Which prop? Merc 350 & 14-foot runabout

[Stangman]

Hi, Newbie question here. I just bought a 14-foot runabout with a 1967 Mercury 350 (25hp) motor. The guy told me the prop on it is totally wrong and I should replace it if I plan on doing more than just trolling. Any suggestions on size / pitch / etc?

Thanks for the help.

 

[jimmbo]

Re: Which prop? Merc 350 & 14-foot runabout

Welcome to iboats!

First does your boat have a tachometer? You need to know what your rpm is at WOT(wide open throttle). WOT range for your engine is 4800 - 5300 rpm. If the rpm is higher, a prop with more pitch is needed, if lower, either the pitch is too high already or the engine is not up to snuff.

My personal guess is some where around 11 or 12 in pitch would be close for your setup.

 

[Stangman]

Re: Which prop? Merc 350 & 14-foot runabout

I've now discovered that the prop that is on there is a 48-380098A1, which appears to be a 10-3/4x11 3-blade aluminum. I'll have to wait until Friday to see how this performs with a 35hp on a 400lb. boat

 

[Stangman]

Re: Which prop? Merc 350 & 14-foot runabout

It didn't run well with the 11-pitch, but a 13-pitch I borrowed seemed to work much better. I managed to get about 21MPH with the motor pushing an estimated 700 lbs. Does that sound reasonable?

 

[steelespike]

Re: Which prop? Merc 350 & 14-foot runabout

I would say 21 is in the ball park.Is that gps or a speedo?Speedos are notoriously inaccurate. You need to confirm your rpm.if your running too low or too high at wot(wide open throttle)you can damage your motor.
Your 35 is probably rated for about 33 stock under ideal conditions.
Depending on the style of your boat I think you could get more when set up right.Careful adjustment of trim and verticle height could easily get more than 2 or 3 mph depending on where you are at now.
Start with the antivent plate about even with the bottom of the boat and raise it 1/4 inch at a time until just before ventilation becomes a problem.
Use 1/4 inch sticks on the top of the transom.Just be sure your firmly clamped on and it continues to pump water.If you can gain one inch it could equal close to 3 mph.Get yourself a tinytach; easy to install and pretty reasonable.A must for motor health.

 

[Stangman]

Re: Which prop? Merc 350 & 14-foot runabout

That is GPS speed...and the speedo pretty much reads the same thing!
I will have a tach on it soon to validate the RPM's, etc. According to a 3rd party manual I read, the operating range for this motor should be 4800-5300 @ WOT. Does that sound right?

The motor height had already been adjusted about 3/4-inch when I purchased it. I don't think I'll mess with it for now. It appears to run OK at that height.

Thanks for the insight.

 

[steelespike]

Re: Which prop? Merc 350 & 14-foot runabout

That is GPS speed...and the speedo pretty much reads the same thing!
I will have a tach on it soon to validate the RPM's, etc. According to a 3rd party manual I read, the operating range for this motor should be 4800-5300 @ WOT. Does that sound right?

The motor height had already been adjusted about 3/4-inch when I purchased it. I don't think I'll mess with it for now. It appears to run OK at that height.

Thanks for the insight.

The motor developes its hp at 5,000 rpm so your rpm range seems reasonable though it would seem it should at least reach the 5,000 rpm figure minimum.
Height is probably adjusted to a max though I would check the height of the antivent plate.