What a great information stream!

Malcolm if I might ask, I have some follow up questions:

• If a one needs say 50 hp to move a cat at say 10 knots, what are the pros and cons of choosing a 75 hp engine over a 150 hp engine.

-I have read that diesel engines should be run at approximately 70% of their rated horsepower.  Why is that?

-I have noticed the rating (200+ hp) of the Escape model is higher than I, the neophyte, would expect in an economical catamaran design. The graph I have seen shows this boat uses 1.5 liters per engine at 10 knots.  Is it alright to cruise for long times at 10 knots with engines this big?  I am looking for a catamaran design which will cruise a approximately 10 knots and have the ability to "run away" from weather at 17 knots.  With that in  mind, what size engines would you put on  a boat design like the Escape?

What are the design differences between the Escape and Ragtime models?  While they are approximately the same length, the performance specs and recommended engines are different.  Would the hp needs at 10 knots and 17 knots be similar for each model?

Thank you in advance for the support and knowledge you bring to this group.  It is appreciated!

Brian

Arild,

It is 0.238l or 238ml. The system used is immaterial. What is important  is
that a 400hp engine will use roughtly the same amount of fuel to  produce one
HP/KW for one hour as a 50hp engine.

As a designer I use the metric system because it is much easier and  systematic
than Imperial and I use grams instead of litres because I am constantly
concerned with mass [weight/displacement]. Designers generally tend to  have
slightly different concerns to the everyday user.

I hope this helps.

Regards,

Malcolm Tennant.

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Brian Knesal.

I am not an expert on diesel engines, I rely on the advice of those who
know. However I will try to anwer your questions.

Generally you would choose an engine with just a little, say 10% more
horsepower than you need for whatever is your top speed. This makes
allowance for wind and waves but of course you usually slow down in the
rougher conditions anyway. There really is no point in chosing an engine
that has 3 times as much horsepower as you need.

It is not uncommon for people to get a much larger horsepower engine than
they
need and then run it a very reduced revs in the belief that they are
prolonging the life of the engine. This is not a smart move. it may
invalidate your engine warranty and probably damage your engine. If you are
doing long hours, long distances etc get a commercially rated engine. All
this means is that the engine will then be governed and injected to develop
its maximum horsepower at lower rpms with the result that the maintenance
intervals will be longer.

Aside from the "hand grenade" engines of dragsters, reciprocating engines
cannot be run at full throtle for ever.

The engines in the Escape are the size they are because it does 25 knots top
speed in full load displacement. The HP, gearbox, reduction, propeller
parameters etc are generally all based on top speed in full load. 10
knots is probably the worst speed you could possibly chose for the Escape.
It is right on FN.4 for the hull ie: the point of maximum hull resistance.
You would use less fuel at 15 knots. The Escapes ideal cruising speed
appears
to be 19 knots at which speed I believe it is using a total of 2.2 litres
per nautical mile.

The Escape is a more recent design than the RagTime and as such has a more
refined hull form[ie more efficient] and has our contemporay wing deck
structure and shape.
Any differences are largely attributable to the Ragtime having 4 tonnes more
full load displacement. They probably have different speed/length and
displacement/length ratios.

I hope this helps but you will need to consult a diesel engine expert for
more detailed [accurate] information.

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