Repower cruisers yachts 360

[Richard Khawam]

Hello i have a 2009 cruisers yachts 360 express currently equipped with mercury 8.1s with zf63iv ratio 2.5 and 1.5inch shaft i am repowering with twin diesel yanmar 6lpa stp2 and keeping same gearbox. The boat came from factory as an option with the following setup: yanmar 6lpa with zf63iv ratio 2.5 and 1.75inch shaft my question is : should i upgrade my shaft from 1.5 to 1.75 or i will be safe keeping my old shafts?  

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[pwrobert]

When I re-powered with 6BTA’s at 355 HP each from 200 HP Detroit 8.2L engines and went from 3 blade to 4 blade Nibrals (22X20)I kept my 1.5 inch Aq19 shafts after getting different suggestions.  My 38 ft boat is likely heavier than yours and carries 300 gallons of fuel and 80 gallons of water – about 24,000+ lbs. Transmission is 1:1.75 so shaft RPM’s may be similar but also consider mine are diesel and yours are gas.  Not sure of the torque difference but usually the diesels have more and would put more “twist” to the shafts.

I run it normally and am not a throttle jockey and don’t slam into forward/reverse like I see some of the other fisherman do and have not had any issues whatever.

Shifting at the 700 RPM sugggested limit of my ZF220A’s is aso followed.  My cruise is between 2200 and 2450 RPM but mostly around 2300.  Running up and down seas puts a lot of stress on the running gear but this setup has never shown any signs of issues like loosening props, spun lock keys, etc.

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Richard Khawam

[Rob Schepis]

As Tony notes, If you do not prop “up” you will not need to up your shafts.  Based on my post I think that if anything you will be propping down from current specs.  If you up your shaft diameter you will need to deal with the struts unless upgrading shaft material to Aq22 to gain the needed strength for same diameter, check these out and run some numbers:

https://www.aquamet.com/sizes-and-diameters

https://www.pacificmarine.net/engineering/propeller-and-propeller-shaft/how-to-size-a-propeller-shaft.htm

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Richard Khawam

[Richard Khawam]

Ok great infos, thanks for the help. I will definitely keep my current 22 23 prop and we will figure out during sea trial what’s the best pitch to use. Hopefully i will not  have to change my props.

My other question is what do you think about my shaft Diameter? Should i upgrade to 1.75inch or keep my current shaft which is 1.5inch.

And in the event of changing to 1.75inch should i also change my shaft strut??

[Rob Schepis]

That spreadsheet is helpful.  Good to see it appears they were running with full fuel and water.  But only 2 persons and no gear, not even canvas as stated right on the report.  Add extra persons, canvas and all the various gear and personal effects and the fact that boats get heavier with age that 3,880rpm they achieved is only going to get worse.  And that 3,880 didn’t even satisfy Yanmar’s spec as per the sheet Tony posted.  You’ll end up at 23 pitch or less IMO.

[Richard Khawam]

I totally agree with the sea trial process before spending money on new props.

To be honest i contacted Cruisers yachts and they gave me the infos and nunbers concerning the same boat like mine but with yanmar 6lpa and from the factory they installed 22 25 so i am confused.

attached u can find the spec sheet

Really appreciate your help and guidance 

 

[Tony Athens]

You are getting bad info as to properly propping your 6LPA. You are close to 200RPM low on paper.     Your 8.1 is “more engine” than your 6LPA’s.. IMO, I would not touch your props until after seatrial and the boat has a few trips on it fully loaded. You may end up removing pitch.

[Richard Khawam]

Thank you for your reply.

I am upgrading my props from 22 23 to 22 25 to be able to achieve a max rpm of 3880 and a cruising speed of 21 knots at 3100 rpm with the yanmar 6lpa setup.

Currently with the Gaz setup I am cruising around 21 knots at 3500 rpm with 22 23 4 blades props

[Tony Athens]

If you are staying the with same prop, the load on the shaft will not change. If you “up” the prop, they you have to make a judgement  call. Another possibility would be to stay with 1.5″ and USE certified AQ 22 material as I doubt you vessel would have some with that plus new shafting always adds a safety factor.. 

What EXACTY are the specs of the props you have now? IMO, you may already be marginal in my book.

What RPM do you run at now to get your “happy” Cruise speed?

 

 

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Richard Khawam

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