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Author Topic:   Anyone Use Jack Plates??
donel posted 05-09-2002 10:52 AM ET (US)   Profile for donel   Send Email to donel  
I need to change the mounting of the outboard on a 22-Outrage (thanks to tech advice on this forum ) and wonder if I should add a transom jack plate at that time. It would give a 5.5-inch setback and 5-inch vertical adjustment.
OutrageMan posted 05-09-2002 11:13 AM ET (US)     Profile for OutrageMan  Send Email to OutrageMan     
Try the reference article @


jimh posted 05-09-2002 11:39 PM ET (US)     Profile for jimh  Send Email to jimh     
Also see the now separate article about adding setback brackets to standard transom boats:

Cut Transoms and Brackets

I have expanded the article based on my own experience; I am in the process of adding 10-inch setback brackets to my 20-foot Whaler.

thebone12 posted 05-19-2002 05:22 AM ET (US)     Profile for thebone12  Send Email to thebone12     
speaking from an ocean mans view, I say stay away, on a calm lake it will do, but on a rough day, you are just giving a 400 pound outboard motor a level to snap your transom.
Wild Turkey posted 05-20-2002 10:25 PM ET (US)     Profile for Wild Turkey  Send Email to Wild Turkey     
So thebone12....
How many Whalers have you seen with their transoms snaped off due to setback plates? If you have, there are a large number of forum members (with setback plates installed on their Whalers) that would like to hear your stories.... including me.
jimh posted 05-20-2002 11:15 PM ET (US)     Profile for jimh  Send Email to jimh     
I have been thinking carefully about the idea proposed by thebone12, that is, the threat posed to the transom by the engine weight being suspended a foot or so from it. The 400 pounds of engine operating at a 1-foot lever arm has a 400-foot-pound moment on the transom.

Using this same analysis, I have discovered that there is a 25-pound anchor suspended 25 feet from the transom, on the bow pulpit. The 25-pound anchor operating through a 25-foot level arm has a 625-foot-pound moment on my transom!

Effective immediately I am going to move the 25-pound anchor off of the bow pulpit and move it to the rear of the engine, where, on a one-foot lever arm, it will only exert 25-foot-pounds of moment on the transom.

After that I am going to take a chain saw and start cutting off the front of the boat so I can reduce the bending stresses that it puts on the transom until I get them back to a safe level, safe enough to go out on the ocean.

Seabrook posted 05-21-2002 02:24 PM ET (US)     Profile for Seabrook  Send Email to Seabrook     
I have a hydraulic jack on the back of my it. No worry about the transom. If it is that rough at the rear of the boat (the smoothest place), your body could not take the pounding mid ship. I truly feel that with a transom stand off or not, Whalers can take more of a beating than our bodies can.

BTW, Montauks make great skinny water boats. I have my jack plate mounted on the highest holes and can still run the lift all the way up. Can not go full throttle or it will cavitate, but mid throttle is fine and real skinny on plane. I use a Stingray tale as well.

Seabrook posted 05-21-2002 02:26 PM ET (US)     Profile for Seabrook  Send Email to Seabrook     
One more point I forgot to mention, the worst beating your transom will ever take will most likely be on the trailer, not in the water.
Wild Turkey posted 05-21-2002 03:52 PM ET (US)     Profile for Wild Turkey  Send Email to Wild Turkey     

What about the 1000 foot-pound moment on my Montauk transom caused by me..... (250 lbs approx. 4 ft from the transom)

thebone12 posted 10-24-2002 05:46 PM ET (US)     Profile for thebone12  Send Email to thebone12     
I have not seen any broken transoms but have seen alot with stress cracks Bad. Seen 3 whaler, 2 of them 13fts and 1 17ft,
newt posted 10-24-2002 08:45 PM ET (US)     Profile for newt  Send Email to newt     
What I do not know about brackets I make up for in a general understanding of statics, and for what it's worth...

the moment calculations above are so flawed they can only be taken as sarcasm. Don't quit your day jobs. :)

Wild Turkey posted 10-24-2002 09:06 PM ET (US)     Profile for Wild Turkey  Send Email to Wild Turkey     

It would be pretty hard to quit my day job considering the fact I am a registered professional engineer. :)

I has been a long time since my last Statics and Dynamics course though.

newt posted 10-25-2002 07:39 AM ET (US)     Profile for newt  Send Email to newt     
Ha HA HA Touche! :)
OUTRAGEOUS22 posted 10-26-2002 06:08 PM ET (US)     Profile for OUTRAGEOUS22    
My 1994 30" shaft 250hp yamaha weight is 525 lbs+jackplate 21 lbs+SW ss Prop 15 lbs+ seastar hydraulic steering 6 lbs = 567Lbs on the back of the 22'outrage which replaced a big block Johnson 200hp close to the same weight jumping wakes at 40 to 50 mph. No cracks yet from 1988 ???? Guess I got a good one.
jimh posted 10-28-2002 12:59 PM ET (US)     Profile for jimh  Send Email to jimh     
While there might have been a hint of sarcasm in my analysis of the moments acting on the transom, perhaps it bears a closer look.

