Need prop starting point.

[roscoe]

Hi guys.
History:
I bought this little 16' Alumacraft a month ago. 2 bench seats, and a single seat console helm. Book says it weighs in at 380#. Rated for 65 hp !! !!
Total weight boat motor batts fuel gear, and me = 1000#.
It was powered by a 1977 Evinrude 35 hp. I replaced the damaged prop with a "stock" 10x13p and brought the rpm up from 4300 to 4700. Then I raised the motor 3 inches and got the rpm up to 5100 at 28+ mph.

Problem:
I really needed power tilt for my back and shoulder.

Solution:
I bought a 2001 Mercury 50 hp 3 cylinder 2 stroke with power tilt and trim.
And the engine is rev limited to 5700 rpm.

Question:
Where do I start to prop this new motor?
Boat, new motor, batts, gear, bigger fuel load and me = 1100#.
I went to the Merc site and punched the info into their propfinder program.
It recommended a 13p prop. Doesn't seem right.

If the old 35hp and a 10 x 13p prop is right.
Then shouldn't the 50 hp need a 10 x 17p or maybe even a 10 x 19p?

I realize I may need to buy a second prop, but I want to be close with the first purchase.

Your thoughts on a starting point?

 

[Silvertip]

Re: Need prop starting point.

The Merc has a 1.83:1 ratio lower unit so you might try a 15. I agree that a 13 is a little short on pitch. My fishing buddy has that exact engine on a 17 foot Lund Explorer and it runs a 13P. On your much lighter boat I would think a 15 would be in order and you might even try a 17. A 19 is a way too tall in my view however if the engine can twist it, you would have a little hot rod.. I run a 19 on my 75 HP which has a 2.3:1 ratio. If it were mine, I'd start with a 15.

 

[Crownie2]

Re: Need prop starting point.

I don't know what you entered into the calculator but I entered 16' aluminum console and came up with a 15" recommendation.

In fact it showed 15.5", 5,250 RPM's and 35.5 MPH as the calculated results.

Have fun and be safe!

Bob

 

[Silvertip]

Re: Need prop starting point.

I didn't use a calculator, I used known factors. However I just ran a calculator and with a 15P cupped prop on that light boat the calculator showed 39 mph using a 10% slip factor. I suspect, properly set up the boat could use a 17P provided it was run light. My reasoning is that with a 15P running a light load, the engine may actually hit the rev limiter.

 

[roscoe]

Re: Need prop starting point.

What calculators are you guys using?
I would be interested in comparing the results from several.

Which one did you use to calculate the speed?

THanks for the input so far.

I think I will borrow a 15" to try out, before I buy one.

 

[Silvertip]

Re: Need prop starting point.

Here are two that I use.

http://www.rbbi.com/folders/prop/propcalc.htm
http://www.csgnetwork.com/marinepropcalc.html

 

[Crownie2]

Re: Need prop starting point.

Silvertip: I was responding to Roscoe, not you.

Roscoe - I used the Merc Calculator you refered to.

Bob

 

[roscoe]

Re: Need prop starting point.

Thanks guys.

 

[Silvertip]

Re: Need prop starting point.

Give us some numbers when you get it "fine tuned". Its always good information to file away should someone else have a similar situation.

 

[walleyehed]

Re: Need prop starting point.

I punched in 1500lbs total weight because it says 50HP is too much for 1000lbs, and it came up with a 15P, so I have to agree on the 15-17P range.

 

[roscoe]

Re: Need prop starting point.

thanks.
Will post back with results.
May be in a few weeks, or next April, as time is short and we have had hard freezes 65 miles to the north.