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[John Gardner]

Today I set out on a windless day, so engine on.  I'm sitting off to the side so don't see the knotmeter, but after a while the penny drops that with the engine rpm at my normal 2300, I'm only doing 4.3 kt.  Normally it's 5 point something, and it seems to me, now that think of it, that a few years ago it was more like 6.  I anchored and dived on the propeller - nice and clean and no dings. 
I checked the transmission oil level - right on the money.  The speed sensor is nice and clean and although it was reading lower than the GPS, it was only half a knot off.  The bottom was painted this winter and is nice and clean.  I haven't checked whether anything is wrapped round the keel - the Chesapeake is so murky, you can't see the keel from the propeller, and the possibility didn't occur to me till later, but in any case it's unlikely.  There's not many possibilities left.
Comments and wisdom would be much appreciated.ner
John Gard

[John Langford]

One possibility is that your tach is off and you are actually running the engine well below its normal cruising rpms. Can you get access to an electronic tach to check the boat tach?

[David Sanner]

I guess your transmission could be slipping...

I might start with a few tests:  

At wide open throttle, in neutral, what is your tack reading?
At WOT underway (no wind/seaway)  and what is your tack reading?
Should be close to the same... approx 3k.  Typical cruising rpm
is 80%+ of max rpm and if you're prop is sized well, your hull
is clean you should easily get above 6kts.

Do you notice any lost of thrust in high load situations...
such as accelerating with quick starts or reverse?

Assuming no major vibration, a cleanhull, prop looks good and you can get
close to the same RPM in neutral (around 3k) as you can under load but
still your SOG in quiet water isn't getting much above 5kts any where
near WOT you've certainly a problem.

... two things that come to mind are an under
pitched (damage) prop or a slipping transmission.

But I'd start with checking your tack reading and making
sure you're getting proper and accurate rpm under load.  

[gwp]

Start simple.....current or headwind? 

[Jim Hardesty]

I also slow down with a head wind.  Into a strong 25kn wind I don't get much more than 5kn of boat speed.  Not what I expected with 35hp.  My previous boat, a 30 footer with 15hp, I never noticed a loss of speed.  Haven't done any troubleshooting but a slipping transmission would explain it.
Jim

[lazybone]

Slipping trans would soon turn the fluid dark/black.

[John Gardner]

Thank you for your replies.
Tacho calibration:  Was done a few years ago when I upgraded the alternator.  At that time max rpm was just about 3000, so I'll check if that's still the same.  I'll check David's other speed/rpm thoughts at the same time.
The 2-bladed prop is clean and appears fair.  It was professionally repaired, faired, and balanced about 3 or 4 years ago after it hit something, so I think it's alright.
The only wind was the wind due to my motion, and current there was a little, possibly half a knot, which may explain the difference between GPS and knotmeter.
And the oil was clean.  Not sure if I'm pleased or not - if it was black that would probably give me the answer, expensively, but being clean leaves me still in doubt.

I will have a go at it this weekend.  I'll run across the wind in a current-free area and check knotmeter, gps, and tacho in both directions.  See what that turns up and let you know.
Thanks
John Gardner

[Ralph Masters]

On Ciao Bella at 2000 RPM I normally show 5.7 - 6.1 on the GPS, and I max out at WOT at 2400 roughly  At WOT it normally pushes up to about 6.5 - 6.7 fast enough to make the old girl squat down and get that engine exhaust mark on the stern above the bilge discharge opening.  Messy to clean off, and hard to get to.

Ralph

[Ron Hill]

John : I'm sure that you changed Xmission oil this spring and it's at the correct level.  If you check the shifting linkage at the transmission and you get the full throw of the lever(at the transmission) everything should be OK.
 
Now if you changed to an Edson stainless shift lever at the helm then you need to adjust the throw below.  You might need to put the shift lever cable in the lower hole (more throw) from the top hole (more leverage).  A few thoughts

[Ken Juul]

Ralph,
Either your tach is way off or you are way overpitched.  The engine should get close to 3000 at full throttle in gear.  Suggest you calibrate the tach first.  Either borrow/buy a digital rpm counter or use the florescent light method.  Pretty sure the Florescent light method is described in the wiki, if not search for engine rpm or tach or similar.  Last time I saw anything about it was several years ago.

The tach has two adjustments, one on the back of the tach for the needle and the other a series of dip switches to match the tach to the alternator.  Both should be described in the wiki

[dgill]

Hey John,

There is a tach calibration tool in the Fleet 12 "Tool Box" that you can borrow. 

D. Gill

[Ralph Masters]

Ken,
From the survey the prop is a 2 blade 15 X 14.  Not sure what the manufacture puts on as standard equipment.  I do know that I need to replace the tach because the hour meter does not work, has not moved since I bought the boat and the PO, who had it two years, said "oh yeah that's never worked".  So go figure.  I do know that if I go too fast then the admiral has me on the swim step scrubbing the black stuff off the transome back in port.

Thanks,

Ralph

[Ken Juul]

I think that is the standard for the 2 blade. 

Another test you can do...how fast will the engine run in neutral?  If it is close to 2400 then you know the prop pitch is ok and it is all tach.  If more that a couple hundred different then you have a couple problems.

If the loaded/not loaded rpms agree, then you can figure that your cruise rpm should be 18-1900 or 75-80% of max.  These engines don't like to be run for long periods at full throttle, the black on the transom is one sign you are working the engine too hard.

When you do replace the tach, it will be necessary to set it up for the alternator you have.  If instructions aren't provided, you know where to find them.

[Ralph Masters]

yep, the no load is about 2400 too.  Normal running is right about 1900 - 2000 when we motor.  If we go up to the 2300 - 2400 the temp starts going up, so I know the engine is loading there.  If we keep it at 1900 - 2000 it doesn't squat down and get the mark on the transome.  Great little engine really.  With the problem with the hour meter I just figured it was not reading correctly.  Some day that will be replaced, it's just down the list a bit.

[Tom Soko]

Ken and Ralph,
I'm pretty sure that a 15x14 is way too much prop.  The standard prop supplied with the C34 and C36 has been a 15x9 three blade or 15x10 two blade.  The tach might also be off, but probably the reason you are getting a dirty transom is the prop.