Propellers

[Michael Soloway]

I am seeking to replace the prop on my CD 31. Anyone have the specs on what might be ideal for a CD 31-36. I also have the original 2 blade prop that came with the boat in 1984. Is there any reason that that would not be a good choice? Since I have it - I would not have to buy anything and could just put it on the boat. The prop that was/is on the boat when I bought it last year is way too large. There is barely ant y clearance at the top and I want to replace it.



m@msoloway.com

[Tom]

You didn't say which engine you have, but if you have the Universal, you will probably find you're under propped if you go back to the two blade. Mine as I recall was an 11 X 11 two blade. It's been 10 years since I changed to a three blade larger prop for more push so I don't remember exactly. Most people want the most push they can get so they change to the largest prop they can fit in their opening with the corresponding reduction in pitch. I can't remember exactly but I believe I have a 13 X 9 three blade on there right now after trying about 4 different props. What makes you say the one you have on is too big? Do you have a lot more power than you think you need and have to run the engine at lower speed than the top of the power curve? If you have the Universal Model 25, the top of the power curve is around 1800 rpm. Can you make hull speed (more or less 6 knots) at 1800 rpm even in a little chop? People who change to larger props have usually found that they can't maintain hull speed in a chop even at higher rpms than the most economical. So they switch to larger props and go from the two blade to a three blade. No problem going back to the original two blade, but you will have a noticeable decrese in motoring performance as a consequence. If your current prop really is too big this won't be a problem for you. You could always try it for a year and see what you think. But don't throw the current one away, you might want to come back to it. Other engines than the Universal turn at higher RPMs so these remarks would not apply to other engines. JMHO

I am seeking to replace the prop on my CD 31. Anyone have the specs on what might be ideal for a CD 31-36. I also have the original 2 blade prop that came with the boat in 1984. Is there any reason that that would not be a good choice? Since I have it - I would not have to buy anything and could just put it on the boat. The prop that was/is on the boat when I bought it last year is way too large. There is barely ant y clearance at the top and I want to replace it.

TomCambria@mindspring.com

[Michael Soloway]

Universal 25. This prop is so big that there is less than an inch clearance at the top and I feel that it is making the motor work too hard. We have to motor at 2000+ to maintain 6 knots. The only reason to switch back to the 2 blade was to save money this year. There are a number of CD's in the boatyard - all up in the air - all with similar props - all about half the size of mine - but all 3 blades. Also I cannot get the nuts off the end of the shaft because there is no clearance for 2 regular wrenches

Thanks

[Boyd]

Hi Michael..

Sounds like someone put way to big a prop with not enough pitch on your boat. The size and pitch are usually stamped on the propeller somewhere. The pitch of the propeller may be way off. If you go to a three blade then the diameter should be less.

There are minimum clearances required between the propeller and the opening in the rudder necessary for the propeller to perform as designed. I think the tip clearance is 15% of the propeller dia. I am working from memory here and will check my references when I get home tonight. There are several texts (Nigel Caulder is one) which have formulas and tables which give a good place to start for prop sizing. Another good source is the propeller manufacturer's. Several web sites (Michigan Wheel) have calculators that will give recommendations also. Some of the propeller shops will also have computer programs that size props.

Just as a point of reference my CD 30 MKII has a 14x9 two blade. The Westerbeke 21A has a max rpm of 3200. As a general rule the crusing rpm should be 80% of max rpm (2500)and the boat should make hull speed at that rpm or less in flat water. There should be no black smoke (indicating excess fuel and the engine working too hard) at that rpm. I checked on getting a three blade and the recommendation was for a 12" dia.

In order to get the prop puller on my boat I had to turn the rudder full to one side. I am not sure what you are refering to when you say that there is not enough clearance for wrenches? Getting the nuts off was the easiest part of pulling the prop.

Boyd

Universal 25. This prop is so big that there is less than an inch clearance at the top and I feel that it is making the motor work too hard. We have to motor at 2000+ to maintain 6 knots. The only reason to switch back to the 2 blade was to save money this year. There are a number of CD's in the boatyard - all up in the air - all with similar props - all about half the size of mine - but all 3 blades. Also I cannot get the nuts off the end of the shaft because there is no clearance for 2 regular wrenches

Thanks

boyd@wbta.cc

[Tom]

If your engine is working too hard you'll be getting black smoke out the exhaust when you're cruising. If the plan to save money has to do with fuel consumption, you'll use less fuel with a two blade running at 1800 rpm, but you won't be doing any 6 knots. You might make 5 in calm water, in a chop less than that. With the two blade you might burn a tenth of a gallon less per hour so you'd have to run 10 hours to save a gallon, and don't forget you won't be going as far in that hour because with less push you'll be doing maybe 5 knots rather than 6 knots so you'll have to run longer to get to your destination. The fuel saving is going to be minimal unless you're planning to do a lot of motoring. You will, however, sail a little faster because you'll have less drag with a two blade. So it depends on the kind of sailing you'll be doing. If you're going to sail a lot and just motor a little at the ends a 2 blade might make sense. If you live in an area where you have to motor a lot to get to the sailing part you might be better off with a prop that has more push when motoring.

Be cautious comparing props on other boats. You have to verify that they have the same engine and the same size boat. The Univesal model 25 in a CD 30 is quite a different proposition than the same engine in a CD 31. 31 is much heavier and more wetted surface, thus a prop appropriate for a 30 wouldn't necessarily be OK for your boat. Other engines than the Universal turn at higher rpm so use a smaller prop. Thus be sure you're comparing other 31s with Universal engines before you make prop decisions based on what others are doing, otherwise it's apples and oranges.

I don't understand the nut problem. You push the rudder over to the side and lock it with the brake on the pedestal. You then have total access to the end of the shaft don't you? If you can't get the nut off how are you going to get the prop off?

Universal 25. This prop is so big that there is less than an inch clearance at the top and I feel that it is making the motor work too hard. We have to motor at 2000+ to maintain 6 knots. The only reason to switch back to the 2 blade was to save money this year. There are a number of CD's in the boatyard - all up in the air - all with similar props - all about half the size of mine - but all 3 blades. Also I cannot get the nuts off the end of the shaft because there is no clearance for 2 regular wrenches

Thanks

TomCambria@mindspring.com

[Michael Soloway]

Thanks for the help so far. This has nothing to do with fuel consumption which is negligible as an isuue. What I meant by economical is that I already have the 2 bladed prop because it came with the boat when I purchased it from the original owner - I wouldn't have to buy another one. The one on there is a 15/10. I got the nuts off. The stern nut it not just a nut - it is a whole long peice with a "nut" built in on it's forward end. Therefore you had to fit 2 wrenches into the area between it and the aft end of the prop to hold one and turn the other. Finally I used (with help) two of those skinny adjustable stuffing box wrenches side by side and it worked. I had to get the boat yard to pull the prop off, however. I understand that I will go slower with the 2 blade, and I do have to motor a lot where I live up the Hudson from New York Harbor. I operate at a maximum of 2000 RPM and I want to make hull speed at that RPM.

Any help on what hull spoeed is for my CD31?

Anyone with a CD31 and a Universal 25 recommend their prop exactly?

Thanks for all of the help.



m@msoloway.com