12 foot powercat

[riskmore]

I love my high performance inflatable thundercat, A 12 foot pvc tunnelhull that has replaced my avon searider as a poor man's ocean racer. At 150 lb. each for the hull and a tohatsu 50 two stroke I can go 50 over 2 foot seas and can carry it in my SUV.
Almost perfect sport boat and yacht tender except I was just turned away from the Hells Canyon river gorge because the material was not rigid. Now I want a rigid boat that is this size and I prefer that it it be collapsable.
If anyone knows of a production boat that might work for me it would be appreciated. Seing as the coast guard has gone to Aluminum for this mission and I have the equipment to fabricate with Al I am thinking of transferring the thundercat design to metal. Besides the improved rigidity of the structure and toughness of material I envision an opportunity to have the cross members and floor boards detachable for transport but still be robust enough to address these connections being the major weakness in the inflatable design.
I am boning up on the fundamentals of hull shape and volume for this type of service and had expected the fine vee bows flatter chimes to give me better performance. Although, from what I have read here so far there seems to be merit to the cylindrical bottoms when using lifting strakes.

[Zappi]

Good luck on the project. I had a thundercat myself and absolutely loved it. The saltiest most fun dingy in existence!!!!

[riskmore]

Thanks for the reply. trying to attach photos now.

[srimes]

I've been thinking of making one out of plywood. Cheap and easy and it won't get popped by a fish hook. Also, if built rigid there can be a lot more interior room.

I never considered a round bottom, don't see the point if building with sheet material. Pontoons are round 'cause it's cheap to fabricate.

I lean back and forth on bottom dead rise and width. Narrow and flat like the thundercat, or wider with some slope? I dunno, guess it just depends.

[riskmore]

it would be great to experiment with plywood but if i build one i don't want to experiment if it could be helped. I will post a google sketchup model in case we can get a discussion going.

Although, I would gladly pay for a production boat if it exists so if anybody has seen such a thing a lead would be much apreciated.

[Adam Younger]

Not collapsable - but this an interesting little catamaran project (11' 6") that we were involved in some years ago. Worked really well, even if the deck was a bit 'angular'.

Moulds are available if interested, also a more modern deck was drawn up.

Over the years I've been involved with lots of large offshore boats - but often I think the most fun comes from small boats!

[HJS]

Just a scetch

JS

www.sassdesign.net

[riskmore]

thanks for the interest. The Younger design hulls seem to be a prospect for me but one feature of the inflatable that I am sold on is a wash through transom. I will post a photo but it is an aluminum box tube frame that compliments the bow bridge or "wing" having a small profile to a wave washing over the boat without swamping.

This allows beach launching through big surf even alone.

My first random thoughts about the ideal design:

The minimum weight of the inflatable pontoons tapering to zero along with the minimized weight of the wing and floorboards reduce the volume needed up front. In fact the shape is wave piercing if you stuff it but you can actually recover without pitchpoling but just be strapped in to a few points so to not to be washed over.

The wing has a top angle creating a needed downforce to keep a pitch trim counter to the tunnel lift. the crew has an active job of weight shifting.

Of course the cabin structure is an important structural element in a molded boat as the top of a box frame parallel to the bridge deck but the perhaps I could take the Younger hulls bare and then seal them flush at the gunnels. Adding enough bearing surface across 4 gunnels attachments would eventually provide the torque and racking strength needed to keep the boat rigid.

With a sealed hull the top sides and total volume could be minimized for reduced weight seeing as dryness is not an issue. Interestingly, this is a very dry ride if you are driving correctly.

The Transom connections could be improved free of the inflatable considerations and also there seems to be an opportunity to use fasteners to connect bolth these cross members to the hull without compromise. Collapsable was not a requirement but it seems a bonus when remake-able connectors are used.

My drawing so far has a flush deck for simplicity but this trades the security of a cockpit if this actually a real benefit rather than emotional. The bridge under-deck height to the waterline seems to be an important consideration.

I should search for a formula or guideline for this dimension. In my unfortunate experience with an overweight sailing Cat, I pounded a 40 footer to pieces in Mexico because it was overweight. I suppose that there is a sweet spot between too much windage and getting pounded.

Seing as this can be designed to run only at a top speed of say 50 mph the dynamic and static moments could be calculated for a normal pitch angle. I am talking out my ass here but it seems that in theory that the air compression in the back of the tunnel may also be considered in the model.

These things are obvious to anyone with experience I would suspect but since the only wetted surface is one prop blade in steady state this is more aircraft design. maybe the hull shapes could perhaps be much less a factor than I thought after on plane.

There are 2 major problems with this boat design as it is. First the weight sensitivity. If you add a 3rd person in this boat it will not clear a one foot chop but between adjusting prop trim and weigh you can take 2 guys up over 4 feet of wind wave. The key is having extra power to control the pitch attitude for clean entry.

