Power-speed graph displacement catamaran

This is the same displacement cat as above, but with some nice graphs and fuel data right down to 4km/hr, which should give an idea of how resistance increases from 4 to 8 km/hr.
http://suzukimarine.com.au/assets/Uploads/2xDF90A-Argus-E35-Costal-CruisingCat2.pdf

 

Thanks.


Does anyone know a way to easily approximate the effect of motoring into a headwind? Ie, at what wind speed would the boat be unable to make headway.

 

As always google is your friend eg

http://books.google.ca/books?id=_ti...q=ship windage resistance calculation&f=false

Richard Woods

 

Thank you Richard. But... Math? That's why a went to law school instead of engineering.

 

Windage and air drag are a big deal, especially on powerboats, yet are often ignored. Unlike sailboats they go straight into the wind and may well have 12-15 knots apparent wind even in a flat calm

So you always have to calculate the drag, and as Molland (my old head of college BTW) states, it can be as much as 6% total drag even on a ship

Some years ago I was motoring my 32ft Eclipse sailing catamaran down the ICW past Cape Canaveral. I was motoring at 4 knots into a 30 knot head wind - flat water of course. Ahead of me was a larger "Grand Banks" style trawler yacht. He was motoring at the same speed. By chance we moored next to each other at the end of the day. So I asked him "why were you going so slow" he said "too much windage, I had my (55hp) engine at full throttle" I had a 9.9hp and had it not been such a narrow channel I could have sailed. The trawler yacht had no alternative and at some stage would have come to a stop.

I have been stopped when motoring a 35ft sailing catamaran, again with a 9.9hp outboard, into a 50knot headwind. Of course headwinds are actually easy - you just come to a halt - it's the diagonal winds that cause problems when underpowered as they blow the bows off and you end up doing donuts.

That is all flat water of course, waves slow the boat even more

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I would have never guessed that a trawler would have been so limited in speed at those headwinds, at least not without some wave action to make it worse.

My issue here was to try and figure out at what headwind the proposed cat would be unable to make headway. Given such a paltry amount of available power (4.4kw propulsive power from the 8kw nominal engines) it seems that any decent headwind would park the boat.


The more numbers I run (not necessarily correctly of course) the sillier this seems. It's the decision to specifically exclude sails and the refusal to install backup dc generators that boggles my mind. Either alone or in combination make this a much more reasonable proposal. It may still have some major downsides, but at least I could see it work.

 

Moral of the story is boats that have high and wide superstructure are particularly susceptible to headwinds, and anything with pretensions to effectiveness with low power just can't have them. Even in strong crosswinds, a boat whose below water side profile 'centroid' is out of whack with the above water one, will be retarded by the helm required to keep it running straight.