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...

[cory]

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]

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]

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]

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.

[cory]

Here's the pics of the bottom of the boat that I promised. Sorry for the late response. I tried to take the best pics I could with the boat on the trailer. Let me know what ya'll think about where to put the angle aluminum and how wide the pad should be. I have a bunch of .080 aluminum around the house and I could get some 1" or 2" angle to make the "air entrapment" that ya'll speak of. Thanks.

Cory

[cory]

My strakes are about 6" apart center to center. I am gonna weld 1" aluminum angle to each runner immediately outside the center making my air trap 12" wide. Then weld my flat piece of 1/8" aluminum plate between the two pieces of angle all the way to the front where the the hull starts upwards. What yall think? Think that pad would be too thin?

[Cubman]

TTT

Any more advice on setting up an aluminum boat? I've got a 1648 with about 7* V on the bottom with a 70hp Johnson tiller. How much setback should I start with? I need to run a prop with a hub since this is a duck boat, but should I run a bow-lifting or stern-lifting prop?

[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/p95/1995US1/?action=view&current=good4.mp4

http://i126.photobucket.com/albums/p95/1995US1/IMG_0326.jpg

[Cajun]

And now the motor with her stripes back on:
http://i126.photobucket.com/albums/p95/1995US1/good-1.jpg

[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

[Cubman]

Sorry for the elementary question, but if I operate in water no less than 3' deep would I be better served to just raise my motor on the transom until I loose water pressure or cavitate?

[Cajun]

You could, but it doesn't give you the adjustability of a jackpklate. Most jackplate have 3" lift adjustability built into them. If you are wanting to go faster and faster, use every little advantage you can get. Because I do, and have tested so many 60hp and under props for a certain manufacturer, I have a hydraulic lift, so it's easy for me to test diferent props and report accurate data. My 10.25" diameter Quicksilver Vengeance will run about 2" above the transom, my STOCK 10.25" diameter Yamaha will run about 3" above the transom, my modified 10.25" diameter yamaha will run about 6" above the transom as will my Quicksilver Trophy Sports, my 12" diameter quicksilver cleaver will run 8" above the transom.
Di

[Fastjeff57]

Cubman:

You need to run your boat for a few days with the motor raised WAY up there. If you do, you'd find that the simply act of planing off takes on a whole new meaning! The prop will cavitate like mad, shaking the entire boat (and doing no good for the transom and suspesion components) until you finally nurse her on top. Do that a few dozen times and you'll be delighter to lower her back down there, even if you have to give up a few miles per hour!

Jeff (been there, done that, changed back)

[Cubman]

How was the yamaha prop modified to allow you to go from 3" above to 6" above?

[Cajun]

How was the yamaha prop modified to allow you to go from 3" above to 6" above?

I do not bend propellers correctly or on purpose. My prop was modified by someone that modifies propellers for a living. Just like I did when I raced and he did my props, I told him what I wanted to accomplish and let him use his knowledge to attain that. I have zero cavitation, no motor shaking, no torque? I wanted as close to an all purpose propeller as we could get. I run this prop when empty, and I run this prop with 3 other guys and gear in the boat. Only time I don't run this prop is to tow another boat, I use a smaller pitch spare I keep in the boat.
So to answer your question, I do not know everything he did to the propeller.
D

[88workcar]

6" above the transom as will my Quicksilver Trophy Sports, my 12" diameter quicksilver cleaver will run 8" above the transom.
Di

How does this translate to propshaft hight in relation to the bottom of the hull?

[microbream]

Here's the pics of the bottom of the boat that I promised. Sorry for the late response. I tried to take the best pics I could with the boat on the trailer. Let me know what ya'll think about where to put the angle aluminum and how wide the pad should be. I have a bunch of .080 aluminum around the house and I could get some 1" or 2" angle to make the "air entrapment" that ya'll speak of. Thanks.

Cory

I recently made a pad to suit my shallow vee bottom. The dimensions were very much guess work and scaled from bigger boats. However it is 240mm wide at the stern and tapers down to 60mm at the bow. All edges are sharp, straight and it is perfectly flat.
Trust me i picked up 600rpm at the top end straight away, probably need another repitch to make the most of it. I can now hit 33mph with just 8hp on the back!
as far as porpoising goes, i just drive right though it!!!
cheers
kael

[Cajun]

How does this translate to propshaft hight in relation to the bottom of the hull?

