[PCW] planning hull at displacement speed

[Candy & Gary]

Dear all,

What would be the impact of a projected planning cat hull if we set up
engines for lower speeds? Is there loss of efficiency? Confort? Any
other points?

Roger

Yes.  Great losses.  Planing hull shapes -- monohull, catamaran or any 
other -- incorporate large flat horizontal surfaces on the bottom and 
power plants many times larger than those found useful in displacement 
hulls.   They use this great power to gain enough lift from those flat 
hull surfaces to raise the whole boat up to the surface of the water and 
then climb it up over their own bow wave crest, commonly termed 'getting 
up on the step' or 'getting up on plane.'  Once this planing is achieved 
they are free to operate at any speed they can afford above the wave 
trap speed limit imposed on displacement hulls.  They literally skim or 
fly along on the surface of the water.  Purely planing hulls achieve 
tremendous speeds up there on the surface, but of course they depend on 
having very little weight to lift, great whopping engines to lift it and 
a small shallow hull to put it all in.  This small shallow hull shape 
must be carefully optimized for stable performance on plane, given the 
high speeds involved, and the usual presence of high testosterone levels 
in the operators.  These design tradeoffs sacrifice almost every other 
design consideration deployed in displacement boats to achieve the 
highest possible speeds and adequately thrill their owners.  The 
propellers are particularly optimized for performance at high speed.  
Since these designs rely on keeping the flat hull surfaces cleverly 
oriented just so to the surface of the water they cannot plane in rough 
water --  where their other hull shape compromises, such as low 
freeboard, short length, sleek lightweight hulls with heavy engines 
perched way too far aft for balance when not up on plane, etc. etc.-- 
their off plane performance is appalling.  The only speeds that are 
worse than displacement speeds are seen on overloaded or under powered 
planing boats, where they are struggling to almost but not quite get 
over the step.  At displacement speeds they are frequently threatened to 
be swamped by their own stern wake.  They squander fuel, rock and roll, 
and are inordinately threatened by rough water.  They have little room 
for living amenities and seem to be best suited to water skiers and bow 
wenches.  That of course is just fine for them, and they have exactly 
the same rights to be on the water as anybody else.  I sense that those 
priorities are not particularly important for us Power Catamaran folks.  

Purely displacement boats from canoes to supertankers can be designed 
today with considerable freedom to provide fuel economy, rough water 
capabilities, weight /cargo capacity, etc., but must always operate 
nicely below the wave trap hull speed.  

There is one other practical way to minimize the hull speed limit, 
frequently termed 'slender ship theory' performance.  These designs rely 
on exceptionally long slim hulls with power increased somewhat over the 
common displacement model, but not nearly as much as the planing hull.  
These hulls have in-the-water shapes with lengths anywhere from ten to 
twenty or more times their beam.  They work by minimizing the depth of 
the so called wave trap trough so that they can climb over the 
diminished bow wake crest to a considerable degree.  Catamarans are 
particularly well suited to this ploy, as they have two parallel hulls, 
and the beam of each is less than half the beam needed in a monohull to 
carry it's weight (weight capacity, or displacement goes up at a rate 
like the cube of its linear dimension, such as beam). 

Of course,  modern boats, particularly catamarans, are very seldom pure 
displacement hulls (like ancient Polynesian paddling/sailing cats) and 
almost never pure planing hulls (example racing hydroplanes running with 
only two sponsons and the prop in the water).  These are compromises 
between the extremes, often referred to as semi displacement hulls. 
Power catamaran hulls  are almost always going to have some slender ship 
stuff going for them.  They are usually a varied mix of these design 
approaches, beginning with a displacement look, adding some flat bottom 
aft and employing widely spaced slender hulls.  My beloved PDQ is a 
particularly good example of this approach.  We get pretty much 
equivalent performance and stable behavior at any speed up to about 
three times hull speed, without any discernible sweet spots and with no 
difficult speeds.  Thanks to very sophisticated modern designs the 
displacement behavior seamlessly transitions into the planing and 
slender hull boosted high speed behavior.   We are not penalized by 
needing huge thirsty engines and have lots of usable space available in 
a stable (and to some eyes at least, attractive) hull. 

There is another approach common enough in power catamarans today, one I 
call the tunnel hull boats.  These hulls much more closely resemble 
familiar planing hulls with a slender tunnel down the middle.  They get 
some fine advantages from this tunnel, much softer high speed 
performance, somewhat wider deck plans, better roll resistance and 
such.  They do not harvest all the advantages of the slender hull power 
cats, but they don't take all the hits of the modern monohull planing 
boat either.  They have much higher top speeds than their slender hulled 
brothers, but at the cost of higher fuel consumption, much heftier 
engines and of course the advantages and costs of the shape and size of 
the hull. 

Selecting any of these kinds of boats is entirely a matter of personal 
preference.  Fishers seem to like to get to and from the fishin' spots 
quickly.  Coastal cruisers want comfort and economy.  Party patio 
pontoon boats and lake houseboats appeal to folks with socially powered 
agendas.  Your particular choice is unlikely to line up perfectly with 
mine.  That's fine, there's room for us all. 

Returning finally to your question Roger, I want to suggest that to the 
extent that any boat is designed for real high speed planing it becomes 
very difficult to recapture very many of the low speed displacement or 
semi displacement boat attributes, and impossible to get them all.  You 
have to have big, heavy, thirsty engines to plane, and props optimized 
irretrievably for high speed.  The real planing hull has so much flat 
horizontal surface on its bottom that there is no hope of getting 
slender ship advantages, and at displacement speeds it will have to 
shoulder out of the way huge amounts of water to proceed.   The shape of 
a planing catamaran offers less space for the open amenities coastal 
cruisers want.  I cannot claim that I entirely escape my own preferences 
and prejudices but having said that I will say that I think the slender 
hull option offers the best of all possible worlds.  YMMV of course.

Warm regards,
Gary