Planing Power Cat plan and hull type question

[jcros]

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.

[Alik]

Generally asymmetrical hulls are intended for higher speeds.

[jcros]

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.

[Alik]

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

[u4ea32]

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.

[Mr Efficiency]

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.

[Richard Woods]

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)

[Alik]

the whole idea of using a multihull platform is that you have long thin hulls to minimise wave drag.


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.

[Richard Woods]

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

[Mr Efficiency]

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.

[Mr Efficiency]

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.

[IMP-ish]

I should have added that intended use of the boat is for recreational fishing, cruising, and skiing/tubing. Inshore and some moderate offshore.Can you narrow down the water conditions, wave heights, chop or swells that you will want to boat in with this boat?

[Stumble]

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.

[Mr Efficiency]

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.

[whitepointer23]

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)

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.

[tunnels]

Generally asymmetrical hulls are intended for higher speeds.

Totally agreed !!:D
thats were they are at there best !!going fast !![/U][/B]The height above the water of the tunnel and the way it compresses the air between the two hulls lifts and holds the boat in surface effect !
Have you heard about the term surface effect ??

In choppy water they are like nothing you have been in before !!! the worst thing drivers can do is ease off the throttle !!
Thats the time you give it all its got trim to its optimum and hold on . The boats are designed to fly!!! litterally .

Cats are on the other side of the coin and the hulls and configeration do not work the same way . Two hulls ,two motors ,and a big gap between (high above the water tunnel roof ) the hulls are normally identical in shape but some are not . They are stable plateforms for fishing , ride well in seas and do not need to be 100 % aerodynamic .
Althought they can and will travel fast given enough horse power and balanced properly they do not behave the same as a tunnel hulled boat !!. The tunnel of a cat does not perform the same function as a true asymmetrical tunnel hull boat !!
Know and fully understand what you are looking at !!
Its important also while you are just looking that you come to grips with the major differances between the two differant boat designs .
You need some seat time in both !!! if you dont know much about either then riding and driveing in both in the same sea conditions .
Reading and trying to come to grips with whats good and whats not can only come from first hand exsperiance and getting out there and doing it !!
What you like and how you drive will not be the same as what the other guys does so it then becomes personal .Remember also about weight and balance in either of these hull forms .
Weight kills speed !! and in the wrong place can really upset balance and stability and safety at sea !!.

Me i am a true and tried Tunnel person hence my name TUNNELS . :P

[Mr Efficiency]

OK Tunnels you're a tunnel hull man, any successful ones in the size range and intended usage mentioned above you can show us ?

[FMS]

he is asking about power cats, not
...
converted sailingcats will of course be more efficient but they cannot carry much load.
Based on what jcros is looking for, you are right. But I'm glad that Richard put Skoota into the mix for the extreme fuel efficient option.

[Stumble]

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.

The 26 is around 4500lbs, the 24 is about 3800. But I am pulling this from an admittedly poor memory :D. I know the 24 was 4000 including the duel axel trailer without fuel, for shipping weight but I can't remember the exact specs off hand, and Twin-Vee has redesigned the boats too much to reliably pull the data from them now.

Their boats are true planing hull cats,with a reasonable high clearance in the tunnel, and only minor sneezing. Heck, I loved the 26 so much I bought the 24 as a dive boat for down in Jamaica where it routinely run 20 miles out with 5 divers and 8 tanks at 30kn or so. Into the open Carribean sea (headed mostly south from the south coast of Jaimaica.

I am looking for my fuel flow charts for the boats. If I can find them I'll put them up later today.

[tunnels]

OK Tunnels you're a tunnel hull man, any successful ones in the size range and intended usage mentioned above you can show us ?

Used to work with and for Scott Robson and we made a production 5 mtr Tunnel Called a KONA with a pair of 55hp suzuki and used to go well ,the same boat with a pair of Yamaha 60s was really good ,and with a pair of 70hp omcs was boardering on dangerous .
The 22 footer we were making had 2X 140 hp Yamahas and went good but i always felt the 175 hp twins set up would have been the ultimate . At that time Scott had a 30 footer with a pair of 200 yamahas and that could almost fly . All these boats in choppy water were so comfortable to ride in even the little 5 mtr boat .
We took a 5 mtr out in the open sea and played for hours jumping waves just like a jet ski . Even a novice boatie in less than 5 minutes was jumping waves !! Fantastic !! Ever since then i was hooked .In a moderate sea it was possible to simply set the boat going at a comfortable speed and not touch the wheel and it would track in what ever dirrection you set it at . In the calm water of a harbour the same stand on one side and it would slowly turn a little stand on the oher side and it would come back again . BALANCE !!
TUNNELS DONE AND SET UP PROPLERLY ARE EXSTREMELY USER FRIENDLY even a 10 year old kid can drive one safely !! The 5 mtr was a family boat after all !!:D



Scott Robson power boats . tunnels not cats !!!!
Last i heard he was still based in Christchurch New Zealand , possibly has a website ! www.robsondesign.co.nz/ - Found this but the site is underrepair . Has a boat called a stealth 22 thats worth a look at !!

