Still Confused on Prop

[Reming870]

I have been having a good summer with the boat, but I do not feel the prop is still right. I think I only have a band-aid and am curious to the ups and down sides here from the prop gurus.

First, a little background. New to me, purchased in early July. 1984 Starcraft 191 Islander. 3.0L Mercruiser / Alpha 1 drive. Had the wrong carb on it / I replaced with the proper one. I upgraded (or some may say downgraded) to a Pertronix electronic ignition. Confirmed gear ratio at 1.98:1. Boat had a 14 x 17P alum prop on it. Confirmed timing OK. Confirmed Compression in 1,2,and 4 at 120 PSI / #3 at 130 PSI. Changed plugs. Motor was loading up with old carb.

Boat was a slug with trouble to plane. 3000 RPM WOT / 15 MPH after what seemed to be eternity to get there. I had the luxery of trying a friends 16 x 13P on the boat and got to 4000 RPM / 20 MPH WOT with the boat jumping out of the water to plane. So, to get the kids tubing and me fishing I ordered a 16 x 11P prop and now have the boat at 4200 RPM / 24 MPH WOT. MPH's checked with GPS. RPM's confirmed with shop tach.

From some of the research I have been doing, this is an insane prop for this set-up. Others with similar set-ups have a 14x15P or 17P and are over 30 MPH WOT at 4600 RPM.

What are the negatives of this prop, where else can I look for possible problems that this prop is band-aiding.

 

[eavega]

Re: Still Confused on Prop

Could you be schlepping around a lot of extra weight (saturated foam, too much stuff onboard)? Verify your hull weight, as that may be wreaking havoc on your prop calculations. On my little 15' runabout, about 100 lbs was the difference between the motor turning at its recommended 5500 RPM and planing the boat at 25 MPH, and struggling to get on plane and only spinning the motor at 4200 RPM with a top speed of about 15 MPH.

-E

 

[Bondo]

Re: Still Confused on Prop

Boat was a slug with trouble to plane. 3000 RPM WOT / 15 MPH after what seemed to be eternity to get there. I had the luxery of trying a friends 16 x 13P on the boat and got to 4000 RPM / 20 MPH WOT with the boat jumping out of the water to plane. So, to get the kids tubing and me fishing I ordered a 16 x 11P prop and now have the boat at 4200 RPM / 24 MPH WOT. MPH's checked with GPS. RPM's confirmed with shop tach.

From some of the research I have been doing, this is an insane prop for this set-up. Others with similar set-ups have a 14x15P or 17P and are over 30 MPH WOT at 4600 RPM.

Ayuh,... You Are leaving the trailer in the parking lot, Right,..??

Donno What, but something sounds Very Wrong....

 

[steelespike]

Re: Still Confused on Prop

Confirm timing is advancing as it should.Are you sure the carb is jetted right?
Is the bottom clean and smooth.Does the boat respond to trim
Bow should come up and speed increase.Does it run flat like its glued to the water.Throttle plates opening all the way?Are you sure all plugs are firing all the time. Polarity on the coil right?Any distortion in the bottom especially just ahead of the transom? imo while some aluminum boats aren't the fastest on the water.
You should be able to try a 21" prop and make over 40 mph.but not much hole shot.Something is really screwy.

 

[Reming870]

Re: Still Confused on Prop

Confirm timing is advancing as it should.Are you sure the carb is jetted right?
Is the bottom clean and smooth.Does the boat respond to trim
Bow should come up and speed increase.Does it run flat like its glued to the water.Throttle plates opening all the way?Are you sure all plugs are firing all the time. Polarity on the coil right?Any distortion in the bottom especially just ahead of the transom? imo while some aluminum boats aren't the fastest on the water.
You should be able to try a 21" prop and make over 40 mph.but not much hole shot.Something is really screwy.

Timing Advance - Have yet to check
Bottom Smooth - Paint is removed and is just aluminum. Little dings here and there, nothing big.
Boat responds to trim. After market tabs on the sides also work and respond.
Not flat at all. Boat actually rides pretty high and sits high as well.
Plugs are firing correct, polarity is correct.
No distortion.
Throttle plates - Have yet to check.
40 MPH would be nice!!!

So, I am down to:
Saturated foam (I think I can rule this out by the way the boat sits in the water, it sits very high).
Timing not advancing: How can I check this?
Throttle plates: Is this as simple as just watching them with the engine off? Should they open a full 90-degrees to full closed?

 

[steelespike]

Re: Still Confused on Prop

Motor off in forward look down carb throttle plates shound be fully open at idle
plate are close to comepletely closed at idle.Use a timing light to set the timing
and to check the advance.Be sure the choke is opening all the way.
You could try carefully closing the choke a little by hand at speed to see how it responds.You could be running lean.
Water in the foam usually only affects hole shot and isn't likely to result in severely reduced speed.