[PCW] gas or diesel on Prowlers

[Candy Chapman]

A lively discussion, so far.... 

I would like to add that perhaps the most persuasive issues in my 
estimation  are weight and more importantly weight distribution.  All 
boats are subject to performance penalties of various sorts when their 
weight increases significantly from that intended by the designer, and 
more importantly, when the balance the designer built into the boat is 
dramatically degraded by adding more and/or different weights out at the 
fore or aft ends.  Cruising catamarans are much lighter than their 
monohull displacement peers as they neither have nor need ballast.  
Great efforts are expended to minimize their overall weight both 
throughout the design and in later admonitions to the buyer.   First, 
catamarans (particularly the semi-displacement kind) have very slender 
hulls and trade off great roll resistance at the cost of very much more 
pitch sensitivity.  And second, these catamarans with their slender 
hulls have tiny flat planing surfaces on their bottoms aft, and the 
orientation to the water surface of this lifting surface is pretty 
critical.  Both these considerations make the fore-and-aft distribution 
of weight really critical. 

The worst sin is to add too much at the ends, such as heavy diesels aft 
and/or a heavy genset forward.  The designer has worked long and hard to 
get the weight concentrated amidships and also to make the fore and aft 
ends as light as possible in order that the hull can react better to 
pitching in rough water.  It is a matter of rotational inertia, or 
moment, and at the same time a lever problem.  Imagine if you will a 
seesaw with a pair of sumo wrestlers on it.  If the wrestlers both sit 
way out at the ends of the seesaw plank it could indeed be in balance, 
but their large inertia would make it hard to move the seesaw ends up or 
down.  The big guys would have to move their considerable weight a great 
distance up or down to get the plank to move -- say maybe twenty 
degrees.  Now place our imaginary sumo-dudes right next to the fulcrum 
(pivot point).  It could still be in balance but now they don't hardly 
move up or down at all to get the plank to tilt our twenty degrees.  The 
sumo-moms propelling this activity from out at both ends of the plank 
don't have to work nearly as hard.  Same goes for pitch sensitive 
catamaran hulls.  Their bows need to rise and fall easily to track the 
waves.  I suppose that wave piercing hulls might work with long skinny 
hulls containing massive inertia in the bow and/or stern, but so far 
that is just my speculation.  I have no direct and very little indirect 
information about the pitching performance of a full sized wave piercing 
catamaran.  However, does anybody know what happens when the speedy 
little beach sail cats bury a bow (or two) in a wave?  It's called 
pitchpoling, and is not very pleasant. 

The point of all this jabber is that the Prowler designer probably 
worked pretty hard to properly balance the fore-and-aft trim of the hull 
in order to get optimal pitching performance and to achieve the desired 
attitude to utilize the planing surfaces on the bottom.  Simply 
increasing the weight of the engines is bad, unless they can be moved 
far enough forward to restore the original trim.  Clearly the outboards 
hang way out behind, and are not exactly lightweights themselves, so 
perhaps it could be done.  Sorta like moving the heavier sumo wrestler 
much closer to the fulcrum than the sumo baby he replaced.  Gensets (and 
sumo wrestlers too) need to be carefully placed in a boat to get optimum 
performance. 

This stuff may or may not be apparent to everyone, but I for one think 
it is a significant issue

More from that old drone Mister Science
AKA Gary Bell