[C320-list] Boat Speed

[Scott Thompson]

If you have a 3 blade prop then it is almost certainly the factory 
option and it will be appropriately pitched for the 320.  At least mine 
was on hull number 653 before I switched to a folding prop.

You cannot compare RPM / speed correspondences across different boat 
designs with any reliability.  There are two many variables in hull 
shape, bottom condition, prop, and gear ratios.  So the fact that those 
other boats were doing well at the same RPMs isn't very relevant.

For the Yanmar 3GM30F cruising RPMs are technically 3100.  3400 is the 
max RPM (the "one hour" rating).  However both of these are considerably 
greater than what most owners actually use.  The tachometers are not 
necessarily very reliable, and the speedometers aren't very reliable 
either.  You should be able to get up to 7 knots through the water, but 
you need to make sure that your speedo is calibrated to be sure you have 
an accurate reading.

If you can't make 7 knots on the GPS in flat water and no wind or 
current then you do have a problem.  The most likely culprits (assuming 
the engine runs well and you aren't towing anything or carrying a heavy 
load) then the most likely problem is a fouled prop or a fouled bottom.

I have found that it takes very little fouling of the prop to cause a 
very noticeable decrease in max speed and max RPMs, and rough running at 
anything above 2000 RPMs.  If your engine is putting out any black smoke 
at all then a fouled prop would be my first suspect.

Scott Thompson

pjaarch wrote:
> I just purchased my 320 several weeks ago and am concerned about the boat speed while under power.  I have a 3GM30F and cruise at about 5.4 knots at 2700RPMs.  It seems slow to me.  I am currently cruising with other boats, a (hunter 28, beneteau 331, and an Irwin 38.)  They all cruise easily 6 knots at 2700 RPMs and I can't keep up.  I have a 3 blade prop.  Could I be under proped?  What is the cruising RPMs for the boat?  I have no ID plate on the engine block.  I assume it is rated for 3400.
> 
> Paul Adison
> C320 #722
> "Rough Draft"

-- 
Scott Thompson
Surprise, #653