Standoff-Extension Boxes, Pros and Cons

[Ratickle]

Well, Ryan is talking about putting extension boxes on his Black Thunder, http://www.seriousoffshore.com/foru...cx-4-black-thunder-46ec-750hp-ryan9154-2.html, and I've always wondered about the pros and cons of different boxes. There are several out there, and I am starting this thread to get some feedback on them. It is something I've considerd on the Marlin, but even with the minus 2 Imco Shortie, I'm still a tad deep. So, any box on there would also have to be one of the ones that raise the height.

Here are some of what I know is available...

 

[Ratickle]

The ITS box from Mercruiser has a 5" setback I believe and no height change. It is the box that Doc runs on his Fountain SVL I believe. It comes with all internal steering I think, and I've been told has even less play than full external hydraulic steering.

 

[Ratickle]

There is a Mayfair 12" setback box. I don't know much about it at all.

 

[Ratickle]

There is a Latham 12" setback box.

 

[Ratickle]

There is also a 7" setback box from Latham.

 

[Ratickle]

IMCO Makes a 12" setback box. Which I believe is currently the most popular. Especially among the SVL guys.

 

[Ratickle]

And then there is the Stelling 12" setback box that is fully adjustable and actually has u-joints and a carrier bearing from what I recall so it can be adjusted up and down.

 

[Ratickle]

So, what do you know about setback/extension boxes. How the heck do you decide what to try? And how the heck do you even decide when to try it?

Which option is better?

12", 7", 5", nothing? If there are all of those different lengths available, why does no one make a 8", 9", or 10" box?

If you have a 20 foot boat do you use a 12" and if you have a 45 foot boat 12" is overkill and you should use a 7"? Or is the rule of thumb just the opposite? Longer boat, longer box because the water has been discomboobalated more at the stern so the drive should be farther back to find cleaner water?

????????

 

[TYPHOON]

This should be a good one. All I have heard was therory's and no real test data. There are several issues to talk about. Top end speed,turning quality's and getting on plane. Fountain may be the only company that might have done these test back in the day. If there ever was a time it would have been in the factory racing days but box's were not aloud with the exception of only a handfull of boats. The cost to test them would be insane unless you were a manufacture of the boat or the parts and at that every boat has a differant bottom,weight and motor. Heck, now Im even more cofused. Im with Paul anyone out there????
MD

 

[torrent]

The Mercury ITS is a 7 inch set back. The stelling is no longer available which IMO is a bummer since it seems to be the easiest to adjust with minor changes.

 

[MOBILEMERCMAN]

In general boxes and notched transoms do the same thing. They change the leverage point, make the boat act lighter and allow the drive to be raised. The lighter the boat acts the the faster it goes in flat. The opposite is true in the rough. The further back the drive is the more it is exposed to leverage when a boat spins and increases the likelihood of parts getting damaged. SVLs like the setback largely because it is relatively long for a single engine and the increased setback lightens the boat. Most SVls like all the weight as far back as you can get it. Since there are few races in rough water that is been the trend.

I think it is hard to find real info as far as what is best for an individual boat. There is so many variables acting out at once. Prop, tabs locations, operators, different CG's, and personal preferences. To get a real test the boat needs to be tested at its limit and I believe few actually operate their boats at the boats limits. Most often it is operated at the riding crews limits. When the tests performed are based on an individuals operating limits the true test of the set up change can not be evaluated.

With the past F2 boats only straight bottom boats were allowed to run boxes so long as the engine centers remained the same. Just like you can adjust them up and down you can also adjust them in. That had a positive effect on most twin engine boats. Not sure how the rules applied to F1.

Even consecutive boats in production with ever intention of being identical could be different and benefit from different set's. Partly because of crews operating preferences and in part because it is so hard to exactly duplicate a boat in every measure.

Test test test each and every boat is the only way to know for sure. It is pain staking. Every change needs every variable tested again. Few have persistence and time to test to that extreme and fewer have the means. It is so much easier to copy. Then the funny part to me is after you go through all the effort and expense to change something the natural human tendency is for many to justify their decision and say it is better. So it is hard to really know for sure when talking to people.

The easiest thing to do is just add power and run some tab like training wheels to increase your speed.

I am sure there will be many different opinions on this topic.

 

[MOBILEMERCMAN]

Paul, on your Marlin the best approach would be to raise the engine and use the parts you have. If your starting point is too low all the subsequent changes you are trying to make will be compromised. If you go back and up from a too low setting you are still too low.

