40-passenger pontoon boat - 2 hulls or 3?

Hello all... my first post in these BoatDesign.net forums...pleased to be aboard.

My efforts to sway my sailing club from displacement diesel to an electric pontoon-type craft as a replacement for our aging Club tender (40-passenger private ferry boat) have been for nought, so this has ended up somewhat academic... but here goes -

I have set up a little Yahoo Group thingee at:

http://groups.yahoo.com/group/QCYCTender/

where you can read the "design parameters" (I guess they're called) for the Clubs tender, but there's one aspect of pontoon craft I have no clue about. Here's a message I posted there (that Yahoo group), to explain:

Gets back to figuring whether a third hull provides a net
benefit, as the 48-passenger thingee had a third, which I hadn't
imagined - thought it'd be still two-pontoon thingee..

Thought at first, three hulls bad, but have re-thought. How `bout
making the outer hulls a little bigger, but keep the middle pontoon smaller, so when lightly loaded, the middle hull lifts clear of the water?

Seems to me that'd put a little nuckle in the "bouyancy curve"
(something like that)? Cut some wetted surface when you can, maybe save fuel? And the extra bouyancy/wetted surface would kick in when the outer hulls got loaded up, so to speak. The centre hull would still help keep the boat stiff. Maybe the place to set up the batteries - centerline?
===== end of my message ======

It's this idea of a smaller centre hull, which doesn't come into play until the craft is loaded up to a point, which I am curious about. Has anyone ever heard of such an approach? Are there obvious disadvantages? Nothing to gain of any significance? I'm just a recreational sailor, but have been sailing beach cats and reading many things about Multi's for years. This might be something new and good, but I'm suspicious - `specially since I dreamed this one up muself <grin>

Thanks in advance, for any replies!

Cheers
Lock
Tornado KC234 "High Heels"
QCYC
Wards Island, Toronto harbour

 

Dear Lock,
What you need to design are two (or three pontons) with the minimum wetted area. As the drag is proportional to the wetted are of the boat. Hence for any displacement the minimum wetted are is the most efficient design considering all other things are equal. In parctical terms, to get the wetted are determine the displacement and LWL (Loaded Water Line or the Design Water line in your case). Then determine the length of the circumference x length of the pontoon to determine the wetted area. Iteratively change pontoon size, shape and number (2or3) until you determine the minimum. Good Luck.

 

In general, big things have less surface per unit volume than little things. On a constant displacement, one hull will have less wetted surface than two, and two will have less than three. Offhand, I can't think of any powerboats that designed for low-speed operation that are multihulls - except for pontoon boats really designed for easy construction and zero speed operation. The power cat ferries that I have seen (in Alaska) are designed for planing speed operation, mid-teens, I suppose.

 

Dead right seadrive. However not knowing the full story the 3rd hull may be required to house propulsion or provide the extra bouyancy due to other unknown criteria or limits. OPn abother matter, do you have a generic design for pontoons that comply wiyh USL Code (Australian shipping laws requirements but should be equivalent in the USA) for 45 metric ton displacement of 20x8.5m Hire and drive hoyuseboat.
Jaguar