AeroCat power cats, opinions?

[Krauthammer]

They are in PA and since they build them over male frames they can accommodate custom sizes and configurations and they can build them light.

http://www.aerocatboats.com/

Any opinions?

[Mr Efficiency]

The claim of fuel savings of 50-75% over conventional v-bottoms is dubious at best. From what I can see the tunnel height is not that great, and tunnel slap is not a pleasant thing.

[Krauthammer]

Good points.
All head-on photos show the "airfoil tunnel" where the roof of the tunnel appears to be at water level when the boat is at rest. Are there any other cats with similar tunnel designs?

[Jimboat]

Originally Posted by Krauthammer

Good points.
All head-on photos show the "airfoil tunnel" where the roof of the tunnel appears to be at water level when the boat is at rest. Are there any other cats with similar tunnel designs?

It is quite common for the tunnel roof to be wetted when a tunnel hull is at rest.

[Krauthammer]

Thank you Jim.
What is your general impression of this design and have you ever heard of the designer?

[Mr Efficiency]

Originally Posted by Jimboat

It is quite common for the tunnel roof to be wetted when a tunnel hull is at rest.

Common maybe, but desirable, maybe not. Depends on the waters the boat will traverse, but the greater the clearance the less chance of impacts. The temptation to keep the clearance low accords with styling considerations where high boxy structures are less aesthetically pleasing, and it also has the desirable effect of lowering COG. The underway clearance is a function of the lift generated by the sponsons, so planing cats typically have lower clearance anyway. I would definitely prefer no tunnel roof immersion at rest, though.

[Jimboat]

Originally Posted by Mr Efficiency

The temptation to keep the clearance low accords with styling considerations where high boxy structures are less aesthetically pleasing, and it also has the desirable effect of lowering COG.

Actually, planing cats best considerations for tunnel roof height is based on potential of aero lift from the tunnel wing aerodynamics. This aero lift is, in part, influenced by 'ground proximity', so smalller tunnel roof height above water increases lift (ground effect). Of course, the determination is a compromise with potential for tunnel roof wetting at operating planing velocities. As with all hull design, there are many compromises for optimum design for desired conditions.

[Alik]

Tunnel roof normally should not be wetted at rest, at least for cats with symmetrical demihulls designed for FnV<5, that is the case. I recommend minimum vertical clearance of 2...3% of L for such planing cats, this provides proper gap for running condition for cats of normal proportions, considering running trim.

Tunnel should not be used to generate hydrodynamic lift though it is normally wetted by spray. Otherwise excessive slamming might occur on flat tunnel surfaces.

We use wetted tunnel design only on landing craft at fully loaded condition - this is reasonable compromise to obtain required load carrying capacity.

The pictures below show tunnel flow at FnV=2.5, 3.4 and 4.1. There is clear gap visible, especially at higher speeds. This is how it should be on such cat with symmetrical hull.

[Jimboat]

Originally Posted by Alik

Tunnel should not be used to generate hydrodynamic lift though it is normally wetted by spray. Otherwise excessive slamming might occur on flat tunnel surfaces.

Agree, definitely.

Drag generated by wetted tunnel roof will always degrade performance. should always avoid where possible.

Originally Posted by Alik

Tunnel roof normally should not be wetted at rest, at least for cats with symmetrical demihulls designed for FnV<5, that is the case. I recommend minimum vertical clearance of 2...3% of L for such planing cats

Don't usually worry so much about tunnel roof wetted at rest on planing cat.....more attention to overall freeboard and static stability for at-rest analysis.

Usually use tunnel height (h/c) 4% - 6%, so 2-3% minimum certainly agree.

[Alik]

Originally Posted by Jimboat

Don't usually worry so much about tunnel roof wetted at rest on planing cat.....more attention to overall freeboard and static stability for at-rest analysis.

Usually wetted at rest means wetted at planing speed FnV<5.0. As another approach, one can do Savitsky analysis and calculate stern sinkage to evaluate submersion of aft part of tunnel at running trim.

Freeboard and stability are no worry at all, on cats they are excessive excerpt 'wind and wave' stability criteria to ISO12217 category A.

Usually use tunnel height (h/c) 4% - 6%, so 2-3% minimum certainly agree.

You mean vertical clearance to length? Or?

Note: I understand we are discussing quite common pleasure cats, not racing ones.

[Jimboat]

Originally Posted by Alik

Usually wetted at rest means wetted at planing speed.

not in my experience.

Originally Posted by Alik

Freeboard and stability are no worry at all, on cats they are excessive excerpt 'wind and wave' stability

don't agree. (oh, well...)

Originally Posted by Alik

You mean vertical clearance to length?

yes.

Originally Posted by Alik

I understand we are discussing quite common pleasure cats, not racing ones.

correct.

P.S. - Alik - i like your designs on your web site!

/jim

[Alik]

Jim, I would say 4.5-6% of clearance is good minimum number for displacement and semi-planing cat. On planing cat, it is a bit excessive in my opinion due to hydrodynamic lift. With increase of vertical clearance accommodations are likely to suffer, we get higher CG, etc.

As interesting comment, German Lloyd treats tunnel surface lower than 5% L as 'wet deck' with very high slamming loads; higher surfaces are treated just as topsides. So I believe my numbers are correct.

There is another approach - look at minimal freeboard requirements to define vertical clearance (assuming that proper freeboard provides absence of wetness) - proposed by Doubrovsky in his book. Thus, tunnel on cat is usually higher than minimal freeboard (excerpt planing cats that are lifted underway), this actual freeaboard on cat is always higher than minimal. We looked at freeboard numbers deriving from ISO12217 and they match well with above recommendations.

I would like to see the sample of good seaworthy planing cat with submersed tunnel at rest; there might be some but we never design them this way.

Stability issues - we look at them from retrospective of rules, and never had any issue for categories B, C, D for pleasure cats. For category A, cats are getting obvious disadvantage because roll angle formula used in ISO standard is for monohulls. Cats have area of stability curve shifted to small angles of heel, so...

[Mr Efficiency]

The stability shortcoming of planing power cats is of the dynamic, rather than static variety. At rest and slowly underway they are excellent, but pushed hard at certain angles to the wrong kind of sea state, they can go over surprisingly easily, especially if relatively light-weight, with high COG and/or sporting fat sponsons with a lot of lift.

[Alik]

Originally Posted by Mr Efficiency

The stability shortcoming of planing power cats is of the dynamic, rather than static variety. At rest and slowly underway they are excellent, but pushed hard at certain angles to the wrong kind of sea state, they can go over surprisingly easily, especially if relatively light-weight, with high COG and/or sporting fat sponsons with a lot of lift.

I believe You are talking about corkscrew motions and bow diving. The first is a matter of habit and comfort; the second one can be the issue but same as monos. For planing powercat of normal proportions AVS is about 60 deg and maximum of stability curve is at 15...20 deg (but never less than 10 deg). This is quite similar to wide monohulls and provides enough reserve stability.

[Krauthammer]

Nice designs on your website Alik, compliments.

Some transom photos of the AeroCat show that the tunnel may not be immersed at rest though very close to it with unknown loads.

And another question, do pleasure cats benefit from drag reduction from the non-compressible air in the tunnel and if so would the high bow of the AeroCat generate increased lift and reduced drag compared to constant tunnel height cats?

Good discussion, much appreciated.