Effective setback on short-shaft motors

[2fast4mom]

Guys,

It's been talked about here more than once, about how a short-shaft motor bracket has a built-in or inherent 2 to 2.5 inches more setback when compared to a standard (20 inch shaft) motor.

Well, I spent over an hour on the phone with Darris Allison a few days ago and the issue of setup & setback came up as regards my XB-2003. I am going to let Darris, Todd & company have at it when the rally is over. They are going to be doing quite a few things.

On the topic of setback, let's call it "virtual setback" if you will, Darris surprised me when he insisted emphatically that the short-shaft motors have LESS setback by 5/8 inch than the 20 inch motors. The reasons are as follows:

1. The transom is not a vertical, 90 degree mount.
2. The "arm length" of the short-shaft motors is shorter, so
3. the "lever point" or pivot providing the same angle with the same amount of propshaft height will be different.

The net effect is that the SS motors lose about 5/8" of "virtual setback" on the 2003's when compared to a standard-length shaft, all other things being equal. Darris went on to say that about 15" setback is appropriate for the 2003 with a short-shaft motor. Currently, I am running 14.5".

Now this seems to fly in the face of what others have said, and included here is what I've seen Badbait do with his boat and what Froggy has told me. Badbait (Bill) is running a 250XS on his 2003 with almost NO setback and has had excellent results.

I'm very interested in the physics of the setback equation, and so thought I would begin a thread for this as a topic for discussion. Specifically, I'm interested in hearing from those with SS motors and what their experiences / experiments have yielded. Thanks!

Lou

 

[JR]

There IS a small setback loss that is built in to the inward angle of the transom as the engine drops, wether from the inital mounting height or sliding up/down the jackplate. There is also more setback [supposedly 2.5"] built into the 15" transom bracket. We have always known that the 15" needs more setback due to less leverage from the shorter "arm" [of leverage] that the shorter mid generates. The net effect is; less bow lift from the 15" mid compared to the 20" mid [at the same setback].
The guys with more "ponies" use less setback due to the greater thrust and aero lift that lifts the bow. The extra HP has more effect on lifting the bow, even with less setback.
I like the extra setback since it helps me cruise faster [at less applied HP] without have to trim up beyond the most efficient angle.
Every setup is a compromise to hopefully perform best where you "get your jolly's".
JR

 

[badbait]

I think the 2003 hull was designed to run with 375 to 400 pound motor on the back. Darris has been completely against putting anything heavier than that on the 2003 for the reasons that I found. So comparing my setup using a 510 pound motor may be an apple and orange comparison. I started with 12 inches of setback and was satisfied with the speed which was in the 95 to 96 mph range if I remember right. But that was all I was satisfied with. I had a big problem when shutting down having water coming up on the back deck and the hole shot stunk. So for me it was an easy decision. I needed to make some changes or put the motor on another boat. I changed the setback 6 times and each time I came in a inch or two I saw some improvement in the problems I was having. I did loose bow lift but through trial and error I figured out how to get that back. I

 

[RedAllison]

WOW Bill those are interesting numbers and really makes me feel better about the prospect of someday having a 250XS on RDW!!! :twisted:

LLDoggg if Darris says it then I guess it must be true, but he is the first one that I've ever heard touting that the bracketry in the SS motors actually yields SHORTER internal setback than the 20"ers. It's always been my understanding that the supposed 2" setback increase in the SS motors was there to make it less likely to strike the front of the cowl/powerhead on the top side of the transom when raising it fully (not really an issue tho with jackplates well over 1' back on boats like ours). I'll be taking a tape measure too a 20" 2003 vs my 15" 2003 at the rally (with identical Allison setback/hyd jackplates) I think with hulls leveled and driveshafts perpendicular too the ground a measurement should be taken from the driveshaft too the leading edge of the transom. That will tell the tale!

