Help with Prop Size on 31 Duffy w/370 Yanmar

[Shortlob]

Does anyone have this combination. I have a 2005 Yanmar 370. Im running a 22x20.7 4 Blade (No Cup) and am a underpropped at this time. What other sizes are out there. Thanks

 

[BillD]

Nothing wrong with being under propped !
Your Yanmar will thank you fit.

How much under propped is the engine?

1st things 1st are the WOT rpms you are recording as "under propped" accurate as is a "phot tach" or a "digital tach" ??

 

[Shortlob]

Hey Bill, Definately plan to photo tach the engine in a few weeks when I go back in the water. The prop has been changed around some with the prev owner and we think it came from ABC with a 22x22 but wanted to see if there are any other 31 Duffy owners on here with the same power. When Im finished I want it to hit 3400 or 3450 on the pins with the hand held tach

 

[Bill_N]

Sounds like you bought Huntley. Pretty boat..

Captainlarry had a 370 Yanmar in his JC. He might have some insight on this.

 

[captainlarry84]

On a guess and assuming the boat has a 2:0 gear the wheel should be anyone of the following:
22 X 22
23 X 21
Maybe a #4 cup.
If the boat is real light with no F/B & less that 200 gallons of fuel
Maybe a 23 X 22 #4 cup.
You need to photo tach. Yanmar panels are usually off by about 100 RPMs to high. Also always photo tach your boat with all fresh fuel filters. A dirty Racor or primary can cost you 100-200 RPMs.

 

[BillD]

Adding to Larrys post
Make sure the boat has full fuel full gear full water and as many adult guys you might caryy on a trip.

Then make your WOT runs. If you are under propped then perfect.
Leave the prop as is and leave 3-5% of rated hp on the table.
Your emngine will thank you in the long run.

Keep us posted