Prop size and pitch O'Day 30

[James Rohr]

Does anyone know what prop size and pitch should be on a 30, 1979 vintage w/Yanmar 2Qm15? mine orig had a 2blade fixed 16/10. changed to martec 15/12 which is what they recommened. if i try to get any higher rpm out of motor than 2300rpm all I get is a lot of black smoke. engine acts like its being overloaded. Orig I attributed this to a very tired engine. however last season when the old girl gave up the farm, i found another motor completly rebuilt her and quess what. same old problem. at 2300rpm she moves her at 6.5knots but ocassionally I need more. I.e your late getting to the starting line and need her to move at 7.0knts. from the hp.curve in yanmar book it appears that a 15/10 or 15/09 should be on this engine. by the way motor is rated 15hp@3000rpm. at 2300 its rated at 10.5hp. should this be operating higher on the curve? what props do other sister ships have on them? Sure could use some feedback. THANKS.

 

[Justin - O'day Owners' Web]

Props

James -The waterline lenght of your boat is 25 feet, 5 inches. That gives a theoretical hull speed of about 6.75 knots. If you are finding that you are getting 6.5 you're almost there. It is going to take _a lot_ of engine to get to 7. Before you go too crazy with props, is the bottom immaculate? Is it fair? As you start to approach that hull speed these things really do start to matter. It does sound like you're boat is somewhat over propped - the rule of thumb I was taught to use is that you should be pushing hull speed at 80% of maximum revs. If your redline is 3000 then that would be at 2400 rpm. Almost right. You're only about 4% perecent low on the revs. If you were to change to the 09 prop, you would be making a 25% change in pitch. That's a lot and may throw you out of the rev range you want to be in.I would suspect that the reason the engine acts overloaded when you try to push beyond 2300 is that it is becoming overloaded - but by the boat, not the prop. Are you sure the speed you're reading is correct? Could you really be going more like 6.75 knots? Is that bottom perfect? I'd address that first - better for winning races anyway. What is the boat like dockside? Does the engine rev enough for close quarters maneuvers? That information would be helpful to a diagnosis.Justin - O'day Owners' Web