You are right. A variable speed prop is always an advantage. But why we don't see them is more about economics. Ask your buddy what that variable speed prop of his costs relative to a fixed prop.
I have seen variable pitch props on outboards, but it isn't common. Mercury used to make on that adjusted all the way from reverse to full forward via a lever. My assumption is it never got popular because outboards are normally made to be the most inexpensive way to power a boat. And because outboards operate over a large rpm range, they can use variable rpm instead of variable pitch to deal with the problem of getting power to the water. For reverse, there is either a gearbox or else the whole outboard can be reversed.
An outboard is all about cheap and simple. It turns out that using a fixed pitch and varying the rpm is cheaper and easier than varying the propeller pitch.
Now an inboard diesel is more elaborate (and expensive) setup and more like the Cessena. An inboard diesel has a smaller usable rpm range but has torque to spare. For that setup there are lots of variable pitch propellers on the market and quite a few boats use them. That's when a variable speed prop is the obvious solution in spite of the expense.
That's how I see it. But I think you are correct in that a variable speed prop is always an advantage. And they are out there, just expensive.
Roger L.
----- Original Message -----From: Kirk McLorenSent: Friday, May 31, 2013 8:59 PMSubject: Re: [Electric Boats] Trolling motors ==> electric outboardsI remember taking a ride in a Cessna 180. That variable pitch prop made a huge difference.Prop failure in a boat isn't as catastrophic as an airplane so why don't we see them?
From: Dennis Wolfe <wolfeboats@gmail.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Friday, May 31, 2013 5:19 PM
Subject: Re: [Electric Boats] Trolling motors ==> electric outboards
A trolling motor that produces, say 50# thrust at zero boat speed would produce zero thrust at 6 or 8 mph. Double the prop pitch and thrust at zero speed will drop while thrust at 8 mph will increase. Motor power is the same regardless.
On Friday, May 31, 2013, danbollinger wrote:To a physicist, they are very different concepts. Thrust is a rotational unit of force. Power is a unit of work.
Energy / Time = Power
--- In electricboats@yahoogroups.com, John Francis <surv69@...> wrote:
>
> I don't understand how someone can say THRUST is not power, since the
> THRUST that an engine, along with gearing and a propeller, is the wash that
> is produced as yourr boat moves forward . . . and of course, what we're all
> looking for is to move our boats forward.
>
>
>
> On Thu, May 30, 2013 at 10:48 PM, John Riehl <riehlj2002@...> wrote:
>
> >
> >
> > By contrast, I attempted to use a 36 lb thrust MinnKota on a 1,800 lb 22'
> > MacGregor. It would move the boat, but couldn't push it against anything
> > more than about 10 kts of wind.
> >
> > *From:* Skip von Niederinghausen <farharbour@...>
> > *To:* electricboats@yahoogroups.com
> > *Sent:* Thursday, May 30, 2013 6:49 AM
> >
> > *Subject:* [Electric Boats] Re: Trolling motors ==> electric outboards
> > **
> >
> > Maybe I should mention to the group that years ago, I used a Min Kota
> > 24lb thrust trolling motor on my International Folkboat which was 26'
> > LOA and displaced about 5000lbs. This was in Annapolis Maryland where
> > there is little tide or currents. The motor was great for getting off the
> > dock at my house, then under the Eastport Bridge and out into the bay. I
> > used her for only overnight trips in the area and this system worked fine
> > for this limited use.** **Skip**
> > **-- **"The sky hides the night behind it and shelters the people beneath
> > from the horror that lies above."** -- Paul Bowles<http://www.goodreads.com/author/show/7659.Paul_Bowles>
> > ****
> >
> >
> >
> >
> >
>
__._,_.___
| Reply via web post | Reply to sender | Reply to group | Start a New Topic | Messages in this topic (54) |
.
__,_._,___
No comments:
Post a Comment