how fast should my boat go?

[BobGinCO]

Re: how fast should my boat go?

Did it sound like 4700 RPM?

Few people can tell you what 4700 RPM sounds like.

 

[frantically relaxing]

Re: how fast should my boat go?

I can. But can't really describe it.

Funny thing about his rpm/speed numbers. They're nearly identical to our Party Cruiser- 32' pontoon boat, 115hp Mariner with an 11 x 13" pitch prop. She plows along about 14 mph at 5000 rpm.

The numbers on the OP's prop may not be size numbers...? Regardless, it almost sounds like he's running a small diameter outboard prop, or at the very least, a prop that's ACTING like one...

As for tach numbers, if you're running less than 13 volts to the tach, the lower the voltage, the higher the tach will read...

 

[jestor68]

Re: how fast should my boat go?

1967-1977 OMC stern drive props(120-245 HP) are 14" diameter and were available in the following pitches: 12,14,16,18 and 20. The biggest HP listed for 1971 is 235. But since you have re powered, it may be more.

One of the few places I know of that still makes OEM replacements is Propco Propellers in Monroe, GA. They carry the 12,14,16,and 18 pitches.

You need to get the correct diameter prop for your(large gear case) stern drive, if you expect it to go anywhere. The 12 and 14 pitch are billed as heavy load props. I suspect with your V-8 power, the 14 X 14 should be a good choice.

These are pin drive props and have no rubber hub to slip.

Marine Engine .com carries replacement drive pins, as well as the other hardware for the prop. I highly recommend that you order some extras from them. When they're gone, they're gone. ( they list like 10 in stock for these little items). Google "1971 OMC stern drive parts" to find them.

 

[QC]

Re: how fast should my boat go?

Few people can tell you what 4700 RPM sounds like.

OK, maybe I'm few people . . . Most can certainly tell the difference between 2500 and 4700, and this equation is in that neighborhood of disparity.

 

[humboldtbrad]

Re: how fast should my boat go?

ok so the prop I was using was a 14-1/4" 18 pitch prop which according to OMC Boat Propellers, Outboard Marine - propco marine boat propellers is a cruise prop. I think I need a heavy load prop, but isn't the lower the pitch the slower it is?

 

[jestor68]

Re: how fast should my boat go?

I think that you need to verify the accuracy of your tachometer before you do anything. I suspect that you are not turning the rpm that you think you are.

I couldn't find a 1971; but a 24ft 1979 weighs(dry) 4200 lbs with a 260 V-8. So I'm not sure how your boat could weigh 7000 lbs.

 

[dingbat]

Re: how fast should my boat go?

His 25 footer could weigh as much as two 20 footers. I bet it has never gone over 35 mph. I would be expect around 30mph at wot.

I agree. A 230HP I/O on a boat of that size and weight (#7k) would be a dog.

 

[steelespike]

Re: how fast should my boat go?

I agree with jestor68 you must confirm the tach is accurate. Its highly unlikely for a boat with a 18" pitch to make only 16 mph at 4700 rpm.