AeroCat power cats, opinions?

We may be both saying same thing. We measure height above water at aftmost trailing edge of tunnel (transom), and as ratio (h/c) of "aerofoil" chord. So at 2% a 20ft hull would have only 4.5inches height - not usually enough. Especially with planing cats, excessive wetting of tunnel roof adds too much additional spray drag, and exposure to water disturbance.

Agree, Efficiency. Dynamic stability is most important, and must be critically designed for.

Yes, and Yes. (although aero lift is dependent on several factors - article on influencing factors here.)

 

I would say that lift generated by tunnel of typical pleasure cat is negligible, compared with displacement - 5% or less. One can run calculations and see the result.

 

sometimes true enough...but even 5% aerolift reduces hydrodynamic lift requirement and associated hydro drag - which is significant. This is why a well designed tunnel hull can be more efficient by generating only a small amount of aerodynamic advantage.

 

Some power cats acquired a reputation for excessive spray mist around the outboard engines, sharply reducing engine life expectancy. I don't know whether a higher tunnel clearance helps or not, but you don't want it if it can be avoided, or at least reduced.

 

Just setting aside the issues of efficiency etc for a moment, I personally wouldn't dream of having a powercat of any variety where the bridgedeck clearance was less than about 3 feet. I've spent time aboard a number of planing cats that have low (or no) bridgedeck clearance at rest and they are the most unpleasant of boats that I have had the misfortune to be aboard. I can't imagine trying to get a good nights sleep in even the slightest slop

 

If true then a single OB mounted between the sponsons would be doomed but a great many small cats seem to keep their single motors alive.

 

Well, I do know at one time some outboard makers would not warrant their engines on cats because of salt spray ingestion. Motors have changed, but I doubt to the stage of solving this issue.

 

Of course on smaller vessels that kind of high clearance wouldn't be practicable, but I agree you don't want water lapping the tunnel at rest. Then again aluminium boats of any kind are subject to such noises. Hate it !

 

Fully agree; cat with tunnel submersed in the water is just a raft with end plates

Sometimes this happens with cats that are badly overweight, or designed 'light, attractive and very efficient'... without genset and air-conditioning in weight schedule.

 

Note that on cats the engines are installed far outside from centreline, so usually it means spray will be blown back from engine; the is no air backflow zone at engine installation position. We had this spray problem on few our cats, but just because swimming platform supports (mounted above tunnel) were in way of flow behind tunnel, at some running conditions. Correction was simple: just to move support mounts a bit outside.

 

The big potential + factors with cats are lower vertical accelerations, a more stable platform, good deck area for boat length, and high resistance to broaching. The downside is twin motors, with the higher costs entailed, and in most designs the turning circle of a ship. On balance, they work well.

 

This might be true at high speeds; but by reducing RPM of inner engine much smaller turning circles can be achieved. We did a lot of measurements of turning circles during sea trials, some results are published.

Advantage - cats do not heel at turn and side acceleration (that is subject to certification by HSC Code) is reasonable. On commercial planing monohull, one should anyway limit speed of turn to comply with these regulations.

At slow speeds cats possess better turning ability compared to monos.

 

They sure turn a treat at slow speed by having one engine forward gear, the other reverse. For those who can afford the extra costs, cats are the way to go, for their greater margin of safety for offshore use.

 

Please elaborate and identify any impact this may have on a single center mounted OB.

Agreed but what factors other than maximum speed and return home capability would prevent the installation of a center mounted OB? Not to underestimate the come home advantage but modern day outboards are far more reliable than their ancestors.

If an issue the tunnel spray against the mid-section of the OB could be addressed with a single pod mounted on the roof of the tunnel while the propeller efficiency running in undisturbed water should show gains.

For a near shore craft, in the sub 30 ft range, the single engine cat has distinct acquisition and operating cost benefits while significant appendage drag and transom weight can be eliminated. Any reasons why not?

 

So far we have launched only two cat designs with centerline mounted outboards.

Usually we use twin installation that is natural for catamarans. Any engine installed in tunnel is a brake, not a propulsion device.