Power multihull

[taniwha]

How seaworthy are power cats?
When will be the first circumnavigation by a serie production powercat? I am not talking of extreme design such as earthrace but just regular affordable powerctas under 45'.

[CORMERAN]

To taniwha :

I have designed and built, both: Power Cats and Power Trimarans.
That are all seaworthy vessels.

That is not the issue.

If you check out all the 'round the world, record holders over
the last few years - sail or power - you will find many similarities.

First: - they are pretty well, all over 45 ft.
( 80 to 100 ft is the general range.)
When you're in the Southern Ocean
......size DOES matter !

Second: - they are ALL, custom made boats.
( See above. You don't want a production boat,
built down to a price, when you are far from land.)

Third: - power boats are thirsty beasts.
( Even multi - hull power boats.)
It is hard to get big enough fuel tanks
into a 45' vessel - to cross an ocean - and still
have room for a crew.

Finaly; Earthrace looks Rad..........However, it follows some
very basic and essential rules:

As above:
- About 80 ft overall.
- Custom made. Very light; but STRONG
construction - out of carbon fiber.
- Very long and thin hulls that will give excellent
fuel efficiency.

Cheers !

[catmando2]

Hi guys,a couple of comments. we're doing a fifty foot powercat at moment that will have a 2000 + nm range. She will be light, around 4700kg dry and low powered,65hp each side, so not that thirsty, doing around 10kn. . We have no intention of doing southern ocean work in her but I will have no problems doing trips out to Vanuatu and New Caledonia in her.

Like Coremeran says, you won't get a boat like this of the shelf, we're building ourselves.

Have a look in photo gallery

Dave

[CORMERAN]

To catmando2:

Your vessel looks fine and your specs. are appropriate
to LONG range travel.
Also, you indicate a reasonable and seamanlike attitude,
to the realities of going offshore.

However, taniwa, used the ambitous word: " circumnavigation ".
So I was gently suggesting, that successfull circumnavigators
- historicaly - tend to lean towards longer craft.

It's true, a few brave souls have gone 'round the world,
in pretty tiny boats.
- And the reason their voyages are celebrated, is the proof
in it's self, as to how risky such an endevour can be.

Cheers !

[catmando2]

Indeed you are right coremeran, cicumnavigation is the word used, but does that mean it has to be a non stop trip or can we coastal hop until getting to jump off point. And yes. I reckon id love to have a forty foot acommidation cat on 80 ft hulls,but gee 50ft was as big as I could get in the back yard. + don't have the bottomless pit of money so 50ft sounds just fine and dandy.

Dave

[CORMERAN]

To catmando2:

We appreciate that you are working to the limits of
your situation.
Also, you are right - what are the terms of reference ?

The 80 footers I was referring to, are mostly, in respect
to people trying to break records. If you take away the
urgency of the a time element - this gives you a lot
more leeway.

Also: 50' doesn't sound much different than 45' to the
layperson.........but it is.
- Given that everything about boats seems to be
mulitiplied by either the square or cube root.......
...and a cat has 2 hulls....

- The load w.l. length can be seen as being increased,
in effect, up to 10 ft.
- and presuming a single hull beam of not more than 10 %
or 12%, of each hull's w.l. beam = maybe 4' ?
- with a draft of approx. 18" to 2' ?
That gives you potential increase in fuel tank cap.
over a 45' cat of:

- At least, 1.5' x 4' x 10' = 60 cubic feet.

So at your conservative projected cruise of 10 knots, that
gives you a lifesaving, increase in range
- over a 45' long vessel.

Good luck in your endevour.

[catmando2]

Did'nt think 80 footers applied to the real world

Dave

[taniwha]

When I say circumnavigation I actually mean longer distances. I have no problem in circumnavigating in 2 or 3 years. Or in 25 weeks such as a Nordhavn 40 Off-The-Shelf or such as the Buehler 50 footer aluminium dieselduck Idlewild with 50 hp !!! Look at trawler web.com or dieselducks.com and you will find plenty people circumnavigating under power. From the multihull builder I hear only that they boat CAN do it or will do it. I only wonder who is going to do it. I do not trust the brochures when the Nordhavn rally crossed the atlantic with 19 trawler they really made their point. If I see 19 power multihull crossing the Atlantic I might consider buying one.

[CORMERAN]

To taniwa:

I was born in South Africa. Therefore, I have,
I flatter myself, an understanding of the South African mindset.
So, I will not be so foolish as to stand in your way
- or try to discourage you - if you have set your sights
on an adventure.

As a designer, I concurr with your unwillingness to
accept at face value what people's brochures say.
Many lay people do not realize that there can be
enourmous differences between one multi - hull
and another.

