Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 11-03-2009, 10:00   #16
Senior Cruiser
 
Cheechako's Avatar

Cruisers Forum Supporter

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 19,369
30hp X 2 on a 42 cat was not enough in some situations. OF course that was the Yanmar high rpm HP rating which really is BS. Consider what HP you are really getting by looking at the HP curve at about 2600 rpm...
__________________

__________________
Cheechako is offline   Reply With Quote
Old 11-03-2009, 16:37   #17
Registered User
 
C--man's Avatar

Join Date: Mar 2009
Location: Noosa, Australia
Boat: 9' Zociac (fastroller)
Posts: 60
Images: 1
Mark-

when you say "hull speed" how many knots are you talking?

on the Winwndspeed 40 with fine hulls and daggers the pair of 27s

would push it easily to about 7knots even with one engine without much throttle

but when I increased the throttle more all it did was force the stern down

and did not go any faster, is that normal?

By the way, Catana is a very nice Cat.

C-man
__________________

__________________
C--man is offline   Reply With Quote
Old 11-03-2009, 17:02   #18
Senior Cruiser
 
Cheechako's Avatar

Cruisers Forum Supporter

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 19,369
The problem is not flat water speed, but when the wind pipes up with a cat's windage, you need plenty of power. Some squat is normal on the upper end of the hull speed curve, cat or mono. Actuall, the cat I mention above was I think 27 hp not 30. All in all it was OK, but if reoplacing the engines I would have went to the 35 that was available. I was a much more robust engine I understand.
__________________
Cheechako is offline   Reply With Quote
Old 12-03-2009, 03:17   #19
Registered User

Join Date: Mar 2009
Location: Morlaix Brittany France blog: theguerns.blogspot.com
Boat: Colvic Watson/32ft/Feels Good
Posts: 461
Images: 4
Send a message via Skype™ to feelsgood
hull speed

Hull speed is deturmined by the square root of the water line length To push a boat above this speed takes alot of hp and burns more fuel. But it doesnt mean the a cat has 2 hulls so will go twice as fast. I agree that the 35 hp engines would be best
__________________
feelsgood is offline   Reply With Quote
Old 12-03-2009, 12:35   #20
Registered User
 
mark_morwood's Avatar

Join Date: Jan 2007
Location: Cruising (Atlantic -> Med -> Carib -> Pacific)
Boat: Vancouver 36, Hobie 33, Catana 48, now all with new owners
Posts: 207
Quote:
Originally Posted by C--man View Post
Mark-

when you say "hull speed" how many knots are you talking?

on the Winwndspeed 40 with fine hulls and daggers the pair of 27s

would push it easily to about 7knots even with one engine without much throttle

but when I increased the throttle more all it did was force the stern down

and did not go any faster, is that normal?

By the way, Catana is a very nice Cat.

C-man
Around 9 knots. Once I get to around 9 knots, more throttle doesn't make much difference to speed on our boat.

The formula (1.34 * square root(waterline length)) suggests closer to 8.5 knots with a 40' waterline length on fine hulls.

I agree with comment that punching into wind and sea is when you really need effective power (prop and engine).

Mark.
__________________
mark_morwood is online now   Reply With Quote
Old 13-03-2009, 17:56   #21
Registered User
 
C--man's Avatar

Join Date: Mar 2009
Location: Noosa, Australia
Boat: 9' Zociac (fastroller)
Posts: 60
Images: 1
that sounds about right, Thanks!
__________________
C--man is offline   Reply With Quote
Old 16-03-2009, 02:22   #22
Registered User

Join Date: Dec 2008
Location: Luxembourg
Boat: Spirited 380
Posts: 29
The needed power mainly depends of the displacement.

Most of the production catamarans have a ratio of 6 to 7 hp per 1000kg maximum displacement, or 8 to 10 hp per 1000kg lightship weight, and this is rather on the high side. This is the total for both engines.

Hull speed is the limit over which the energy needed to displace the yacht is considered to increase dramatically. But the 1.34 *squareroot(waterline*length) limit was calculated by Froude for boats typically having a length:beam ratio around 4:1 (monohulls), and it is of much less importance with multihulls. See explanation here.
__________________

__________________
Esprit Marine is offline   Reply With Quote
Reply

Tags
engine

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Need help with prop shaft sizes Beemer Propellers & Drive Systems 0 21-02-2009 01:12
Appropiate rope sizes for various applications. ribbony Deck hardware: Rigging, Sails & Hoisting 29 07-12-2008 15:41
anchor sizes and chain lengths michaelmrc Anchoring & Mooring 12 21-08-2008 01:30
Mooring rope sizes bg9208 Anchoring & Mooring 3 21-08-2008 00:15
Large Sizes Moog General Sailing Forum 2 07-01-2007 23:23



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 05:46.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.