The Right Prop?

[rmgmag]

I have a 1989 Four Winns Sundowner with a 5.7 Mercruiser, not sure about the outdrive, pretty sure it's an OMC. This is our first boat and we're really enjoying it. Last week we hit a rock or two and destroyed the prop. I'm not sure of the size that prop but the boat accelerated very well with it and we topped out at about 32mph at WOT, I don't remember what the rpm was. I had a spare on hand and had it installed. It is a 13 1/2 x 19. It doesn't get on plane quite as quickly but it's still pretty good, maybe 5 seconds vs 3 seconds with the old prop, not sure how accurate that is, but it's not like we're waiting for it to get on plane. Top end with the new prop is 35mph but I'm only getting about 3,700 rpm's on the tach. It's probably the original motor but from what I've read, I thought that WOT should be around 4,200 or higher.

We haven't towed a water skier or a tube yet, so getting up on plane quickly really isn't an issue. We generally cruise around at 2,500 rpm and that gives us between 20mph and 23mph depending on the current and the winds. The boat weighs about 3,200 lbs.

My question is, am I over propped? Should my old motor be able to run at 4,200 rpm at WOT?

 

[series60]

Re: The Right Prop?

I have a 1989 Four Winns Sundowner with a 5.7 Mercruiser, not sure about the outdrive, pretty sure it's an OMC. This is our first boat and we're really enjoying it. Last week we hit a rock or two and destroyed the prop. I'm not sure of the size that prop but the boat accelerated very well with it and we topped out at about 32mph at WOT, I don't remember what the rpm was. I had a spare on hand and had it installed. It is a 13 1/2 x 19. It doesn't get on plane quite as quickly but it's still pretty good, maybe 5 seconds vs 3 seconds with the old prop, not sure how accurate that is, but it's not like we're waiting for it to get on plane. Top end with the new prop is 35mph but I'm only getting about 3,700 rpm's on the tach. It's probably the original motor but from what I've read, I thought that WOT should be around 4,200 or higher.

We haven't towed a water skier or a tube yet, so getting up on plane quickly really isn't an issue. We generally cruise around at 2,500 rpm and that gives us between 20mph and 23mph depending on the current and the winds. The boat weighs about 3,200 lbs.

My question is, am I over propped? Should my old motor be able to run at 4,200 rpm at WOT?

You may want to go down to a 17P or even a 15P for a better hole shot. RPM's should be up around 4400-4800. Your old prop should have a number stamped on it somewhere, usually around the hub where the prop nut goes.

 

[Bondo]

Re: The Right Prop?

I have a 1989 Four Winns Sundowner with a 5.7 Mercruiser, not sure about the outdrive, pretty sure it's an OMC. This is our first boat and we're really enjoying it. Last week we hit a rock or two and destroyed the prop. I'm not sure of the size that prop but the boat accelerated very well with it and we topped out at about 32mph at WOT, I don't remember what the rpm was. I had a spare on hand and had it installed. It is a 13 1/2 x 19. It doesn't get on plane quite as quickly but it's still pretty good, maybe 5 seconds vs 3 seconds with the old prop, not sure how accurate that is, but it's not like we're waiting for it to get on plane. Top end with the new prop is 35mph but I'm only getting about 3,700 rpm's on the tach. It's probably the original motor but from what I've read, I thought that WOT should be around 4,200 or higher.

We haven't towed a water skier or a tube yet, so getting up on plane quickly really isn't an issue. We generally cruise around at 2,500 rpm and that gives us between 20mph and 23mph depending on the current and the winds. The boat weighs about 3,200 lbs.

My question is, am I over propped? Should my old motor be able to run at 4,200 rpm at WOT?

Ayuh,.... Welcome Aboard,.... To be honest, None of yer post makes much sense....

Mercruiser, 'n OMC were competin' companies,... you've got 1 or the other, not both....

'n a 13" diameter is an Outboard prop, not an I/O prop....
Most I/O props, whether OMC, or Merc are in the 14"- Plus range...

If ya can't actually Id what you've got,.... post some pictures...

 

[steelespike]

Re: The Right Prop?

I too wonder about a 13 1/2" diameter But I think more puzzling Is your low 3700 rpm is producing slip of 30% at 35 mph.
Very high.Perhaps your speedo is wrong.
You need your speed by a gps.I have your gear ratio at 1.32 perhaps that is wrong?It may be stamped on the out drive.

 

[rmgmag]

Re: The Right Prop?

Sorry for the confusion, it's an OMC 5.7 motor. Can't really see the markings on the prop because it's under water even with the outdrive all the way up, but before the marina put it on, I got a good look at it and I made a note of the size and saved it on my phone. Interesting that you guys think it's an outboard prop because I couldn't find a 13 1/2" prop online. So that kind of explains that problem.

So maybe I need a 14 x 17 prop to get the rpm's up? Seems kind of low, but I'm more familiar with RC airplane props than I am with marine props, same concept though.

The speed was clocked using a GPS and not the onboard speedometer, so I assume it's pretty accurate. Maybe the tach is off?