15" mid vs 20" mid???

[scawd the dog]

What are the Performance advantages of a 15"Mid over a 20" mid..on a STV/Mirage, etc type boat?
Thanks
Brent

[jphii]

Lower center of gravity.

[Hot Shot Merc]

One big advantige is getting the engine weight lower to the water.The boat will be much more stable with the powerhead weight closer to the water.Picture a flats fishing boat with a tower and two full grown men atop while running.The weight up higher makes the boat want to rock more.

[Sam Baker]

The 20" is lighter. I've had both on the same boat and I can't tell the difference, and that was on an Allison Super Sport so they would be touchier than a tunnel like an STV, QuarterShot or Mirage.

I think the biggest advantage is the "cool factor" that comes with a 15." However, as far as Mercury's go, I didn't pick up anything speed or accelleration wise going to the 15" from the 20". I'm sure some people have, but I didnt' notice any difference in speed or handling. And theoretically, the gyro effect of the flywheel being placed further from the boat will actually create a higher stabilizing effect than plutting it closer to the center of rotation - similar to a high wire walker walking with a really long balancing pole...the longer pole makes him more stable on the wire. The gyro has more leverage the further away it is from the boat.

I know Wayne Taylor has said moving from a 20" OMC midsection to his 16" midsection is more horsepower and I can believe that because he also improves the exhaust flow. I don't know that there's any improvement in the Mercury 20" versus the 15". I'm just reporting my experience.

[dreamboater]

On larger boats where 2 O/B are used, the lower center of gravity seems to help in rough water.....especially with the 300's additional weight.

[Wolverine]

Didn't make any difference on my Mirage. Looks nice though.:)

[RNM018]

SAM is pretty close to right on with this . Although a true 15" offshore is a much stronger mid then a recreational 20" there is a weight differance .Also the 20" mid provides more leverage to help lift the boat better( BIGGER CROWBAR ). This being said I've found you can also run less setback with the 20" . This is much more noticable between a 12" and a 20" though . My personal prefferance it a 15" just for the built in COOL FACTOR . Rich Martin 018:cool:

[300x Stoker]

"someone" told us that part of the problems (sponson hop) we have are caused by the big 300 being jacked up so far. I was hoping that with a lower C of G the boat would be easier to drive. After reading this it sounds like it's not worth the money:(

[jphii]

SAM is pretty close to right on with this . Although a true 15" offshore is a much stronger mid then a recreational 20" there is a weight differance .Also the 20" mid provides more leverage to help lift the boat better( BIGGER CROWBAR ). This being said I've found you can also run less setback with the 20" . This is much more noticable between a 12" and a 20" though . My personal prefferance it a 15" just for the built in COOL FACTOR . Rich Martin 018:cool:

12" is even cooler!

http://imageigloo.com/images/6324DSC_0026.jpg

[SportJ-US-1]

12" is even cooler!

http://imageigloo.com/images/6324DSC_0026.jpg

What?:eek: No stainless nuts to hold the gearcase on. Shame! Shame!
;)

[Hot Shot Merc]

"someone" told us that part of the problems (sponson hop) we have are caused by the big 300 being jacked up so far. I was hoping that with a lower C of G the boat would be easier to drive. After reading this it sounds like it's not worth the money:(Read my earlyer post.It will help handeling.Not a MORE SPEED MODIFICATION but will bring down the center of gravity to help stabalize the sponson dance your talking about.:)

[jphii]

What?:eek: No stainless nuts to hold the gearcase on. Shame! Shame!
;)

I know, I gotta go to Fastenal on Monday anyways, so maybe this time I'll remember to get them.

[RNM018]

Ya , I know Joe . I've been considering a 12" for the Bass Boat :eek: What you think, Allison Basser with a 12" mid ?:cool: Rich Martin 018:D

[SportJ-US-1]

I know, I gotta go to Fastenal on Monday anyways, so maybe this time I'll remember to get them.

Have you concidered stop drilling the cracks? If you don't, they just get longer, which is bad.

[jphii]

It's not really a stressed area but I do plan on doing something about them after Jasper. That whole extention just hangs out the back. I wish I could find an OMC racing case for it. Then I could lose the adapter plate.

[beer30]

I think it does help with the COG, but thats about it. And they are really cool looking!!!!:cool: ;) Chuck

[RedAllison]

Power difference? ..... Nope
Speed difference? ..... Nope
Handling difference? ..... Nope (not on typical lake boat)
Idle quality for a lake motor? ..... SS sux ass gotta trim out constantly, but DAMN that idle is sweet and lumpy!!!
Bottom end difference? ..... SS bottom end sux ass, limited to chopper style for quick holeshot.

Sound difference? ..... HELL YEAH!!!
Looks/"coolness" difference? ..... HELL YEAH!!! Motor really kicks ass with an Allison wing on it. My lower and plate are painted black the whole "squatty bitch" appearance looks like something that Darth Vader would've had.

