Am setting up to build my own "taffrail generator" as like most things marine
, the cost of a commercial
unit is (in my opinion) way out of line with the cost of a diy version, not to mention way out of line with my budget
. The Solitaire is being refit
with the goal of absolute minimum power usage. Your mileage may vary. With only a small kicker
for aux power, some sort of electrical
generation is required. While solar
prices have come down to a level that makes their use as a power source while on the hook extremely practical, space requirements and susceptibility to damage make them impractical for use off-shore. I used a wind
powered generator for 15 years on board my previous vessel, but suffered usual complaints (noisy, poor performance at anchor
/ downwind, etc) but was better than running the diesel
. Having gone from a 14M vessel to an 8M vessel, no longer have the space for a wind generator
, and as max power use underway on the Solitaire is designed at less than 3 Ahr a taffrail generator seemed an obvious solution.
This is a work in progress. Proof of concept
prototype expected to be complete within 2 weeks for testing behind a friends power boat
. Will finalize design after that. I thought I would share the design here along with some additional information I have found during my research
that could answer some questions posted in this thread and elsewhere. Thanks to Eric onboard the s/v Sarana for his suggestions and pointers, as well as the folks at windynation.com for their DC motor
/ generator calculations.
The design is a "standard" towed prop configuration using 1/2" line to drive a permanent magnet DC motor as a generator. The components may change after proof of concept
Prop End Details and Calculations
The prop end is based on the design used by Everfair Enterprises as a modification to their wind
generators. The last reference to this I was able to find was in 2002 so seems to be no longer in production. I was able to find a great photo
which is located as an attachment at the very end of this post.
One of the most interesting aspects of this design is the "downrigger foil" which is mounted to a free turning bushing on the prop shaft. Eric says he can reduce his line length to 25' using this gadget rather than the 75-100' usually required to keep the prop from "skipping"
The prop used in this design is a 8 1/4 x 5 plastic prop. I am using an aluminum
prop, same diameter and pitch
. Prop is mounted backwards as you are towing it, not driving it. I picked mine up at a local outboard
shop, used, for $20.00
A short primer on pitch/rpm. The following calculations will allow you to determine the rotational speed (in rpm) of a towed prop of a given pitch (in inches) Diameter doesn't figure into rotational speed, but anything over 10 inches is going to be hard to handle and stow, and anything under 8 inches will probably lack enough power to spin the generator under load. Prop pitch = the distance through the water the prop will move in one revolution
rpm/knot of towed propeller
= 1215.36 divided by pitch of prop in inches
Example: 5" pitch prop 1215.36 / 5 = 243 rpm/knot
So our proposed prop being towed behind a vessel travelling 4 knots will (in a perfect world) spin at 942 rpm
(243rpm/knot times 4 knot). Real world factors such as slip and drag will reduce this theoretical output. How much? my WAG is about 20% leaving us with ~800 rpm
Motor / Generator End Details and Calculations
A permanent magnet DC motor makes a great little generator when you spin it. Unfortunately while e-bay used to be a great place to find these motors cheap
, now days every PMDC motor even close to suitable is marked as "perfect for a wind generator" and priced accordingly. They are still not terribly priced and with some searching you should be able to find a suitable candidate in the $40-$90 dollar range. BUT!
the key word here is "suitable" . Its really pretty easy to determine if a given motor will give you what you need, but if the seller does not provide enough info to plug
into the following calculations, search on .... Especially problematic with "treadmill motors" Widely available in the $30-$40 dollar range they rarely have the rated rpm at rated voltage as they are designed as a variable speed motor. While some are very well suited for our needs, some are absolutely useless. No rpm = no buy.
That being said, here is how to figure out whether a given motor will work as a suitable generator. (Thanks again to Windy Nation )
First assumption is our generator will need to put out at least 15 Vdc at 700 rpm (speed our towed prop is turning at a little less than 4 knots) So how to we translate the motor specs DC voltage DC rpm into that information?
Divide DC voltage by rpm.
As an example if we have a motor that says on the nameplate, 100Vdc 3000rpm. its volts to rpm ratio is 0.033 (100 divided by 3000)
To determine its output at 700 rpm (our above prop towed at a little less than 4 knots), (700 rpm) x (0.033 Volts/rpm) = 23 Volts
In the real world the motor when being used as a generator is only 80-85% efficient so we correct the above by multiplying 23 x .8 to arrive at a real calculated output of 18 Vdc at 700 rpm.
This motor would be suitable for our setup.
Generally what you are looking for is a high voltage, low rpm motor. Anything with a voltage/rpm ratio above 3 should be good. Below that, your mileage may vary
Amperage Rating This is a quote from Windy Nation's article. It pertains to wind generators but also applies here. The higher the amp rating the larger (physically) the unit will be.
The next item is the amperage rating of the motor. This provides information regarding how much current the motor will put out as a generator. From our experience, it is very difficult to predict what type of current your motor will put out as a generator. We’ve seen motors that expel more amps than that for which they are rated. However, one thing remains true: The higher the amperage rating, the better. You should be looking for a motor with a minimum amperage rating of at least 5 Amps. Anything above 5 Amps and you are good to go.
The power that a wind generator produces is directly proportional to the amps and voltage:
Hope this helps someone else out there, open to comments as to somewhere I might have gone seriously astray... Picture of old Everfair Enterprises/FairWinds system is below.
Onboard s/v Solitaire