Like Tree0Likes

Motor setback on OB Cats...

Reply
Old 06-08-2005, 10:52 AM
  #1
Registered
Thread Starter
 
X-Rated30's Avatar
 
Join Date: Mar 2001
Location: Monroe, LA
My Boats: 1997/2010 Spectre Catamaran w/ twin 250XS Sports
Posts: 3,882
Default Motor setback on OB Cats...

I think all of the cats I have seen with twin OBs have an adjustable jackplate bw the motor and the transom. But I have noticed that they all seem to be about 4 or 5 inches thick. This is my first cat, and my first OB boat, but I would think that some setback would would help the boat from both speed and handling standpoints. It also seems like it would be much easier and cheaper to do with OBs than I/Os and they do it all the time. Why doesn't anyone run more setback on 28 - 32 foot cats???
X-Rated30 is offline  
Reply With Quote
Old 06-08-2005, 10:55 AM
  #2
Registered
 
FOUNTAINATLAST's Avatar
 
Join Date: Aug 2004
Location: Sunny, AZ
My Boats: 2001 Fountain Fever
Posts: 1,182
Default Re: Motor setback on OB Cats...

If it isn't broke don't fix it!!!
FOUNTAINATLAST is offline  
Reply With Quote
Old 06-08-2005, 11:02 AM
  #3
Registered
Thread Starter
 
X-Rated30's Avatar
 
Join Date: Mar 2001
Location: Monroe, LA
My Boats: 1997/2010 Spectre Catamaran w/ twin 250XS Sports
Posts: 3,882
Default Re: Motor setback on OB Cats...

Not about to, but I am puzzled by this. I know in smaller tunnel hulls, they do it to help make the porpoise go away.
X-Rated30 is offline  
Reply With Quote
Old 06-08-2005, 11:37 AM
  #4
Registered
 
Tantrum's Avatar
 
Join Date: Oct 2000
Location: USR, NJ
My Boats: Stray Cat Statement Marine
Posts: 4,000
Default Re: Motor setback on OB Cats...

Does your set-up have the jack plates (set back).
Things to consider.
Does your transom have a notch?
Where is the clean water?
Whats worked on similar boat set-ups?

28 Skaters run no set back and mount direct to the transom where most of the 30 Spetre's I see run them.
Tantrum is offline  
Reply With Quote
Old 06-08-2005, 12:00 PM
  #5
Registered
Thread Starter
 
X-Rated30's Avatar
 
Join Date: Mar 2001
Location: Monroe, LA
My Boats: 1997/2010 Spectre Catamaran w/ twin 250XS Sports
Posts: 3,882
Default Re: Motor setback on OB Cats...

Yes, I have the jack plates, but they are only 4 to 5 inches thick. I have heard of smaller boats running 12 inch setbacks like bravo extention boxes give you.
X-Rated30 is offline  
Reply With Quote
Old 06-08-2005, 12:39 PM
  #6
Registered
 
Tantrum's Avatar
 
Join Date: Oct 2000
Location: USR, NJ
My Boats: Stray Cat Statement Marine
Posts: 4,000
Default Re: Motor setback on OB Cats...

I suspect the smaller boats dont have a notched transom. The purpose is to get the prop into clean water and for speed you want it mounted as high as possible.
With the notch the water displaced by the hull is that much further forward requiring a smaller setback that a strieght transom.
The set back on my AMT is about 5 or 6 inches as well.
Balance would also come into effect is you were to push it out an additional 6 inches I think.

Fine print.
Im not an expert on this but find it an interesting topic. Hopefully we will get some more experienced opinions and keep this topic going.
Tantrum is offline  
Reply With Quote
Old 06-08-2005, 02:13 PM
  #7
Registered
 
dreamboater's Avatar
 
Join Date: Apr 2002
Location: Southern Maryland
My Boats: 2000 28 Skater
Posts: 1,234
Default Re: Motor setback on OB Cats...

YOu will hear a lot of mixed reviews about this. I run a 28 skater. I bought the boat with no setback. After tossing it around all winter I decided to add 6" of setback and am spinning 15x34's instead of 14.5x32s. I have gained about 7 mph from this and believe I can gain another 2 with light fuel. The boat does tend to fly nose high more than last year. The props break loose more just because they are higher in the water. The more setback the more strain on the transom. I can tell you the 28 has a few more Knee bulkhead braces on the transom than the 30 spectre does (unless the newer boats are different). If you are running 2.5's with offshore mids you already have 2" of setback. If you run in rough water going to a 10" bracket in my opinion would be to much strain on the transom. Are your speeds comparable with other 30 spectres with your setup??? Setback moves the center of gravity back farther which will cause your handeling to be worse. What is your current setup and what speeds are you seeing.
dreamboater is offline  
Reply With Quote
Old 06-08-2005, 03:46 PM
  #8
Registered
 
Shane's Avatar
 
Join Date: Oct 2000
Location: Niskayuna, NY
My Boats: 2009 FORMULA 292
Posts: 5,543
Default Re: Motor setback on OB Cats...

Typically, most OB cats, including 28 Skaters use CNC Stern Jacks. Most are 4 inches but some provide 6 inches of set back. The reasons for this are many. In no particular order some are as follows:

Jack plates allow you to accomplish several things at once. First, you can intricately "dial in" or adjust the engine height to more exacting specifications. This also allows you to add what I call "ficticious" length to the boat. As was stated earlier, when set back is increased, the center of mass, or balance point is moved farther to the stern as well. Some people have moved the rear seat forward (like I did) while others move batteries and the like, to compensate for the shift in an effort to maintain correct angle of attack (attitude). Typically the boat will "loosen up" in this scenario and often times significant increases in speed are the result. If the balance point is adjusted to compensate for the setback, you will more times than not experience significant increases in speed, with little reduction in handling characteristcs.
Shane is offline  
Reply With Quote
Old 06-08-2005, 09:08 PM
  #9
Registered
Thread Starter
 
X-Rated30's Avatar
 
Join Date: Mar 2001
Location: Monroe, LA
My Boats: 1997/2010 Spectre Catamaran w/ twin 250XS Sports
Posts: 3,882
Default Re: Motor setback on OB Cats...

Quote:
Originally Posted by dreamboater
What is your current setup and what speeds are you seeing.
I am not interested in changing my boat, so much as I am with Tantrum and find it an interesting topic.

Or at least that WAS true until you said you picked up 7mph.
X-Rated30 is offline  
Reply With Quote
Old 06-08-2005, 10:22 PM
  #10
Registered
 
Join Date: Dec 2002
Location: chesapeake bay md
My Boats: 30 motion ob
Posts: 959
Default Re: Motor setback on OB Cats...

All things being equal more setback will usually result in more speed. [ I said usually} The problem becomes, you have a fast boat that will not run in rough water. I guess one needs to balance the speed and rough water ability for the conditions you run in. With todays boats I don't think transom strength is a big issue. Look at what wins in the super stock classes. The 28' skater would be the fastest hull but you do not see many of them run anymore. Fast in calm water and fast around a race course or across the bay are two different things. But I could be wrong!
mmwalters is offline  
Reply With Quote
Reply

Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Michael1
Drives and Lower Units
6
10-19-2007 10:45 PM
big poppa pump
Skater
6
03-05-2004 02:43 PM
big poppa pump
Skater
3
11-24-2002 12:37 AM
Skaterfast
Skater
3
05-13-2002 04:11 PM
Skaterfast
General Q & A
1
05-05-2002 01:07 PM



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 On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 03:51 PM.


Copyright 2011 OffShoreOnly. All rights reserved.