New Prop selection

[Gilaroo]

I recently installed a new inboard diesel engine 3YM20 21hp yanmar on my boat. Max rpm is 3600rpm at the crank and 1689rpm at shaft. Displacement is 1900KGs fully loaded, waterline is 16ft. I needed to change prop as previous prop was anti clockwise as old engine was a Volvo. I sought a lot of advice from various prop shops and on line and I bought a 13X10 prop after the guy I bought it from convinced me this was optium, unfortunately it's not. Having run the boat a WOT I am actually getting too many rmp, 4000 rpm which gives me about 7 knots. My goal is not to achieve better acceleration but is to achieve the best crusing speed I can get at about 3300=3400 rpm, hopefully 8.5-9.5 knots.

My instincts tell me that a 14X11 should probably best deal with reducing the 4000rpm to close to 3600rpm on the basis of 1 extra inch in Dia should reduce rpm by about 150rpm and 1 inch increase pitch should reduce rpm by a further 200rpm totalling in bringing 4000rpm down to about 3650 rpm closest to recommended rpm?

Anyone got advice on this ? Are my assumptions on relationship of increasing D and P correct ie 150rpm and 200rpm respectively. I really want to get the optimium prop for the boat and don't want to have to buy a third prop. Any advice would be welcome. Again my goal is improved speed for cruising as my fishing grounds are a good hour out in the bay so want to get there as fast as possible.

[Guest625101138]

The data I found on the motor shows different gearing:
http://www.maronier.nl/3YM20_TechData.pdf
This gives ratio of 2.21 and rated prop shaft speed of 1629rpm.

At 4000/2.21rpm, 7kts and a 3-bladed 13X10 prop the prop will absorb 12.8kW. Going to a 14X11 prop you could expect 7.4kts at 15.3kW and motor at 3690rpm.

Going for 14X12 will reduce top engine speed to 3450rpm and the top speed could be a bit slower because motor may be a fraction below full power. I expect 14X11.5 would be close to the best possible.

If the gear ratio is as you advise then the motor will be developing more like 14kW so there is very litlle room for higher top speed although you can reduce the engine rpm at top speed by going to the 14X11 prop.

Hull speed for a 16ft LWL hull is 5.4kts so you are pushing it well into heavy wave making at 7kts. Most of your power is going into making waves. If you want to achieve 8 to 9 kts economically you need a boat about 36ft long.

Rick W

[Gilaroo]

Rick

many thanks for that, you are right the 2.21 gear reduction will deliver 1629rpm at shaft not 1689, typo by me. Given the engine guidelines would be to run motor at generally 90% of top rpm i.e 3240 with occassional bursts of WOT , I think 14X11 might suit best. My understanding is that you should always try to get prop to match Max rpm as close as possible to get the most power from motor, so the 14X11 should get me there. The boat is a semi displacement which give me hope of improving on your correct calc of 5.4 knots for a displacement boat.

One other piece of info I came across is that increasing 1'' in Diameter is = to a pitch increase of 2.5''. If this was correct this would imply increasing by 1 inch dia = 2.5 X 200 rpm = reduction of 500rpm on my current situation = 4000-500= 3500 which is close to max 3600rpm. Your assumptions looks more accurate to me so I think 14X11 will give me the best option, I just want to make sure I don't get to a under reved situation with 14X11. Therefore 14X10 or 14X11 look like better choices ?