I know when I bought mine Jaco had the plate as far forward as it would go. I don't know if he did that to prevent me from going faster with intentions of later moving it rearward after I learned to drive it or what. But the first thing Glenn Reynolds did when I carried it too him a couple years later was to move it all the way back. I know I have pretty much NO problem lifting just about any load I can put in the boat except having someone over 200# sitting up front with me when heavily loaded for fishing. If they are under that weight then any decent running prop I've tried (choppers, RE3s, Trophy's, Quad IVs, High Fives) picks up and carrys the load fine.

Bill is your pad stock? I'm glad to hear about a 250XS running so closely too 2.5 #s. I was concerned from a few other reports that the 250 wasn't going to yield anything but 225 ProMax numbers. Bill do you think your boat would carry an honest 2 man tournament load 95gps?


RA

 

[2fast4mom]

I've seen Bill's boat in action most recently at the delta in September and I have NO doubt that he could carry the 2-man, tournament load at speeds of at least 95.

I was (and am still) shocked :shock: at the way that boat performs with that huge chunk of a 250XS on the transom. Yes, Bill HAS done some pad work, and this is an interesting subject that he can shed more light on, but I'll give it a shot here.

As a baseline, Bill did not just go out and start carving on his pad lip. He got information from some very reliable sources before he started out. Bill definitely wanted to "measure twice and cut once", and I believe he did this.

In 1995 or 96, the "rumor" is that the lip on the XB-2003 pad was increased from 60 thousandths to 90 thousandths of an inch tall. This change was apparently done to increase bow lift for lower horsepower motors; as usual, the factory credo is efficiency gains* (evidence the 150 max hp sticker on your boat). Increasing the lip is like lowering the flaps on an airplane; Bernoulli lift effect increases and therefore the bow of the boat is raised or tilts upwards.

Now, Red, our 1998 boats have the larger pad lip and so did Bill's (I think his is a 1999). But the more power your motor has, the less need for the heighth of the lip. So Bill did some painstaking experimentation, removing the lip heighth in increments of 10 thousandths or so at a time, all other things being equal, and then taking the boat out and running it each time. Such experimentation is something that most of us cannot or do not have the time and resources to do, but Bill is semi-retired and lives near the Oroville afterbay, a glass-smooth, debris-free waterway about 4 miles in length, with easy public access.

Bill can correct or elaborate on this, but when he finished up (after taking TOO MUCH lip off and restoring it a bit, I think?) he "dialed" the lip in to a lower value than the stock 0.090". I think it was 0.040" where he found the "sweet spot" that the 250XS liked.

I can testify that its holeshot is amazing for a 3 liter, and also that the big problem was the slow-down after being fully strung out.

You bet it will be interesting to take the tape measure to the boats at the rally. That many boats, set up differently, is another great resource that doesn't happen every day.

Switching gears, I can't see the extra bracket setback as necessary for up-trim to prevent the cowl from "crashing" into the transom. The heighth difference is BELOW this pivot-point in the midsection, so how can that affect it?

Darris had me "seeing the light" on the virtual LOSS of setback with the SS motor on the 03's during our phone conversation. I suggest anybody that wants to, call him up and have him explain it. It's probably something where a picture is worth more than a thousand words, but I know of nonesuch in existance. Maybe we need one! The bulk of the argument is that the pivot point on the trim is reduced vertically by 5 inches, and this wreaks havoc on the obvious conclusion that the added setback at the SS bracket decreases the need for more static setback at the engine mount distance.

Darris says that 15" setback is around optimal for 15" shaft motors. Plus, he says there are a few modifications he can do to my 03 (or anybody's) that will dramatically improve holeshot even with this setback. Some of these changes have been learned by his work on the XB-21 hull, and some from the research he is currently so excited about on the new larger hull he is experimenting with.

