[MHml] Prop Pitch for Multihulls, and spray rails

[Bill Gibbs]

How about spray rails also for a bow up vector at speed, or just a drier
ride?
 
Some of the big monos have faired the top of the spray rail into the hull,
more of a chine than a rail I guess.  This eliminates the "water on top"
problem with waves etc..
 
Bill
Afterburner

  _____  

From: multihulls-bounces at steamradio.com
[mailto:multihulls-bounces at steamradio.com] On Behalf Of Silas Spence
Sent: Sunday, March 15, 2009 10:25 AM
To: multihulls at steamradio.com
Subject: [MHml] Prop Pitch for Multihulls, and spray rails


Hi all,
Sorry for doing a hit and run with this thread, I spent the last few weeks
dealing with the worst project I've ever had and was just too burnt out to
think about more boats once I got home. Tough problem to have, I know.

But thanks to all the people that responded with their experiences. It was a
bit tough to get anything useful out of this propellor program we have at
work since it wants to know a whole lot of pretty obscure information about
the prop in question. But, I made it up as best I could and found all the
prop diameters and powers quoted for a given boat displacement seemed to
show the same trend as the bigger boats I've been testing: higher pitch and
slower rpm = more efficiency.

However, there are the practical problems, namely low speed maneuvering. I
think Peter Nice hit it on the head pretty well when he mentioned a gearbox.
I've talked to our resident prop guru about this a few times and he
maintains the only ways to get the high pitch efficiency along with low
speed maneuvering are variable pitch (best option and what the big boats
use), or a gearbox. Either way, weight, complexity and cost. When I brought
up outboards he just threw up his hands and walked away. I think, given the
the better gear ratio on the high thrust Yamaha's, I'd be inclined to try
one with the standard prop. All in all, not much of use to most of us
unfortunately. I do hold some hope for use in a diesel-electric setup where
the low RPM torque of electric motors might eliminate the need for a
gearbox.

One question, a few people - I think Paul Nudd was one - mentioned speed
gains with prop A vs prop B. Was this at full throttle, half throttle, a
constant rpm...? Selecting a prop for improved thrust, and thus speed,
(without the luxury of also choosing your gear reduction) is usually a
different consideration than selecting a prop for improved efficiency. I
guess to determine efficiency gains one would have to look at something like
fuel consumption for X knots boat speed.

To hijack my own thread, one interesting thing that has come up at work
recently is an internal study on spray rails. Conclussion, is that spray
rails have no positive effect under Froude number 0.85-0.9 (about 18-19
knots for a 12m boat, faster for bigger boats), and don't really show much
gain until about Fr=0.95. At slower speeds the spray rail actually increases
resistance by a couple percent unless it is completely clear of the water.
And the nose diving that happened if water managed to flow up over top of
the rails at the stem! I think some of the Farr Open 60s had spray rails
this time around? Hope they spent enough time at 24kn+ to get some benefit
out of them...

I'm also trying to track down a public version of the report from our tank
testing of the Gokstad, a reconstruction of a fairly well preserved Viking
ship. If I find it I'll share with the group as I think there was some
debate awhile ago regarding performance of these boats.

Cheers

Silas Spence



  _____  

Tell the whole story with photos, right from your Messenger window. Learn
how! <http://go.microsoft.com/?linkid=9650732>  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://steamradio.com/pipermail/multihulls/attachments/20090315/eba3c890/attachment.htm