wrote:
On 21 Aug 2006 07:03:54 -0700, wrote:

Turns about 4800 - 5000 rpms... figure that's about right. Doesn't
seem to lug or over-rev. Wasn't sure about several things, but guessed
4:1 on the tranny, 13 pitch prop, and 0.15 on the 'slip'.... sound
about right? Those figures came out 13mph.... so if my guesses were
right sounds like that speed is about right.

Brother-in-law is looking at a new one and the dealer is saying 32 with
a 90hp. Now I'm thinking that's way too high?

You have a merc 40 EFI, non big-foot?

My book says you should be turning 5500-6000 WOT so you are over
propped
The reduction is 1.83:1 and merc recomends 12¼ x 9 or a cupped 8,

The 60HP seems to be a fairly good compromise between speed and
economy on my 20. I had a 75 2 stroke (about the same weight) and it
was more like 24 MPH but a lot more fuel. At a certain point you are
just churning water and not going much faster. My neighbor has a 90
Yammy 4 stroke on a heavy 24' and he gets about 26

Yep, two stroke 40, little foot. Now we're talking - if I plug in 5500
rpms, 1.83 ratio, 9 pitch, 2200 lbs, 21', 0.15 slip (whatever that
is... cavitation?), the calc
(http://www.boatpropellers.com/propcalc.html) comes up with 21.8. Even
if it was close to that it would be tolerable. I thought it should
turn a little more than 5k... may swing by a prop shop. I think
there's one here in town that'll sell ya one and let ya bring it back
if it's not kosher.

Wow, if your neighbor's 24 is hitting 26 with a 90 the dealer may be
closer than I thought on the 32mph statement. It's a 21' and a fishing
pontoon so maybe those are a little lighter (less furniture) and the
90 is a four-stroke.