- Are they:
- Hard chine, planing hulls ?
- Round chine, displacement hulls ?
- Fat beamed hulls that get filled up, with heavy " essentials " ?

However, if I am personaly involved, the vessel
designed by me - WILL be capable of off - shore voyages.

i.e.
I designed a SEAWORTHY multi - hull ferry, that was intended
to take people through very rough seas - to a former prison.
Formerly the residence of a famous S.A. president.
I'm sure you have heard of him.

Speaking of said, rough water:
I would strongly suggest that you put the brochures aside,
turn off the computer - and get some of the reps.
of the various vessels considered - to take you for some
demo rides off - shore, in those same waters.
Not far from your front door - is some of the worst conditons
a vessel will ever face.

I have never been reluctant to prove the capability
of MY boats to potential clients.

One memorable time, after jumping waves at twice the speed
of any other vessel brave enough to go out to sea - that
stormy day - my client became an instant beliver.
As soon as we tied up at the dock - and his knees stopped
shaking - he quickly wrote out a cheque to retain our services.

As our American cousins say:

Money talks - and B.S. walks !

Cheers and good luck !

[tgwhite]

A great read is the setsail website by Steve and Linda Dashew. Lots of good information showing the design parameters of the monohull WINDHORSE. I posed the question to Steve, "Why not a cat design hull?" His answer was about lilke this - "For circumnavigation the hull connections are in the big waves and receiving loads that require massive and heavy structures to keep the boat from breaking up."

Further he says, a circumnavigation designed power cat is not fuel efficient thus requiring big tankage as compared to a long thin monhull. Steve puts his money where his mouth is and has greatly advanced the business of hull design, sail vs power and safety at sea. I daresay he has more miles under his bottom than any other power cruser of the last few decades.

PS -I own no stock in his enterprises.

[taniwha]

well at least Steve Dashew went at sea with his own design now if he could make his design affordable it would be perfect. Again I wonder when the multihulls guys are going to go around. Cormeran shall we take uo the challenge?

[FAST FRED]

"now if he could make his design affordable it would be perfect. "

Smaller boats cost less , how big does it really need to be?

FAST FRED

[taniwha]

around 44ft, big enough to liveaboard, small enough to handle with two and kind of affordable.

[CORMERAN]

To tgwhite:

One thing Mr. Dashew and I agree on is:

- LONG and THIN !!

.....wether 'tis mono or multi.

I will admit I have a fondness for multi - hulls.

However, I also agree with Mr. Dashew - re:
the interconnections he is concerned about.
- They are also of great concern to me, as well.

I feel that the design I'm presently working on
- addresses these issues.

Given the proprietry nature of this work in
progress I'm not quite ready to show what I'm
doing..............just yet.
But I will soon.


To taniwa:

My intention IS - to produce the design I refer to
above - and take up the CHALLANGE.
Of some serious off - shore voyages.

Cheers.

[taniwha]

I am definitely looking forward to see your design. I would tend more toward monohull due to their experience but I am definitely an open minded person that why I would love people to demostrate me the offshore capability of a powercat. In 6 months time I will join a delivery of a powercat from Cape Town to the Caribbean which should make me change my mind on multipowercats .....or not.

[Frosty]

The thing about power cats --and all cats is the bridge deck and its height above water. Even when the height is considerable the banging when the waves exeed this hieght is unbelievable. My 44 foot 14 ton semi displacement with surface drives can cruise all day at 17KTS-- 24max,- but when the seas get 1 meter you start to hear the occasional 'slap' on the deck. Then at 2 meters its just intollerable. You feel like the boat will be destroyed, it is not a soft ride but a very severe banging. Apparantly this is normal, and when you think about -- well I suppose it is, unavoidable!!!

2 meters is not what I call rough and when I say 2 meters I mean from trough to crest, this is what the weathermen call a 1 meter wave.

[CORMERAN]

To jack frost:

your comments are appreciated - as they hit on
an area that I consider critical.

Possibly,one of the most neglected areas, in power
multi - hull design - is the tunnels between the hulls.

- Why, so often - so many FLAT surfaces ?!
That beg to be pounded ! In heavy seas.

- Why, so often - are the tunnel ceilings so LOW to the water ?!
That beg to be pounded ! In heavy seas.

Partly because people like to sell " roomerans ".

- Suffice to say, my off - shore designs, have deep tunnels
with forgiving curves to reduce the incidence of " thump !!"

To taniwa:

I might make a convert of you yet.
When I finaly get sailing.