*(This quest for efficiency is a two-edged sword. On the one hand, Darris builds some of the fastest hulls in the world, and prides the company on using less horsepower to go X mph. On the other hand, who, when buying a Ferrari, wants to put a 4-cylinder motor on it? Most of the customers hang motors that are way in excess of the hull ratings...so given the hulls are designed for lower power, it makes sense that changes are appropriate to provide suitable performance.)

 

[John Richied]

I guess when Mercury incorporated the extra 2”to 2.5” of setback into the 15" bracket on the short shaft it was done to bring it close to the same as the 20” shaft Mercs.

I’m not a short shaft owner or user, but I have always heard that when you put say a 12”setback plate on with a short shaft you don’t add the 2” incorporated into the short shaft and call it 14” of setback.

As JR pointed out “There IS a small setback loss/gain that is built in to the inward angle of the transom as the engine drops/raises.”
For those of you who have a hydraulic jackplate and raise it up while underway, you are increasing the setback. We need a running test, not an on the trailer measurement. Now who are we going to get to measure this while the boat is underway?

Man I’m glad I don’t have a short shaft or hydraulic plate… :lol:

Also wouldn’t the measurement since this is what’s in the water need to be from the back of the pad/lip to the front of the nosecone or gearcase.

And when you run the propshaft above or below the pad you are increasing this distance. But when you run above the pad and run less trim you are actually trimming down/in so there is less distance. Followed by if you are running the propshaft below the pad you are trimming up/out more and increasing the distance from the pad to the tip of the gearcase.

Then all nosecones are not the same lengths, so if you have stock Sporty or a Bob’s, Hydromotive, or whatever nosecone the distance from the pad to the gearcase nosecone may be more or less.

Or if you have a Sportmaster and blunt the nose

 

[RedAllison]

LLDoggg you say the powerhead height isn't effected by setback and is the same on either motor. But with the gearcase being 1/2" below the pad on each engine the powerhead on the SS WILL be 5" lower than on the 20"er (thus the propensity to dunk the engine if you kill the power and sitdown in a hurry). With the the engine at this same setting now trim it out. If it wasn't hanging 14" off the transom the front of the engine WOULD hit the transom on a 15"er and wouldn't on a 20"er. Thus the "need" for additional setback on the SS.

My 03 is a 99 but if I get a 250 put on her someday (still haven't ruled out the 300XS!!! :twisted: ) I WILL then have to get the pad sliced like deli ham!
RA

ps
That bein said I am goin out too the garage with a tape measure. I'll return shortly!!! >>,<<

 

[John Richied]

Last time we seen RA he went out to the garage to measure sum ting… He had a beer in one hand, a turkey sandwich and tape measure in the other and we ain’t seen him since… LOL

Must be a refrigerator stocked with beer out there :mrgreen:

 

[2fast4mom]

DANG i got McDogg to head to the garage with a tape measure :lol: :lol:

 

[RedAllison]

Alright you sumbeechez I dun it!!! :roll:

(and yes I DO have a stocked beer fridge and a freezer fulla game meat standin right next to it in the toybox!)

Here's what I came up with:

I have the Allison hyd plate and it is all the way back. From the front edge of the plate (obviously flush with the transom) it IS 14.5" so ya know what plate I got.

I then leveled the trailer, I know this is flatter than the hull is at running attitude but it's pretty level. The bottom edge of the frame rail both just behind the tongue and then the rearmost back of the trailer was 14.75"

My boat wasn't snugged on the bow roller, it was "floating free" like they do with no force downward from the strap. (Boatmate fixed my boweye mount a few years ago so the "free" hull is barely 1/2" from touching the roller.)