[Mr Efficiency]

The 26 is around 4500lbs, the 24 is about 3800. But I am pulling this from an admittedly poor memory :D. I know the 24 was 4000 including the duel axel trailer without fuel, for shipping weight but I can't remember the exact specs off hand, and Twin-Vee has redesigned the boats too much to reliably pull the data from them now.

Their boats are true planing hull cats,with a reasonable high clearance in the tunnel, and only minor sneezing. Heck, I loved the 26 so much I bout the 24 as a dive boat for down in Jamaica where it routinely run 20 miles out with 5 divers and 8 task at 30kn or so. Into the open Carribean sea (headed mostly south from the south coast of Jaimaica.

I am looking for my fuel flow charts for the boats. If I can find them I'll put them up later today.

That probably is on the light side here, I know a popular 23 footer here that could easily go 4 tons over the weighbridge on trailer, they weren't made to break.

[Wavewacker]

I had trim tabs on a V hull and used them often while people moved about but I never noticed any fuel efficiency, I didn't notice fuel efficiency at all, just filled up. Do tabs help with fuel efficiency?

[keysdisease]

Yes. Boats are at optimum trim on an even keel and at a particular bow up angle. Due to loading and conditions like stuff and people a vessel may not be in optimum trim. People may also move around while the vessel is underway.

Trim tabs allow the operator to adjust trim while underway and to keep the boat trimmed properly. They can also increase fuel efficiency and range by allowing a vessel to stay on a comfortable plane at a slower speed than it might be able to without tabs. This allows the vessel to travel on plane at a lower throttle setting.

Boats float and move fine without trim tabs, they don't "need" them. But they will get on plane faster, stay on plane at slower speeds, and stay at trim regardless of changes in load and conditions with trim tabs with the touch of a button.

Steve


I had trim tabs on a V hull and used them often while people moved about but I never noticed any fuel efficiency, I didn't notice fuel efficiency at all, just filled up. Do tabs help with fuel efficiency?

[Wavewacker]

Thanks, I thought so, but had not really noticed, but that was when fuel was cheaper...lol. Then what would the effect of tabs be on a cat? Not to get OT, but if it was a sailing cat modified as a motor, would the tabs make up for the rocker of the sailing hulls? Design longer or wider tabs?

[Richard Woods]

a few quick points

I try to design efficient boats, not just boat that float and move along. So a sailboat hull can never be a good motorboat hull, or vice versa of course

So for this application I would try to maximise the speed/hp ratio rather than try for maximum speed

My power cats have totally different hulls from my sailing cats

A 20ft geosim of the 24fter you mention will have less than 60% of the displacement of the 24. So it can never have the same size engines

I have written it many times:

A planing boat needs a lifting surface, so to be efficient needs a high AR. A powercat hull has a very low AR, typically say 10:1. So it can never be an efficient planing boat UNLESS it develops lift in some other way, most obviously by using the air lift from under the bridgedeck. But that only sensibly works at high speeds.

And after this debate, what does the OP think now?

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Jimboat]

jcros - check out this web site (http://www.aeromarineresearch.com/stbd2.html).

[whitepointer23]

Based on what jcros is looking for, you are right. But I'm glad that Richard put Skoota into the mix for the extreme fuel efficient option.

i agree, i like richards designs too and one day i will change over to an easily driven hull myself.

