Tunnel Hull - PAD'S

pad design seems to be a fairly well worked out subject for fast monohulls but I have yet to find anything on pad design for tunnel hull/catamaran hull applications


the balancing act is a pad issue with monohulls which wouldn't apply to a twin hull...but Im thinking a directional stability issue would be likely due to the increased possibility of lateral 'skidding' being easier on pads than on the knife edge of each hull.....but at the same time it could reduce likelihood of catching an edge in a turn

bad sketch attached for reference....

Its something im thinking of trying on my recently acquired 1983 built ply 21ft Lorne Campbell boat - Witch of Endor - still current UK Coniston speed record holder in E5000 @ 92.34 mph....which unfortunately cant run there again due to safety cell requirements that just cant practically be built into her

that doesn't stop me wanting to go faster safely! where the UIM super nannies don't have sway....could pads help achieve that

 

Make it fly man !!

Ok what are you trying to do exactly just go faster ??

A pad at the back will make the boat ****** ???
Why not double the length and make it twice as wide and really make it work !!


Why make it just a add on wedge ?
why not make it longer and go forward more ??
The boats we made the pad started almost at the chine at the front and by the time it was 2/3 along the hull was flat and went all the way to the transom was about 100 mm wide at the 2/3 mark by memory that was on a 16 foot boat .
if you want to go faster and you want to reduce water drag and make the boat fly !!
how would you go about making it fly and lift out of the water ??
The tunnel is were I would be concentrating all my efforts !
the shape and the height play a really important part in all of this !! .
what is your impression of the function of the tunnel ??
what does it do exactly ???
What is it capable of doing ???
do you understand what its job is mainly ??
so how fast is it now and how fast are you hoping for ??

 

by points

I don't know what ****** ??? but Im assuming its referring to not making the performance good

doubling the length and making it wider...if you say so.....its a sketch not a detail....my thinking is simply that I don't need the kind of pad lift required for a monohull

Im getting lift generated by the air pressure in the tunnel

Aerodynamic lift is minimal with this design....

will a longer pad help to arrive more quickly at a point of constant equal stability on each hull....

the drawing of the shape and proportions of my tunnel hull are not at hand so I cant add details ....two pics I have to show the design are added

flat roof and flat sides

The boat has some tendency to skew/drift slightly at about 85 mph and up

what I want it to do!....I do not want it to do that!

So I know a pad (or in this case 2 pads) is not going to offer me any magic speed increase.....

what I'm hoping for is that using a flat pad on each hull will help to maintain two contact points of 'more' constant equal pressure than is presently provided on the contact surface of the angled dead rise at the transom of each hull

Thereby reducing or eliminate the skew/drift caused 'potentially by the fact that drag is more easily increased on a constantly angled dead rise as a result of slight but increasing forces being applied 'unequally' to the hulls...prop torque or aero or both

So in the case of this boat it may be that at speeds increasing over 85 the drag caused by unequal immersion is greater on one hull than the other ....causing it to skew/drift

which can be summed up as....It isn't running flat port to starboard

its reliable up to 85mph now....I want 100mph on near flat water

this is a straight line 1000m lake Coniston record boat....it doesn't need to turn at high speed....

 

http://aeromarineresearch.com/

This is the guy you need to communicate with Mr Jim Russell !! have a look see on his site and deal direct with him !!

 

We studied pads on a 4 m asymmetric cat with similar proportions to your hull. Bad result; it tended to plane slightly yawed and was not stable. The reason we found was that the transverse flow into the tunnel increased, resulting in an unstable wetting of the aft tunnel sides. The boat yawed and heeled slightly until the flow along one tunnel side or the other was stable.

Course instability in planing, asymmetric catamarans often occur due to too low trim (too much lift aft) and absence of even the slightest keel or fin lateral area aft. So, I can't see the pad as a solution to your problem. Rather a short, additional step aft might do the trick.

 

thanks to all

thanks for input to this.....

I knew I was struggling to remember the correct word to describe the behavior

course instability 'yaw' = my 'skew/drift'

just a thought on beckmo observation of transverse flow into the tunnel caused by the pad could be negated by a fin running alongside the pad vertical to the inner tunnel side

but I guess that's just adding appendage drag even if it did stop transverse flow into the tunnel