Capt. Neal® wrote:
As a sailor you should be more concerned with trimming sails.

I can do that, too.

DSK

Perhaps you did not realize that Doug is a just a motor boat driver now.
He has some sort of motor boat with nasty osmosis problems -he's
posted lots of pictures of it.

Cheers

Capt. Neal® wrote:

As a sailor you should be more concerned with trimming sails.

Leave the motors to the likes of Captains Shen and otn.

Captain Neal
(a member of an elite group)

"DSK" wrote in message ...

Farr1220 wrote:

Doug,

There's no need for the complication of variable pitch.

Well, sure. There's no "need" for the boat at all.


... A reversing
motor (electric or hydraulic) is all that is needed.

And that's simpler than a CPP?


... Hence for this
size vessel you see symmetric 4 blade props.

Sometimes 5 blade.


... The small increase in
efficiency gained by having variable pitch is not worth the extra
complication.

Bart wasn't asking about efficiency, he was asking about
controllability. An electric drive may be on par with the controlled
pitch for quick & easy reversing, but they're much less common... the
ones I know about have a bad reputation for being troublesome.

As for a hydraulic motor, if you think these are efficient and
trouble-free then I suggest you get some experience with them. It would
be at the bottom of my list for almost any power application.

Actually CPPs are slightly less efficient, ideally, than conventional
props becuase of the bulky hubs. However, in real world conditions, they
often achieve greater efficiency by being able to trim the prop to match
sea/wind conditions to engine load.

Fresh Breezes- Doug King

DSK wrote:
Farr1220 wrote:

Doug,

There's no need for the complication of variable pitch.


Well, sure. There's no "need" for the boat at all.

... A reversing
motor (electric or hydraulic) is all that is needed.


And that's simpler than a CPP?

... Hence for this
size vessel you see symmetric 4 blade props.


Sometimes 5 blade.

... The small increase in
efficiency gained by having variable pitch is not worth the extra
complication.


Bart wasn't asking about efficiency, he was asking about
controllability. An electric drive may be on par with the controlled
pitch for quick & easy reversing, but they're much less common... the
ones I know about have a bad reputation for being troublesome.

Well we disagree there. Look at all the boats in your boatyard -what %
do not use electric thrusters? I think electric is the most common
installation on yachts. I've not seen any alternative to electric and
hydraulic. What have you got in mind?


As for a hydraulic motor, if you think these are efficient and
trouble-free then I suggest you get some experience with them. It would
be at the bottom of my list for almost any power application.


I said that efficiciency is not an issue for a thuster. Now for
reliability, hydraulic are are no worse than electric motors in hostile
(SW) environments (look at what working fishing boats use to retrieve
gear if you want insight to reliability). That reliability is why
hygraulic power is used so much on mega yachts for furling gear, winches
and rams. But think on this: The hydraulic power will certainly be
there on this size yacht so why not use it? There is no reason to throw
money away when such a simple solution presents itself.


Cheers

... An electric drive may be on par with the controlled
pitch for quick & easy reversing, but they're much less common... the
ones I know about have a bad reputation for being troublesome.


Nav wrote:
Well we disagree there. Look at all the boats in your boatyard -what %
do not use electric thrusters?

Ah, I see the problem. You're confused about the subject.

Again.

The discussion is not about aux thrusters, which can be a trivial
fraction of main engine horsepower, but rather the main propulsion itself.

DSK

Crossed purposes I see. I was thinking about the thrusters here. What is
the origin of term "back down" when talking about going astern come from
anyway -I was misled by it into thinking about manouvering with the
thrusters...

Cheers

DSK wrote:

Farr1220 wrote:

Doug,

There's no need for the complication of variable pitch.


Well, sure. There's no "need" for the boat at all.

... A reversing
motor (electric or hydraulic) is all that is needed.


And that's simpler than a CPP?

... Hence for this
size vessel you see symmetric 4 blade props.


Sometimes 5 blade.

... The small increase in
efficiency gained by having variable pitch is not worth the extra
complication.


Bart wasn't asking about efficiency, he was asking about
controllability. An electric drive may be on par with the controlled
pitch for quick & easy reversing, but they're much less common... the
ones I know about have a bad reputation for being troublesome.

As for a hydraulic motor, if you think these are efficient and
trouble-free then I suggest you get some experience with them. It would
be at the bottom of my list for almost any power application.

Actually CPPs are slightly less efficient, ideally, than conventional
props becuase of the bulky hubs. However, in real world conditions, they
often achieve greater efficiency by being able to trim the prop to match
sea/wind conditions to engine load.

Fresh Breezes- Doug King

DSK wrote:

Bart Senior wrote:

2 points

After hitting the lottery for big bucks, you have nearly completed
your 150' new sailing mega yacht. Included on your new toy are both a
bow and stern thrusters--for side-to-side control..

What sort of prop would you want if you were concerned about precise
speed control, and being able to back down nearly instantaneously?

Money is, of course not a concern.


Hydraulically controlled variable pitch.


For main prop definitely CPP. I'm not sure I'd want just hub hydraulic
control though. Might be better to have a servo that can be overridden
by human muscles -just in case.

Cheers

No props. A set of Kamewah water jets.

Rick

Rick wrote:
No props. A set of Kamewah water jets.

If you want to quibble, the water jets substitute impellers for
propellers. In some cases they are harder to access too, but in general
they're less troublesome IMHO... just like a big pump... in fact that's
what it is.

The trade-off is that they're less efficient at low speed because the
intake velocity gives the impeller/nozzle a boost just like apparent
wind for a sailboat. I don't know from my own experience, but I've heard
it said that they don't give as much kick for maneuvering at low speed,
but don't have prop walk so that's good.

One of the interesting water jets I saw was on a 21' light trailerable
sailboat. The water jet came out the back through an extended nozzle
about 4" above the water line.

Fresh Breezes- Doug King

DSK wrote:

If you want to quibble, the water jets substitute impellers for
propellers. In some cases they are harder to access too, but in general
they're less troublesome IMHO... just like a big pump... in fact that's
what it is.

The trade-off is that they're less efficient at low speed because the
intake velocity gives the impeller/nozzle a boost just like apparent
wind for a sailboat. I don't know from my own experience, but I've heard
it said that they don't give as much kick for maneuvering at low speed,
but don't have prop walk so that's good.

No need to quibble, much less troublesome. Lots of "kick" at low speeds
since they accelerate a huge weight of water in a very short time and
put it anywhere you want it. Twin jets and a bow thruster are lovely.

The OP specified a 150' boat, rapid reversing capability, and precise
speed control ... all the attributes of jets in a boat that size.
Efficiency is not the highest rung on the ladder in that installation.

Rick

Rick wrote:
.... Lots of "kick" at low speeds
since they accelerate a huge weight of water in a very short time and
put it anywhere you want it. Twin jets and a bow thruster are lovely.

That does make sense but I've head a couple of people say that jets will
not give as definite a push when gunned at low speeds as will a
conventional prop.

If you look at it as a problem in differential pressure, it seems to me
a pump & jet nozzle could generate higher delta-P much quicker though. I
guess to make sure I'll have to try it for myself some day.

I have driven a couple of small jets, the biggest one a 25-ish foot
center console... heavy boat, flat bottomed, badly prone to side slip
but a lot of fun in the shallow water it was designed for. It had a
fixed jet nozzle & conventional rudder.



The OP specified a 150' boat, rapid reversing capability, and precise
speed control ... all the attributes of jets in a boat that size.
Efficiency is not the highest rung on the ladder in that installation.

Yep. People that are mega-rich have different priorities!

Fresh Breezes- Doug King