175-Merc Prop Pitch Help

[White_Caps]

Hi Everybody,

Looking for some input. 1998 Mercury 175hp EFI sitting on a 19' Ranger Reata. Currently turning a 3 blade 25p prop 5000rpms WOT and getting 52mph. Is there more speed available in this rig by stepping down in pitch? Have heard these rigs are capable of running around 60mph..

Thanks,
White_Caps

 

[Sea Rider]

Welcome to Iboats,

5000 wot rpm seems kind of low for that engine, check min-max wot rpm range to know how much more revs will need to be dialed by dropping pitches.

Happy Boating

 

[White_Caps]

Engine rated at 5000-5600 rpms max.

 

[Sea Rider]

Now the issue is should I drop just 1 or 2 pitches to be at 5600 ? According to the "rule of thumb" will need to drop 3 pitches down, but that rule ain't cast on stone so watch out with it, probably just one or max 2 will suffice ?

Happy Boating

 

[White_Caps]

Think I might give a 23 a try and see what happens. Engine appears to be mounted at correct height. Can't believe going down in pitch would give me more than 1 or 2 mph on top end but who knows....

 

[Texasmark]

Think I might give a 23 a try and see what happens. Engine appears to be mounted at correct height. Can't believe going down in pitch would give me more than 1 or 2 mph on top end but who knows....

Work the numbers. If everything is constant, the percentage transfers. Looking for 8/52 more = 15% increase in speed. Engine from 5 to 5.6 = 12% increase in rpms. Nope, won't reach.

However, if you can get your rpms up to 5600 with your current prop........"Go-fast.com" Bam Prop slip calculator: Assume a 1.87:1 gearbox for a 175 EFI with 5000 rpm, 25P and 52 mph = 17% slip........way too high. For a Ranger pad that ought to be 12 or less.

Looking at a Reata, seems to be the typical Ranger pad hull. Engine obviously has PTT. First of all are your instruments telling you the truth? Speed by speedometer or GPS is the first weak link. Rpms are usually accurate. Seems to be only one gear ratio offered for that engine. If the prop is stamped 25 then you can rely on that.

Not being sarcastic here but do you know how to run your boat on the pad? When you are looking for max mph, what is your boat load.....you alone, loaded up with friends and gear? Hull clean and slick, no slime or barnacles? Wind and water conditions? Going against a current?

 

[White_Caps]

Speed is from a GPS, RPM is from the tach. I got those measurements at WOT with a light load, calm water, with motor at optimum trim.

Stopped by local dealer, they had a 21p Enertia to try so I'll give it a shot.

 

[Texasmark]

Just keep in mind that different brand/model props have different characteristics. You are looking for a good cupping and lots of rake for good bow lift and get the hull up and out of the water....making the best of your pad.......just like you see the guys doing on the equipment commercials.

 

[White_Caps]

Well some more results. Picked up a 23p Laser II prop as well. That one gave me 5800rmps and 55mph with a tailwind for %19 slip. Never even tried the 21p as I could tell it would put my RPMS way above the 5600 max.

Checked engine height and the cavitation plate is out of the main water jet so appears to be at proper height. Perhaps one hole too high if anything. At this point I'm thinking a 23p Tempest may be the way to go as I've heard the Tempest don't spin as fast as the Laser II.

Any input?

 

[Texasmark]

If a Tempest doesn't spin as fast it means that it bites better and as a result loads the engine more....course that should equate to less slip and more mph. I wouldn't worry about the 200 extra rpms. I run all my Mercs where I want. My 115 tower loved 6000, last engine a 90 triple ran 56-5800. I see you took the initiative and ran the Bam slip calc as I got 19 slip too. Man that's too much slip for that boat. Something is dreadfully wrong somewhere. Picking up 3 mph and 800 rpm tends to tell me that you had more rake on the Laser (besides being ported) giving you better bow lift and less wetted area (drag) to assist the pitch reduction lighten the load on the engine (rpms up) and increase the mph.