Not to give too much away - lets just say - that various navy
designs, the Earthrace boat and I, are moving along similer paths.

As one British Naval proclamation stated:
It's a "..... stabiliized monohull." Apparently, they were loath to
say outright, that it was a trimaran.

However, there is a certain wisdom to their statement.

```````````````````````````````````

Gentlemen; it's been fun.........however.........

I must take my own advice, and turn off the internet computer
- and turn the design computer back on.
Those annoying clients are getting demanding....

....will get back to you when something hits the water.

Cheers!

[Frosty]

I can park the dinghy underneath, with the outboard on and I can wash the boat lying in the dinghy--- a yardstick I use to check bridge deck height!!. At 44 feet it already looks high, and out of the water it starts to look odd and looks very high. Other cat ownwers have said to me with a chuckle in there voice ' well its a cat thats what they do, didnt any one tell you they do that'.

Well no they didnt and me and the wife had an awakening on our first trip. However once you know what it is and you know its not going to break.

In calmish waters my Seawind Ventura rides like you were walking on a trampolene. before in other boats I would turn into ferry wakes ,now I dont feel them. Unless they are more that a meter then "bang"

No matter how big the bridge deck is it can not exeed the wave hieght of all seas. You just have to come off the plane and criuse at 12kts. For me with surface drives that is such an uneconomical speed so I have to drop to 6, which is tick over for the engines. At that speed it looks like I have jets. This is an acceptable speed for a sailing boat but after doing 17 it feels like I am stoped dead.

BUT on a sunny calm day and the engines at 3400- (max 4200)-17KTS and the props are starting to dry out It drones like a Lancaster bomber,-- its the buisness. Oh 35 liters per hour 20% threretical prop slip.

Just one more thing surface drives dont vibrate --ATALL they cannot cavitate but airiate, air is pulled from the surface at atmospheric pressures and are not holes ripped out of the water as is cavitation. The shafts are only 4 feet long and 2.5 inches if alligned there is nothing to vibrate.

[catmando2]

Seems we're agreeing that 10kn is a pretty good speed for all conditions and that forty foot cats look pretty tall, which is why i'm doing 50ft with 40ft accom. and only chasing a 10kn cruise,smaller motors,more range less $$$/nm.

Dave

[CORMERAN]

To jack frost:

There are multiple design considerations around the
configuration of bridge decks and tunnels.
It sounds like your tunnel height is not too bad.
- A rule of thumb used by some designers is
that said height should be at least 7 % to 8 % of
the vessel's waterline length.
- In your case, that would require a clearance of
at least a meter - I'd guess.

However, there are other concerns - like how wide
the tunnel is in proportion to the depth.

- Another rule of thumb is:
that MOST boats are most happy in waves of
10 % of their waterline length - or less.
( Note: I said MOST boats. This does not includ
foilers, flarecraft or hovercraft.)
Your experience seems to demonstrate this rule.

A rule of seamanship is; if a vessel is taking a
lot of punisment: - Slow down.

So given all the above - and putting aside rebuilding
your boat......just now - a few solutions to consider:

A. Perhaps a different prop.
All surface drive propellers are not the same.
With a different geometry - suited more for the
mid - range and lower - you will lose a few knots at
the top end, but your cruise speed will not suffer
too much.
Most important, this will make your vessel more
flexible and therefore, more seaworthy - as you
will be better able to fine tune your speed to
suit prevailing conditions.

B. Quartering the seas.
By turning the vessel, to an angle away from
- head on into the waves - slamming can be reduced.
It's a bit like a sail boat, tacking back and forth,
going upwind.
It might seem like you are losing headway, if you
have to veer off your course.
However, if you " tack" back and forth - with less
slamming - your can cruise at a higher speed.
So your actual time to go, a given staight line distance,
can be less - by doing this.
If nothing else, your passengers will be more relaxed.

Cheers !

[Frosty]

Cormeran, Well yer Ok but at 13-15 she is coming off the plane,theres not a boat in the world can fight that hump, your either on or off. Although it is semi displacement there is deffinately the obvious feel of off or on.
yes I do 1/4 the waves but you know she doesnt move like a speed boat, at 44 foot and 14 tons with 1 1/2 tons of fuel Jeeees you would be fighting it trying to dodge waves. But come off the plane and slow down is all you can do. I generally head for an Island somewere where and put the kettle on.

[yipster]

it is not a soft ride but a very severe banging. Apparantly this is normal, and when you think about -- well I suppose it is, unavoidable!!!
wonder what difference boats with a third flying hull make in a chop
http://www.hydrofoildesign.com/Images/Photos/Inovation/A5_3.jpg