what size prop to use

[whiskey wings]

Changed my engine from a gear reduction A 4 to a direct drive A 4. Never changed the prop so now when I'm in gear can get no rpms. Prop on now is D 16 P 10 . Any recommendations on what size prop is needed . Size of my boat 36 Columbia 1968

[Al Schober]

Most direct drive A4s take a 12" prop with 2 blades, pitch about 7". Pitches were higher in the past (9" or so) but current thought is to use a lower pitch for more RPM. Some folks are using 3 blade props - perhaps one of them will give some insight.
Not sure what direct drive will do for a 36' Columbia - that's a pretty heavy boat. Doubt if you'll be thrilled with the results. Why did you change from the reduction gear?

[joe_db]

Second that - you are going to suffer some loss in power, maybe a lot.
I wish I had a reduction gear, my 35 foot boat is pushing the limits of the A4.
That said, 13x7, 12x6, 12x7, and 12x9 two blades have all been used on A4s. I am happy with by 10x7 three blade Indigo and I donated my 13x7 to another Afourian and he is happy with it.
I would highly reccomend staying with the reduction gear. If you can't, try the Indigo prop.

[whiskey wings]

Changed my A4 gear reduction because of cracked block and Trans bolts were striped. When I bought the boat this was one of the problems. Any chance I can take this prop to a shop and have it fixed?? . Thanks for the info will try to find some different props to try.

[joe_db]

No - you will need a new prop.
I would start with this:

http://www.atomic4.com/propeller.html

[ndutton]

Do you still have the old engine or is it long gone? I ask because if it's still around, the reduction gear can be grafted onto the newer direct drive engine, you can use the original prop and benefit from the greater torque.

[joe_db]

This will give you some idea of how much power you will be losing. Most of us max out someplace between 2200-2700 RPM with direct drive.

[edwardc]

Loss of available power aside, conventional propeller wisdom is that you want to swing as large a prop as you can fit as slowly as you can in order to achieve maximum efficiency*.
The reduction gear lets you operate your A4 higher up in the power band while still spinning your prop more slowly. A further benefit is that the torque loads on the engine are lower with the reduction gear, reducing bearing wear and increasing its lifetime.

My 12,800 lb Pearson is quite happy with a 3-blade 15RH8 prop and a 2:1 V-drive. It pushes me to 5 kts @ 2000 RPM in flat water, but allows me to get up to 3100 RPM at WOT. This gives me a nice power reserve for storms and such.

* Hydrodynamics for Ocean Engineers , Prof. A.H. Techet

[marthur]

If you want to stick with the prop and power you had originally, I have a reduction gear that I would be willing to part with for a very reasonable price. Email me if you are interested: zephyr4 at comcast dot net.

[Mo]

I have a 12x6 three blade on a C&C 30...it will push her 6.2 kts, flat calm, and she does well in choppy seas. 12x6 or 12x7 3 blade are about your best options to stay in the power band...I don't think it will push a 36 ft Columbia very well at all and I think you'd be lucky to get up to hull speed on a good day, if it will do it at all. In a stiff breeze my thinking is that she won't hold her with the direct drive. My advice is consider getting the reduction gear back in there because you can spend money trying props and still be no further ahead.

[Mo]

I have a 12x6 three blade on a C&C 30...direct drive, top speed 6.2 kts, flat calm. She does well and can hold 3-5 kts in choppy sea though, open ocean. 12x6 or 12x7 three blade are about your best options to stay in the power band...I don't think it (direct drive) will push a 36 ft Columbia very well at all and I think you'd be lucky to get up to hull speed on a good day, if it will do it at all. I think you will notice serious loss of boat speed in a stiff breeze and she might not even make way. I've seen a C&C 34 with the wrong prop that could barely get off the dock and was just terrible to try and do anything with on a good day...and that was with a reduction gear and wrong prop. My thinking is consider getting the reduction gear back in there because you can spend money and still be no further ahead...you need to know she will hold her own if you need it.

[lat 64]

Dubya dubya,

Let us know just at what stage of the repower you are at because Neil is right, the old reduction gear can be fitted to the new engine with a only bit of trouble but not as much as with eliminating it completely. That job would need:

1 new prop
2 new shaft (down to 3/4" dia. and longer length
3 new shaft seal
4 new cutless bearing

I general, a bigger wheel(prop) is always a tad more efficient with all other factors being equal. This is true with our Col. 36.
The Col. 36 does just fine with a 16x10 or 16x11(mine). We have the luxury of having a big place to swing that wheel. I did a lot of reading, mostly Dave Gerr's Propellor Handbook, and it turns out that our boats were just fitted quite well in the first place. Restoring the original setup would be the best.

Mine bites good, and because it is aft of the rudder, backs like a forklift.(that's a good thing)

Russ

[capnward]

I like my Indigo prop (3-blade, 10x7.5). My boat is 33 feet and about 12,000 pounds. At 1950 rpm I can get 6 knots on calm water with a clean bottom, vacuum around 7. Hull speed (6.7 knots) might be achievable at higher rpm, haven't really tried that. I doubt that it would go past 6.2 knots. I have a heavy boat with what may be a tired engine. Compression is good, though. With a 2-blade 12x7 prop I could get maybe 5.5 knots at 1750 rpm, wouldn't go higher without working the engine too hard with little increase in speed. They say higher rpm makes for more power to push through chop and wind. Or a reduction gear, your call.