 

[ryan9154]

I'm putting boxes on due to cavatation signs on props want to get props in cleaner water also good rule of thumb for every 12" back you can go up 1-2" so in my case getting the surface drive to surface more and use less trim a at wot

 

[Ratickle]

I know it's true they increase leverage. But I also thought the extension boxes were primarily developed to replace/make-up-for, the notched transom developement in boats. So my theory was a notched transom boat wouldn't need an extension box. I'm just really curious. If extension boxes made a 5 to 7 mph improvement, and you weren't drivng a race boat where fast cornering mattered, why wouldn't every manufacturer provide them that way?

 

[MOBILEMERCMAN]

good rule of thumb for every 12" back you can go up 1-2"

The water rises as it leaves the transom. When you go back 12 and up 1or 2 [ depending on the speed and a few other variables] the water actually hits the gearcase in the same place. If an increase of speed is realized it is from the balance and leverage change. Not so much from the more surfacing effect. To benefit from a more surfacing effect it needs to be raised more. That is provided you have the power and the boat is in a speed range for its size to benefit from a surfacing set up.

The handling will change for sure but the speed may or may not increase in every case. The boats that will benefit the most are the ones that run heavy in the water and respond little to trim. If the boat is already light and responsive to trim the change could be less desirable.

To Pauls point. Once a boat is molded its is what it is. How many manufactures actually go back and change the molds? Some of the high end boats improve their molds over time. Notches come in many different sizes. Plus a boat that is going to run at say 80 mph will likely enjoy a different set back then a boat running 100. Most boats are available with a range of engine options.

There are times when no matter what you do the boat runs the same speed. Different drives, ratio's heights ect make no difference. Maybe it does go faster but is twitchy and hard to control. Have you made an improvement or not. All depends on the individual. A race boat that is fast but, hard to control wins few races.

Ever wonder why the Fountains always have sand marks and touch up spots on their race bottoms? It is because even the slightest low spot has a huge effect on high speed boats. Point is the bottom is most often over looked when people are trying to make their boat go faster. Any short coming in the bottom integrity, surface or design has a huge effect on how fast it will go. In such cases changes to the set up may not change anything the speedometer does.

Say your designing a boat to race a particular class. The length limit is 30 feet. You know you want the bottom to be as long as it can be. You know the boat will appreciate a lot of set back but, you are working with a length limit. The extension box is not measured in boat length.

Possibilities and preferences are countless. My opinion is if the boat likes set back it is a better boat if it is built in to the hull. If the drives are in the basement too low re cut the transom and raise the engines.

It is hard to stay on point I will stop now.

 

[torrent]

I know it's true they increase leverage. But I also thought the extension boxes were primarily developed to replace/make-up-for, the notched transom developement in boats. So my theory was a notched transom boat wouldn't need an extension box. I'm just really curious. If extension boxes made a 5 to 7 mph improvement, and you weren't drivng a race boat where fast cornering mattered, why wouldn't every manufacturer provide them that way?

This is a really interesting thread and I too am just really curious. To your point most manufacturers that rig a boat with over 700hp (which in Ryans case that’s where we are currently) use #6/NXT drive which incorporate a set back into the transom assembly. Is this by dumb luck Mercury Racing decided to do this, probably not? I wonder if the #6 had no set back what type of effect there would be.

 

[MOBILEMERCMAN]

Ryan,
Have you tried different drive heights already? Are you looking to make it ride different or just searching for speed?

 

[MOBILEMERCMAN]

I am curious Ryan how did you arrive at the point you are now. Was the boat set up at the factory the way it is? Did it get re powered and or new drives hung on the back? If that is the case what was the boat cut out for originally? How fast did it go and how fast is it now?

To another point. If a boat is running well, handling well, and already going pretty fast incremental changes can make a recognizable change. For example changing heights 2"s or set back 12"s are big changes. Those changes may require a propeller change to realize the full effect. There are also times where lowering a 1/2" is in order. We have read where some new boats are set up for top speed from the factory and owners eventually enjoy a lower height. It could be from the way the boat gets loaded with gear, where its used, individual cruise speed preference, or a matter of feel.

 

[Ratickle]

This reminds me of Brooks's Outerlimits, the one that Dr Pete has now. That boat ended up with the 12" extensions, regular Bravo drive length, and a 1" spacer to run the props even deeper. When he got over 110 or, the boat acted weird and this setup corrected all of that from what I understand. I'll try to get ahold of Brooks and see if I recall that correctly.

 

[offshoreexcursion]

All I know is my Baja LOVES the Stellings boxes on the back! It feels so "dialed in" I am scared to change it. The boat handles on rails, eats up the rough, and runs around 100mph, all in a frickin BAJA! :hurray: Helping those boxes out are IMCO SC drives and 380 K Planes.

My previous 33 outlaw had 454s, bravo drives, NO boxes, 280 k planes. Nice running boat but felt like a normal boat.

My current 33 outlaw feels like A MILLION DOLLARS.

I don't know what boxes do but I truely believe in them. Whatever they are doing, it works on my boat.