is 6300. Are you able to trim the motor up some at speed or does the prop cavitate ?

If you can trim out to get the bow up and less wetted surface you should pick up some speed compared to the engine being trimmed all the way down. If that rpm is with the engine trimmed for best speed you need to drop down in pitch, not go up.
New development.... spoke with my mercury mechanic and I think it may be more of an issue with the throttle bottoming out on the console not allowing it to get to WOT. Hopefully I'm propped right and just need to shim the binnacle or adjust the throttle cable....
 

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not a prop question, but you guys running a 115xs, what lower are you running ? command thrust ? especially interested on the marquesa
 

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not a prop question, but you guys running a 115xs, what lower are you running ? command thrust ? especially interested on the marquesa
I am running the standard (not command thrust) at the strong suggestion of Hells Bay. It turns out I have full range of throttle, so I am now back to what is hopefully a prop issue, not a motor issue.
 

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No offense by this question but are you trimming up to get the bow up and boat aired out? This should be the right prop for you. I run the regular Merc 115 with 18p 4 blade and hit 5.9-6k rpm on my Marquesa. Also run 21p 3blade PT same rpm range, higher top end. Paul at HB didn't just randomly pick a prop for your boat, he wet tests every one before delivery. They have sold several others with the ProXS with that same prop with no issues to my knowledge. Where are you located? I have two spare power techs for my Marquesa with Merc hub you could try if you want. 4 and 3 blade.
 

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Discussion Starter #12
No offense taken, this one has been a head scratcher for me. Yes, I have fooled a bunch with the trim. I am confident now that it is the right prop after discussing the issue with HB. I am in Placida. When they wet tested the boat it turned 6000 rpm with 49mph.I can not come close to that. I will be exploring further with my mercury mechanic to rule out throttle cable issues. Thanks for the offer, if you're close by I may take you up.
 

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I am in Jupiter but will be going through St Pete to pick up a new Tohatsu Thursday morning next week. If you have an hour in the morning early to run it shoot me a pm.
 

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Discussion Starter #15
Fresh water. I've been to my mechanic who has confirmed that I am getting wide open throttle as well. Taking the skiff back to HB in a couple weeks for them to do further testing.
 

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New development.... spoke with my mercury mechanic and I think it may be more of an issue with the throttle bottoming out on the console not allowing it to get to WOT. Hopefully I'm propped right and just need to shim the binnacle or adjust the throttle cable....
Ok now I’ve got to see pics of your console
 

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I am seeing 44-45 at 47-4800 rpms. I should easily see 49-51 with the right prop.
These numbers blow me away. I get why it was suggested that maybe your throttle was not getting full range. With that kind of max rpm it would have to be something wild, for instance my other two initial questions were does this guy have so much weight he’s about to sink it or is he mistakenly running with the tabs all the way down. I get that those are extreme but topping out a 4800 rpms because of incorrect prop is extreme. For instance I’d have a hard time believing that if it was a prop issue hampering rpm that much, that you even be able to get it on plane. Then add the fact that you are within 2-3 mph of what most fishing loaded marquesas are going to top out at.

Please update when you get it figured out. This ranks right up there with unsolved mysteries for me.
 

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These numbers blow me away. I get why it was suggested that maybe your throttle was not getting full range. With that kind of max rpm it would have to be something wild, for instance my other two initial questions were does this guy have so much weight he’s about to sink it or is he mistakenly running with the tabs all the way down. I get that those are extreme but topping out a 4800 rpms because of incorrect prop is extreme. For instance I’d have a hard time believing that if it was a prop issue hampering rpm that much, that you even be able to get it on plane. Then add the fact that you are within 2-3 mph of what most fishing loaded marquesas are going to top out at.

Please update when you get it figured out. This ranks right up there with unsolved mysteries for me.
I've been a guiding since 1994, this is my 4th skiff and 2nd HB. No excessive weight or tabs, lithium-ion battery rig (very light) and minimal gear. I weigh 160 pounds soaking wet...I am totally vexed. I will let you know after the crew at HB gets to the bottom of it.
 

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I've been a guiding since 1994, this is my 4th skiff and 2nd HB. No excessive weight or tabs, lithium-ion battery rig (very light) and minimal gear. I weigh 160 pounds soaking wet...I am totally vexed. I will let you know after the crew at HB gets to the bottom of it.
To only be getting 70-75% of max rpm at wot it has to be something major. That’s what I was getting at. The 30-40 lbs of weight savings in a 24v lithium setup won’t affect that. If it pitch were that’s easily a 3-4 pitch drop.
What are you using for you measurements of mph and rpm? With a 19 pitch prop you’d need closer to 5800 rpm’s to achieve 45 mph. That would be with pretty minimal prop slip. Maybe it’s just a faulty gauge?
I wasn’t implying you don’t know what you’re doing... just trying to show how extreme those peak numbers are. Like even if that prop were made of tungsten, I’d think that engine would still spin it faster than that. Like I said earlier, I don’t really have a clue what could cause those numbers, but I guess if it were me I’d with a new warranty I’d hope for a faulty gauge or computer.
 
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