Prop Size

[townsendme]

I need some help please. I have a 87 Grady Offshore Pro with full Grady drive bracket. The Boat has been repowered with a single 2001 yahama 250 ox66. I have a 14.5/ 17 pitch prop now and just not getting the job done. I know the prop is wrong but before I go try a hundred props I am hoping someone has the same setup and can help with the correct prop Size. I am burning way to much fuel right now with this prop.

Some examples with this prop
5500 top RPM = 30 miles top speed
4700 RPM = 25 miles (Fuel here is round 13.8 - 14 gph)
4500 RPM = 21 miles (fuel here is around 15.5 gph)

any thing lower that 4500 i really can not plane out. At 4500 I am planed out but it really don't feel like i am on top of the Water i feel like im still plowing.

 

[wrxhoon]

Does your tacho work ok? Your engine should have 1.75 gearcase, 17" pitch prop should get you about 42-45 MPH at 5500. Something doesn't add up . Could be the tacho over-reading and the engine not 100%. Looks like you have clean bottom as well. How much does the boat weigh including gear , persons on board and fuel?

 

[townsendme]

If guessing correctly 5500 lbs with fuel passengers gear.

Tach works
Motor is been checked out it runs great

Just went thru all the o2 sensors
Water pump
Thermostat
Spark plugs
Checked vst
Check throttle position sensor

 

[DennisG01]

Something else is going on. I have the Yamaha 17" prop and the boat literally screams out of the hole. The boat planes almost effortlessly - maybe 1/3 throttle, if I choose. My top end is 40MPH and can stay on plane in the low 3,000RPM range.. Either you're not operating the boat (trim) correctly or there is something else happening. Is the bottom of the boat clean and running hardware in good condition?

 

[townsendme]

what is the Size of the 17? I have a 14.5/17 ? is your a 15/17 prop? do you know what Degree cup you have on yours? Bottom is clean don' t know if you can see the pic I posted of the boat? Here is my prop I have now I have attached photo

 

[townsendme]

 

[DennisG01]

I did see the pic, but wasn't sure how long the boat had then sat in the water.

I'm not sure of the prop diameter, offhand. It's a Yamaha "17M" - I'm sure google can help on that, though. However, there's no way that a little bit of difference from one 17" prop to another (not even to a 19") is going to account for the difference you're seeing. I've run, literally, hundreds of boats and the issue you're seeing is NOT due to minor prop differences.

Are you measuring speed with the speedo or GPS? The motor's pitot can/does get clogged and skews the MPH. If memory serves, the GPH you're seeing at the various RPM seems similar to mine - but my speeds are much higher. But, that still doesn't explain the hole shot issue.

 

[seasick]

What is the exact model number of that motor? (Does is have the correct shaft length for that hull)

Also, when you try a hole shot, do the revs increase very quickly? ( is the prop slipping? At some point in time, the skeg hit something as the photo shows. That prop may have been damaged)

Have you always had this issue with planing and top end speed? If you didn't have these issues but do now, we would look at a smaller group of possible issues.

Your fuel burn for that motor is not way off for a Saltwater series motor
It could be a simple thing like bad LP fuel pumps or a VST issues. How did you check it?
It also could be a spark issue, plugs, wires or coils.

 

[dbiscayne]

I've got the same boat but w/the single bracket, 1997 OMC 225, 15x17 prop, tired bottom paint but no growth, my numbers are
4000, 23
4200, 25
4500, 29
5200, 33
don't remember what I got a wot but it would reach 5600, this old engine is gettin tired.

 

[wrxhoon]

5500 RPM doesn't work out 30MPH with 17" prop regardless of prop diameter . My guess is your tacho is not accurate . 17" prop is about right for your boat at 5500 lb.
The old girl looks nice and clean, especially for a very old boat.
1997 OMC 225 has 1.86 gearcase if memory serves me. In this case 33mph at 5200 works out . Your engine at WOT should be 5-6000 rpm , like you said a bit tired and bottom paint doesn't help.