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11-29
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DairymanNM

Active member
My 455 Buick now has a new Edelbrock carb and intake. I picked up about 200 rpm and lost about 75 lbs. with the new intake. I still have some pitch to put in the prop so I would like to generate some more power and use all the pitch. My boat now seems to run on plane significantly better but still struggles to plane out until it hits shallow water. My driving skills are on the increase and I am finding my boat to be more functional now. It seems to me that my biggest problem is the weight of my motor. It is just too heavy to get the performance I am looking for. What can I do to lose some more of the weight off of it? Ultimately I am looking to run dry with a direct drive if that is possible? On a different note, I have already been approached by the local Rangers and asked if I can be more quiet when I leave the dock in the morning to hunt ducks. It seems the campers are upset about the 5:30 am wake up call when the motor comes to life. I am trying to be as compliant as possible but I can see some real battles coming up for me next year. Any ideas on how to handle this type of situation? Keep in mind, I have the only airboat in Southern New Mexico if not the whole state. People here are not used to them.

Lost in the desert
 
Check out the post by nebraskaairboater towards the end of the running in the tall grass thread. He talks about running a new quiet prop from Sensenich on a direct drive 455. He's getting less noise and more thrust.

Jim
 
It's been a long time since I've run a direct drive 455, but I'll throw my two cents in.

We were running a direct drive 455, stock except for the cam, on a heavy 7x16 plywood boat. With no plastic, it wouldn't run dry, but we didn't have any trouble getting it on step and the top speed was somewhere over 60 mph in deep water. Based on this experience, I am not convinced that your problem is weight.

What are you running for a cam?
What prop?
How big is your boat?
How long are your headers?

I have a friend who has been running a direct drive 455 on a 8'x17' boat for years. He has done some tweaks to improve performance. I'll find out what he has done.
 
My boat is a 7.5 x 16 aluminum with poly underneath. I am not sure how long the headers are. My prop is a composite sensenich. I can not remember the length but I believe it is 8.5" wide. As for the cam, I am sure it is stock. I think the set up is solid, it is just needed some tweaks to get in line. It is interesting that everyone seems to be surprized that the boat will not plane in deep water. I swear it will not! Until recently, it would not even stay on plane if it hit deep water. Even with my foot all the way in it. It just has a bugger in it somewhere that I need to find. That is why I keep using you guys and sounding like an idiot and a rookie ( I am a rookie).
 
Just a crazy thought.

Does the nose try to lift when your in deep water like a power boat does when getting up on plane or does it just plow through the water like barge?

Could it be that the angle the engine is set at is shoving the nose down?

If it plows the angle of the engine might be wrong. I know that by adjusting some of the lycoming engines mounts you can take the porpoise out of the boat.

Have seen that happen with outboards, especially some of the new 4 strokes that are sitting at the minimum power range a boat needs. If the trim is off, they never will get on plane.
 
It seems to me that it really tries to push the water out the way instead of trying to ride up on top of it. I have had many other fishing and hunting boats but this one is the first I have seen that will not come up out of the water. The more power I give it the deeper it sits in the water. Run into some ankle deep water and off you go like a striped ape. Maybe not that fast but I think it runs 30-35 mph. The bow does not rise like a normal boat when it is coming up. This may be normal for an airboat though.
 
You need to check with some of the hotrod boys out there. If you always operate at 4200 ft, you probably need a major compression increase, a leaner mixture, and more spark lead. At that altitude, you are only getting about 75% power, (and that assumes proper mixture, which you don't have with the factory jets) and you already have a low compression engine.

Jim
 
Do you guys think that this motor is worth building up or should I just use it as is until I can buy a hot motor? I have a spare 455 also that I can scavenge from which would help me in the wallet if need be. I won't use this boat much during the summer so I hate to put thousands of dollars into this thing. Another problem I see is trying to use it at sea level if I tune it to this altitude. Am I correct?
 
You could raise the compression to the max that will run on premium at sea level. Then jet it and set the timing for 4200 ft. Then when you go to sea level just rejet and reset the timing. You will probably need to do that with any carb engine. Does that make sense?

Jim
 
It does make sense but I must tell you I am certainly no mechanic. I was proud of myself for getting a new alternator put on it by myself. Once I see it done or have it explained in detail I should have no problem getting the job done.
 
