[Rhodes22-list] Hull Speed

David Rasberry rhodes22-list@rhodes22.org
Thu, 25 Jul 2002 20:33:14 -0700 

The accepted formula for displacement hulls is 1.34 times the square root of
LWL, or waterline length. This is the formula for the speed of a wave train
as defined by the distance between crests. As Roger has pointed out before,
it is just a point on a curve. The power required to propel the boat beyond
hull speed increases exponentially. This is because the boat generates a
wave train as it displaces water. At hull speed it is in the trough between
the bow wave and the stern wave. To go faster it must climb up the backside
of the bow wave until it reaches planing speed. For a Rhodes, hull speed
calculates to 5.99 knots. The upper limit of displacement hull speed is
around 1.54*lwl^.5. For a Rhodes this is around 6.8 knots. Average
displacement cruising speeds in moderate breezes of 8-10 mph are typically
.85-1*lwl^.5 or 4 to 4.5 knots. Some well designed hulls seem to be able to
fool the water into thinking the boat is longer than it really is. The
Rhodes 22 seems to fall into this category such that true hull speed is
probably around 6.2 knots.
To illustrate the relationship between speed and power required for a hull
of 20ft LWL and 3500lb cruising load displacement:
3knots, speed/length(s/l) ratio=.67, required Hp=.87
4knots, s/l ratio=.89 req.HP=2
5knots, s/l ratio=1.12 req.Hp=4
6knots, s/l ratio=1.34 req.hp=7
7knots, s/l ratio=1.57 req.hp=11
8knots, s/l ratio=1.79 req.hp=16.5
9knots, s/l ratio=2.01 req.hp=23.5
10knots, s/l ratio=2.24 req.hp=32.3
I play with these formulae quite a bit as a hobby. I would someday like to
build a custom electric auxiliary sailboat, so working through various
design problems is pleasant entertainment.

Razz


-----Original Message-----
From: rhodes22-list-admin@rhodes22.org
[mailto:rhodes22-list-admin@rhodes22.org]On Behalf Of Roger K. Pihlaja
Sent: Friday, July 26, 2002 6:17 AM
To: rhodes22-list@rhodes22.org
Subject: Re: [Rhodes22-list] Hull Speed


Steve,

With a 20 foot waterline length, figure hull speed is 5.25 knots on a Rhodes
22.

Remember that Dynamic Equilibrium is a 1976 Rhodes 22.  It's lighter & has
the old style high aspect ratio centerboard vs. current Rhodes 22.  Also
remember that we have a very roachy fully battened mainsail, a 150% genoa
made of Bainbridge Cruise-Lam (tm) composite sailcloth on a Harken unit 0
roller furler, & we're rigged to fly a tri-radial spinnaker.  With the
current Rhodes 22 configuration & your individual driving habits, your
mileage may vary.

In flat water, Dynamic Equilibrium can approach hull speed 1st on a broad
reach with a wind speed of about 8-10 knots while flying the tri-radial
spinnaker.  Close reaching with the tri-radial spinnaker requires about 9-11
knots of wind to achieve hull speed.  Our standard rig achieves hull speed
on a broad or beam reach at about 11-13 knots of wind & about 12-14 knots of
wind sailing close hauled.

If you read my story about the weekend cruise on Saginaw Bay with my son,
Gary; then, you will note that by exploiting waves & surfing, the Rhodes 22
can significantly exceed its theoretical hull speed.  This cut our transit
time across Saginaw Bay (27 nm) by 20-25%.  However, note that this
intrinsically involves sailing the boat "actively".  It's great fun, but
demands concentration, experience, and close cooperation between the
helmsman & sail trimmers.

It can be done with the boom in the upper position.  We sail with the boom
in the upper position up to about 15 knots of wind speed.  On Dynamic
Equilibrium, lowering the boom is triggered by having to dump the mainsail
traveler to leeward more often than about once per minute due to wind
direction shifts &/or wind gusts.  Naturally, this trigger point is a
function of wind speed/direction, but is also affected by crew weight,
number of crew, & willingness of the crew to function as "live ballast".  In
passive mode (i.e. if you don't want to sail the boat "actively"), you
should lower the boom at about 8-10 knots wind speed.

However, sailing with the pop top up &/or the bimini top is just a big drag
& should be avoided if you care about going fast.  If the order of the day
is comfort, elegance, & entertaining friends out on the water in light air;
then, by all means sail with the pop top & bimini top up.  We do it all the
time.  The speed penalty is greatest sailing close hauled.  With a wind
speed of about 10 knots, sailing close hauled with the pop top up will cost
you about 1 knot of speed & increase the angle of heel by 5-10 degrees.
Under the same conditions & close hauled point of sail, the bimini top costs
about 2 knots & increases the angle of heel by 15-20 degrees.  As you can
see, sail close hauled with both the pop top & bimini top up & you can end
up going pretty slow, heeled way over, with a huge weather helm.  Therefore,
try not to sail close hauled with the pop top &/or bimini top up.  The
speed/heeling penalty from the pop top &/or bimini top is not so bad on a
beam or broad reach & almost negligible on a run.

I've only sailed on IMF equipped Rhodes 22's in light air.  So, I can't
comment on what conditions would be required for one of the new boats to
surf.  I would think a big tri-radial spinnaker would be the sure cure for
any inefficiency in the standard rig.  But tri-radial spinnakers can be
tricky to sail, especially for a novice - the very antithesis of the way you
have your boat set up.

Steve, I hope this discussion gives you some idea of the trade-offs involved
& the order of magnitude of the effects.

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: "Steve Alm" <salm@mn.rr.com>
To: <rhodes22-list@rhodes22.org>
Sent: Friday, July 26, 2002 5:41 AM
Subject: [Rhodes22-list] Hull Speed


> I've owned my R22 for less than a year and I'm still learning a lot about
> the boat and still learning a lot about sailing.  First, what is the hull
> speed of the R22?  I know there's a standard formula to calculate this but
I
> can't seem to find it.
>
> Second, I have the IMF and the 175% Doyle genny.  What's the approximate
> minimum wind speed and optimum point of sail necessary to achieve hull
speed
> under full sail?  I'm guessing a beam reach with winds at about 14-16.
> Would it be easier with the boom in the down ("first reef") position?  Can
> it be done safely with the boom in the up position?  Some of you have said
> that you ALWAYS have the boom down unless it's very light air.  (I prefer
to
> sail with the pop top up.)
>
> Third, without the standard main or a foam luff pad, is it possible for me
> to pop over my bow wave and exceed hull speed?  If so, under what
conditions
> could I safely do that?
>
> Sorry to sound like such a novice...I yam what I yam.  I'd like to take
some
> lessons some day but in the mean time, I'd really appreciate a few sailing
> tips from you experts.
>
> Anybody?
>
> Slim
>
> _______________________________________
> Subscribers, send mail to this address Rhodes22-list@rhodes22.org
> To change your options or unsubscribe from the list click on this link
> http://www.rhodes22.org/mailman/listinfo/rhodes22-list Check out our
> FAQ pages at http://www.geocities.com/blew_skies/topics.html
> http://www.sailnet.com/ -Where Sailors Get It!  http://www.rhodes22.org
>

_______________________________________
Subscribers, send mail to this address Rhodes22-list@rhodes22.org
To change your options or unsubscribe from the list click on this link
http://www.rhodes22.org/mailman/listinfo/rhodes22-list Check out our
FAQ pages at http://www.geocities.com/blew_skies/topics.html
http://www.sailnet.com/ -Where Sailors Get It!  http://www.rhodes22.org