T&T: props

[Mark Richter]

<<1. max rpm is 2450 port, 2300 starboard.  9.5-10 kts with reasonably 
clean
bottom and usually towed dinghy left at dock.  No smoke, higher but  in the
green temps.
2. yes, port tranny is 2.0 ratio and 1.9 starboard.  This is due  to the
failure of the original Velvet Drive tranny running in reverse to achieve
contra-rotation.  Design flaw that is not uncommon for '80s boats.   The
props are reportedly 19 and 20P to make the math work, but I think they are 
way
off.  Further, the prop DARs are very different. >>

When you get the pitch on the port prop measured, you will have your answer 
for the correct pitch, assuming a prop design (blade count and DAR) that is 
similar.  The port engine is turning up to rated engine RPM, which is 
correct for most displacement or semi-displacement boats.  If the port prop 
measures out at 20 inch pitch, then you will need 21 inches of pitch on the 
starboard prop to account for the different transmission ratio (it's turning 
slower, so it needs more pitch).

<<4. a fairly sophisticated program I downloaded from Castle Marine leads me
to further confusion about sizing.  Says I need 24x16P props - I am
(theoretically) running 24x20P.>>

IMHO, prop programs are for new boat designs, for which there is no actual 
speed/RPM data.  You already have a much better idea of the correct prop 
size and pitch than any theoretical program will provide.


<<5. I will probably need to repitch and possibly re-bore any used props I
buy.  I hear you can change pitch by up to 2", but is re-boring  expensive?
>>

Not really, but if you have to both re-pitch and re-bore your used props, 
then you'll have to get a great price on the props to come out much ahead. 
And the amount a prop can be re-pitched depends on its history; it may have 
already been re-pitched as far as possible.  Call a local prop shop and get 
rework pricing before you shop for those used props.  BTW, a good source for 
used props is the local  prop shop.  They sometimes take them in on trade.

Mark Richter, m/v Winnie the Pooh, 80hp Deere, 2.8 ratio, 26X20 3-blade, 
Ortona, FL