is my propellor too large ??
 

Hi,

I have just bought an Osprey 15 foot boat manufactured in Zimbabwe. It
has a Yamaha 115HP outboard with a Laser II 13.5" X 22" prop. The boat
will reach 47 MPH quite easily with the engine running at 4200 RPM but
according to the engines manual the engine can rev at 4500 - 5500 RPM.

Am I correct in thinking that replacing the prop with a 19" pitch one
will raise the revs to 4800 RPM and give the boat a little more
acceleration?

Are there any possible problems in reducing the pitch?

All advice greatly appreciated.

Thanks

On 12 Apr 2005 10:22:17 -0700, (Carl Harrison)
wrote:
I have just bought an Osprey 15 foot boat manufactured in Zimbabwe. It
has a Yamaha 115HP outboard with a Laser II 13.5" X 22" prop. The boat
will reach 47 MPH quite easily with the engine running at 4200 RPM but
according to the engines manual the engine can rev at 4500 - 5500 RPM.

Am I correct in thinking that replacing the prop with a 19" pitch one
will raise the revs to 4800 RPM and give the boat a little more
acceleration?

Are there any possible problems in reducing the pitch?
===============================================

There are a couple of possibilities. One is that the motor needs a
tuneup or is getting tired. Another possibility is that you are
running the boat more heavily loaded that the previous owner, and of
course there is also a chance that it has always been over propped.

I'd have the motor checked out by a mechanic first, and then think
about decreasing the pitch by a few inches.

Carl,
That seems quite fast for that amount of rpm's. I'm not that familiar
with this boat and motor combo, but perhaps you could try to make sure
that your tach is correct about the rpm's. Another thought might be to
find an Osprey dealer and see what info they could give you.
You might want to also check with a prop web site that sizes the prop
for you. ex. http://www.gpcprop.com/propeller_sizing.htm

Paul


Carl Harrison wrote:
Hi,

I have just bought an Osprey 15 foot boat manufactured in Zimbabwe. It
has a Yamaha 115HP outboard with a Laser II 13.5" X 22" prop. The boat
will reach 47 MPH quite easily with the engine running at 4200 RPM but
according to the engines manual the engine can rev at 4500 - 5500 RPM.

Am I correct in thinking that replacing the prop with a 19" pitch one
will raise the revs to 4800 RPM and give the boat a little more
acceleration?

Are there any possible problems in reducing the pitch?

All advice greatly appreciated.

Thanks

Thanks everyone.

I have had a look at http://www.csgnetwork.com/marinepropcalc.html and
according to their calcs the engine must be pulling 5200 RPM to give
47MPH. I will get the tach checked next time I am in Zimbabwe.

Carl

Carl,

Wow. A 15' boat and a 115 hanging on the back running 47 mph!

Got any rubber plugs at the base of your prop? Look up prop ventilation
and see if you think that will help you. Google the group for "prop and
vent" or "prop and venting" and you should get some results from a few
discussions a while back. That might be the simple, and free, solution
you want.

Dan


Carl Harrison wrote:

Hi,

I have just bought an Osprey 15 foot boat manufactured in Zimbabwe. It
has a Yamaha 115HP outboard with a Laser II 13.5" X 22" prop. The boat
will reach 47 MPH quite easily with the engine running at 4200 RPM but
according to the engines manual the engine can rev at 4500 - 5500 RPM.

Am I correct in thinking that replacing the prop with a 19" pitch one
will raise the revs to 4800 RPM and give the boat a little more
acceleration?

Are there any possible problems in reducing the pitch?

All advice greatly appreciated.

Thanks

On 12 Apr 2005 10:22:17 -0700, (Carl Harrison)
wrote:

Hi,

I have just bought an Osprey 15 foot boat manufactured in Zimbabwe.

I owned/started that company. I sold it off for hefty profit only
after 6 months.