Requesting opinions on 4-blade v 3-blade props for 27FC

[DCatSea]

I'm toying with the idea of replacing the OEM 3-blade prop on our 27FC with a 4 blade prop. We have the Peugeot Lehman 4D61 engine, and I would be amazed, and possibly concerned, if we exceeded 9 knots, so speed is not a deciding factor.

We intend some longer cruising in Spring pf 2016, and I am wondering if a 4-blader would be smoother, less "revvy" and hence quieter than a 3-blader, for normal cruising, and if there would any effects on fuel consumption, handling at lower speeds and issues when running close to WOT (etc.)

Also: "Mazboot" is a 1984 27FC without a skeg under the prop. Has anyone retrofitted one of these? If so, any tips on best ways to do it would be welcomed. (Going to do some gunk-holing in the Bay, and would like the protection a skeg provides.)

If you have experience with these 2 modifications I would love to hear from you.

Thanks AOGers everywhere.

Doug

[tego]

Doug, I run a 18x14 4 blade, but I have the nissan engine, so can't comment on your particular setup. I know the 4 blade is smoother than the 3 blades I checked out when I was looking at 27's a couple of years ago and was a definite consideration on the positive side, on my purchase. Some of the guys on here have done the rudder/skeg mod on their earlier models. Hopefully one of them will pipe up. Good luck, Ben

[DCatSea]

Thanks Ben,

Your comments are most welcome, and useful. I am beginning to think that 4 or 3 blades doesn't have too much effect on performance when cruising, but does make a difference in maneuvering; apparently 4 blades have great walk-ability which takes some getting used to but is useful in tight situations. A smoother cruise is also very attractive. I think I will back-burner the switch for now, but keep it in mind, for next season.

The skeg modification is a definite, so I look forward to hearing from those who have made the change.

Doug

[Beta Don]

Propellers are usually spec'ed by efficiency - Most people want the best fuel mileage possible

The most efficient prop is a two blade and that's what you usually find under sailboats with small diesels. If you have the room (diameter) to use a two blade prop with the proper pitch for the engine/trans ratio/hull, then you'll *usually* get the highest efficiency with a two blade prop

When you don't have the room for a two blade with a reasonable pitch, you move to a three blade and give up a little efficiency in order to get a workable pitch within the constraints of the available diameter. Each time you add a blade, you crate extra turbulence which lowers the efficiency of the blade which follows it through the water, so *usually* you don't add another blade unless you don't have the room (diameter) to fit a prop with a workable pitch on the boat

Another 'golden rule' is . . . . larger props turning slower are more efficient than smaller props turning faster. If you have room, you'll gain efficiency by fitting a larger prop. If your 4D61 equipped 27 is like mine, it came with a 15RH11 prop. Not the 'ideal' prop for the boat - I assume Albin got a super deal and bought a bunch of them. If this 15 X 11 is more or less 'correct', (WOT gets you right at the engine recommended max RPM but no more) then if you want to reprop, I would suggest another 3 blade, but a larger one. A 16 will fit easily and even a 17 will still give you almost (but not quite) the recommended 15% clearance between the blade tips and the bottom of the hull. I have a 17 on mine and the blade clearance is 2 1/4" which is between 13 and 14% of 17. 15% is recommended but you can go a bit lower - I know of 27FC's using 18 inch props with no adverse symptoms

http://www.yanmarhelp.com/i_propclr.htm

Adding an inch of diameter is roughly equal to adding an inch of pitch, so if you were really happy with the performance of your 15 X 11, then a 16 X 10 or a 17 X 9 would give about the same performance, but with a slight increase in efficiency - You're pushing a larger column of water. If you want a bit more low end grunt at the expense of high end horsepower, you could add an inch or two of diameter and keep your 11 inch pitch - You probably would not be able to push it to the max recommended engine RPM (and it might smoke a little if you tried to) but you'd have better 2500 RPM performance *and* you would then get your normal cruising speed at a lower engine RPM, which is what it sounds like you're looking for

The one case where you might consider going from 3 blades to 4, even though you have plenty of room to fit a correctly pitched 3 blade of suitable diameter is . . . . if you find your 3 blade prop is 'noisy' or 'thumpy' - If that's the case, a 4 blade will be bit smoother and quieter, although with a slight loss of propeller efficiency

http://www.propellerpages.com/?c=articl ... _of_blades

I added the skeg to my '84 FC and made some modifications to the rudder as well. If you'll PM me your e-mail, I'll send you a couple pictures and a brief explanation of what I did to mine

Hope this helps,

Don

[sail149]

Don ,
What a lot of yes full information you have really got a good handle on this.
My boat came with 2 props , can't wait to check out how what I have compares.
I think for me max performance is what I would want since the efficiency is so good anyway ( or am I confusing efficiency with fuel economy).

Doug, re skeg. I don't think the skeg was fitted to the Albin to improve the protection for the prop but most likely to simplify construction. The early skeg less boats have a much stronger and reinforced rudder post/ stuffing box so it is not needed to strengthen the rudder. The skeg models have a much simpler rudder mount.
Will the skeg improve the ability to deflect crab pots and the like...... I doubt it will make much difference.
There must be other things you could better spend you time working on.
Any body have real life input on the need for the skeg on the Albin?...there are hundreds of boats out there with completely exposed props.....???

[Beta Don]

I installed mine not to protect the prop - It's up behind the keel and fairly well protected as it is

When I had my rudder removed to add some to the leading edge, I was really surprised to see how thin the rudder tube is. Coming from Sailboats, I've never seen a tube this thin on a rudder which had no bottom support. The rudder draws the same as the keel and I could just picture what would happen in thin water if I was backing up and grounded the rudder - Instant bent rudder shaft I fear, so I added the stainless skeg to strengthen the rudder

Later model FC's used a metal rudder with a good bit of it forward of the shaft - A more balanced rudder than the early models with their huge fiberglass 'barn door' rudders. I added 4 inches to the leading edge and tapered it more to a point (the OEM rudder leading edge is very blunt) so my modified rudder should present a little less drag, plus adding to the leading edge and balancing the rudder will make it turn in a tighter circle

The skeg is a 2 inch by 1/4 stainless flat bar with a piece of stainless 3/4 angle welded on the top side of it. It attaches to the rudder with a long stainless bolt I epoxied in the bottom of the rudder (I cut the head off the bolt and drilled a larger hole in the rudder, filled it half full of epoxy and dropped the bolt in the hole) with washers and a stainless aircraft nut. It's through bolted to the bottom of the keel with 3 bolts spaced a couple inches apart with a wooden backing block coated in epoxy on the inside of the keel - You can just barely reach down through the deck plate on the floor of the aft cabin to get the nuts on the top of the bolts. My boat doesn't have an inspection/drain port on the side of the keel like some of you guys do

Don

[DCatSea]

Thanks Don,

That's pretty much what I had in mind. I'm very glad to see that someone else has added a skeg so successfully, and has come up with a way to attach it to the rudder. Do you have other pix or a sketch? If you want to email me, please see my profile.

Also, thanks for the great input on prop sizes. Very useful indeed.

Doug