prop sugestion

[yellowbelly]

1996 lund angler dlx (rated 75)
with all my gear the weight is around 1600lbs, the boat has 2007 40 efi on the back..right now its spinning 3000rpm's wide open @26mph..prop is 10 1/8"x15p..the cav plate is even with the bottom of the boat..what can i do to bring the rpm's up

 

[MikDee]

Re: prop sugestion

A 10"X 12" should work, if they even make that size increment?

If not, a 10"X 13"P. If you don't get enough RPM, you might try venting the prop. There's plenty of info on this forum. This would improve hole shot as well.
I think with a 10" X 11" you might over rev your engine!

 

[yellowbelly]

Re: prop sugestion

so looking at my manual my options are 10 3/8x13 or 10 5/8x12

 

[anzomcik]

Re: prop sugestion

Are you sure your RPM are correct, or that the speed is correct? My feeling is a boat of that size and weight doing 26mph is pretty good for a 40hp. I say this because I had a 40 on a much lighter 14' aluminum boat that could get 33 maybe 34mph and I had it setup with proper prop and a cmc power tilt trim that set the motor back 6" or so and I jacked the motor up so it didn't sit to low in the water...

What are you using to measure speed and RPM? Because I feel that one may be correct but not both.

 

[yellowbelly]

Re: prop sugestion

tach is dead on its a mercury smartcraft and speed is using a lowrance lcx 111 gps

 

[jestor68]

Re: prop sugestion

To be clear, we are talking about a 2007 Mercury 4 stroke efi 40 hp with a 2.0 gear, right?

What does your manual tell you the WOT rpm range is? ( 5500-6000?)

Your stated numbers represent a 7% slip value; not likely for that type boat.

I have a feeling that you're going to need to go lower than a 12 pitch, if the rpm range is what I believe it is.

There are props available all the way down to 7 pitch for that motor.

 

[QC]

Re: prop sugestion

you might try venting the prop

Venting is ONLY for hole shot. If it was to improve top end RPM you would only be increasing slip. Hardly a good plan

 

[yellowbelly]

Re: prop sugestion

yes those are the specs

 

[jestor68]

Re: prop sugestion

See if you can find a dealer/shop to let you test a 9 and 10 pitch prop.

You gotta get the rpm up to at least 5500 or you're going to ruin the motor.

Dropping that much pitch will not necessarily result in much lose of top speed. Remember, if the motor is turning faster, so is the prop.

 

[yellowbelly]

just a little update.. buddy had a 12p sitting around..im u to 4300rpms now so it looks like 10p or a 9p might work out

 

[buckinrut25]

Just curious what was your speed with the 12p prop? You can go to Mercury's website and use their prop selector and just see were it puts you. It looks

 

[yellowbelly]

27 i went up 2 mph

 

[yellowbelly]

now i get this beep warning at wot and trimmed up

 

[jestor68]

With your 2.0 gear ratio and 12 pitch, using a 10% slip, it comes out to about 5300 rpm at an actual 27 mph. And you're showing 4300?

 

[yellowbelly]

just ran down to the boat and pulled the tach out..its setting is 6p,turn key to on it goes to zero start the motor its idles at 775rpm's..took it out and wot was 4300rpm

 

[steelespike]

I agreed with Jestors very reasonable figures It appears your tach is off by about 1000.
Note the position on the selector and operate it a few times to clean the contacts.

 

[jestor68]

A 2.0 gear ratio turning a 12 pitch prop at 4300 rpm equals a theoretical 24 mph. The actual speed has to be something less than the theoretical speed when corrected for slip. Corrected for a 10% slip equals 22 mph.

Just because the tach seems to be reading correctly at idle doesn't necessarily mean it's reading correctly in the higher ranges.

Either the speed is off or the rpm is off.

 

[yellowbelly]

just got back from the boat and i double checked the setting and turned it a few times..tach is new i took it out of the box when i started this thread..same readings..maybe i need to raise the motor up a hole or 2..boat is very stern heavy

 

[jestor68]

The prop selector recommends ( with your stated 1600 lb weight ) a 12 pitch and says you should be able to 27.8 mph @ 5750 rpm, which represents a 13% slip(normal).

You're showing 4300 @ 27 mph.

Everything matches except the rpm reading.

 

[yellowbelly]

how would the tach be dead on at idle but not at wot? this has been bugging me for the last few weeks ..i think im gonna repower the boat with a motor closer to the max rating..its rated for a 75,i only bought this package cause my buddy sold me boat motor and trailer for 2500 a year after he bought the motor cant say no to a one year old 40efi motor for 2500