Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 27-08-2012, 06:39   #1
Registered User
 
GreggL's Avatar

Join Date: Feb 2012
Location: NYC
Boat: Hunter E33
Posts: 194
Largest Auxiliary Engine Possible

So i got a 33 footer with a 29hp auxiliary; gets me going at roughly 6 knots without putting the hammer down on the throttle. Supposedly my hull speed it about 7 knots. Would a larger engine speed things up or is it still determined by the theoretical hull speed? Not like I can change anything but just curious.. Thanks!
__________________

__________________
GreggL is offline   Reply With Quote
Old 27-08-2012, 06:47   #2
Senior Cruiser
 
skipmac's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 12,373
Re: Largest auxiliary engine possible

Quote:
Originally Posted by GreggL View Post
So i got a 33 footer with a 29hp auxiliary; gets me going at roughly 6 knots without putting the hammer down on the throttle. Supposedly my hull speed it about 7 knots. Would a larger engine speed things up or is it still determined by the theoretical hull speed? Not like I can change anything but just curious.. Thanks!
In a word, yes. Once you start approaching the boat's hull speed, trying to get another knot or half knot requires more and more horsepower and you will reach a point where you can achieve no more speed unless you have enough power to lift the hull out of the water to plane.

I think 29 HP is a reasonable power for your boat. The only potential benefits that you might see are the ability to maintain cruising speed against a strong head wind or waves and possibly faster acceleration or braking in reverse when docking.
__________________

__________________
The water is always bluer on the other side of the ocean.
skipmac is online now   Reply With Quote
Old 27-08-2012, 06:54   #3
Registered User

Join Date: Jul 2011
Location: Stuart Fl
Boat: Cabo Rico 38
Posts: 692
Re: Largest auxiliary engine possible

<LI class=mitem><LI style="DISPLAY: none" class=jsb><LI style="DISPLAY: none" class=jsb>Search Results





  1. Hull speed - Wikipedia, the free encyclopedia

__________________
casual is offline   Reply With Quote
Old 27-08-2012, 07:22   #4
Registered User
 
jeremiason's Avatar

Join Date: Oct 2010
Location: Punta Gorda, Florida
Boat: Sea Ray 270
Posts: 1,427
Images: 2
Send a message via ICQ to jeremiason Send a message via Yahoo to jeremiason Send a message via Skype™ to jeremiason
Re: Largest auxiliary engine possible

Before spending $12k or more on a new engine you might want to play around with yours a little. You should eb able to achieve hull speed with your engine.

I assume you have the Yanmar 29 hp which should have max rpms of 3600. Does you current engine reach 3600 rpms or exceed it? It could be that the propeller is pitched wrong or that you need a three blade instead of a two.

When you are measuring your speed, are you doing it as through the water speed or GPS? As you probably know, but many forget, your GPS speed is relative to you movement over ground, which is effeced by current and wind.

Was your bottom clean when you measured the 6 knots? A dirty bottom and/or prop will slow you down drastically (A full knot or better).
__________________
Tom Jeremiason
Punta Gorda, Florida

jeremiason is offline   Reply With Quote
Old 27-08-2012, 07:29   #5
Senior Cruiser
 
Sailmonkey's Avatar

Cruisers Forum Supporter

Join Date: Sep 2009
Location: Houston
Boat: '76 Allied Seawind II, 32'
Posts: 5,766
Re: Largest auxiliary engine possible

Quote:
Originally Posted by jeremiason View Post
Before spending $12k or more on a new engine you might want to play around with yours a little. You should eb able to achieve hull speed with your engine.

He's not even trying to go faster. The OP stated that the 6 knots is easily achieved.
__________________
Sailmonkey is offline   Reply With Quote
Old 27-08-2012, 07:47   #6
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,737
Re: Largest auxiliary engine possible

I understood that you're not planning to actually change your engine -- you're just curious.

As others have said, hull speed is hull speed -- laws of physics. Somewhere around hull speed, you boat will squat and starting climbing its own bow wave. Power required goes up steeply. Naturally, fuel consumed also. It's not worth it.

6 knots cruising speed is good for a 33 footer. In my experience, you need to be a knot or so below your hull speed to have good efficiency. In the three years I've had my boat, I have gradually slowed down my cruising speed as I got to know the efficiency characteristics. My hull speed is about 9.3 knots; I now cruise at 7.5 to 8, having reduced my cruising speed RPM from 2800 to 2500 finally to 2250, every time getting big gains in economy and noise for small reductions in speed.
__________________
Dockhead is offline   Reply With Quote
Old 27-08-2012, 08:31   #7
Registered User
 
jeremiason's Avatar

Join Date: Oct 2010
Location: Punta Gorda, Florida
Boat: Sea Ray 270
Posts: 1,427
Images: 2
Send a message via ICQ to jeremiason Send a message via Yahoo to jeremiason Send a message via Skype™ to jeremiason
Re: Largest auxiliary engine possible

Quote:
Originally Posted by Sailmonkey View Post
He's not even trying to go faster. The OP stated that the 6 knots is easily achieved.
Opps silly me... your right!

Many people mistake theorectical hull speed as a brickwall, which cannot physically be exceeded, which of course is not true.