Only thing that doesn't fit the MO is the slip increase. However, the ports are supposed to seal off once you get enough water pressure along the barrel of the prop. But if you are running a high plate at a high trim angle on the pad and maybe not as much setback as you need (you have some but a jack plate would give you more), you could be eliminating a lot of the sealing effect of the water and your ports may be opening up somewhat and allowing exhaust gasses across the prop like occurs in the hole shot, helping to increase your rpms (800 for a 2" reduction in pitch is at least 2x more than you should be getting).

One way to find out. Get some blank plugs and plug the holes and try it again or epoxy them closed for a test. You can take a hammer and punch and knock the epoxy out when finished with your experiment.

 

[White_Caps]

First off I want to say it seems like you really know your stuff so thanks for taking the time to help me out.

The ported prop theory seems plausible, although the original Tempest is also ported so that may not explain the difference in slippage.

On the last test I observed cavitation plate to be approx 2-3" above water line at cruising speed and moderate trim. Getting around 15psi water pressure. Too high of an engine could result in increased slippage right? Although I'm not sure if this set-up is too high or not...Wish I could easily adjust engine height, may need to look at a jackplate.

Boat is currently at dealer getting a motor inspection (not sure when it was last inspected). So no tests for at least a while, weather pending of course.

 

[Texasmark]

With the setback your notched pad gives you and your attitude while on the pad, that 2-3" is probably about right. One way to test for blowout is to tuck it in from your normal trim angle while at WOT and watch your rpms and mph. If you are blowing out at the normal trim you run tucking in slightly will drop the rpms and up your mph. That was my yardstick for trim angle....push her out till rpms start to rise and mph start to fall then back a notch.

On the Tempest, how big were the ports? Bet they were round and a lot smaller area than the Laser II and last time I looked they were square also which adds a fair amount of extra opening as compared to to a circle of the same size.....a x b vs pi x radius exp 2.......I just ran the numbers and for a ⅜ dia hole vs a ⅜" square the square has 29% more area........0.140" vs 0.11"

Yes too high could increase slippage but again watch your rpms and speedometer as mentioned above. If your engine is a factory installation with the engine clamp bracket sitting right on the top of the transom I doubt you are too high, especially on a Ranger of all boats!

That's a good idea and may uncover something that would correct some of the error you are having. See you later.

 

[White_Caps]

I'll test the method you mentioned about trimming in at WOT and watching RPMS and MPH. Also will try plugging the ports on the next run and possibly talk to local Ranger dealer to see if they have input. Thanks for the help I'll post an update next chance I get.

 

[Texasmark]

Do that because it helps me to understand if I'm helping others or not.

My education has been self inflicted (Hard Knocks U.) so I am just relaying what has happened to me in my 50+ year boating tenure. I had an 18' Ranger Fisherman once (same pad hull like yours with similar setback built in) with a 115 tower and had a lot of fun with it; one heck of a boat and brought me home numerous times when I didn't think I was going to make it. I guarantee you I used the high rake of my Laser 19 numerous times as I could walk the bow up and down about 6' on the big waves just by manipulating the throttle at a reasonably high trim angle and prevented submarining. When the bow plows into an oncoming wave and the water rises to the gunwale it makes you pay attention and do something different! Texas has big lakes and big winds. Sometimes it makes getting back to the dock a chore.

One thing that bothers me about your numbers is that I consistently ran 6000 rpm at WOT trimmed to the sweet spot (rpms started up and mph started down) in calm to light chop, alone in the boat with tackle and gas, 3 batteries and all. 49 was my number and I couldn't nudge her above it. I ran the first edition Laser which was ported; round and ⅜" as I remember. Course 49 from a 115 on an 18' boat isn't too shabby. I guess I could have dropped another $400 and upped to a 21 maybe getting there or a tad better since I had 500 rpm I could consume, but it wasn't worth it.