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Speed and RPM Question on 2013 Yamaha, Q25, Dual Elliptical.

Kells

Kells

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Commerce Twp, MI
Here is the dilemma; 

I'm maxed at 29 MPH, Cannot get RPM over 5,000 no matter what trim level.

I'm not unhappy with the speed, but I'm concerned over not being able to get above 5,000 RPM's when i'm reading 5400 - 6000 posts seeming normal..

It has a stainless steel reliance prop on it.  

It's a 25' QCL, no waveshield.

I'm not familiar with this side of things (Engine / prop / power) so maybe lower RPMs is normal because the boat is heavy?

Does this seem right?  Any thoughts or suggestions from anyone?

I appreciate it.
 
Need to know what motor, what size prop, 2 tube, 3 tube, or ESP.
 
Yea, that info might be important.

It's a 2013 F150 (Yamaha).  2 tube Ellipticals.  I'm not sure the prop size.  Do just measure it with a tape, or is it printed somewhere on it?  It says Reliance, i'll check it out when I get home tonight on the prop size.
 
Prop info is stamped on the prop
 
Kells,

While you get the prop info to better understand your setup....also note, I have a 2013 RCL with a 150 Merc, 2 toons, and get 35mph top speed.  However, I have learned how a very small build up on the toons will create a lot of drag.  With your elliptical toons, you have more surface area so your situation will be magnified.  Our water is fairly clean, but it only takes about a week in the water and the growth can knock several mph off.  Also, was your speed during the summer (warmer) weather or now?
 
I have not seen anyspeed north of 30 miles an hour or rpms about 5000. I'm in a shallow lake very clean mostly sand as well. When I pulled the boat out for the 20 hour maintenance it didn't even need cleaning. I mentioned this to the dealer today after posting on this thread, he believed I should see 6000 RPM's at WOTso he's going to call me back, although I'm not sure what to expect.
 
Im running high 30's at 5000rpm and break 40 between 5500 to 5750 which is redline for my 150 Mer Pro XS.  But you 25 is a lot more boat then my 2275RCW.  All depends on load and water conditions

What hole is your mounted in?  Mine was all the way down, Ie in the top hole.  I moved it up 2 holes since taking delievery in 2011.  picked up a couple mph but need to watch the trim when turning.  It will blow out if you don't trim down while making a tight (well, as tight as you can make in a twin elip tube setup) turn.

Need pitch / Diameter of prop, height on transom?  F150 I assume 4 stroke and not HPDI.

Let us know

Thanks

Todd
 
14.5" x 15" Reliance SS Prop Mounted all the way down.  Not sure on Transform height.  It's the F150LA
 
Service dept. said this is the right prop for the engine, and mounted in the hole that Bennington recommends.  Any other thoughts?
 
I have a 15 pitch on our 2275rl and it's a lot lighter than yours. Have them try a smaller prop.
 
So a bigger boat would perform better with a smaller prop?  I'm clueless when it comes to this, but that doesn't seem right....

The dealership is pulling the boat today to check it all out, hoipefully i'll have it back next monday...

I'm really hoping I can gain the extra RPMs and crack the 30mph mark :)
 
Smaller as in pitch. Not smaller as in diameter. I think that's what the previous post referred to.

Here's the scoop on props. A prop is like a screw. Turn it one time and it will move a certain distance in one revolution. A 15" pitch, in perfect conditions, will move forward 15" in one revolution. A 14" pitch will move forward 14" in one revolution.

An engine has X horsepower. Your engine has 150. There is also a torque band, but that's another conversation. Engines operate in their HP band when revs are high. So what you are finding is that your engine doesn't have enough horsepower to push your boat faster, once rpm's hit 5000.

Engines are designed to be optimized with a certain WOT (wide open throttle) rpm range. I believe your engine has a WOT of 5850 - 6000. So to maximize efficiency, you want to prop the boat so that it will hit 5850-6000. If it won't get there, your prop has too much pitch and the engine isn't operating within its engineered optimum range. All sorts of things happen under water when you are not propped correctly. Your holeshot isn't what it should be. Acceleration isn't what it should be. Slippage is more than it should be. You're not as efficient as you can be. 

Slippage - all props slip. Meaning in reality, a 14.5pitch prop won't move the boat forward 14.5" in one revolution. You might have 6% slip. You might have 15% slip. You might have 20% slip. The less slip the better.

