315 Yanmar-Low RPM's

[Seamoor]

I've got a new 25cc with a 315 Yanmar inboard. The engine runs out to 3,650rpm at w.o.t., which yields a top speed of about 28-29 knots. The prop is a clean Acme 18x23 4 blade which came standard from the factory, and I get no black smoke. Cruising at 3200rpm, I go approx 22 knots, burning about 10gph by Floscan. The tach is calibrated to "14" from the alternator sending unit, per Mac. Also, these numbers are with a clean painted bottom, half fuel and 2 people. Does this seem normal for a 315 Yanmar/25cc or should I be looking at checking out the tach, prop etc?

[Laukia]

not.......

You want that engine to turn 3950 at WOT fully loaded.

[Hey Doc]

Check your tach

I would check your tach first. If that is accurate, you will need a prop with less pitch. 3650 with such a light load indicates you are over proped.

[Seamoor]

Yes, I've seen 3900-4000 as the target w.o.t. rpm. I'm wondering if the other indicators such as cruise speed, no smoke, fuel figures, etc, indicate that I actually am getting the right rpms, but that the tach is off.

[Split Decision]

I would check the tack first. Volvo is another that have in accurate tacks

[THE SHARK HUNTER]

I Have 315 In My 28 I Get 3950 W O T I Had Some Problems With The Tac Check Out Te Sending Wire First You Just Might Have To Do A Quick Clean And Try It Again This Year I Had Some Bouncing On The Port Tac Same Thing Had To Clean

[Seamoor]

Check the obvious

Thanks for the replys. After all was said and done, it was as simple as a loose linkage. Hmmm.

[RUNKELDC]

What numbers are you getting now that you have the problem corrected? I'm interested to see the difference between your inboard and my jackshaft version with similar power. I have a D6 310 in my 25.

[Seamoor]

3,950 w.o.t. gives about 30 knots. Cruise 22-23 knots at 3200, 2.3-2.5 nmpg; 25 knots at 3350 gives about 2.1-2.2 nmpg. All figures are averages, moderately loaded,3/4 to full fuel, using Floscan 9500. One thing that seems clear is that in this setup, the 315 Yanmar uses less g.p.h. than the fact sheet indicates. How does this compare to the Volvo/jackshaft?

[RUNKELDC]

I reach about 35 knots 3500 rpm (wot) and cruise 27 knots at at 2900 rpm. Those numbers are 1/2 fuel, gear and 3-4 people. I reached 36.4 knots once with 1/4 fuel and just me on board. Yesterday we ran 80 nautical miles on 25-30 gallons. About 15 of those miles were trolling.

[Seamoor]

Wow. I wonder how much is the motor, and how much is the drive.

[Bluefin 32]

I had a jackshaft version of the 25 with a Yanmar 6LP. I could also hit almost 37 knots lightly loaded at 3875 rpms. I normally cruised at around 27-28 knots at 3200-3300 rpms. Outdrive boats are definitely more efficient users of power due to straight thrust angle from propeller. But don't have any regrest whatsoever. I would never recommend anyone to get the Yanmar/Bravo III setup. The engine is great and reliable, but the drive simply cannot handle the torque. Mine was a 2000 model and the guy who bought the boat from me is just putting in his second replacement drive. Even the so-called heavy duty "X" version of the BIII disintegrates under the load of the Diesel. By contrast Volvo seems to have gotten this right since from all I hear their Duprop drives do hold up with Diesels.

[Seamoor]

Thanks for the encouragement. I'm actually quite happy with the speed and economy of the inboard Yanmar package, even though it clearly gives something up to the duoprop/jackshaft. At the end of the day, there's something to be said for simplicity and reliability, especially offshore.

[JuliaGCC25]

25 top speed

Seamoor:

How are you measuring your top speed? GPS or "paddle wheel"? My 25 w/ yamar 315 tops out at 3900 rpm and 27 knts.

Anyone think I should be going 1 -2 knots faster?

[Seamoor]

I'm measuring by GPS. In my opinion, you should be going faster by about 2 knots.