Yet another propping question

[DCatSea]

I currently have the bog-standard 15x11 on our Lehman 4D61 / Hurth 2:1 drive train.
I may have found a 16x14 bronze prop in excellent condition.
I am not of a mathematical bent.
SO
Can anyone advise on whether repropping to the larger dia/pitch would have any significant effects on performance - GPH, maneuverability at low speed, cruising speed etc?

Thanks

D

[dkirsop]

Adding an extra inch to the diameter of a propeller is the mechanical equivalent of holding your boots up at the end of a broom stick. Or think of a figure skater holding the arms out to spin slowly and tucking them in to spin fast. Your boat performance will only improve if your original prop was undersized to start with. In this case you will be increasing both diameter and pitch so don't be surprised if you can't bring your engine up to full RPM.

A company like Michigan Wheel can probably tell you if you are already reasonably matched. You also need to check if there is clearance for a larger prop.

[motthediesel]

Ah, a prop discussion -- let the games begin!

With any such question, you first must consider what kind of boat you're dealing with. I would say that propping is much more critical on a planing hull than it is with our full-displacement type boats. I have a lot of experience with planing outboard boats, even racing outboards, and even a tiny change in pitch there can make a huge difference in performance. With our A27, I find that it is not nearly as critical.

Our boat has the same engine and gearbox as yours. From what I've been able to find, these boats came equipped 15" dia props, with 11" or 12" pitch. When we got ours though, it was swinging an enormous 19" x 12" three blade. The boat preformed well enough with that wheel that the previous owner had used it that way for years, including his original delivery cruise from Portland ME to Kingston ON by way of Long Island Sound, Hudson River, NY Barge Canal, and Lake Ontario. The downside was vibration, caused chiefly (I believe) by too little tip clearance to the hull, and by a slightly bent shaft.

The up-side of that big egg-beater was that the boat would cruise at 7kts at only 1800 rpm. This engine is pretty loud at 1800 anyway, but it is way louder at 2400 or so, where you would probably have to be with the original sized prop for the same speed.

I decided to try a little less prop, and it just so happened that I had a 16" x 16" RH 1-1/4" (from an old PennYan tunnel drive) shaft on hand, so we gave that a go last year. We were pretty pleased with it, here are some numbers for RPM, speed (slack water), and exhaust gas temperature.

1400 -- 5.4 kts -- 400F
1500 -- 6.2 kts. --450F
1700 -- 7.1 kts. -- 490F
2000 -- 8.1 kts. -- 550F

The engine feels busy and strained at 8 knots, and I would never drive it at that speed for long, even though the EGT reading is quite moderate, and we saw no sign of black smoke. At 1700 though, it's quite a pleasant shipmate, loud, but not annoying. As to it's effect on fuel efficiency, I don't know. It would be fun to do a series of runs with a graduated fuel vessel to see how several different props compare in that regard.

Anyway, I think you would do just fine with that 16" x14", and I would like to see what numbers you would get with it.

[DCatSea]

Looking through the forum, there are many, many thoughts and near-academic essays on appropriate prop size for the 4D61, ranging from 18xScary to the OEM 15x11 and all ports in between. So - I've bitten the bullet and bought the brand new, unused brass 16x14 (for $123.00) and intend to install it this spring, just to see what happens when it makes contact with the drive train and the Bay (What can possibly go wrong? (FLW ).
I'll keep the OEM 15/11 and if it all goes pear-shaped or if Mazboot becomes uncontrollable, I'll revert, and have a lovely shiny brass paper weight.
Watch this space.

[motthediesel]

That's great Doug. When this pesky winter finally ends, you should do some runs with that new wheel and record some numbers -- I'd like to see them and I'm sure others would too.

Tom

[DesertAlbin736]

16X14 is near the same diameter and two notches steeper pitch than what I have on my A25. Will be interesting to hear how that works on a larger boat with a more powerful engine.

[DCatSea]

I will be sure to post some figures once I get them. I'm particularly interested in the effects of the new driver on low speed maneuverability in close quarters, and any lessening of noise levels at the usual cruising speeds. My regret is that with the current 15x11 all the data is in my head, not recorded.

[DesertAlbin736]

The up-side of that big egg-beater was that the boat would cruise at 7kts at only 1800 rpm. This engine is pretty loud at 1800 anyway, but it is way louder at 2400 or so, where you would probably have to be with the original sized prop for the same speed.

It's an apples to oranges comparison since the discussion here revolves around the A27 and not A25, but for us it's been serendipitous that we got the boat we have equipped as it is with small 24 HP Yanmar & AquaDrive CV joint/thrust bearing setup. Our initial exposure to to Albin boats back in 2012 was a 1978 A25 DeLuxe that had been repowered with a 2005 vintage 40 HP VP engine. I don't recall the model number of that motor now, but when we got the owners to take us out on a check ride during purchase negotiations we were shocked at the noise and vibration levels such that we couldn't run it above idle. The vibration issue had something to do with the engine installation, either motor mounts and/or shaft alignment since I think it was the first time the owners had it out since the engine had been installed. But it still would have been loud without the vibration problem. We ended up walking away from that deal. Our current boat on the other hand runs very quiet and nearly vibration free at our normal cruising RPM of 2,350, quiet enough at that speed to hold normal conversation at normal voice levels. That other A25 had no name and sold to an out of state buyer soon after we looked at it and we never saw it again. Another issue we had concerns about was how low the exhaust outlet was as can be seen in this photo.

As an aside, if anyone recognizes this boat as having come from Arizona and has a two burner HillerRange propane stove with oven installed in the galley I'd be interested in hearing about it.

Albin (1280x960).jpg

[Dieselram94]

The up-side of that big egg-beater was that the boat would cruise at 7kts at only 1800 rpm. This engine is pretty loud at 1800 anyway, but it is way louder at 2400 or so, where you would probably have to be with the original sized prop for the same speed.

It's an apples to oranges comparison since the discussion here revolves around the A27 and not A25, but for us it's been serendipitous that we got the boat we have equipped as it is with small 24 HP Yanmar & AquaDrive CV joint/thrust bearing setup. Our initial exposure to to Albin boats back in 2012 was a 1978 A25 DeLuxe that had been repowered with a 2005 vintage 40 HP VP engine. I don't recall the model number of that motor now, but when we got the owners to take us out on a check ride during purchase negotiations we were shocked at the noise and vibration levels such that we couldn't run it above idle. The vibration issue had something to do with the engine installation, either motor mounts and/or shaft alignment since I think it was the first time the owners had it out since the engine had been installed. But it still would have been loud without the vibration problem. We ended up walking away from that deal. Our current boat on the other hand runs very quiet and nearly vibration free at our normal cruising RPM of 2,350, quiet enough at that speed to hold normal conversation at normal voice levels. That other A25 had no name and sold to an out of state buyer soon after we looked at it and we never saw it again. Another issue we had concerns about was how low the exhaust outlet was as can be seen in this photo.

As an aside, if anyone recognizes this boat as having come from Arizona and has a two burner HillerRange propane stove with oven installed in the galley I'd be interested in hearing about it.

Albin (1280x960).jpg

Looks as if it may have had a issue with the rudder skeg? Looks as if something is clamped together?