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Old 04-08-2006, 17:56   #1
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Propeller generator

Does anyone out there use thier free spinning prop to generate power? I have an old paragon transmission that I believe is designed to freewheel when sailing(Cascade 42 with perkins 4107). There is a pulley attached to the shaft that leads me to think that my boat was once so equiped.
I would like to do a few test to see the effect of linking a generator, how much speed lost vs power generated. Does anyone have an idea as to what size DC motor or generator I should be able to spin with a 19 X 12 LH prop. Also, what is the simplest (cheapest, least problematic) way to supply this power to the battery?

Any info on this subject is welcomed
Steve
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Old 04-08-2006, 18:39   #2
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Towed genrators are a serious power source. I should think they would be a better solution.
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Old 04-08-2006, 18:52   #3
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Aloha Steve,
It used to be a pretty common practice. Do you let your prop spin while sailing? Does Paragon say that it is ok to do that? Shaft bearing and seals are the things that would suffer much from the experience if the transmission is self lubricated and doesn't depend on input from the engine for lubrication. If it were me, I'd start experimenting with small DC motors or old car generators and see what performance you can get out of them.
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Old 05-08-2006, 15:39   #4
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Free wheeling the prop is a function of the transmission. Some can and most can't. Many messages posted here on the topic. You really have to check with the transmission manufacturer. My Hurth is a put it in gear and in reverse when sailing type.
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Old 05-08-2006, 16:57   #5
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I understand that the boat's freewheeling prop will slow you down more than one locked in place. Some people have had problems with their trannies.

It was common to tow a prop on a line with sail boats. The rope twisted a small generator on the stern of sail boats. People reported that they worked well. One unhappy sailer had his generator prop bitten off the end of the rope by a large shark. Since then, people paint their props black to reduce the chance of loosing them.
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Old 05-08-2006, 18:23   #6
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A prop slowing you down locked or not, is a subject of debate depending on several factors. Firstly, the efficiency of the prop when water is pushed against the front profile of the blades. In a different description, then the engine is driving, it woudl be how efficient the prop is when in reverse. The secon is the amount of torque required to turn over the gear box. If there was very little resistance, then the prop would turn over with ease and little drag would result compared to a large torque load, causing the prop to drag through the water more. A prop locked solid still produces a certain amount of drag. But the blades are in a stall mode and thus tend to slip through the water. As soon as the prop start to spin, a point in RPM is reached where the water flow over the blade becomes such that the prop comes out of a stall mode and the effect of lift is created. Once that happens, the prop can take a lot of energy.
I have even heard of someone that can actually jump start their engine by getting a decent sailing speed up and dropping it in gear.
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Old 07-08-2006, 12:14   #7
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Thanks for the feedback, however, my question was specifically about a shaft driven gen. My understanding is that paragon transmissions are no longer made so, info straight from the horse's mouth is not available. As the model transmission in my boat was built without a keeper on reverse ( you have to hold it in reverse, by design) the choices are freewheel or forward. I have been told that this model is designed to freewheel. ( A repairman quite familiar with the model) Assuming that, I can let it spin or harness the power. I think I would opt for a windgen over the towed water model, but if I could slap a small gen on the shaft, low cost electric even if only a small amount. If anyone has specific info on the really old paragons, I'd love to hear from you.
Cheers
Steve
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Old 07-08-2006, 14:22   #8
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Aloha Again Steve,
You can do it. Get an old auto generator and hook it up. You'll have to adjust pulley sizes on your generator to get the most out of it but it does work. Many folks used to do it while cruising. I'd opt for small cheap old generator to start with and see how it works.
My old boat had a towed propeller hooked to a generator that worked well but I was always afraid of loosing the prop to a snag or toothy critter. I lost a Walker knot log that way and didn't want to chance loosing a bigger piece of equipment.
My concern about your proposed system is cutlass bearing wear when it is freewheeling and shaft seal bearing wear on the side you'll be tightening a belt for the generator while freewheeling. That's why I'll opt for solar.
Let us know how the experiment goes.
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Old 07-08-2006, 14:58   #9
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I have to pipe in on the whole debate about spinning props slowing you down more than ones stuck in place (aka in gear). I have to strongly disagree.

