Yanmar 6LYA 370 Boost Question

[michael sanchez]

Hi Tony/Corey,

Thanks for all of the great information that you post online. It’s a huge help for us.

We are working through a prop issue on a straight inboard center console (29′) with a 6LYA 370 that we bought used recently and have been refitting and had a question regarding boost. Some background:

– ’04 6LYA 370; 1.96 gear ratio
– Boat had A LOT of black smoke when powering up to cruise and when powering on the trailer.
– Bought EGT and Boost gauges from Seaboard (woo hoo!).
– EGTs at cruise prior to prop work were around 1100-1150 when loaded.
– Slight black smoke at cruise (and on transom).
– The prop was stamped as a 19 x 25 and had only 1/2″ clearance from the hull.
– After reading many of Tony’s posts, we decided to borrow a digital tach to confirm the accuracy of the Yanmar tach. At the dock, Yanmar tach was about 10% over (Yanmar: 1000 – Digital: 890; Yanmar 2000 – Digital: 1830; Yanmar 2500 – Digital 2248).
– Yanmar tach was showing about 3400 at WOT (while underway) which converted to 3060 actual; didnt test WOT at dock.

So, we knew the boat was overpropped. Had the prop scanned, and it was actually a 19 x 27.5. Had the prop repitched and resized to 18 x 25 which is the size that the prop shop recommended based on our specs and info provided and the exact size that another party has with the same boat, motor and tranny ratio.

– we lost about 2-3 knots at cruise and top end with the reconditioned prop, but MUCH less black smoke at start up, no black smoke with powering up on the trailer, boat feels smoother/quieter, EGTs down to 950-1050 at cruise, and now have some clearance between the prop and the hull.
– Tach showed 3600 at WOT which converts to 3,240 assuming the same ratio as above applies at WOT.
– Boost at 2800 (actual) cruise was about 18 psi.
– Max boost we saw was 24-25.

We are considering changing the prop to a Michigan DQX which has more surface area and we’ve been told by several that increases performance (the other guy with the same boat has an 18 x 25 DQX and is getting a few knots more than us), but were wondering if we should see higher boost (Yanmar says max is 32). Unfortunately, I didnt take notes of the boost with the larger prop. Prop shop says the prop is a cut down from an even larger prop and has a larger diameter hub than is normal for a prop that size which reduces blade surface area and efficiency.

Any thoughts and feedback would appreciated.

Thanks

Topic

[Tony Athens]

Boost is what it is…..If it were me, I shoot for an accurate 3400-3450 at WOT and use a AMCE prop.. IMO, way better technology …………….

Tony

1 user thanked author for this post.

michael sanchez

[michael sanchez]

And should I be concerned with not seeing more boost at WOT, or is lower boost ok with the correct prop?

[michael sanchez]

Thanks Corey. What is the “Tach Setting”?

[Corey Schmidt]

Have you referenced your actual numbers against the spec sheet?

See attached…

[Author]

Replies