New motor prop and rpm questions

[SmokyMtnGrady]

Anybody here running a 4.2 liter F250 and what prop are you using?

I have 8 hours and the motor won't turn more than 5400 RPM. Tried a power tech OS4 15.25 x17 and saltwater series 2 15.5 x17 and both topped at 5400. My old motor would spin up at 6k RPM with both props.

Motor is mounted on second hole from.the top. Max RPM 5400 and Max speed 42. Grady claims top speed of 48.3 and top RPM 5800. They claim 5k RPM at. 42 and I get 42 at 5400 or 400 RPM more than their performance bulletin.

So, what gives? I am considering testing props from propgod . I love the 4 blade on this boat. I am going to drop down to 15.25 x 16 OS4 . Ken at Propgods told me at 15 inch pitch the power tech prop washes out some and slips a bit..

Any insight from others is welcome. Mid Carolina says the boat is now over propped . Other friends here have said the same yet Grady runs it at 15.5 x17 sds prop.

I am puzzled and would like input. Thanks

 

[artodea]

You're not horribly over-propped. I thought the range for that engine is 5k-6k wot. Where is your anti-ventilation (aka cavitation plate) relative to the water surface when trimmed out on a plane? If you are buried the revs will go up slightly when raised. I do hear lots of good things about propgods - good luck!

 

[SmokyMtnGrady]

The plate seems at the right height. The tail end of it is at or above the surface. The peak HP for that motor is around 5500-5800 RPM, so I am told . I am not so concerned about top end speed as I rarely run it WOT, but it is an indicator in regards to having the proper prop and such on the boat. Also, if it is over propped at the top end, it is over propped through all RPM ranges. Ken did a prop slip comparison to what Grady says they got versus what I am getting. I am getting 15% slip and Grady claims 8%. That is significant.
I like the motor so far but this is vexing me.

 

[DennisG01]

I tend to agree with your line of thinking of wanting to be at the upper end of the RPM range. As Art mentioned, you're not hurting anything running the way you are, but it's good to start at the upper end for those times when you're loaded up and it brings the RPM's down. Personally, I like to even go a step further and prop it for MAX RPM with my normal load, not a light load. When I'm running light, I can control the throttle through the use of that skinny lever thing to the right of the steering wheel.

A couple basic questions that always need to be answered since we don't know the answers ahead of time...

-- Are you trimming the engine up as high as possible?
-- What is the bottom of the hull condition? Dirty/slimy?
-- Are you running an excessively heavy load?
-- Is the trailer still attached to the boat?

You've got plenty of power for that boat. I'd really be surprised if you had to start dropping pitch. To pick up 600 RPMS, you'd likely have to drop to close to a 13" pitch, which definitely doesn't seem right. Since it's a new motor, I would probably also have a conversation with your selling dealer.

 

[SmokyMtnGrady]

Oh, crap! Wait, I have to remove the trailer from the boat while running? For 15 years I have been doing it wrong? Why didn't somebody say something?

Bottom is clean. Motor trimmed up. Load is 3/4 tank or about 100 gallons of fuel. Four people in board and Gold bricks stowed in the bow. I pulled the boat yesterday from the water and it was just me and still the RPMs did not change. I prop for normal conditions and normal for me is 3/4 tank and 6-8 people cruising around . Fishing it is typically a full tank and 4-6 on the boat. My old motor would spin up in normal conditions to Max rpm. I don't see why it does not spin up to Max with my new motor? I will keep you posted. Next week I am getting a couple test props from propgods.

 

[Parthery]

I'm with Dennis, disconnect the trailer. :grin:

Seriously - what happens when you trim it out? On mine, I pick up 300-400 RPM when I trim it up. Assuming yours acts similarly that would put you at 5700-5800 RPM. If that doesn't get you there then I would raise the motor one hole.

I find GW performance numbers on their website to be wildly optimistic.

 

[DennisG01]

Cash under the mattress, now gold bricks in the bow...

If you're operating the boat correctly (which I assume you are), then it really doesn't make sense that you should have to start propping down. It just doesn't seem right. I would seriously have your selling dealer check it out. If you start messing with props you may be masking an engine issue.

 

[SmokyMtnGrady]

Brian,
I only get 5400 trimming it all the way up before you start to blow out the prop.

Dennis, I am thinking something is off with the motor. Maybe it does need to go up one hole ? It is mounted on the top hole of the bracket and the second from top on the motor. I think I will take it back to Mid Carolina next week.

 

[DennisG01]

Yeah, I think that's probably your best bet at this point. A couple things stand out... the anti-ventilation plate seems to be at the right height and the RPM's didn't change, regardless of load. Could you raise the motor and solve the issue? Possibly, but from what you've written here it seems correct.

I don't know a whole lot about the computer programming on that engine... is it possible that the computer is limiting RPM's while it's still in the "break-in" period? Is there anything in the manual about that? Just a wild guess, though.

 

[chrisk]

SMG, I have the 4.2 litre 250 on my 246 G Explorer and I had the same issues. I am running a 17 pitch prop and was only able to get 5400 RPM. I raised the motor to the top mounting hole and now I can get 5600 - 5700 RPM and 48 MPH. Might do the same for you also, cheaper than buying a new prop!

Also saw your post about going to FL to fish. FYI, the weather (wind) down here has sucked for the past month. Spent a week in the keys and the best day was 2-4 foot seas with several 3-5. This weekend and into next week they are predicting 3-5 and 4-6.

Chris

 

[Parthery]

Just as an aside, I heard from SMG this morning....his motor was stolen last night while he was in Titusville, FL. Unreal....

Thoughts go out to you Bob....hope you get back up and running soon.

 

[Ky Grady]

Sorry to hear about the theft Bob. :sorry

Hope you're back up and running soon!!

 

[Legend]

Sorry about the theft SMG - unbelievable...can't even imagine that happening

 

[DennisG01]

Stolen?!? Sorry to hear that, Bob. Thieves are amongst the lowest scum of the Earth.