Planing Power Cat plan and hull type question

Hello All-
First post, so please have patience. I am interested in plans for a 20-22' planning power cat. Ideally engines around twin 150 hp. I've looked around and seen some plans from Glen-l and others but nothing that really seems quite right (and no real performance or handling data).

Ideally I think something like a miniature version of the Freeman 33 (http://www.freemanboatworks.com/) would be perfect. Maybe a bigger version of the Tidelines 18 (http://www.tidelineboats.com/). Prokat 21 is another example.

One thing I've noticed is that most racing cats seem to use asymmetric hulls whilst most other cats (Glacier Bay, Sea Cat, and even Freeman) use symmetrical hulls.

What are the benefits and downside of symmetrical vs Asymmetrical?

Thanks in advance for your suggestions on plans as well as info on the comparison of the various hull shapes for a power cat.

 

Generally asymmetrical hulls are intended for higher speeds.

 

PLaning Hulls

Hi Alik-

Thank you for your response. When you say higher speed, what range- 50+, 75+ or greater. Are there negative handling characteristics at slower speeds- say cruise in upper 30's to mid 40's and even trolling at very slow speed?

I should have added that intended use of the boat is for recreational fishing, cruising, and skiing/tubing. Inshore and some moderate offshore.

What hull type would you recommend- I am pretty positive I want a p[laning and displacement cat.

Thank you.

 

I used to say one needs 'split hull' at speeds over 40kts. For 20' cat, even less.

 

High speed power cats are quite a different breed: over say 100kts, they are primarily air cushion effect vessels. At more sane high speeds, say over 30 or 40 knots, they have dynamic lift from the bottom of the hulls like any planing craft, but they also are supported by the white water packed under the bridge deck, as with the original Sea Sled designs nearly a hundred years ago. While the original Sea Sled had the V on the inside and vertical topsides, today people have learned experimentally to do it the other way around, with the hull deadrise to the outside, and mostly vertical sides to the tunnel, with the height decreasing from bow to stern to compress the white water and/or air for lift.

A disconcerting effect of the square tunnel/outboard V is they tend to lean out in turns. Lots of people hate that feeling.

At lower speeds, say 15 to 30 knots, the speed regime of Glacier Bay and similar, the more-or-less symetrical hulls tend to allow for flat turns or even a bit of inward leaning, depending on the deadrise: less deadrise or round bottoms seems to allow an inside lean, more deadrise seems to result in an outboard lean.

The odd thing is that the wave drag of these moderate speed catamarans is very high because the wave trains of the two hulls add together. However, the rough water ride can be very good, but only if there is substantial bridge deck clearance. At the higher speeds, the riding-on-white-water effect starts to kick in so low bridgedeck clearance is not a problem.

At all speeds, the small waterplane area due to narrow waterlines reduces the vertical accelerations due to seas. That's good. However, this also makes the boats very sensitive to the amount and location of load.

 

I'd say an offshore power cat 20-22 feet with twin 150 wouldn't be cruising any faster than high 20's (knots) to stay reasonably fuel efficient, so considerations of speeds over 40 knots is superfluous. Unless you are thinking some ultra-lightweight construction, which I believe will scare the pants off you. I always think it pays to look at what hard-nosed commercial operators use, those who venture out when recreational users might not, because they have bills to pay and have to get on with it. In Australia, you won't find split-hull tunnel hulls being used by such people, and the asymmetrics are generally shunned for boats with symmetrical, hard chine sponsons. Just a case of what works best, gets adopted.

 

One problem you have to consider is the weight of the engines. A typical 150hp outboard weighs over 400 lbs, plus controls and fuel. And you will have two of them. So they may well weigh more than the 20-22ft hulls (certainly they will weigh more than my complete Skoota 20 does, ex engine)

Furthermore, this weight has to be at the back of the boat

Think how wide a 20ft monohull speedboat is in order to carry that engine weight. But the whole idea of using a multihull platform is that you have long thin hulls to minimise wave drag. That's not what you are going to get if you use monohull sized engines on a small powercat. And that's why so many powercats are so inefficient

I would suggest you either look for a longer boat, or scale back your speed requirements to under 40 knots, so that you can use lighter, more fuel efficient engines

Richard Woods of Woods Designs

www.sailingcatamarans.com

(who has just returned from a 250 mile trip on his Skoota 20, average speed 10 knots, at 9mpg)

 

Not for planing cats; wave drag is already at minimum at high planing speed and we minimize friction drag.

I see only one problem of cruising on such small cat at 40kts: it will be uncomfortable due to high vertical accelerations as such cat is too small for 40ks of speed at seastate 2. This is all about weight-speed-seastate triangle.

I owned a 21' cat with 2x75HP that could run 34kts top speed; I used cruise 20-25kts. Some owners use this model with 2x120HP and I am sure they not cruise any faster.

 

I would agree that twin 60hp should sensibly be enough on a 20ft powercat, whether you have a planing or semi planing boat, because you'll never use more power to its full potential, yet have to carry around the extra weight of the bigger engines, which adds drag so you have to have bigger engines etc.

Motoring fast in a sea way is as uncomfortable as sailing fast. Most people "lift their foot" from the pedal in any waves.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Planing cats in the size range quoted by the OP typically need 2 x 100 hp or more for best performance, a lightweight planing cat is a bit of a menace offshore, the legacy of having two widely separated lifting surfaces that can produce some unnerving rotational forces in certain sea conditions. The antidote is a fairly heavy boat that doesn't bounce around so much, but ploughs along more like a semi-displacement mono might, thereby needing more power.

 

I doubt he sees it as a passage-maker, just a day-boat. A good cat in that size range can easily maintain 20 knots with sit-down comfort in some pretty horrible water, at most angles to the sea they leaves even the best monos in the shade. But they have their problems, ingestion of spray into outboards being one.

 

Can you narrow down the water conditions, wave heights, chop or swells that you will want to boat in with this boat?

 

I don't know about you guys, but I own two power cats, a 24 and a 26. The 24 is powered with twin 115 yamahas, and the 26 with twin 140 Suzukies. Both boats routinely run offshore in waves, and normally at speeds that would rattle your teeth on a monohull. At 4500rpm, on the fast end of cruising efficiency we get 34kn, at 2.45mpg on the 26, and slightly slower, but better fuel economy on the 24.

I routinely run offshore at high speeds easily over 30kn while running out to fish, and it is amazing how many monohulls are trolling out while we are running at speed.


Take a look at the Twin-Vee power cats.

 

Any weights for those boats, Stumble ? The hairy-chested offshore cat brigade here adhere to the high horsepower/heavy boat preference, though it virtually guarantees these vessels are incapable of rising on plane on one engine, even by flogging them mercilessly to get enough headway for a bar crossing return to port. Cats are good, but expensive to operate, which has kept them to a limited market segment. But I can guarantee there are people who are alive today who likely wouldn't be, but for the superior seaworthiness of them.

 

he is asking about power cats, not your sailing cats with an outboard on the back. there are plenty of shark cats with twin f115 yamahas in the 18 to 22 ft range and they are very efficent load carrying boats, converted sailingcats will of course be more efficient but they cannot carry much load.