27FC Prop sizes - Again

[DCatSea]

I know there have been hundreds if not thousands of posts on this topic, but could I ask one question after a statement of requirement?

I am perfectly happy with Mazboot's current performance both for fishing trips, close quarters handling and no wake zones/rivers etc.. Last winters prop reconditioning and balancing seemed to work minor miracles in these regards, and with the "apparently OEM" 15x11 RH 3-bladed propeller attached to the standard Lehman 4D61, she bops happily along in open water at 6.5 - 7 knots at ca. 2500 rpm. Fuel usage is in the 1 - 1.5gph range. At 3500 rpm, done more for kicks than anything else, she can race up to 8.5 knots (wind and tide notwithstanding). All is well in "Mazbootland".
BUT
I would really like to cruise at 8 - 9 knots at 2500rpm (or less???) with an emergency burst of speed to 10 - 11knots (Phew!). All this without affecting the good points mentioned above (fishing speed, close 1/4s handling etc.) I am aware that the hull design is not "built for speed". As you may have gathered - We are not speed freaks.

And finally, he gets to the question:

Do Albineers have any (preferably bullet pointed) suggestions on whether or not I can put a different prop on to achieve my modest needs? Maybe a cupped 15x11 or cupped ?x?. As many of you know, there's not a lot of space between the prop and the hull.

[dkirsop]

Going faster will require more power so it will require either a higher rpm, if your engine has sufficient power to turn higher revs, or you will need an engine capable of putting out higher power at the design rpm of 2500 if this is your design point. There is no free lunch.

With a displacement hull boat the power requirement increases proportionately with the cubic power of the speed increase. In your case an increase from 7 to 9 knots is a factor of 1.3 so your power requirement will increase by a factor of 2.2. In other words, to achieve your stated goal of 9 knots power would have to be increased by 220%. Either that or get a planing hull.

[hetek]

My 27FC was a near carbon-copy of yours.

6 knots all day long and a top speed of 8.4 kts at 3750 rpm. The poor 4D61 had an outrageous redline of 4,000 rpm but I think it wouldn't live long there.

I seriously looked into a repower but curcumstances told me otherwise. I was happy enough at 6 kts.

And for all you young kids out there, carbon paper was used to make copies with a typewriter.

...And a typewriter was like a mechanical word processor, but it didn't do much "processing".

[TorreyWP]

Doug, our '85 27 FC fits a 18" diameter prop easily with about 2" between the blade and the hull.

Prop diameter translates too how much load the prop can move effectively.

In your situation, I would try going up on both pitch and diameter if you can. figure 150-200 rpm(up or down) for each pitch increase or decrease but make sure after the switch that you can still read your engine's max rated WOT rating or it is being lugged at all rpm.

Im not convinced cupped blades would be effective on our displacement type hulls as those are typically seen on high speed (30knot +) boats to reduce cavitation.

Prop-ing is a real rabbit hole....

[motthediesel]

Ah, prop talk...

Our boat (same engine, 4D61) came with a giant 19x12 three blade wheel. That translated to 7.2kt at 1800 rpm. That was great, but with tip clearance from the hull of only just over an inch, it made for a very shaky ride.

As luck would have it, I just happened to have a 16x16 RH wheel that I bought off our local C-list years ago. I think it was from a PennYan tunnel drive We've got that one mounted now, we'll see & report how it works.

Tom

[DCatSea]

I think 19x12 would be interesting; somewhat shaky, and perhaps not a little scary. I'll look forward to see what you think of the 16x16. I'm thinking of a 16x12, just to perk things up a little without being accused of being a speed demon.

Also don't want to overload the Mighty Ol' Lehman Peugeot, perhaps just P it off a little to keep it on its toes. At the moment we are the best of friends and it's a relationship I value and nurture, (No, it's not looking over my shoulder as I write, but.....)

[motthediesel]

I have also fitted a exhaust gas temperature gauge, so we’ll see what effect this new prop has on engine loading. I sure do want to overload the engine!

[DCatSea]

Be interested to know which exhaust gas temp gauge you fitted.

[motthediesel]

Well, we've had the boat in the water for almost a week now, and we have about 7hrs steaming time on the clock, so a report on the 16x16 prop is in order. To refresh, this is on a 1985 27FC with it's original Lehman 4D61/Hirth gears.

Overall, we are quite pleased with it, we can cruise at about 6.8 kts @ 1600 rpm, with reduced noise and much less vibration than previously with the 19x12 wheel. Now, some of that is likely due to the straightened shaft and the added intermediate bearing, but it's all to the good anyway.

Here's a link to a 10 second video clip of the gauges: https://youtu.be/Obr4fV13GCU

You will note the SMX exhaust gas temperature gauge is showing only 450F. Even at 2000 rpm/8.2 kts (max speed) it only goes to 600F, and with no visible black smoke or coolant temp rise, so I don't think this prop is overloading the engine in any dangerous way. You will note that the GPS is showing 7.2kts, but we are running down stream there. Up/down average at those revs is about 6.8 kts.

So my worries about excessive pitch with this wheel seem unfounded, but I do miss the "kick" that we got for maneuvering with the former 19" prop. Too bad that one just didn't have enough tip clearance to the hull. Of course, there is a good prop shop near me, so I could see about them taking an inch off the diameter of that one. Is there ever any end to prop guesswork?

Tom