Prop Depth

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Shawn Gibbs - 16 Oct 2003 07:34 GMT

Could anyone tell me or point me in the direction of info regarding how to
determine the optimal & minimal operational depths for props?

Warm Regards
Shawn

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D MacPherson - 16 Oct 2003 12:59 GMT

Shawn:

The answer to that question depends on the application - type of propeller,
power loading, operating speeds. Can you tell us a little more?

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
http://www.hydrocompinc.com
tel (603)868-3344
fax (603)868-3366

> Could anyone tell me or point me in the direction of info regarding how to
> determine the optimal & minimal operational depths for props?
>
> Warm Regards
> Shawn

Reply to this Message

Shawn Gibbs - 19 Oct 2003 01:31 GMT

>The answer to that question depends on the application - type of propeller,
>power loading, operating speeds. Can you tell us a little more?

I'm playing with the base design for a hydrofoiling semi-wing-in-surface-effect
craft; electric, solar-electric, hybrid-electric; semi-kayak-ish main hull with
proa-like outriggers. The motors (probably Minn Kota but haven't ruled out
using more efficient motors used in electric vehicle competitions) will be
located at the base of either V-type wave-piercing or T-type fully submerged
foils, so part of my current task (and the basis for my admitedly
vague/nebulous question) is to make sure that the props stay at a functional
depth as the foils rise. Current design allows me to vary the depths of the
foil-prop assemblies. I'll ask the question again after I've figured out how
light I can safely make the frame/hull, how many batteries I can pack and still
stay safely buoyant, which will finally lead me to foil, motor and prop
choices.

Warm Regards
Shawn

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matt colie - 20 Oct 2003 23:20 GMT

Shawn,

Boy, you gadda set don n' figur o' What cher doin.

A propellor will stay working until it aspirates (sucks air in from the
surface). This is not a real big problem for trolling motors and not
even a big issue to IC outboards until SHP gets up there (<~50). Not to
mention that you are trying to mix three completely different animals.

Now,
Are you investigating 1-Wing in ground effect, 2-surface piercing
hydrfoil or 3-fully submerged foil with active control??

The motors of electric powered cars are not a lot of horsepower, but the
starting torque is real good and that almost does not matter to the
propellor. Electric machines tend to be "made out of heavy" and
batteries won't help you out there either.

I think you might think about this a little longer.

Matt Colie
An ex-engine professional, licensed marine engineer, naval architech and
builder of very strange things

>>The answer to that question depends on the application - type of propeller,
>>power loading, operating speeds. Can you tell us a little more?
[quoted text clipped - 14 lines]
> Warm Regards
> Shawn

Reply to this Message

Shawn Gibbs - 21 Oct 2003 23:12 GMT

>Boy, you gadda set don n' figur o' What cher doin.

Wish that were the first time I'd heard that! :)

>A propellor will stay working until it aspirates (sucks air in from the
>surface). This is not a real big problem for trolling motors and not
>even a big issue to IC outboards until SHP gets up there (<~50). Not to
>mention that you are trying to mix three completely different animals.

It gets worse. The design is a prelim to making it submersible.

>Now,
>Are you investigating 1-Wing in ground effect, 2-surface piercing
>hydrfoil or 3-fully submerged foil with active control??

Yes! This version is a hydrofoiler, probably surface-piercing for control
simplicity's sake, using a WIG structure on the struts as an adjunct only. The
wing-foils are inflatable (think closed-cell parafoil/parasail) and will use a
foil profile incapable of actual lifting the craft from the water (true WIG)
except under high wind velocities when the wing would be deflated. With the
foils inflated, the craft will only be practical on fairly smooth water unless
equipped with fully-submerged foils due to the changes in angle of attack that
would result when it encountered a swell. The wingfoils are also intended to
act as hydrobatic diving planes for the submersible when that time comes.

>The motors of electric powered cars are not a lot of horsepower, but the
>starting torque is real good and that almost does not matter to the
>propellor. Electric machines tend to be "made out of heavy" and
>batteries won't help you out there either.

My goal is not a high top-end velocity, but one of efficient use of batteries
... eventually with the goal of reaching a dive site using the least amount of
energy and without having to have a surface tender. In that respect, EV motors
are a good choice in terms of weight and efficiency. For that matter,
everything has to be considered in terms of weight. When it comes time to
consider the final transition to submersible, I'll unquestionably have to
convert to a hybrid electric due to a relatively substantial increase in weight
(ballast and mixed-gas/rebreather tanks, etc.).

>I think you might think about this a little longer.

Me thinks thou art being kind and tactful. I'm of the mind that I'm going to
be thinking (and dynamic modelling and calculating and recalculating and ...)
about this one for a *lot* longer! :) Truth be known, I'm still not completely
convinced that it can be done as it's currently envisioned -- especially since,
as you so poignantly pointed out, I'm melding several different
technologies/principles not normally mated (and possibly for good reason).
But, since I love designing for designing's sake, that's okay. Sometimes
science is advanced as much from the mistakes/failures as it is by the
successes. Although I very much prefer success, it's still just different
sides of the same coin to me.

Warm Regards
Shawn

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Backyard Renegade - 27 Oct 2003 16:47 GMT

> >Boy, you gadda set don n' figur o' What cher doin.
>
> Wish that were the first time I'd heard that! :)

<SNIP>
Sometimes

> science is advanced as much from the mistakes/failures as it is by the
> successes. Although I very much prefer success, it's still just different
> sides of the same coin to me.
>
> Warm Regards
> Shawn

Hey shawn, I think it is a great idea. Watch this forum for info on
skin on foam construction. Very light, very strong. Also maybe
consider a mast type appendage with the ability to generate power
during dive time. Like a solar panel and a windmill? Maybe even put a
couple of mirrors and tube in there and have a periscope?? Sounds like
fun. I remember how much sh.t I took last year with a boat with
negative rake and tumblehome shear, now there seem to be a bevy of
boats coming from the larger designers with just this "look".. Don't
let em' get you down, just keep buildin' it.
Scotty from SmallBoats.com

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Brian D - 27 Oct 2003 17:10 GMT

It's funny how the ones that are resistant to change (or anything new) whine
the loudest and longest ...I'm not *even* going to bring up the topic of
traditional boat building versus 'composite' ...doh!!! I appreciate all
kinds, and enjoy all kinds.

Brian

> > >Boy, you gadda set don n' figur o' What cher doin.
> >
[quoted text clipped - 18 lines]
> let em' get you down, just keep buildin' it.
> Scotty from SmallBoats.com

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Donald Phillips - 16 Oct 2003 14:34 GMT

> Could anyone tell me or point me in the direction of info regarding how to
> determine the optimal & minimal operational depths for props?
>
> Warm Regards
> Shawn

Thanks an interesting question. Submarines have been known to go fairly
deep and the props kept working. Maximum down angle is usually
determined by the engine specs, somewhere around 5 degrees, but then
hover craft and helicopters have around 90 and their props work. You
should try to maintain about 2 3/8 to 2 1/2 clearance from the hull.
Other than that, most props will work to some extent. On the more
serious side, David Gerr has written a good bit of information on props.

Good Luck,

Donald

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Boat Forum / Building / October 2003

Prop Depth