2005 Merc 40m mods for river racing

**cory** · 10-02-2006

Guys, got a 2005 Merc 40m 3 cylinder (about 1000cc). Runnin on a 14' 44" bottom semi-v aluminum boat. Boat weighs about 300# without me in it nor the motor (another 400#), set up for running fast with all my fishing crap pulled off. The motor has stock reeds, 60hp reed stops and 60 carbs. Rev limiter has been disconnected. A local prop shop shaved the leading edge??? of a Power Tech semi-cleaver 14" and it turns about 6000 rpm according to my Teleflex tach, consistently. GPS shows about 42 mph with a little porpoise. I have the motor up on a 4" setback plate. Prop shaft is running about 2" below bottom of boat. Trim tabs are set to neutral. On a cool day with a little tweaking, I can get about 200 more RPM and about 1 or 2 more mph.

My question is twofold.

What else could I do to the motor? I don't really plan on turning over 6500 or so. I want to cut the tuner and relieve the exhaust but don't really have an idea of where. I know a basic rule of thumb is 2 1/2 times the diameter of your exhaust port for the end of your diffuser cone. Does this apply to outboards as well? Do I need to go with Boyesen reeds? I know alot of guys add a little exhaust duration but I'd rather not cut on ports until it grenades.

For you Ron, what prop works well on these hulls? Most I've seen run anywhere from semi-cleavers to round ear Vengence style props. A few guys with some mods run full on Sport C cleavers. It would seem that I would want more transom lift to minimize porpoising, but some say I need to get the bow elevated? I am planning to send the lower unit to Bob's to get a nose cone installed. So the prop may see a little more surface in the future.

THanks to all and sorry for the big post!

**Mark75H** · 10-02-2006

Originally Posted by cory

I want to cut the tuner and relieve the exhaust but don't really have an idea of where. I know a basic rule of thumb is 2 1/2 times the diameter of your exhaust port for the end of your diffuser cone.

I think the diffuser cone size of an expansion chamber is tremendoulsly different from an outboard's internal tuner ... I'm pretty certain you will find the tuner is already much larger than that. I am also certain that you will find no power increases from cutting the tuner or relieving the exhaust

I think you will have more success with trying better props and adjusting the motor and boat to work optimally with each prop. If you are not willing to accept "hand grenade" reliability, you can't really proceed with with advancing the timing to the maximum each tank of gas can stand, reducing possible rich carburation or any other "tuning"

**Ron Hill** · 10-02-2006

I'd bet there is more speed in the bottom than the motor...

A good flat bottom, maybe with some air traps, and a larger set back, with bottom water pick up nose cone, see Fast Fred's.... then a larger diameter and bigger pitch prop could help...do the engine mods LAST or NEVER!!!!

**cory** · 10-03-2006

Ron, the air traps you refer to....is that just hook in the hull or are you talking about extensive fabrication? And do you think I would benefit from more that 4" of setback? I'm trying to keep the trim tabs in a neutral or possibly a negative position. However, I would think that more setback would increase the hull's tendency to porpoise. Will surfacing the prop offset the porpoising? Most of the guys around here porpoise until WFO then the hull settles. Am I in the right ballpark with the semicleaver, or should I change to a round ear for more bow lift? I have sanded the welds, however imperfections in the bottom are inevitable because of the aluminum's malleability. Thanks y'all for the input. I will cool it on the motor until I get the maximum out of the hull and prop!

**cory** · 10-03-2006

Oh, by the way. The bottom is a 3 degree vee with no pad. I have seen a few "race" flat boats with a small pad welded to the bottom center about 8 to 10 inches in width running the latter half of the boat. I'm guessing this would benefit (like the Hydrostreams) in top speed but make my boat turn like a rocket? THanks!!

**Ron Hill** · 10-03-2006

Where do you live?

Take some pictures of the bottom....and transom area...

As Gritch says, "Speed cost money, how fast do you want to go?"

There is a lot of stuff that can be done, cheaply, by yourself...

If you can't boat in the winter, turning it over wold be the first step...PAD bottoms work...

**cory** · 10-03-2006

I will get you some good pics when I get home in a few. Area where I live is southern Louisiana.
Turning the boat over is an option. I have already sanded some welds to help with hydrodynamics. Also, I do have the resources to weld a pad, if that is a suggestion. What are the pros and cons of a pad on this type of hull? Would I benefit from sanding the paint off the bottom and polishing? I have noticed that water still exits the rear of the hull "distorted". Could sharpening the rails and ribs on bottom clean this up? Thanks.

**Roy Hodges** · 10-03-2006

This reminds me of "the chinaman , John Lowe" in Vallejo, Calif . back in the 1950's . He first had a very liteweight plywood rowboat with a MK 30 . it would go 36 mph. But ,i would have been afraid to ride in it. he had placed the steering only 18" from the bow ! I never saw a boating fanatic as him . his first summer with the Mk30, he ran that mk30 a good 25 running hours per week ! No b s . Later ,he got a Mk58 on an aristo craft, had tons of fun , till he died, of cancer, i think . The Mercman might remember him . He, along with Brac Kramer , were two of the nicest guys i ever knew in boating .

**cory** · 10-03-2006

David, I do have dibs on the stock trim and tilt for this particular motor. Around $700 bones will get me trim and tilt plus I can keep the jackplate. I have heard "to go fast, one must trim". Hell, $700 is only a few more than that raggedy CMC you're talking about, and I have vertical adjustability. Whatcha think?

What kinda bottom mods are ya'll talking about, other than the pad? I know I removed hook and sharpened rails for my old Vector, but this is a different. Just curious what some of ya'lls "tricks" are...

**iwanajohnson** · 10-10-2006

I also run a flat bottom aluminum. Although it's a custom built .63 gague with no v. The river I run isn't deep enough for prop so I'm forced to outboard jet. The question I have is... Are setbacks and cmc t/t units for props only or would they work for a jet also. I was wondering about it cavitating on takeoff with a setback.