Prop Question for 71' 280

[murraycentral]

I have a 71 280, this vessel originally came with a Chry. 318 and was repowered with a Ford 351 before i purchased it. this is mated with a Paragon HD7L 1:1 trans. I had to replace the prop on it a few months back and we fitted it with a 17x16 with a #3 cup LH which was close to what was on it when i got it.

I finally got the last few items all squared away and had her out for the last few weekends see what she will do. My max RPMs have been pretty consistent but low, 3400 max and running 16.4 MPH (according to GPS). this was with 40 Gals of fuel and 4 adults.

Last weekend, we had 6 adults and 2 kids out for a cruise, same amount of fuel and we were maxing out at 3000 RPM and 12.6 MPH.

Now i know this is no race horse, but she should be able to give me about 15 MPH at cruise and about 21 MPH WOT (according to the specs i have on it).

does anyone have any idea what prop I should have on this? according to a couple calculators i used, its getting 68% slip at 3000 RPM which is just horendous. I can have this prop repitched down to a 17x14 with no cup but am unsure if that would be helpful or if I should change out the trans for something with a better ratio like 2:1.

any insite or help would be greatly appreciated.

[CrazyIslandMoFo]

for each inch of pitch you will gain or loose 200(+/-) RPM , sounds like the pitch is too great...causing the motors to work hard... by reducing the amount of pitch will allow the motor to increse RPM. pitch = inches travled in one rotation. your WOT rpm should be 4200-4600. check your shop manual...

[murraycentral]

thanks for the insight. I contacted a local prop shop before i posted here, you would have thought i stuck a dagger in his heart when i asked him to re-pitch my prop down 2 inches and loose the cup. told me i would be wasting my money.... technically, i would only be going down 3 inches (including the inch they give you for the cup) which is only about an additional 600 rpm but that would at least get me closer to 4000 WOT, at 12 MPH, im not completely on plane. what are your thoughts on going to a 4 blade 17x11. still kinda new at the prop game, tryin to figure out where the sweet spot is.

[7228sedan]

I just purchased a 72 280 with a 454; I have no specs at all on her. I'm interested in concrete #'s LOA, Beam, dry weight etc. I have to rebuild my transmission. I was concerned with the ratio as well. I have a velvet drive 72 series V drive 1.5:1. I have not yet checked out the prop yet. I know the 28 Silverton sedan of similar vintage came with a 1.5:1 gear as well. I think our hulls are heavier though. I was considering going to something closer to a 2:1 ratio & proping accordingly. I need to know some numbers though before I can make an educated decision.