Don't know, I'll shoot a pic later today. The transom is 21" tall, the motor is mounted 3" above the transom at it's lowest point, and the lift plate will rise 6" while underway. So I was mistaken, the 12" diameter will run 9" above the transom for a total of 30" height from the bottom of the boat to the clamp bracket on the motor. I am water pressure limited. It drops from 27# of pressure to about 5# at that height. I do not care to modify the gearcase and water intake. Like I said, I'm gettin to old for those impromptu swimming trips.
D

[Cajun]

How does this translate to propshaft hight in relation to the bottom of the hull?
I originally thought I could run much higher when I first saw the bottom design on this hull. After testing the hull, it rides so high when up on plane, that limits the height the motor can run. Watch the video, it runs very high, more like a padded hi perf bass boat than an aluminum hull.
http://s126.photobucket.com/albums/p95/1995US1/?action=view&current=good3.mp4

He ya go. Please note, that is a fishing prop on the motor, not my 12" diameter cleaver. With the plate raised to it's full height, the cav plate is 4.75" above the bottom of the V on the boat. The prop you see on the boat runs about 2-3" lower than the cleaver.
http://i126.photobucket.com/albums/p95/1995US1/101_3238.jpg


http://i126.photobucket.com/albums/p95/1995US1/101_3237.jpg

http://i126.photobucket.com/albums/p95/1995US1/101_3239.jpg

[Ballistic]

Note that the more setback you have, the higher you can raise your engine.

[Cajun]

Note that the more setback you have, the higher you can raise your engine.

You are exactly correct. The point of diminishing return for us was over 10". We gained height fairly proportionate as we went up to 10". When we went past 10", we could not raise much more until we went to right at 18" setback, then we began to gain more height. 18" is not really acceptable with a tiller, around 12" it starts to get difficult to reach the tiller. We were testing a 3 cylinder Merc.
D

[Cubman]

Cajun,

I have read a post of yours on another forum that said a 16' boat will run 3-4mph slower than a 15' boat. What can I do to minimize this? I assume it is mostly the weight distribution?

[Cajun]

Cajun,

I have read a post of yours on another forum that said a 16' boat will run 3-4mph slower than a 15' boat. What can I do to minimize this? I assume it is mostly the weight distribution?

Don't know. we have tried everything and canot make them run the same. Don't know if it's the weight or the fulcrum point is further forward tha n the fulcrum point on a 15' boat.
D

[Fastjeff57]

Interesting answer. I would have thought extra weight of the sightly longer boat was responsible, but that's more "logical, Captain".

Jeff

[Cajun]

Interesting answer. I would have thought extra weight of the sightly longer boat was responsible, but that's more "logical, Captain".

Jeff

We have tried the same brand boat, a 1648 and a 1548, the 16 weighs about 20# more, we added 20# to the 15, still won't run the same, 16' is slower. That is why I run a 1556, I wanted the extra room but not the added length. And to me, square inches are square inches, be it width or length.
D

[chris3298]

We have tried the same brand boat, a 1648 and a 1548, the 16 weighs about 20# more, we added 20# to the 15, still won't run the same, 16' is slower. That is why I run a 1556, I wanted the extra room but not the added length. And to me, square inches are square inches, be it width or length.
D


dam 15x56?? my alweld is a 15x52 and deffinitly feels wide enough

[Cubman]

Cajun,

I've got a problem with my jon boat porpoising. It is a 1648 with 7 degrees V on the bottom. Motor is a 70hp Johnson with 19 pitch omc SST prop. The cavitation plate is 1.5" above the bottom of the center chine and 0.5" above the bottom of the boat.

There are 5 chines across the bottom of the boat. I added an 1/8" hook to the two outside chines. The boat still porpoises at neutral trim. Should I add more hook to the outside chines, hook the middle pair a little, or something else?

Thanks

[Cajun]

Cajun,

I've got a problem with my jon boat porpoising. It is a 1648 with 7 degrees V on the bottom. Motor is a 70hp Johnson with 19 pitch omc SST prop. The cavitation plate is 1.5" above the bottom of the center chine and 0.5" above the bottom of the boat.

There are 5 chines across the bottom of the boat. I added an 1/8" hook to the two outside chines. The boat still porpoises at neutral trim. Should I add more hook to the outside chines, hook the middle pair a little, or something else?