[Mr Efficiency]

Used to work with and for Scott Robson and we made a production 5 mtr Tunnel Called a KONA with a pair of 55hp suzuki and used to go well ,the same boat with a pair of Yamaha 60s was really good ,and with a pair of 70hp omcs was boardering on dangerous .
The 22 footer we were making had 2X 140 hp Yamahas and went good but i always felt the 175 hp twins set up would have been the ultimate . At that time Scott had a 30 footer with a pair of 200 yamahas and that could almost fly . All these boats in choppy water were so comfortable to ride in even the little 5 mtr boat .
We took a 5 mtr out in the open sea and played for hours jumping waves just like a jet ski . Even a novice boatie in less than 5 minutes was jumping waves !! Fantastic !! Ever since then i was hooked .In a moderate sea it was possible to simply set the boat going at a comfortable speed and not touch the wheel and it would track in what ever dirrection you set it at . In the calm water of a harbour the same stand on one side and it would slowly turn a little stand on the oher side and it would come back again . BALANCE !!
TUNNELS DONE AND SET UP PROPLERLY ARE EXSTREMELY USER FRIENDLY even a 10 year old kid can drive one safely !! The 5 mtr was a family boat after all !!:D



Scott Robson power boats . tunnels not cats !!!!
Last i heard he was still based in Christchurch New Zealand , possibly has a website ! www.robsondesign.co.nz/ - Found this but the site is underrepair . Has a boat called a stealth 22 thats worth a look at !!

I see pics of the 22 on the net and it's not what I'd call an offshore boat. Think bar crossings might be hairy in that one.

[tunnels]

I see pics of the 22 on the net and it's not what I'd call an offshore boat. Think bar crossings might be hairy in that one.

Yeah i totally agree ! Its a scream machine . But the tunnel thing is a beautiful ride in the chop like nothings i have ever experienced before and the ease of driving !!. Tunnels don't have to have twin motors all the time . they go just as well with a single motor The same 5 mtr hull was also a open center console boat as well and with a single Yamaha 90 was a pleasure to drive and very economical as a fishing going far away places boat .Still had the same twin fuel tanks so had a long range for fishing and just cruising all day long !!I have some pictures buried some where must have a look see and see what i can find !!:D

[APP]

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. .......
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.......
.

Very good!
By the way do you know what is the typical or preferable range of the Deadrise Angles in degrees (for symmetrical hulls)?
Let's assume speeds 20-30 knots, V hull short power cats of 9 to 12 meters, offshore cruising, a less rough sea and the minimum requirements of comfortable riding.
I also heard that variable deadrise mid to aft is not much recommended.
Thanks
APP


{PS: It seems a good compromise could be a 20 to 25 degr. deadrise}

Added: 5-Dec-2011
-------------------
From a couple of articles about deadrise
1) From: http://www.navaldesign.co.za/articles/Planing%20Hulls%2007%20Oct%202006.pdf

2. Deadrise
Deadrise is the angle a hull bottom makes with the horizontal plane viewed from ahead or astern. The right amount of deadrise gives a boat directional stability, a softer ride and reduces wetted surface drag as the boat rises on a plane. Deadrise is said to be “constant” if it stays approximately the same from midships to the transom. Deadrise is “variable” if it changes from a deep angle at midships to a shallow angle at the transom.
For inshore crafts, deadrise can be about 10 – 12 degrees from the midships aft, increasing from midships as you go forward towards the bow.
For coastal craft, deadrise should be 15 to 20 degrees from midships aft, increasing as you go fwd towards the bow.
For offshore boats, deadrise should be 20 to 25 degrees from midships aft, increasing as you go forward. Some very high speed offshore boats use deadrise in the afterbody as high as 26 – 30 degrees. This is to soften the impact of reentry when the entire boats jumps clear of the water and slams back down, at speeds in excess of 50 knots
In general, the deadrise angle determines at what speed and seastate a planing boat can best power.

2) From: http://www.discoverboating.com/newsroom/faqs-advice.aspx?id=11&qid=57
Deadrise is the amount of V-shape on the bottom of a boat’s hull, measured in angles at the transom of planing powerboats. In general terms the larger the degree of deadrise the more V-shape of the hull, so a flat bottom boat will have a deadrise of 0 degrees, while a boat with 24 degrees of deadrise will have a deep V hull.

In terms of performance the greater the V-shape the better the boat will slice through the waves. The shallower the V the more it will push over them. So a big deadrise is better for rough water conditions and a small deadrise is better for smooth water. The more a boat slices through the waves the more energy it will use. Thus boats designed for rough waters have a greater degree of deadrise and are slower. Boats designed for smooth water have a lower degree of deadrise and are faster. For example, a ski boat designed for speed and water sports, used on a lake will have between 10 and 17 degrees of deadrise. On the other hand, an offshore sportfishing boat used on the gulf stream off the coast of Florida will have a deadrise greater than 20 degrees.

[Krauthammer]

Morelli & Melvin designed a 22 ft Foiler that was built by Corsair in San Diego. It run with both singles and twins, with or without the foil and is regarded as the best small cat ever. The molds were sold but M&M would know their whereabouts.