[C320-list] Boat Speed

[Dave Anderer]

When I tested my tach a couple years ago, I got the following numbers:

Indicated RPM     Actual RPM           % Difference
1000 1000 0.00%
1500 1500 0.00%
2000 1970 1.50%
2400 2350 2.08%
2600 2520 3.08%
2800 2630 6.07%
2900 2730 5.86%
3000 2840 5.33%
3200 3030 5.31%
3400 3260 4.12%
3600 3300 8.33%
4000 3700 7.50%

..which means when I thought I was running 2800 RPM I was actually doing a
bit less.

To pick a nit, I believe the 1-hour rating is 3600; 3400 is max continuous.


On Tue, Jul 7, 2009 at 4:00 PM, Scott Thompson <surprise at thompson87.com>wrote:

> 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
>