Effective setback on short-shaft motors

[RedAllison]

lol OH YEAH, I just remembered sumpin (like this topic needs sumpin else added too it ::>< )

If the SS motors aren't farther back than 20"ers why does Darris rig the hulls for SS motors with the steering line exiting the hull/transom farther back than 20"ers???

:mrgreen:
RA

 

[2fast4mom]

What's the diff between 375 lbs 15" off, and 500 lbs 3" off?

Exactly 343.75 ft.-lbs. more for the SS, captain.

Spock

 

[badbait]

Lou, I'm not sure I understand your 343 pound difference. Can you explain what that number means.

 

[badbait]

I would of understood it if the number you came up with was around 175.

 

[froggy]

Lou,

Two replies to you:
1. Your calculation, of the leverage difference, between a 500# engine and a 375# engine may be flawed. The actual lever is longer than the setback. The lever arm needs to be calculated to the fulcrum. Normally, the fulcrum is somewhere near the end of the pad, not the transom. If you add about 1' to the 15" and 1' to the 3", then calculate, you will get a more appropriate answer.
2. Of course, Darris' "surfboard" is patented, but the patent is flawed. There is demonstrated, "prior art", that was in use well before his patent. We had a drag boat, in these parts, using a "surfboard", exactly like Darris', to help holeshot (helped by .25 seconds on an Allison drag). The surfboard worked well for holeshsot, but not too good for top speed, but that didn't matter in the 1/4 mile.
1+. A 400# engine 8" back is about the same "leverage" as a 530# engine 3" back.

froggy

 

[2fast4mom]

Bill and John,

My calculations were based upon static ft-lbs of torque applied at the transom. John, you're right, I didn't compensate for what the difference would be at the pad; nor do I know how to do this. Since the pad lip is inset by several inches, and I am uncertain of the geometry and how to mathematically compensate for the vertical length of the transom and the inset, maybe you can clue me in as to the formula.

Bill, the way I got the numbers was as follows:

375 lbs x 1.25 ft (15") = 468.75

500 lbs x 0.25 ft (3") = 125.00

468.75 - 125.00 = 343.75

Therefore the SS motor, even at a lighter 375 lb weight, applies 343.75 ft-lbs more static transom torque on the boat than the 500 lb motor set at 3 inches back.

 

[2fast4mom]

If you add about 1' to the 15" and 1' to the 3", then calculate, you will get a more appropriate answer.

But would this not be at the top of the boat, not the bottom? Or is there no difference? Doesn't the vertical (well, angled) transom add some leverage of its own too?

 

[John Richied]

Oh MY God, Lou is off his medication AGAIN!

Lets see if we can get RA or some one else to climb under their rig at 2AM again…

 

[2fast4mom]

RA are you still awake? Get off that depressing news site and go back to the garage!!

 

[hack02]

On my XB2002 hull----with the pad level, the transom is at a 20 degree from vertical angle, measured with my trusty protractor/angle finder. Also, with a level on the transom, the transom angle changes 4 inches for every 1 foot of drop.
So, if the engine mounting point is the fulcrum of the tilt tube, for every six inches the fulcrum goes up it goes back 2 inches; or vice versa; if the fulcrum goes down six inches it goes in two inches.
Remembering the mids are 5 inches different, not six. But six is easier for a math challenged me to work with.
So, when it goes down it goes in 2 inches, I guess thats why they add 2 inches to the static setback of a 15 inch engine.
Food for thought. (Measures at 7:30 in the morning before work, crazy ain't it.) :grin:

 

[GotMyAlly]

Neal once we get the liquid steering in mine we'll take em both too Pickwick (heck we might even go too Clifton and run on that actual world record stretch of water) and we'll do the "empty em and run from a little battery" thing and see just what we can get out of them.




RA

Sounds good to me. I need to pick up a 30" wheel first to really see what she'll do. :twisted:

 

[RedAllison]

You leftcoasters are FREAKIN NUTS!!! :roll: Muh haid isa hurtin... :?

Einstein didn't own an outboard, neither did Pythagorious (sp?... like I give 2schitts) and I wasn't aware the Stephen Hawking had began rigging boats on the west coast (Maybe he's Smartcraft literate?)

