Empathy List Archives

PR

Pat Reischmann

Fri, Sep 7, 2007 1:03 PM

I couldn't agree with Malcolm more, the inherent fuel efficiency and
particularly the prop protection are high priorities. As far as I am concerned
one of the big advantages to going catamaran is you DO have the possibility of
true prop and shaft protection. We have had customers hit the bottom at 15
knots with no damage other than bottom paint scraped off. I think hull
fineness has a lot to do with what speed you are looking to be most efficient
at. Clearly with a longer waterline at our current beam on the Manta we would
be more efficient in the pre-planning range of 9-12 knots. The Manta has an
11:1 ratio currently. The only other variable I see is how to shape the bridge
deck. I think most designs would benefit from more clearance than they have,
but you are fighting interior head room and dock appeal. I like Pete Brady's
thoughts on vapor dampening with the multi chine underwing.

I couldn't agree with Malcolm more, the inherent fuel efficiency and particularly the prop protection are high priorities. As far as I am concerned one of the big advantages to going catamaran is you DO have the possibility of true prop and shaft protection. We have had customers hit the bottom at 15 knots with no damage other than bottom paint scraped off. I think hull fineness has a lot to do with what speed you are looking to be most efficient at. Clearly with a longer waterline at our current beam on the Manta we would be more efficient in the pre-planning range of 9-12 knots. The Manta has an 11:1 ratio currently. The only other variable I see is how to shape the bridge deck. I think most designs would benefit from more clearance than they have, but you are fighting interior head room and dock appeal. I like Pete Brady's thoughts on vapor dampening with the multi chine underwing.

M

mrchuckjohnson@aol.com

Fri, Sep 7, 2007 8:24 PM

I have the impression that the CS hull is a full displacement hull and
does not plane. Do I stand to be corrected?

-----Original Message-----
From: Pat Reischmann preischmann@msn.com
To: power-catamaran power-catamaran@lists.samurai.com
Sent: Fri, 7 Sep 2007 6:03 am
Subject: [PCW] cs hull

I couldn't agree with Malcolm more, the inherent fuel efficiency and
particularly the prop protection are high priorities. As far as I am concerned
one of the big advantages to going catamaran is you DO have the possibility of
true prop and shaft protection. We have had customers hit the bottom at 15
knots with no damage other than bottom paint scraped off. I think hull
fineness has a lot to do with what speed you are looking to be most efficient
at. Clearly with a longer waterline at our current beam on the Manta we would
be more efficient in the pre-planning range of 9-12 knots. The Manta has an
11:1 ratio currently. The only other variable I see is how to shape the bridge
deck. I think most designs would benefit from more clearance than they have,
but you are fighting interior head room and dock appeal. I like Pete Brady's
thoughts on vapor dampening with the multi chine underwing.

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I have the impression that the CS hull is a full displacement hull and does not plane. Do I stand to be corrected? -----Original Message----- From: Pat Reischmann <preischmann@msn.com> To: power-catamaran <power-catamaran@lists.samurai.com> Sent: Fri, 7 Sep 2007 6:03 am Subject: [PCW] cs hull I couldn't agree with Malcolm more, the inherent fuel efficiency and particularly the prop protection are high priorities. As far as I am concerned one of the big advantages to going catamaran is you DO have the possibility of true prop and shaft protection. We have had customers hit the bottom at 15 knots with no damage other than bottom paint scraped off. I think hull fineness has a lot to do with what speed you are looking to be most efficient at. Clearly with a longer waterline at our current beam on the Manta we would be more efficient in the pre-planning range of 9-12 knots. The Manta has an 11:1 ratio currently. The only other variable I see is how to shape the bridge deck. I think most designs would benefit from more clearance than they have, but you are fighting interior head room and dock appeal. I like Pete Brady's thoughts on vapor dampening with the multi chine underwing. _______________________________________________ Power-Catamaran Mailing List ________________________________________________________________________ Email and AIM finally together. You've gotta check out free AOL Mail! - http://mail.aol.com

MT

Malcolm Tennant

Sun, Sep 9, 2007 12:17 AM

Chuck Johnson.

