Power Driven 'Freewheeling Prop'

Well, that or a single large stepper motor. Though of course hand wheels are mondo retro sexy.

Pulleys can be chromed.... Hmmmmm....

I should confess, I've joked that the way to stay out of pictures is to have the best camera hardly anyone knows how to use. My camera is a Mamiya C330S TLR for medium format film.

 

Agree.
My 20 ton 50' planing boat,at about those speeds uses about 30l/h at about 900 rpm IIRC.

A friend's Sunseeker Manhattan 74 can ghost along at 7 knots or so at about 60-70 l/h.

 

20tons/50'/30ltr at about 900revs/6 kts would look like about 0.9 mpg.

74' at 7kts/70ltr is 0.46 mpg.

Sounds about right

 

8.5 knots or so.
At 7 knots she's about 3 mpg. From 15 to 30 knots on plane,stays at about about .6 mpg.

Yes,SS is about .5 mpg. But at 28 knots =

I mis-typed that one.
IIRC the pounds of drag from what I've seen is not a lot of difference 30 vs 20...but it's not just the prop that's involved,opposite lock on the rudders to stay straight should create more drag.

Again,on my boat and friend's boats-get better mpg running both rather than one.
Better would be to swim under and take one off

 

Id like to throw the Cehrubini 45 into the mix.

Someone on the trawler forum is repowering his cummins 6c 450 single with only 1000 hrs on the clock for a smaller engine. It makes a nice comparison to the Gb42 and St 44.

http://www.trawlerforum.com/forums/s32/my-short-haul-refit-19105.html
Post No: 9

The Cehrubini is nearly identical in length and dipl, and is a similar semi-d hull form to the Gb42 and St44.

The whole idea was to convert a big twin engined boat to run on one engine and a 'powered freewheeling prop' to gain single like efficiency.

But...
The Cehrubini 45 is a single big engined 450hp single with S/D hull and only manages 'less than 2mpg at 7 kts.' ( this guy is spending a fortune to change a perfectly good engine to get better mpg with a 100hp replacement)

Some questions:

How does the new design swift trawler 44 with twin 300hp engines manage 2.8 mpg at 8 kts and yet the Cehrubini only 2mpg?

 

The fuel numbers on the st 44 must be a misprint.
you would expect a 300hp Volvo at 1500 to use at least 2-3gal/hr.

So a twin would use at least 4gal/hr at 7kts and less than 2mpg; so should read 1.8mpg not 2.8mpg.

 

Velvet Drive and several Hurth don't do well freewheeling.

 

I think I can help resolve that.

Displacement is a big part.

But also beam to length and draft to beam, both of which affect wave making.

The Cehrubini seems like it has, or would have if the bow were plumb (which it probably isn't), an LBR and BDR ratio of about 3. The lighter boat looks like it could actually have a 3:1 LBR ratio but also a 4:1 BDR. So it is a much more svelte boat ... a me in college vs a me now sort of comparison.

 

If all you want to do is spin the prop at a no thrust speed a modest DC motor should do it.

All you would be "paying" for is the skin drag of the prop surface , a couple of sq ft at best.

 

I like this idea; have you got a ball park idea of how many watts would be needed?

Is there a formula to work out how much power is needed to cancel out the drag of a freewheeling prop?

 

There's gotta be some rules of thumb used by people who combine sail and power.

 

yup, but what you really need to do is tell us exactly what you have (engine, tranny, shaft size, and prop specs), precisely how you plan to operate the boat (the number of hours at each speed per year), and any performance data you have on one and two engines using the current arrangement.

You may be entirely missing the biggest problem and devoting your resources to an approach with very limited potential. Unless you show us exactly where you are starting from, it is impossible guess what the benefits may be vs spending the money elsewhere.

For instance, operating on one prop doubles the thrust coefficient of the working prop, and that might wipe out the gains from other things.

Suppose you put a feathering prop on one side and get the drag down to a couple hp at the speed you want to cruise at. Suppose the twin props were operating at 75% efficiency. Operating singly, the efficiency might drop to 60%. It would be very difficult to make mpg gains facing that kind of headwind merely by improving the fuel map operating point.

So as a starting proposition, I think you should plan on replacing both props no matter what you choose to do if you want to see any substantial gains. Knowing the details of your drive train and what props would fit would be needed to hang numbers on the cost effectiveness of what you want to do.

 

What we need is the numbers for a twin with one prop removed, and then compare it to a freewheeling prop to see what losses in drag there is at fixed revs up through the speed range.

Then it would be possible to work out how power the freewheeling prop would need to cancel out drag?

 

If the pony motor can make 3% of the diesel's peak torque at whatever rpm equals 0 thrust, you would be looking at something like a 6:1 ratio of the amount of power reduction at the prime mover vs the amount of power consumed by the pony motor. Which says absolutely nothing about the comparison between running two wheels vs only one wheel. These are VERY general sorts of numbers.

<<edit>>So in the above example, if the efficiency of the pony system was 66%, you would see a savings ratio of 6 - 1/.6666 = 4.5.

 

Thanks. My interest is a practical one.

I was comparing the a Mainship 34 mk1 at 6tons 34' by 12' beam that can make 15 kts cruise with a cummins 270 hp 6b unfortunately not available inEurope, with a Broom sedan 35' by 12' beam of similar vintage which weighs about the same but has twin Perkins 145hp engines.

http://www.newsnow.co.uk/classifieds/boats-for-sale/broom-35-sedan-for-sale.html

I know the MS 34' will chug along on a couple of gallons at 7kts.

I'm unable to establish the fuel consumption of the broom 35' twin 145hp, so I was thinking of ways to overcome the drag on the second prop.