[C320-list] Boat Speed

[Robert Seastream]

My manual for the 3GM30F in my 2002 C320 states 3200 rpm continuous,  
3600 one hour rating.
I have the three blade prop and have seen 3700 at WOT, never more.  I  
usually run at either 2400 or 3000 rpm.

Bob Seastream
Intuition # 906


On Jul 7, 2009, at 5:01 PM, Dave Anderer wrote:

> 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
>>