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Boat specs.

DairymanNM

Active member
Some of you will remember that I am new to airboating and that I am having problems getting my boat to plane. I took some measurements to see if any of you can spot a problem by looking at them. My hull is alum. with poly. It measures 80" wide by 15' long. The powerplant is a 1973 Buick 455 direct to a narrow 2 bladed composite sensenich. Overall prop length is 75" set at 2.5 pitch mark and is turing at 2800 RPM. When boat is set level the motor is 1/4" higher in the rear than the front. Distance from prop tip to cage is 7", tip to hull bottom is 13.5", transom to prop is 8.25". When the boat is floating the angle at which the motor sits, compared to the level surface of the water, is increased ( downward) due to the stern of the boat sitting much deeper than up towards the bow. The boat plows pretty bad in deep water and will not plane. It seems to plow worse as power is added. Once the boat encounters a shallow location, you can feel the stern rise up out of the water and it will come on plane easily. If run across deep water for an extended period of time after being on plane the boat will settle back into the water begining in the stern and slowly sink back until it is once again pushing water in front. This especially true if people move around and shift their weight. This will usually cause it to fall off plane quicker. Any insight or comments would be greatly appreciated.
 
Level your boat bow to stern, place the level on top of the chines. Make sure your engine or crank is parallel to the chines. Where is your gas tank mounted, where are you and your passengers?? Weight distribution may be a problem.

Randy
 
If I can figure out how to do it, I will post a couple of pictures. I think weight has something to do with my problem but I think there is more to it. On my boat the driver and one passenger sit high in front of the motor then there is a bench down low in front. The fuel tank is in the usual location under and slightly in front of the motor.
 
We have a 16 foot x 8 foot wide fiberglass boat with a direct drive 455 Olds engine. We are turning a 74inch Sensenich Q blade Prop set on the middle (#2) setting at 2800 rpm. our boat has no trouble getting on plane and can even do some limited dry running. Our other boat is a 15 foot x 7 foot aluminum boat with a direct drive 350 chevy. It turns a 72/30 paddle prop at 3000 rpm. This boat also has no trouble getting on plane and limited dry running. You said you are turning a narrow blade prop. And only able to turn 2800 rpm. you migh have engine problems. I plowed the water with our aluminum boat until I leveled the engine and put floor jacks. But I never had trouble getting on plane. My father in-laws boat did something similar to yours but he had carb problems. put a new carb on and the problem was solved. My aluminum boat seating is set up just like yours.
 
Motor seems to be fine as far as I can tell. I put a new Edelbrock carb and intake maifold on it. That seemed to give me a few more RPM's and lost a little bit of weight w/o the old intake. It still may be short on power though. I have no snap or quickness unless I'm in a couple inches of water. Then I'm scared to let it sit for a few seconds b/c of my past experience and lack of faith that it has any power to help me get out of trouble. It has no ability to run dry. I am still thinking that it may be the angle of my motor when the boat is in the water.
 
the only thing about the engine angle statement is that all airboats are heavy in the back and sag. the way the boat sits should help it plane out if the engine is level because it will be pushing up the hill to get ontop of the water. Is your poly dammaged on the bottom? If you get a break in the poly it will suck your boat down. Or if your bottom has a hook in it it may do the same thing.(concaved bottom)
 
Dan,
I see what you are saying about climbing up hill but how much is too much? At some point the thrust forward will be compromised in an effort to raise the stern which will push the bow down and reduce straight line thrust. Correct? As for the poly, I am sure it is not in the best of shape as far as bottoms go. I don't have anything to compare it to but yes, it does have some cracks in it if I remember correctly. When I have time I will pick it up with my loader and photo the bottom for you guys. You can tell me what you all think. I have thought about the jacks I hear about but I am not sure how to go about it and where to put them. Looking under the boat trailered I don't see it being concave. What about getting a wide three blade? Then I'm afraid I won't have the hp to turn fast enough. Right? I'm not doubting you, just trying to understand this whole thing.

Cracker,
The plugs and pipes look fine. The shop I bought the carb from said it should be pretty close on the jetting from the factory. I'm sure I could do a little better but it seems good. Maybe I should take it to a pro here and have them really work it over as far as the set up. That can't hurt.
 