The second is getting blown bow over flipping. Clearly if you get popped up with 10 degrees added angle of attack or come over a windward crest or get a gust the delicate weight balance of the bow gone and you are out of control.

I don't know if it was an accident of design but there is a built in ultimate stability in that the volume of the sterns which has to displace enough to keep the motor from sinking static does not extend much abaft the prop. The result is that as the bow lifts the stern sinks more rapidly and a normally surface piercing prop produces increasingly less thrust so you almost alway stop just short of over on your back. This is helped by that deep prop being a terrific brake with the power off.

[srimes]

Originally Posted by riskmore

I don't know if it was an accident of design but there is a built in ultimate stability in that the volume of the sterns which has to displace enough to keep the motor from sinking static does not extend much abaft the prop. The result is that as the bow lifts the stern sinks more rapidly and a normally surface piercing prop produces increasingly less thrust so you almost alway stop just short of over on your back. This is helped by that deep prop being a terrific brake with the power off.

like this?
http://www.youtube.com/watch?v=X26f9rcKlJc

Also, how comfortable are these things? You just sit on the floor, right?

[riskmore]

The driver usually sits legs across pressing his back into the pontoon maybe sitting on a cushion. As it gets rough he picks his butt off the floor so ending the day with a sore back and neck. If it gets very rough you have to go faster to get above the chop, I try squat with a knee under a strap with one hand on a bow line after my back is trash but if you crash at speed while up like that you could get thrown over the bow.

Faceguard helmets, Kevlar suits, and PFD to hold your unconscious head up are required by racing regs.

100 mile offshore endurance courses will result in a week of bruises, abrasions, and spasms but a deep vee rib will be just as painful in different places at twice the weight. The difference being you can slow down an shlog thru while resting in the RIB. Tunnel hulls is all or nothing but good driving also means soft landings.

While solo, I dream of having a straddle helm station with foot straps like the avon searider rescue model but this adds to the complexity and weight. The key to going fast as you know is to control the pitch and roll to enter the water flat but not having trim tabs you have only throttle, steering, and weight shift to work with.

This sort of helm would allow me to lurch forward and back while in flight as a good crewman will do. His knees and elbows will be bloody after a race.

[sabahcat]

Heres a small powercat dinghy made from duflex (epoxy balsa)

http://www.mahnamahna.com.au/January%202007.html

[Adam Younger]

Originally Posted by sabahcat

Heres a small powercat dinghy made from duflex (epoxy balsa)

Neat way of building - interesting website too - wil have a proper look through when a bit more time. Always had a big interest in cruising cats - looks great.

[Adam Younger]

Originally Posted by riskmore

Of course the cabin structure is an important structural element in a molded boat as the top of a box frame parallel to the bridge deck but the perhaps I could take the Younger hulls bare and then seal them flush at the gunnels. Adding enough bearing surface across 4 gunnels attachments would eventually provide the torque and racking strength needed to keep the boat rigid.

Many good / intersting points / details.

The Feline 3.5 (as per previous photo) hull could certainly be used as a basis I guess and would be pretty easy to get stiffness back into the hull - maybe use just a forward section of the deck (before the cockpit) and incorporate with a frame. Could also be sealed and add fixed bouyancy forward.

We could either ship out a hull or as said the moulds could be purchased pretty cheaply!

I'm also doing a 4.2m version of the catamaran as well - always have to have a project on the go!

A bit of a different beast as more tunnel width / less tunnel clearance - and really a cross between the Feline 3.5 and a circuit / inshore racer.

[jonr]

It may be hard to beat the weight of an inflatable tunnel hull, but the idea of different materials is interesting. Roto molded plastic would be most similar in terms of flexibility (which softens the ride in the inflatables).

[rosbullterrier]

I shall be building a catamaran in the next few months but for a much less powered machine - Seagull racing!
If this concept causes bodily damaging mirth, please change topic now . . .
I used a 12ft cat for our Christmas race which was an ordinary deep V hull Hoby type with steered engines behind each hull. Went quite well (yes only 12+ mph) but I need to design and build a lighter purpose made from ply and just sail the Hoby.
Two concerns about the above designs.
The boat from Sabahcat looks superb. Our regs require minimum two crew and is to be none planing. So I think a longer design, say 16ft - and how about the crew sitting at the front (to balance engines) - in the hulls. My concern about being on top of the middle platform is the tunnel would not be high enough to allow clearance for the two bow waves. (at displacement speed) Steering would be easily done by steering wheel.
I got the impression that the engine needs to be in the water flow behind a hull in a displacement boat.

However, there are other Seagulls races which allow planing boats. So the hulls would also need to accomplish this; possibly 8" flat bottom hulls might do this?
I don't understand the advantage of the tunnel vertical hull sides with sloped outer sides?