Power multihull

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.

 

Rough Water and multi - hulls

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.

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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!

 

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.

 

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

 

Tunnel Troubles

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 !

 

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.

 

wonder what difference boats with a third flying hull make in a chop