You can use a long straight edge or even a string I guess to check for hook

Jim
 
Check for hook on the inside of the hull. Quite often the plastic will hide hook on the outside. It will be most noticable right at the transom, and yes, hook could explain much of the problem you are having.
 
DairymanNM,

Good to see you back here again, bad to hear you have not solved your problem.

You mention passenger-s as in plural. That's a load for any direct drive boat. Your hull is very narrow by today's standards and you say the poly is busted. It is almost assuredly hiding a hook - and hooks destroy any airboat's performance potential.

Direct drive boats have better luck with more hull beam. The RiverMaster, built by Hoffmann in Inverness, FL. and Freedom Craft also built there, are 8 feet wide at the transom and are very popular choices at 14' or 15' for direct drivers.

These wider boats plane out with Cont. GPUs - so I'd begin to maybe look at a new platform. If your engine metalworks are on 36" standard centers, it is usually a 6 bolt deal.


Plenty of these set ups are for sale on Airboattrader.com
 
DairymanNM,

A boat "hook" describes a very common condition to airboats and other powerboats - specially those with age. The stringers - lengthwise braces that stiffen the bottom - either break loose (fiberglass boats) or bend and flex (aluminum boats) allowing a pocket to develop just in front of the transom - when power is applied. This creates suction between the boat and water and prevents the bow from lifting and the hull from planing off.

New aluminum boats are wider than those built in the past typically have at least five and more commonly, seven stringers to accommodate the more powerful engines and increased loads of today's airboats.

Auto powered boats are notorious for hooking. There is a fix that works most of the time. It is called a hull jack. Marshmaster Pat will describe this better than I can - but it involves securing a threaded shaft to a cross member just below the harmonic balancer of your engine, running it down to another cross member that rests on the stringers and applying doubled nut force downward to create a slight bulge in the hull just below the engine. This will transfer the engine torque from the hull surface to the engine stand and metal works.
 
Will try to get a picture of one posted tonight, there is a Panther just down the road with one in it. I will call and see if they mind me getting a photo. I have one picture somewhere and if they are not around I will try to find it in my digital camera files.
 
I posted a few pictures of hull jacks in my gallery. One is mounted behind a gas tank and the other photo with the battery cables in the photo is mounted just in front of the gas tank. Both are on a 17 footer.

Cracker - You may want to copy those pics of the hull bracings on the Kline and Stossel. I will probably delete them this weekend.
 
Marshmaster,
Thanks for the pictures. That really makes it quite clear how they are set up. It will take me a week or two but I will get one put in my boat and see how it affects it.
 
What Bigdaddy is saying is exactly what I had happen to my old fiberglass hull. I am running a 500 ci Caddy direct drive. And it WAS on a 12’6� fiberglass hull. When we inspected the stringers we knew we had major problems. They were all rotten and separated from the bottom. At 2500 rpm we would only plow water. The hook problem is like licking a suction cup and sticking it to a piece of glass, same effect. No amount of power will unstick it. What I did is take their advice and called Mrs. Hoffman at Hoffman Airboats. I bought a 15’ Rivermaster and new rigging. We put the Caddy on it. We are working the small bugs out now but I can tell you now I am very pleased. This boat will plane in one boat length and run dry about 100 yards. We are working on that now.
 
What kind of price are we talking for a new hull and rigging? If I have a hook and get it sorted out shouldn't the boat be fine? Although I am in favor of a lighter boat and maybe a little wider. I think 16X8 would suit me just about right. With the problems that could be wrong I wonder what kind of set up I really have. I assume a direct 455 has enough power or it would not have be set up that way. I don't need a 1st class airboat. But I do need one to plane and it would be great if it would run dry a little just to cross over to more water. I don't know how drastically a hook can affect a boat but it may explain some of my other problems when I have been stuck. I'll get a jack or two on it then go from there I guess. I really don't want to start dropping more cash if I don't have to. One other question I had was if that stock 455 could turn a wide three bladed prop? Then again, maybe I don't need one.
 
Don't think it will turn a 3 blade wide prop but, a guy in our boat club has a 455 Buic Direct drive that turns a 72" Sensenich Quiet 2- blade adjustable pitch prop just fine. Our 455 Olds turns a 74" 2-blade quiet sensenich to 2800 rpm on the middle setting. (it has 3 marks). If you went with a 3-blade you wouldn't be able to put enough pitch on it to make it work like it should.
 
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