The plate was all the way up and my engine has one hole left below the mounting bolts holding the engine onto the plate. I leveled the motor (measured both on the cav plate and across the prop with a 3 angle carpenters bubble) and measured the center of the propshaft too the floor. 13.75" Interestingly enough I got the exact same measurement when I measured from the center of my nosecone. (JCs Sportmaster)

I them measured the pad height from the floor. At 6" ahead of the rearedge of the pad I got 14.25" (so the MOST I can run is 1/2" below, no wonder I've never done bat turns!!! :mrgreen: )

I then measured from the rearmost edge of the steering shaft housing (ahead of the driveshaft on the motor) and from there too the rear edge of the jackplate I got 9.25"

So that means the rear of the steering shaft is 23.75" from the transom. There is less than half an inch of clearance between that tube housing and the driveshaft housing so if someone will duplicate these levels and measurements as I did (again with a 14.5" plate except on a 20" motor) then we'll once and for all end this "discussion".

Now for the more important question... WTF am I doin crap like this for at 2am in the mornin??? >>,<<
RA

ps
Oh yeah, for fartsNgiggles I measured the thickness of the motor bracket where it bolts onto the plate... 2"!!! :?

 

[2fast4mom]

At 6" ahead of the rearedge of the pad I got 14.25" (so the MOST I can run is 1/2" below, no wonder I've never done bat turns!!!

So this means the HIGHEST you can get is 1/2" below the pad? Hell, no WONDER you cain't go fast! :lol:

BTW there is a slight difference in the angle of the cav plate vs. the propshaft. Git yo azz out there and take off the prop and lay the level ON THE SHAFT. You'll see a difference.

 

[John Richied]

Sorry RA, it’s after 2:00 AM there, so we are going to have to cut you off ()()() ()()() ()()()

Lucky me it’s only midnight here! :mrgreen:

 

[RedAllison]

LOL LLDoggg even at 2am in TN we "unnerstan" better den yall duz in Fruitopia at midnight!!! :roll: :mrgreen:

The center of the gearcase was 13.75" above the ground.

The center of the pad was 14.25" above the ground.

Where I went to school the pad is 1/2" higher than the propshaft. Thus I can't run any higher than 1/2" below!!!

Sok, least I knowed whutta mean!!! :shock:
RA

 

[2fast4mom]

RA i just edited my post, I am dyslexic any time of the day!

Now get out there and get a TRUE level on dat shaft! :lol:

 

[RedAllison]

lmfao, I think we ALL is "dyslexic" at ANYTIME you ken think of. (Musta been all dat dayum turkey we dun et up today!)

Only "prop" I'm fixin to remove and level a shaft on is already asleep in the bed!!! :shock:

Have a goodun
RA

 

[2fast4mom]

LOL git on dat rear hatch boy

nitey night RA

 

[catfish123]

I don't know about anyone else but I find these types of discussions to be very interesting. I do agree that some pictures here could be worth a whole lot in understanding this issue. As a side benefit to these discussions, after reading all this stuff, I feel a lot better about myself being more sane than I normally give myself credit for. Had I read this around 2AM, I would have probably been out in the driveway with my son holding the flashlight trying to get measurements from my setup. I think it's fair to say that some of us border on being NUTS! Please understand that I am including myself in that group.......LOL

 

[RedAllison]

lmao cat, after sleepin on it, then re-readin this post this mornin I can only come to one conclusion. It's the boats!!! They MAKE us crazy, un-able to sleep, nitpickers and unreasonable. Maybe we are beginning to see now why Darris is like he is. Oh chit, does that mean in a few more years I'm gonna be reclusive, untrusting and paranoid??? Oh yeah, I already am... >>,<<

I'd do it again in a heartbeat!!! :shock: :mrgreen:
RA

 

[catfish123]

Yep RA.............I am sure if you were to rebolt on you motor in a higher hole so that you could raise your motor another 1/32", you would gain another 5-7mph from hitting that sweet spot..........After you try this, let me know how it worked............LOL........almost forgot..........you have a short shaft so getting to the sweet spot may be somewhat difficult unless you approach it from a different angle or position.........anyway, let me know how you make out with it.

 

[RedAllison]

Us "short shafters" are more creative.

I've always been more partial to rear hatches anyway!!! :shock: :mrgreen: !-! $);
RA