So direct answer to your question - yes - a smaller pitch prop can make a boat go faster. 
 
Oh...and for the service department - If nothing is wrong with the engine, limiting rpm's to 5k...then they're wrong...it is not the right rigging or prop. One or the other needs to be changed in order to get setup within WOT specs. That a book somewhere says 2nd hole and a particular prop, doesn't mean that every boat will hit RPM WOT spec with that rigging. That's why there are different holes and different props. Books don't drive boats. And service departments shouldn't rely on books as though they do.
 
Great response!  I was not familiar with "pitch" so I was only thinking diameter.  Great explanation, that helps a lot.  I'll make sure if nothing is wrong with the engine that we give that smaller pitch a try...  

Thanks again.
 
typical gains in RPM by a 1" pitch addition / reduction can be as little as 150 or as much as 200.  Remember, all engines have WOT max hp rating rpm.  For example, my 150 merc Pro XS says 5250 to 5760.  I want to prop so the my lighest load on the boat (ie me and fuel) will hit the top of that RPM range.  That way when I add people, coolers, etc, I am still within the lower limit of the RPM range.  "lugging" a motor at a WOT below the lowest number on that range, can do significant damage to the motor.

Its all trial and error and any good dealer will work with you until you get the right prop for your application.

So for my example above, if you change from a 15P to a 14P, your RPM will go up by 150 to 200.  And vice versa when going up in pitch from 15 to 16.

Hope this hellps
 
Good explanation mtudb24. I think next spring I should take a closer look at this. I never paid much attention to my RPM as it is extremely rare I ever hit wot.


Since I usually cruise at 8 - 15mph would my engine still lug at that speed due to the prop?
 
So I can get the 5800 RPMS if I trim the motor up to the halfway point.  But if it's trimmed all the way down, I can't get above 5000 RPMs.  I'm still not cracking 30mph when trimmed appropriately to get the RPMS up.  Does anyone think a different prop will make a difference for me?

Thanks,
 
Kells

Sorry If I over simplify this (engineers tend to do this at times).  So when your "tucked in" ie trim all the way the way down, the angle of the lower unit / prop push the front of the boat down.  This accomplishes 2 things.  1.. You push the front down to get up on plane quickly.  2.  While you push the front end down, the majority of the logs are in the water.  This is maximum hole shot but yeilds a low top end because you have 3/4 to 100% of the tubes being a wet surface with the water. On a V hull boat and to some extent a pontoon, a smooth ride into a head on sea as the you "cut" thru the waves letting the hull or the toons do the work.  When you start to trim up, you raise the bow and you increase both RPM and speed as the resistance of the hull or tubes get airborn and there is less "wetted" surface.  Friction goes away  and you go faster.  This also increases the "bouncing" of the boat as you are riding on top of the waves. Over trimmed can also lead to "porposing" in which the fornt end will bounce wildly and its not a good thing to have.

When you say you are trimmed up properly to get the RPM's up, what setting on the gauge is this?  Halfway?  On all modern outboards, there is trim and there is tilt.  The Trim of the motor you can hear and see the motor rising slowing.  In Tilt mode, the motor rises quickly and is only meant for trailering or shallow water operation. You want to get near that that "fine line" between the max trim and angle just before it goes into Tilt mode.

Where are you at with speed with the trim at this level?

What hole is your motor mounted in?  Is it in the top hole which mean more of the gearcase is in the water or is is in the 2nd hole etc?  That could be a factor as you may have too much lower unit drag.

LINK - running slow RPM's with a big pitch prop will not lug or damage the motor from what my mechanic tells me.  Its only if you try to open the throttle and you only hit under the WOT RPM your model of engine is made for.  Lots of the big offshore fishing guys do this for fuel economy.  They over prop knowing they will never try to hit WOT but stay in the mid range (say 3-4K rpm) and they can get great MPG numbers.

Todd
 
Great info here. Thanks Todd.

I'm mounted at the top hole. I've never encountered porpousing. I don't think have enough power with this heavy of boat to worry about that.
 
wow great info. thanks. I just ordered a 2550GSR with the salt water elliptical pkg and a 150 Yamaha. Sounds like the dealers mechanic better know his stuff. Taking delivery early December and have to hope all is mounted and propped the best for my boat.
 
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