Ifn ya gots a 2.5 litre series motor on a 03 that is a lake/fishin boat then ya needs 14.5" of setback. Ifn that same boat has a 3.0 then ax Bill, ifn/when I puts a 250XS on RDW I'm gonna remove the 14.5" hyd plate and put on a 4" Rapid Jack.

Oh and I'm gonna remember to slow down... a lil "slower"!
RA

 

[Darth VMAX]

Very interesting and good reading thread.

RA, are you giving up on a XB21 and just going to repower RDW???

 

[2fast4mom]

You leftcoasters are FREAKIN NUTS!!!

Cracker, I see you have a firm grasp of the obvious. If we weren't nuts, why would we be here? :mrgreen:

Just be glad we haven't introduced String Theory into this discussion.

 

[RedAllison]

Cracker, I see you have a firm grasp of the obvious

WHO you callin crackuh crackuh??? :mrgreen: Need I remind you who is from Alafreakinbama in hereuh?

String Theory? Is that kinda like the string that our wives has around our johnsons (and I AINT talkin SLOMCs) that they vigorously keep handy while we are at the lake around all the 'kinis???

DMax I wouldn't say I've "given up" so much as I just changed my plans for the time being. Since I bought the deckboat lastyear my needs for more room for the family is no longer an issue. Moneywise for 07 I'm gonna concentrate more on buying some land (been looking at 250-500acre places) and equipment like a tractor and implements for foodplots!

Repowerwise, I think I'm gonna just run the lil SS until she pops. Then if/when she pops I'll probably go with a 250XS (though I gotta admit I REALLY like the prospects of the 300XS now). As for any new boats, I think I'm safe unless someone wants to seriously get that "10 XR2002 orders" going then I WOULD pull the trigger on that one ASAP!!!

!-!
RA

 

[2fast4mom]

String Theory? Is that kinda like the string that our wives has around our johnsons (and I AINT talkin SLOMCs) that they vigorously keep handy while we are at the lake around all the 'kinis???

RA, you are on the money.

You r plan sounds good. More A-boats. More. MORE!! :twisted:

 

[froggy]

Lou,

Just to make it simplistic assume the pad fulcrum to be 1' forward of the transom.

375# X 2.25' = 843.75 ftlbs
530# X 1.25' = 662.50 ftlbs
difference = 180.95 ftlbs

now to get an offset that 'levers' close to BadBaits

375# X 1.75' (9" offset) = 656.25 ftlbs---this may be a good answer to approximate BadBait once you compensate for the VERTICAL lever that a 5" longer midsection will apply to bowlift. Hint---a 20" mid will apply lift, approximately 33% stronger than a 15" mid so the 2" added offset, just about evens out the 15" and 20" engines---comprende?

froggy

 

[badbait]

Now Froggy is in the ballpark with that 180 pound number. I put a scale under the front wheel of my trailer when I had the 280 on at 14.5 inches of setback I have also done the same thing now that I have the 250XS on the back at 3.5 inches of setback counting the wedge I put on. The difference in tongue weight is approximately 172 pounds more tongue weight with the 250XS. I currently have 234 pounds of tongue weight. It sure tows better. When I first put the 250 on with 12.5 inches of setback I only had around 10 pounds of tongue weight.

 

[2fast4mom]

Hint---a 20" mid will apply lift, approximately 33% stronger than a 15" mid so the 2" added offset, just about evens out the 15" and 20" engines---comprende?

Si comprende, mi amigo... :grin: Mercury "in-built" the added setback into the SS bracket to compensate for the "shortcoming". :mrgreen:

Bill...aren't you supposed to have 7% of total load weight at the tongue for proper trailering? Seems like I read this somewhere, sometime but you know how it is with us seniors.

 

[RedAllison]

Now THATs interesting as hell, I never thought about measuring the true leverage effects by comparing toungue weights. Bill what other plates did you use on your 250XS/03? I'm wonderin if something in the 4"-6" range would work? I'd like to just slap my hyd plate directly on the transom and bolt the 250XS up too it, but I want to eliminate the weight of the hyd unit so that's why I'm thinkin along the lines of either the 4" or 6" Rapid Jack.

I know my boat towed ALOT better with the tonque being about 1" higher than it did when I towed it perfectly level. The nose didn't want to bounce around as much.
RA