A boat on plane often rides on only a few feet (inches)of hull. In extreme cases almost the entire hull is out of the water. In a 22-foot boat, perhaps 21-feet of hull is out of the water and the boat zips along with just a short amount of bottom pad and the outboard engine lower unit in the water.

In this situation, the entire front of the boat, 21-feet of it, is producing a bending moment on the transom, is it not?

jameso posted 10-28-2002 02:04 PM ET (US)     Profile for jameso  Send Email to jameso     
OK, guys, let's stick to the question.

Also Jimh your analysis is flawed, the anchor exerts no force on the transom, only on the bow pulpit, also the 21 feet of boat you are talking about exerts no force on the transom, the high stress area will be just forward of the pivot point of the hull when it is on plane. This pivot point or Center of Gravity will change as the boat comes and goes on plane and varies according to the boat loading ect.

A 450 lb engine on the transom exerts 450 lbs, with a 6 inch setback it will exert much more than that especially in a dynamic, i.e. wave bouncing, wot, enviornment. To accurately gage the forces once would need extensive testing with strain gages ect.

Yes, a setback will put more load on the transom, change static trim and all the above. But I have seen a lot of Whalers, Sea Rays, Bombers, Tritons ect with brackets and few if any cracks and no gross failure of the transoms have occured.

OK, What was the question?

lhg posted 10-28-2002 02:15 PM ET (US)     Profile for lhg    
For all you would-be engineers, etc, what about Whaler's own Whaler Drive unit, with 30" setback? On the 25 & 27 Whalers, this unit is designed to carry twin 500lb engines.
Now that's a real moment.

Have ripped out transoms been a big 10 year warranty issue for the 87-93 boats? My '89 25 doesn't even have a single strees crack in the gelcoat.

thebone12 posted 10-28-2002 02:37 PM ET (US)     Profile for thebone12  Send Email to thebone12     
People think that jackplates are the answer to everything. Jackplates can help but you should take into consideration what you do your boating for, a normal whaler transom was not designed for a jackplate, a whalerdrive is! People have mixed opinions about jackplates, I am just stating that I think that the added stress on the transom is not worth it! It does put more stress on the transom, so why would you do that!
Bigshot posted 10-28-2002 02:59 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Simple.....I run my engine 4" higher than if it was on the transom. If I cross the flats and hit bottom due to not having the clearence, which do you think adds more stress? Hitting the bottom at 30mph and using the lower unit as an anchor or having my engine 5" behind the boat? As faras a jackplate adding more stress I can't see much. The plate is mounted just as well as the engine itself so nothing there. It may overhang and cause more that way but I can't see how much more it could.
thebone12 posted 10-28-2002 03:03 PM ET (US)     Profile for thebone12  Send Email to thebone12     
COME ON! If you hit the bottom, you made a mistake, its not because you where afraid to but a jackplate on the boat. But if I am 32 miles in the n. atlantic and it blows up to 30 kts and 7 to 8fters then I would not want the added stress. And getting stuck in a blow is going to put a BIG stress on the transom and that is the last thing I want to worry about!
newt posted 10-28-2002 03:40 PM ET (US)     Profile for newt  Send Email to newt     
JimH, look at it this way. Imagine that you bolted your boat onto a wall using the engine mounting bolts so that the transom was FIXED solid to the wall, and then took the water or any other supports out from under your boat.

Now, when you put your 25 lb anchor in the bow 25 feet from the transom, you are imposing an additional moment of 625 ft-lbs directly on your transom.

Now, when 21 feet of boat is out of the water, there certainly is more stress put on the transom than when at rest (but WildTurkey will have to run the calcs, I dont have the brain cells!).

Start with one of the basic rules of physics which states that for a body at rest, all the forces acting on it are in equilibrium. Now by "at rest" in this case, I mean not rotating (I can take this liberty because we are analyzing the moment only). So, if your bow attitude is constant, then the transom is "at rest" as far as rotation goes.

The forces on the transom must be equal and opposite. Your engine is producing thrust which will translate into an additional moment on your transom (thrust from prop x distance from prop shaft to center of mounting bolts +/-). This moment wants to lift the bow (and does at first), but is equalized and offset by the managerie of other forces including 21 feet of boat up in the air, drag, aerodynamic lift, bouyancy, and yes, your 25 lb anchor (included in the 21' of boat in the air).

So, when the boat is on plane, I would say there is increased moment on the transom, and it is quite substantial.

And going back to your anchor scenario, I think adding weight forward WOULD increase the moment on the transom even with the boat in the water - but not by a measurable amount, and definitely not by 625 ft-lbs.

I will spare you all the explanation of that one (for now). :)

jimh posted 10-28-2002 06:33 PM ET (US)     Profile for jimh  Send Email to jimh     
An additional consideration: most of these analysis of moments neglect the fact that the engines are running and producing thrust, often with an upward component.

Having 200-500 HP bolted on the transom and producing thrust probably creates some forces that are in opposition to the static moment the engine weight creates on the transom.

thebone12 posted 10-28-2002 07:09 PM ET (US)     Profile for thebone12  Send Email to thebone12     
I'll just have everyone in the boat sit on the bow to counteract the motor on the jackplate!
jimh posted 10-28-2002 08:43 PM ET (US)     Profile for jimh  Send Email to jimh     
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