overloading

[Cape Codder]

Rob,
H&H reduced the cupping when they reconditioned. I have to examine the glued-on label on the prop to see what the cupping is.
You can visually see where they trimmed it.

[halfmoon]

So, at 3800 rpm, Max is turning his prop about 530 rpm faster than those of us with the factory Yanmars. Shouldn't there be a prop out there that would deliver close to 30 kts for the rest of us?

[Cape Codder]

For our Yanmar 315's somewhere around 27 knots may be our upper limit.

RPM should be 3950. Our gear reduction is 2:1. If we could achieve a prop slip of 5%, which is VERY good, you would need a prop pitch of about 19.5". So far, with a prop of that pitch, I have been unable to reach the 3950 rpm. I need to go DOWN in pitch to somewhere near 18.5" to achieve that optimum rpm. Using a prop calculator, if I can get to 3950 with my re-conditioned H&H prop, AND achieve 5% slip (unlikely), then the top speed will be 28.5 kts.

I'm thinking that 8% slip is more realistic (and not bad!) at 3950rpm, a solid 27 knots is possible.(top speed)

It is not a question of horsepower. The Yanmar's may just be shy of the Torque needed to twist that prop enough to hit 30 knots.

Then again, if I could achieve 30 knots, I would probably do that <1% of the time. Cruising at 18-20 knots suits me just fine. My goal is to get there WITHOUT lugging or overloading that sweet Yanmar.

[Serenity]

Bob, the 3950 RPM rule is like securing the Holy Grail. I purchased a new prop, because the original prop that came with the boat in 98 had metal fatigue and had to be reconditioned. I was convinced to go with the new prop because of the 3950 rule. The current wheel was "tuned" twice before I could achieve the magic number, which I have yet to attain. I think the design engineers at Yanmar, were on a Saki run when they decided on the performance specifications for the 315. I have decided that the current performance is great and there is no need for further adjustments. The only issue to address is fuel consumption, which I won't have a handle on till June. Like yourself, I normally cruise at 3100-3200 which gives me 22-23 knots. The engine sounds the same, with no apparent laboring and all instrument readings are normal.

[Cape Codder]

Max,
That sounds great. Fuel consumption numbers will be interesting.

Perhaps those Yanmar engineers are giggling right now!!!

When I splash, I'll see what she spins up to. I was concerned with soot accumulating on the hull near the exhaust, which may be an indication of lugging.

If I can cruise at 18-22 and be around 2mpg......I'm happy.

[RobS]

I have decided that the current performance is great and there is no need for further adjustments.

Knowing your EGT would be good. I don't think you have a pyrometer, do you?

[Serenity]

To Cape Codder, no soot only steam. In regard to Rob's comment about a pyrometer, I do have a Fluke laser temperature instrument. Next time I go out I will measure the heat of the raw water manifold. Anything under melting and lava temps should be ok. Right? Oh, what is a EGT? Does that have anything to do with placing on hand on the manifold while I am running, and my hand won't come off?

[ScubaPete]

EGT is the exhaust gas temperature. It is a measure of how hard the engine is working at any time. It is measured just after the exhaust exits the turbo (or sometimes at the exhaust manifold). I don't know about the Yanmars, but on my Cummins, it should be a max of about 900 degrees F. Higher temps than that indicate that the engine is struggling and working too hard, ie overloaded.
A pyrometer uses a temperature probe to measure this. A external infrared laser temp gun will not give an accurate reading. The temp on the exhaust after the mixing elbow should not be excessively hot and you can put your hand on that.... but check with the laser first.

[Serenity]

I am planning to go out today, definitely tomorrow with my tester. Will publish numbers when I am done. I believe my Fluke will read that high. I seem to recall at the Mac Boring class, they mentioned that the exhaust system was in the range of 800-1000.

[Serenity]

After having the prop tweaked one more time in the fall, and posting my speed vs rpm's I have run through over a 110 gallons of fuel. I first filled up on May 9, I took on 52 gallons and my fuel consumption was 4.33 per gph. She wasn't quite filled to the brim when Serenity was put on the hard. On May 22 I took on 63.3 gallons and my fuel consumption was 3.55 per gph. For the last several weeks I have been fishing which included idling between drifts, which accounts for the low gph. Nevertheless, I cruise at 3100, which I find to be the sweet spot, and achieve 20-21 knts. Water temp, fw side is a160-170 and the sw side, manifold is warm. No black smoke only steam. I expect an EGT to be installed shortly. Bottom line the prop does well. On a side not, in the last two years that I have owned the Albin, I have burned nothing but ValTech fuel from the same ferry dock in Bayshore. The fuel is super clean because they ferry dock runs three ferries to Fire Island, that cruise at 22 knts, and they can't afford to have downtime.

[RobS]

Sounds good. I s/w Tom at B&S a couple weeks back when I had my shaft checked and new coupler fitted. We talked about Serenity. He insists that the Acme prop would give me 2 knots over my current Michigan wheel. A demo prop would be nice.

[Serenity]

I welded Serenity's prop on to the shaft to prevent UDT divers from absconding with my 3 bladed honey.