Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 19-04-2016, 19:07   #16
Registered User

Join Date: Apr 2016
Location: Florida
Boat: Beneteau 36' CC
Posts: 5
Re: HP required to motor at 10 kts?????

At full throttle on BOTH engines you should achieve MAX rpm's (from your engine manual) on both engines. If not the props are pitched wrong.
__________________

__________________
atrometer is offline   Reply With Quote
Old 24-04-2016, 14:00   #17
Registered User

Join Date: Feb 2014
Posts: 1
Re: HP required to motor at 10 kts?????

















The answer lies within the details you have supplied allowing for some small errors.
It is a fact that to double your speed you will require four times the power input all other things being equal. If one engine gives you six knots four engines will give you twelve knots. i.e. from 48hp to 192hp. It is obvious that all things are not quite equal as two engines only give you 7.5hp so either there is an extra drag factor or your figures are slightly wrong. Getting up to 10kts is going to require at least another 100hp and as the speed increases it appear there maybe further additional drag, i.e. 'squatting' (when the bows rise and the stern sits lower) which means your hulls cannot reach 10kts without beginning to plane. My opinion is that the designer/engineer who did the original calculations knew his stuff and appears to have given you an ideal set up, with a good cruising speed of 5knts and a top speed of 7.5knts which is a good top speed without being too thirsty. I have a 50ft Prout Quasar with two 30hp Yanmars and get 5knts cruising and 6-7knts top speed. My solution-- stop trying to reach for the moon- it can be very expensive.


Regards, Peter.
__________________

__________________
Amanzi is offline   Reply With Quote
Old 20-05-2016, 21:34   #18
Registered User

Join Date: Dec 2012
Posts: 650
Re: HP required to motor at 10 kts?????

Quote:
Originally Posted by Amanzi View Post
It is a fact that to double your speed you will require four times the power input all other things being equal.
The rule of thumb is that, below hull speed, doubling the speed of a displacement vessel requires eight times the power, however, hydrodynamics are complex and actual power required to double the speed varies quite a bit.
__________________
Poseidon is an unmerciful god.
mcarling is offline   Reply With Quote
Old 21-05-2016, 04:40   #19
Marine Service Provider

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Marmaris
Boat: FP Orana 2010, Lipari 2011, Hélia 2013, Catana C 47 2013
Posts: 1,030
Re: HP required to motor at 10 kts?????

Yr hull beam to hull length ratio seems pretty good. Helia 44 which is fatter than yours in one engine gives app. 7 kts at 2.200 RPM with 54 HP Volvo. Yanmar engines requires a bit more rev for the max torque, hence, you should be achieving this speed at 2.500 RPM. If not, then you are way too heavy..
10 kts would require min 2X100 >HP and since sail drives are not available at this range of engines, you will need to call for shaft drive and change everything on the boat. Is it worth for couple knots of more speed ?
You'd better buy a motor catamarans; they reach easily 14-15 kts of speed with relatively low consumption.


Cheers


Yeloya
__________________
yeloya is offline   Reply With Quote
Old 23-05-2016, 06:46   #20
Registered User
 
denverd0n's Avatar

Join Date: Jun 2008
Location: Tampa, FL
Posts: 3,948
Images: 6
Re: HP required to motor at 10 kts?????

It's a SAILboat. It is intended to be SAILED. What you obviously want is a POWERboat.

Seriously. Whatever your spend on these monster engines, be aware that when you get around to selling the abomination that you have created, NO ONE is going to want it. People who are looking to buy a SAILboat are going to consider your ridiculously over-sized engines to be nothing but a liability, which will have to be removed and replaced with appropriately sized engines. They are going to want a MASSIVE discount on the boat to account for the hassle you have created. And people who want to go that fast are not going to be dumb enough to buy a SAILboat. They will be buying a POWERboat.

So the only smart thing for you to do at this point is to sell the boat you have, and buy one that is actually intended for powering at higher speeds.

Good luck, whatever you end up doing.
__________________
denverd0n is offline   Reply With Quote
Old 23-05-2016, 17:02   #21
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,452
Images: 69
Re: HP required to motor at 10 kts?????

Given that the OP hasn't returned, or answered any questions, I doubt if there's much point continuing to post in this thread....
__________________
"You CANNOT be serious!"


John McEnroe
44'cruisingcat is offline   Reply With Quote
Old 26-05-2016, 00:35   #22
Registered User

Join Date: Dec 2014
Location: Brisbane, Qld, Australia
Boat: Fastback 43 catamaran
Posts: 179
Re: HP required to motor at 10 kts?????

Gori 2 speed variable pitch propellers might do the trick.
Run in normal for 1 engine, rough conditions or manoeuvring, run in overdrive with both engines, run 1 engine in overdrive if motorsailing.

Sent from my D6653 using Cruisers Sailing Forum mobile app
__________________

__________________
Sardean is offline   Reply With Quote
Reply

Tags
motor

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
Crew Wanted: 2x Deck Hands Required for 39 Meter Motor Sailor Duct Tape Crew Archives 2 01-06-2015 09:14
Best month in SE Bahamas for winds less than 10 kts?? CoopAir Atlantic & the Caribbean 15 14-03-2015 12:44
Confusion over courses required for motor cruising Emason Training, Licensing & Certification 4 10-03-2013 13:00
Motor or No Motor for Long-Term Cruising? boatyard Pirate Construction, Maintenance & Refit 41 02-08-2009 22:13
Can You Believe It? 8 Kts! over40pirate Off Topic Forum 9 01-06-2009 15:15



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 16:06.


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.