Thanks

Try to not disturb the water flow in front of the propeller. Add a little hook to the next 2 corrugations, but, stay away from the center of the hull. You could also try a propeller with less rake. Rake=bow lift.
D

[Cubman]

Do you have a gas tank or batteries towards the front of your boat or do you keep all the weight in the back?

[Cajun]

Do you have a gas tank or batteries towards the front of your boat or do you keep all the weight in the back?

First, I can't tell you how many times I have been asked at the dock "how fast will that boat run"? When I reply , "it'll run mid 40's loaded". I usually get a reply of "well, my rig runs right at 50mph". I then smile and wave as I pass their 50 mph rig in the intracoastal canal with my mid 40's boat.

You are trying to compare two totally different design hulls, won't work. My hull won't bounce with 2 men sitting on the back seat. My motor weighs right at 250 pounds, set back 10" on a lift plate that weighs 47#.

Load configuration depends on what I am trying to accomplish. For that all important, one time, big top end number, tank and battery in the rear, boat completely empty.
For normal hunting and fishing loads, I distribute the load. My start battery and fuel tank in the rear, 2 large deep cycle batteries are against the front deck, my 82# thrust MotorGuide is mounted on the deck, cooler in the middle, all gear in the middle. For duck hunitg, I had 2 guys sitting against the front deck, one sitting on the back seat next to me, dog and all gear in the middle. Again, our boats are nothing similar. I have a hull that lifts itself. I can run near zero trim, that allows me to use all available horsepower to push the load forward. The faster my hull goes, the higher up on the V it runs. You have no V to climb up on.
You have X number of horsepower in that motor, how many of those ponies you gonna use to lift the bow and how many to push it forward. The more positive trim you run, the more ponies you waste to raise the bow.

[Cubman]

First, I can't tell you how many times I have been asked at the dock "how fast will that boat run"? When I reply , "it'll run mid 40's loaded". I usually get a reply of "well, my rig runs right at 50mph". I then smile and wave as I pass their 50 mph rig in the intracoastal canal with my mid 40's boat.
Numbers mean nothing to me. I just want to get the best for performance out of what I have, while using it to do what I enjoy doing- fishing and hunting.

You are trying to compare two totally different design hulls, won't work. My hull won't bounce with 2 men sitting on the back seat. My motor weighs right at 250 pounds, set back 10" on a lift plate that weighs 47#.
From where I sit I see that we are both talking about modified V duck boats. My motor weighs 254 pounds, but I don't yet have a jackplate.

It looks like the edge has some fancy backwards turning chines, and you've got a few more degrees v than me, but otherwise the running surface seems very similar.

http://i126.photobucket.com/albums/p95/1995US1/100_3144.jpg

Load configuration depends on what I am trying to accomplish. For that all important, one time, big top end number, tank and battery in the rear, boat completely empty.
For normal hunting and fishing loads, I distribute the load. My start battery and fuel tank in the rear, 2 large deep cycle batteries are against the front deck, my 82# thrust MotorGuide is mounted on the deck, cooler in the middle, all gear in the middle. For duck hunitg, I had 2 guys sitting against the front deck, one sitting on the back seat next to me, dog and all gear in the middle. Again, our boats are nothing similar. I have a hull that lifts itself. I can run near zero trim, that allows me to use all available horsepower to push the load forward. The faster my hull goes, the higher up on the V it runs. You have no V to climb up on.
Are you saying 7 degrees V is insufficient to cause lift?

I appreciate your advice Cajun, right now I am concerned whether I should even try to get alot of speed out of this boat or if it is futile.

[Cajun]

Sorry, I have about 6 converstaions going at once and my simple mind can't keep them separated. I am asnwering a guy on another forum about his FLATBOTTOM boat also and get the conversations mixed up, sorry.
7 degree V is enough to climb up on if you can get the angle right. The Edge, like most hi-perf boats, has turndown tabs at each rear corner to prevent porpoising. Same effect as hooking the hull. You need to add hook until you can run about 2 degrees positive trim. If you are running negative trim right now, once it goes positive, you should see about 500+ rpm and a bit more speed. If you really don't want to hook the bottom, adjustable trim tabs are in your future. The most I have ever hooked a hull was 3/16ths from the back seat to the transom which was 16". On this particular boat, I had to hook all but the 3 center corrugations to make it lay down.
D