Centaur props

[bobturner]

i need to replace the prop on my westerley centaur.
and would like to take the oportunity to improve the over all performance of the boat.
Current setup gives me too much drive at minimum revs making close quarter maneouvers "Fun"
2000 revs gets me within a gnats of hull speed in calm water
Engine is a Volvo MD 11B which aside from a bit of smoke at full revs is as reliable and noisy as new.
current Prop @12" X 6"
Please Help

 

[srp]

There are quite a few prop size calculators around on the web, such as:
http://www.rbbi.com/folders/prop/propcalc.htm
There is also an Excel file called propcalc.xls - at least I think I got it off the web. If Google can't find it for you I can email you a copy.
It sounds to me that your prop is much too small - a 20' boat with a Yanmar 1gm10 would have something like a 12 x 9 fitted.

 

[bobturner]

thankyou.
I didnt think the prop "looked Right " myself but didnt really have a lot to gauge against

 

[Pye_End]

When you say that you have too much drive at low revs, what is your tickover revs set at?

On another note, 12 x 6 sounds too underpowered. Are you sure that the 6 part of this is correct? Is this measured or stamped, and if measured how did you do it?

 

[bobturner]

doesnt kick into gear till about 500 rpm

measured but in situ not on a datum surface

 

[Pye_End]

?500 rpm is pretty low for tickover. Would expect 850 ish. When you say that you have too much power manoevering is it at this 500rpm? Seems very low. Surprised you say you have too much power at this level.

Perhaps I asked the wrong question - what is the minimum engine speed when in gear - ie the time when you say you are over powered?

Propellor pitch - did you measure this by measuring the distance between the leading edge of one blade and the trailing edge of the next and multiplying by 3? If so, it is rather more complicated than that.

 

[VicS]

How do you measure the pitch of a prop. I've often wondered.

 

[Pye_End]

Well, how do I explain. I'll give it a go.

Measure the width of the blade at the widest part, as a ratio of the full circle - eg it may only represent 66% of the circle at that point.

Now take the already mentioned pitch - ie a 3 bladed is 3 times the distance from the front to the back of the blade.

Next multiply this up by the ratio of blade circle.

So if it covers 66% of the circle, and 2" front to back of a blade, the pitch would be 2 x 3 / 0.66 = 9"

 

[Rum_Pirate]

On (an outboard) prop the size and pitch is ususlly stamped on to the hub.

 

[bobturner]

500 rpm when in gear (according to rev counter)
and i did the font to back x 3 measure for pitch