I tried this after reading this (or the other) thread. I was cruising along at 7 knots, and decided I'd try the experiment. CLUNK! I put it in reverse to stop it from spinning. Probably a stupid idea while underway(sailing, motor off), but it went in.


The boat immediately lost just under 2 knots of speed. No joke.
I lost something like 1.8 with my prop fixed, vs not fixed.

Thought I'd share this real life experience with you all to clear up the mystery.
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Old 09-08-2006, 12:47   #10
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Steve, I think the definitive answer FOR YOU about whether a prop will slow a boat down, is going to depend on the prop (2 or 3 blade? etc.) and the aperature.

In rough rough numbers, you can only pull so many horsepower from the prop shaft before you stall it out and the generator becomes a friction brake. Since I'm not an engineer I"m guessing here...but let's say you have a nominal 18hp engine and a maximum speed under power of 6 knots.

To me that indicates you can pull about 18hp off the shaft before you lock it up, probably well less. let's say, maybe 1/3 of that (6hp) would take 2 knots off your speed. One hp is about 750 watts, so at 1500 watts (14.4v @ 100 amps) you'd lose two knots.

I think in reality you might turn a 40-50A genset while losing those two watts although I wouldn't be surprised to be off by 2x either way.<G>

Ideally you would want a 2- or 3- pulley wheel on the shaft, s the alternator could sit to one side, and an idler wheel could sit on the other side to counteract the side load on the prop shaft. Having that pulley machined up in a machine shop, so you could clamp it down over the shaft (and maybe have two lock screws going into the shaft from opposite sides as well) is probably a $150-250 machine shop job.

Size of the pulley...dunno, you'll have a limited space to work in, but you'll need to check the shaft speed when you are sailing, and then ratio that to the shaft speed your alternator needs, and compromise on the size of both pulleys to get the alternator turning at the speeds IT needs. Most of them put out next to nothing under 1000 RPM, so typically you'd want a really small alternator pulley and a large prop shaft pulley.

If you run those numbers--for the pulley ratios--and check the space you have available around your prop shaft, you may find a conventional alternator just can't be "geared up" enough to work in this application.

Anyone selling kits on the internet?

Or, how about just towing one of the purpose-built water generators that are sold for sailboats? I'm afraid to ask if you could ust stick a larger prop on a MinnKota (electric trolling motor) and clamp it over the transom. AFAIK those are 12VDC motors that shoudl work as gensets, but I don't think they'd live long or prosper in this application.<G>
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Old 09-08-2006, 17:56   #11
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Thanks for the feedback. This is not a project I'm set on doing. The boat already has a pulley on the shaft and I'm at a loss as to what else it would be for. There is no sign of a shaft brake or generator now, only the pulley. So, I thought I should explore possibilties. First I would need to calculate the rpm's then look for something small that would have enough output at low rpms to be worth it. However, if a suitable generator could be found that would keep the batteries topped up and power nav lights and basic instruments while underway, that would be nice. The trade off would be how much speed for how much power. (Un)Fortunately I have plenty of other projects to keep me busy, so this may remain a "what if"

Cheers
Steve
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Old 22-09-2006, 02:01   #12
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hi,

coming in late on this, but if you google prop shaft alternator you'll get some good material. also on fixed versus free-wheeling prop drag.

in the process of setting up such a system on my own boat and so far so good. can provide details if you're interested.

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Old 22-09-2006, 02:26   #13
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A DC generator will tear up the brushes if turned in reverse for very long or fast.

An altenator will work in both directions but needs an excitor and enough speed to produce.

The side thrust to keep the belt tight on the prop shaft could cause excess wear on the shaft at the packing and possibly wear the shaft coupler if not pressed on.

............................................_/)
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Old 22-09-2006, 02:52   #14
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hi again,

usually the drive pulley is bolted between the coupling flanges and my engineer is quite happy that any sideways thrust should not prove a problem. hope he's right.

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Old 22-09-2006, 10:35   #15
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NIck,
I would love to have any insights you have gained from your project. By the way, Ocean Navigator had an excellent article on a cruiser with this set up for 20 yr or so. You can email me a kirvens3@yahoo.com
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