2005 Merc 40m mods for river racing

[cory]

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]

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]

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]

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]

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]

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...

[Cajun]

Cory, right now, you are prop limited. First, if you want to gain the most mph for the $, talk to Ron about a prop. With all respect due to PowerTech, that is not the best prop selection for what you are trying to do. Try a Yamaha Black Painted Stainless Steel on that rig, you will see 2-3 mph difference. If you are planning on surfacing a prop, you will need much more pitch, as Ron could explain much better than I. Playing with aluminum hulls, I have found the optimum setback is 10". After 10" the gain is not proportionate to the setback. From 10" to 18", the gains are very minimal, after 18", the gain rise a bit. That said, 18" is not practical for a tiller.
I know we run different hulls, the Edge bottom is designed to run. For reference, I run an Edge 556, a 15'2" long, 56" wide bottom, aluminum hull, weighs 352 pounds empty. I run a 61 ci Mercury 4 cylinder 4 stroke, EFI, set back 10" with a hydraulic lift plate. The motor has a 60 ecu and makes 64hp@ 6249 rpm. With me @ 205#, a 12 gallon fuel tank, a large starting battery, the hull runs 52mph at 6250 rpm. With another 200# man in the boat, the rig runs 49mph. I run a Yamaha stainless averaging 16.5 pitch. I know if I wanted to play with "race" props, the boat could run much faster, but, I'm gettin too old for unplanned swimming trips.
Please excuse the poor driving, this was the maiden voyage for this rig, took a while to figure out her quirks.

http://s126.photobucket.com/albums/p...rent=good4.mp4

[Cajun]

And now the motor with her stripes back on:

[Cubman]

Cajun,

Will a jackplate on an aluminum boat do much for speed or is it primarily for shallow running? Is there a hub available to run this yamaha prop on a V4 OMC (13 spline, 4.25")?

Thanks

[Cajun]

Cajun,

Will a jackplate on an aluminum boat do much for speed or is it primarily for shallow running? Is there a hub available to run this yamaha prop on a V4 OMC (13 spline, 4.25")?

Thanks

The jackplate is primarily to allow you to raise the engine. They were designed for shallow marsh fishermen. The speed gains are moinimal and a byproduct of raising the engine. I think they sell a yamaha black painted stainless for that will fit the V4.
D