You really need to talk to someone that has run an airboat at 4200 feet to see what will work.

Jim
 
DAIRYMAN,TRY PUTTING A LEVAL ON THE STRINGERS,LEVAL UP THE BOAT,THEN LENTH WISE DOWN YOUR VAVLE COVER.IF THE FRONT OF YOUR MOTOR IS LOWER THAN THE BACK IT WILL PLOW LIKE A BARGE.DEPENDING ON YOUR SET UP YOU MIGHT BE ABLE TO PLAY WITH THIS AND FIND THE SWEET SPOT
 
Dairyman,

Maybe I missed it but are you running this engine as direct drive? If so, your issue MIGHT BE the internal torque curve of that Olds. Many stock GM engines come to life at 3,000 and live till 4,500. An RV cam will move that power curve down in the RPM range.

No airboat prop should ever be turned more than 3,000. That's why the Cadi 472 / 500 '68 / '74 engines are so popular as direct drive airboat power sources. They hit max torque at 3,000. They run 16' fiberglass and aluminum airboats with 600 pounds of friends onboard "up the hill" and across the ground every day, all over Florida.

Also, you might have a busted or warped stringer. If so, that will create a "hook" at the rear of your boat and suck the bow down as engine power is applied. This is a VERY COMMON occurrance on fiberglass hulls that have any time at all on them. If that is the case, the hull must be jacked and decked completely flat before the stringers are replaced. Many times, the fiberglass has stretched at the hook and the hull will never perform correctly.
 
Sounds more like your bottom isn't flat or your engine is pushing the nose down. if you don't get that fixed then the hottest motor money can buy will not do anything but get you wet. We had a boat with that problem and it showered us everytime we came off a shallow spot to deep water and it took forever to get on plane. we found out that the bottom was concaved. The boat I have now had no problem getting on plane but if I tried to run full throttle it slowed the boat down and became very hard to control. I also had a dip in the bottom of my boat. I would level your boat then put the level on the engine. When I did mine I put the level on the prop hub to get the prop angle. I bet that if you get the engine angle/bottom figured out then the boat will be fine even at that altitude.

Just an experience I had.
 
Airboat Dan & Dairyman,

The condition you refer to is called a "hook" in the boating buiz. The bottom compresses in and creates suction - destroying performance and making any boat run on its nose. Since airboat hulls are built so very light and because all bottom impacts are absorbed at the transom / stringer joint; most hulls develop weaknesses at that point. Probably the reason airboat hulls loose about 60% of value once used.

One solution is to install a "jack" to flatten the running surface. This requires mounting a piece of vertical stainless all thread to the engine stand, that runs down to a cross bar running across the stringers. Double nutting with fender washers allows the all thread and cross bar to be jacked down so the running surface stays flat. Very common with heavy auto engine installations.
 
Sounds like I have a place to start. I will put the level to the boat and engine tonight. I will post what I get after I'm done. Thanks guys.
 
If your engine is level you might install the floor jacks and start pushing the bottom down until the nose starts to come up. When I did mine I pushed the bottom down until it started to hop then backed them back down until it stopped hopping. it greatly increased the ability to drive the boat as well as the top speed. My jacks are under the back of the engine. Which is as far back in the boat as I could go. it is just a 2" piece of square tubing welded to the engine stand with bolts that go thru to the top of the stringer and double nutted to lock them in place. We run direct drive on a lot of boats out here. Most of them are 16' x 8' fiberglass boats that weigh a ton. Boats have anywhere from 350 Chevy to 500 Cadillac. All plane out with no trouble... unless they are full of camping gear. My boat is a 15' x 7' aluminum boat with a 350 Chevy Direct Drive.

Let us know if anything helps.
 
Dairyman,

One more thing. You have the premier Cadi performance experts right there in your back yard - so to speak.

Contact Chris support@cad500parts.com

They are: CadCo -8333 Jefferson St. NE , Albuquerque, NM 87113 505-823-9340

After re-reading your previous posts, I see you are running an aluminum hull. Most alum hull manufacturers offer a bottom jack assembly. Diamondbackairboats.com classicairboats.com
 
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