So of course WHY NOT go with an SS since looks and sound are most important??? :D :D :D

;)
RA

[MODXR6]

Definitely more badd azz lookin & even a lil one fitty sounds like a drag mota

http://img.photobucket.com/albums/v490/huntermerc/LowerUnit.jpg

[Little Jon]

a one fitty lol.. Jesus, All those motor's look nice besides that OMC 12" * throws up * lol

[MODXR6]

Yep even a REAL lil one fitty

http://img.villagephotos.com/p/2004-10/851217/XB03back.jpg

[Raceman]

12" is even cooler!

http://imageigloo.com/images/6324DSC_0026.jpg

Sorry, but that OMC mid ain't near as short as a 12" Merc. If I remember right, the one in the picture is 3" taller than a Merc Champ. (a merc Champ that's commonly referred to as 12" is actually a full 5" shorter than a 15" Merc)

Back to the comparison, I don't know how many people have run the exact same boat, with both 20 and 15" Merc mids, but I have. The red STV that was mine, that Yammer has for sale now had a 20" offshore on it when I bought it. Later I put a 20" high perf mid (non offshore, no snout) and finally a 15" Merc SS. The 15" Merc has 2 extra inches of setback in the swivel bracket, PLUS since this particular boat wasn't cut for a short motor, it required 5" of setback to mount the motor without butchering the boat. The extra 7" of setback with the same motor and same gearcase/blade accounted for a fairly significant increase in midrange hop. I believe that in a boat that's purely run top end, there may be a very slight increase with the short mid because of the aerodynamic advantage of lowering the powerhead/engine cowl 5 inches. If I was building an STV or Mirage type boat again now, it would have a Champ mid, but I don't think they're practical for V bottoms in lake toy type applications because the powerhead is too near the water, and it not only jambs the tuner all the way under, but can cause water to wash in the lower pan when getting off plane suddenly.

[Liquid Force]

I must admit I am a little suprised (and possiablya little upset after just spending $6k on two 15in mids and clamps, swivel bracket) to hear they make no difference...

Why would merc make them in two versions if there was no gain?? I Initally had 2x 20 in 300's on my boat (has a 15in transom height) and would have had to Jack them to the moon in order to get them to run at the right height, I am sure in a situation that I am using them in (21ft boat 2x300's) that any decrease in the COG is going to be a good thing for handling overall, only time will tell.

[jphii]

Norris, my mid id 11 5/8" tall. Built in 8" setback. I know you have to have a Champ mid laying around somewhere to take a measurement.

[Raceman]

Norris, my mid id 11 5/8" tall. Built in 8" setback. I know you have to have a Champ mid laying around somewhere to take a measurement.

A Merc Champ is exactly 12" as far as measuring the mid itself, and a SS Merc, which is commonly referred to as 15" actually meastures 17. As far as the OMC/Merc comparison, I was thinking more about transom height and how it mounts, rather than the actual length of the mid housing itself. The OMC mounts about 3 inches higher on the transom because of the relationship between the transom mount and the housing.

Omc cleaverly designed the newer style F1 V8 (which is basically the same as the new generation OMC SST120 housing) to mount on the same transom height as the taller OMC housing in your picture, by drastically shortening the mid (it's like 8" or something close???) to drop the powerhead low while leaving the transom mount up at the same height. This made changing to the new style stuff just a bolt on.

[Jay Smith]

From my experiance drag racing Mirages from 1991-1995 ( Wildcat, Polecat, Jag, Riverracer , and Quartermaster, I raced em all ) I prefered the 20" Bass midsection because of the weight advantadge. I had a "theroy" that a mod vee tunnel air entrappment hull that uses a gulp of AIR when launching to lift and "work" needs the higher rotating mass to be used as leverage. The quicker the hull can take a gulp of air, pack it, lift and head down the track the quicker it would have been, musta worked as I won many titles running a 20" against mostly 15" mids...In all out racing I could have "shiv a git" what something "looks like" as long as I got there first. The optimal "end result", in my opinion with a race boat is to win , without that happening the "bling" factor quickly becomes a mute point. I would think nobody cares how cool the 8th place boat midsection looks ?;)

BUT on the lake, on a pleasure sport boat, they are very popular and seem to be "the thing" in high performance boating circles and demand a very much higher delta in resale attractiveness when buying and or selling.. I machine alot of the 15" down housings for exhaust flowing enhancement and boy are they THICK and HEAVY !!! 15" OS are very heavy and 20" OS are SILLY heavy. Putting that much more weight on the back of a fulcrum effects a boats CG BIG time and has to be put into your set up equation factor for optimal effiency..

Good luck,
all my opinion,

[Raceman]

I had a "theroy" that a mod vee tunnel air entrappment hull that uses a gulp of AIR when launching to lift and "work" needs the higher rotating mass to be used as leverage. The quicker the hull can take a gulp of air, pack it, lift and head down the track the quicker it would have been,

Thats a very good point, and I think it's NOT ONLY a matter of the rotating mass being higher, but the fact that the leverage is greater because the lever itself is longer. (distance from the leverage point of the trim to the propshaft is obviously 5" longer)

[J.P.]

I run a 21 daytona eliminator with a production V8 eninrude,...what do you think the effect would be going to Wayne Taylors 12" mid on it would be with a sport master?