Analysis / comparison
Multihull Construction options - Knots per dollar

by John Shuttleworth Yacht Designs Ltd - web site here

Click links to view... All images! or Dogstar40.gif - Dogstar50_1.gif - Dogstar50_2.gif

Subject: [MHml] Re: Knots/dollar
Date: Fri, 8 Feb 2002 17:20:32 +0000
From: John Shuttleworth <john@jshuttleworth.fsnet.co.uk>
To: multihulls@steamradio.com

Larry Forgy wrote
snip
>Now the question is, suppose you have two possible materials to build your
>boat. One is light and expensive and the other is heavy and cheap. Which
>to chose to get the best performance for a fixed amount of money?
>
snip

This is an interesting question. Not that simple to answer, because a lot of factors would come into play in the design of the two boats. But I have some information to offer.

I have recently been designing a 50 ft Cruiser racer cat that is designed to beat any 40 ft racing tri that is around in the UK at the moment.. And then also at a 40 ft offshore racing tri. As a reference this 50 ft Cat is a development of the Tektron 50 you can see on my site in images at www.john-shuttleworth.com

Both the new 40ft Tri and the 50 ft Cat are in my latest brochure.

We discovered that to beat a 40 ft racing trimaran of all up weight of 2500 Kgs and sail area of 125 m2 ( built in Glass Epoxy and foam) we would need to design a 50 ft cat with an all up weight of less than 4600 Kgs and sail area 160 m2. At this weight the 50 ft Cat would be faster than the tri in all strong wind conditions and the same speed in light airs. So we decided to see what it would take to get the weight under 4000 Kgs to guarantee a faster boat.

For the 50 ft Cat I did the structural design for 5 different technologies. namely (prices in UK pounds)
Glass all foam Materials cost £36000 weight 4200 Kgs
Glass foam with Nomex topsides, bulkheads and decks £41000 weight 4170 Kgs
Glass all nomex £48500 4090 Kgs
Carbon all foam £46500 4070 Kgs
Carbon all nomex £58000 3920 Kgs
All using epoxy prepreg.

the weights are from racing trim including 3 crew.

Then I redesigned the boat eliminating as much surface area as I could while still retaining the accommodation. this meant slimming the hulls reducing the freeboard, and taking out every piece of the boat that I could without affecting the structure.

The cost for All Carbon all nomex prepreg of this new version came down to £ 45200 and the weight to 3540 Kgs.

On the 40 ft Tri design I looked at Wet Epoxy layup versus prepreg and for 4 different constructions. and two types of design. One with a flared main hull and the other just a slim minimum main hull. the range of constructions was
Carbon nomex prepreg
Carbon foam prepreg
Carbon nomex wet layup
Carbon foam wet layup.

The lightest total shell weight was the Carbon Nomex prepreg slim hull version 415 Kgs with a materials cost of £ 14434 The heaviest was the Wet layup Carbon foam flared hull version with a weight of 643.9 Kgs and a cost of £ 11853

The carbon nomex prepreg flared hull version cost £15746 weight 452.2 Kgs So once again the less boat there is the cheaper and the lighter.

The final total weight of the slim hulled 40 ft racing tri would be 2000 Kgs in racing trim including 3 crew.

The approximate overall cost of the 50 ft cat would be about £ 500,000 and the 40 ft Tri about £ 250,000

The problem is that the accommodation of the two boats is not comparable. But what can be seen is that whatever boat you are building it is worth paying very careful attention to eliminating surface area. Otherwise you could find that for the same cost you could build a faster boat in much better technology. For instance in the case of the 50 ft Cat the new design in Carbon nomex prepreg costs less the carbon foam version of the larger volume cat. and weighs 530 Kgs less.

This exercise also shows that the materials cost in the hulls is relatively small compared to the labour costs and the cost of disposables, equipment,deck gear, and rig.

I hope this is useful

I can Email jpegs of the 40 ft Tri and the 50 ft Cat to anyone interested. files will be about 150 K. each.

Click links to view... All images! or Dogstar40.gif - Dogstar50_1.gif - Dogstar50_2.gif

Regards

John Shuttleworth

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