Dear Chuck,

The mathematical definition of planning based on monohulls of course is:- If
the vessel exceeds a speed/length ratio[in imperial measurement]of 2 then,
ipso facto it is planing. By that measure all the CS hulled boats are
planing.

However if you accept the physical definition that to be planing then the
mass of a hull must be dynamically supported by the downwards thrust of the
water deflected off the bottom [newtons third law] rather than statically ie
floating. Then the CS hulls definitely are not planing.

When we are designing a CS hull it is of little consequence whether it
operates in planing mode or displacement mode. What is important is that
when a CS hull is operating on the interface between the two fluids, air and
water, the point of maximum resistance ["the hump"] will be at approx Froude
number 4 and that for any CS hull with a particular set of parameters F4
will be at a particular speed. For this same particular set of parameters we
will need a certain amount of power to achieve a particular speed. And that
there will be a particular value of F number beyond which it becomes counter
productive to try to proceed. Assuming of course that you can get the
engines into the hull anyway.

In other words, it doesn't actually matter, when designing a CS hull whether
one considers that it planes, semi-planes, semi-displaces or operates as a
full displacement hull. What does matter are the numbers.

Regards,

Malcolm Tennant.

MALCOLM TENNANT MULTIHULL DESIGN LTD
PO Box 60513, Titirangi.
Waitakere 0642
NEW ZEALAND
Ph: +64 9 817 1988
e-mail: malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com

Chuck Johnson. Dear Chuck, The mathematical definition of planning based on monohulls of course is:- If the vessel exceeds a speed/length ratio[in imperial measurement]of 2 then, ipso facto it is planing. By that measure all the CS hulled boats are planing. However if you accept the physical definition that to be planing then the mass of a hull must be dynamically supported by the downwards thrust of the water deflected off the bottom [newtons third law] rather than statically ie floating. Then the CS hulls definitely are not planing. When we are designing a CS hull it is of little consequence whether it operates in planing mode or displacement mode. What is important is that when a CS hull is operating on the interface between the two fluids, air and water, the point of maximum resistance ["the hump"] will be at approx Froude number 4 and that for any CS hull with a particular set of parameters F4 will be at a particular speed. For this same particular set of parameters we will need a certain amount of power to achieve a particular speed. And that there will be a particular value of F number beyond which it becomes counter productive to try to proceed. Assuming of course that you can get the engines into the hull anyway. In other words, it doesn't actually matter, when designing a CS hull whether one considers that it planes, semi-planes, semi-displaces or operates as a full displacement hull. What does matter are the numbers. Regards, Malcolm Tennant. MALCOLM TENNANT MULTIHULL DESIGN LTD PO Box 60513, Titirangi. Waitakere 0642 NEW ZEALAND Ph: +64 9 817 1988 e-mail: malcolm@tennantdesign.co.nz www.tennantdesign.co.nz www.catdesigners.com

MT

Malcolm Tennant

Sun, Sep 9, 2007 12:24 AM

Pat,

There is another advantage to the CS hull too. The parallel propeller shaft
on the CS hulls means that the thrust is directed straight aft. Another few
percentage points of efficiency as the thrust varies with the sine of the
angle.

Regards,

Malcolm Tennant.

ps:-Morelli & Melvin and John Marples seem to agree with us too.

MALCOLM TENNANT MULTIHULL DESIGN LTD
PO Box 60513, Titirangi.
Waitakere 0642
NEW ZEALAND
Ph: +64 9 817 1988
e-mail: malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com

Pat, There is another advantage to the CS hull too. The parallel propeller shaft on the CS hulls means that the thrust is directed straight aft. Another few percentage points of efficiency as the thrust varies with the sine of the angle. Regards, Malcolm Tennant. ps:-Morelli & Melvin and John Marples seem to agree with us too. MALCOLM TENNANT MULTIHULL DESIGN LTD PO Box 60513, Titirangi. Waitakere 0642 NEW ZEALAND Ph: +64 9 817 1988 e-mail: malcolm@tennantdesign.co.nz www.tennantdesign.co.nz www.catdesigners.com

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cs hull