Repowering? Bigger is better?

[Andrew Troup]

Originally Posted by Terra Nova

The prop seems an unnecessarily complex project for something that can be so simple, and is so easily damaged.

Maybe I didn't explain it well. It's essentially a fixed 3 blader, with removable blades, where the blades can be inserted at the desired pitch.

Compared to a folding or feathering or (especially) a VP prop, it's dead simple, with no moving parts, but variable pitch all the same, albeit not in real time

It would be Rather Difficult to arrange real-time VP with a retractable drive


With modern techniques it's relatively easy to manufacture such an item:

- 3D print wax masters direct from two only solid models, including integral male and female splines

- use those masters directly to investment-cast metal blades (including some spare blades) and hub

- clean up flanks of spline for propshaft, assemble.


Individual blades can be replaced in case of damage, even offshore, without anybody getting wet.


With a 3 blader, access holes for the allen wrench needed to tighten or remove a blade are spaced between the other two blades; the cap screw for retaining the blade in question being short, ie on the same side of the central bore as the blade is. Fairly neat, it seems to me.

I'm hoping to find someone to cast it in 2205. I'm not sure 316 would be a great option, because even retracted within the hull, the prop would normally be submerged. (I would prefer to avoid bronze if possible - alu hull, etc)

One fringe benefit is that it's quite difficult to get the pitch right, purely by calculation, for the system I propose. This way, I don't have to.

[Terra Nova]

Stu--If something like the last 20% of the charge is the most difficult to replenish, that 20% will be larger on the larger bank. Furthermore batteries self-discharge; the larger bank loses more to self-discharge.

Andrew--The engineering and custom fabrication is one thing, assuming you really want a project. Obtaining spares in remote areas is another. It isn't obvious to me that such complication provides enough practical value.

[S/V Alchemy]

Originally Posted by Delancey

One thing I notice when looking at the performance graphs is at that 1500-1800 rpm range, which is also well below peak torque, any of these engines is running at maybe 1/3 max hp when used as generators and they typically do so for a very long life at those fixed rpms.

That was part of the logic of going with a 60 HP with a double PTO: the "sweet spot" for fuel economy is 1800-1900 RPM, at which I can expect to be driven at 4.8-5.2 knots in flat/cats' paws water with no sail assist for about 1,100 NM with my current tankage.

Which I deem "enough".

What remains to be seen are the respective forward and reverse pitches I need to set for maximum economy forward and maximum stopping power in reverse, the two motoring situations I typically encounter and desire.

If I was redlining it in a light boat to a Wednesday night race around the buoys, I would go with a folder on a turbo-assisted diesel as small as I could manage. I wouldn't expect it to last as long as one run 90% of the time in the same RPM band, however. My experience of diesels is they like to be left running once running, and they prefer to be left at about 75% of WOT.

[S/V Alchemy]

Originally Posted by Delancey

Awesome, thanks for the breakdown! The OEM prop on my boat is 18" but I think I might have room for more, other than cost or clearance are there other factors that limit prop size?

Distance to the hull is a factor. I have heard that tip clearance should be either 10% or 15% of the total distance to the hub.

I have a Gori two-blade folder and, as stated, a VariProp four-blade feathering prop. The Gori is on a 1970s IOR-style racer and is an 11.5 x 8 replacing a 12 x 6 two-blade fixed connected to a direct-drive Atomic 4. The improvement is huge in terms of sailing...I have actually needed to modify the sail changes at various TWSs because the boat visibly accelerates better in a puff. The downsides are the "pause" while shifting from F to R, the loss of the top half-knot of motor speed (which I never used anyway) and the slightly gutless stopping and reversing power.



The feathering prop sits in an aperture formed by the end of the keel and the transom-hung rudder. I was able to go from fixed 18 x 13 three-bladed to feathering 19 x 15 (nominal pitch) four-bladed, which should more than match eight extra horses and the slight loss of thrust per hp Mr. Cate mentions.

But particularly when one is sailing a barge-like tank as we will be, I feel that the feathering aspect under sail will prove even more effective a drag reducer than the folder has on the more racy hull.

[Delancey]

Thanks to all for the feedback and the info. The price came in on the 30 at $10k, a savings of one boat unit when compared to the 38. I also received a price for a 16" feathering prop that Beta sells at around $2,500.

At this point the decision goes on the back burner for a month. Dimensionally, the 30 and the 38 are the same height and width and differ only in length by 6" for one being a three banger and the other a four.

What is interesting to me is that my Universal 5444, also built on a Kubota block, is only slightly larger (1/2" in any direction) than the Beta 38 but weighs 170 lbs more. I am not sure where they are hiding it.

With regards to props, I have spent pretty much all my life, with the exception of a handfull of deliveries or a couple of occasions going cruising with friends, sailing racing boats that all had folding props so fixed props are kind of a new thing to me.

I confess I haven't ever given that much thought to props in general. The Flexofold has my eye, btw. Anyone have experience with the 2 blade version?

I think Andrew's prop sounds like a great idea that has a lot of things going for it, even for someone who doesn't have the option for retracting their drive. Seems like anchors, or engines, or like many things on boats, what's available is often arbitrary when the particular needs of are not.

Andrew's design, based off of two simple parts, allows for a truly one-sized-fits-all system. Granted, the draw-back for most people is the huge PIA factor in the fine-tuning but once accomplished the benefits would be forever.

On the execution side of things, ceramic shell investment casting is capable of of very high definition and accuracy and is at the same time a relatively low tech process that is fairly common worldwide. 3D printing, while being high tech, is become more available and by it's nature is inherently well suited for local manufacture.

I have some experience casting bronze and aluminum, but never steel. Without seeing the part, I would be cautious about balancing the required mass of the tapered shank against the fineness of the blade with concern for shrink back, but would otherwise estimate the project as having a very high probability of success.

I am definitely looking forward to seeing some pictures of the finished product!

[malbert73]

I have a 4 bladed variprop as well. Think it's great. Nice to be able to tweak pitch. If I were to do ICW for example, I could dive with a hookah and adjust pitch up 1 inch.
Difference under sail is huge. 1/2-3/4 knot upwind to my estimate in moderate breeze.
One really nice reason is not having to worry about how to lock prop. Freewheeling is faster but I hate noise and higher chance of snagging lines