There is a great description of Therorectical Hull Speed on Wikipedia...
Hull speed - Wikipedia, the free encyclopedia
__________________
Tom Jeremiason
Punta Gorda, Florida

jeremiason is offline   Reply With Quote
Old 27-08-2012, 08:37   #8
CF Adviser
 
Bash's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: sausalito
Boat: 14 meter sloop
Posts: 7,260
Re: Largest auxiliary engine possible

For the most part, all a larger engine is going to do is make it more expensive to cruise at six knots.
__________________
cruising is entirely about showing up--in boat shoes.
Bash is offline   Reply With Quote
Old 27-08-2012, 08:57   #9
Senior Cruiser
 
skipmac's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 12,373
Re: Largest auxiliary engine possible

Quote:
Originally Posted by jeremiason View Post
Opps silly me... your right!

Many people mistake theorectical hull speed as a brickwall, which cannot physically be exceeded, which of course is not true.

There is a great description of Therorectical Hull Speed on Wikipedia...
Hull speed - Wikipedia, the free encyclopedia
Certainly not a brick wall. I would say more like a soft, foam wall. Initial contact is soft and yielding but the harder you push, the more resistance you will get from the wall.

Excellent article in wikipedia by the way. One very interesting comment in the article, it said that the hull speed concept is not used in modern naval architecture.
__________________
The water is always bluer on the other side of the ocean.
skipmac is online now   Reply With Quote
Old 27-08-2012, 09:16   #10
Registered User
 
Dennis.G's Avatar

Join Date: May 2011
Location: Sea of Cortez and the U.P. of Michigan
Boat: Celestial 48
Posts: 750
Re: Largest auxiliary engine possible

6 knots under power in a 33' sailboat is good performance.
Be happy.
__________________
Dennis.G is offline   Reply With Quote
Old 27-08-2012, 09:58   #11
Registered User
 
GreggL's Avatar

Join Date: Feb 2012
Location: NYC
Boat: Hunter E33
Posts: 194
Quote:
Originally Posted by jeremiason
Before spending $12k or more on a new engine you might want to play around with yours a little. You should eb able to achieve hull speed with your engine.

I assume you have the Yanmar 29 hp which should have max rpms of 3600. Does you current engine reach 3600 rpms or exceed it? It could be that the propeller is pitched wrong or that you need a three blade instead of a two.

When you are measuring your speed, are you doing it as through the water speed or GPS? As you probably know, but many forget, your GPS speed is relative to you movement over ground, which is effeced by current and wind.

Was your bottom clean when you measured the 6 knots? A dirty bottom and/or prop will slow you down drastically (A full knot or better).
Yes I have the 29hp yanmar. I usually run it between 2000 and 2500. This will produce speeds of 5.5 to 6.6 knots respectively. although my throttle will only allow up to 2600 rpm regardless. Is this not normal?

When I calculate speed I favor the gps usually. That is unless I'm going nowhere specific and just like to see my sailing speed over the water.
__________________
GreggL is offline   Reply With Quote
Old 27-08-2012, 10:09   #12
Senior Cruiser
 
skipmac's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 12,373
Re: Largest auxiliary engine possible

Quote:
Originally Posted by GreggL View Post
Yes I have the 29hp yanmar. I usually run it between 2000 and 2500. This will produce speeds of 5.5 to 6.6 knots respectively. although my throttle will only allow up to 2600 rpm regardless. Is this not normal?

When I calculate speed I favor the gps usually. That is unless I'm going nowhere specific and just like to see my sailing speed over the water.
If you are trying to evaluate engine performance the only thing that matters is speed through the water. The GPS will show you how fast you are getting to your destination but if you have a 2 knot current going with you then you will be going 7.5-8.6 kts. Obviously the engine isn't doing all that.

When you say the throttle only allows up to 2600 rpm can you elaborate? Do you know what the max rated rpm is for the engine? If rated max is 3600 rpm then you may be over-propped.
__________________
The water is always bluer on the other side of the ocean.
skipmac is online now   Reply With Quote
Old 27-08-2012, 10:34   #13
Registered User
 
GreggL's Avatar

Join Date: Feb 2012
Location: NYC
Boat: Hunter E33
Posts: 194
Quote:
Originally Posted by skipmac

If you are trying to evaluate engine performance the only thing that matters is speed through the water. The GPS will show you how fast you are getting to your destination but if you have a 2 knot current going with you then you will be going 7.5-8.6 kts. Obviously the engine isn't doing all that.

When you say the throttle only allows up to 2600 rpm can you elaborate? Do you know what the max rated rpm is for the engine? If rated max is 3600 rpm then you may be over-propped.
My throttle level fully down equates to a 2600 rpm reading on the gauge.
__________________
GreggL is offline   Reply With Quote
Old 27-08-2012, 11:26   #14
Senior Cruiser
 
skipmac's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 12,373
Re: Largest auxiliary engine possible

Quote:
Originally Posted by GreggL View Post
My throttle level fully down equates to a 2600 rpm reading on the gauge.
OK. So what is the max rpm rating for the engine?
__________________
The water is always bluer on the other side of the ocean.
skipmac is online now   Reply With Quote
Old 27-08-2012, 11:45   #15
Senior Cruiser
 
skipmac's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 12,373
Re: Largest auxiliary engine possible

If you don't know the spec for the engine then try this. What is the max rpm you can rev the engine in neutral?
__________________

__________________
The water is always bluer on the other side of the ocean.
skipmac is online now   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




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 00:12.


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.