Weight Distribution

[Airhead]

Got a 12'6" X 7 ft hull with a small aircraft engine that I can adjust the position (front and back) of the engine and seat stand etc. I've got about 2" of play to mess with. Right now it seems to me that the boat sits a little front heavy and barges a bit too much on take off. Is there sort of a rule of thumb on how the boat should sit at rest in the water in order to figure engine placement etc and secondly will moving everything back say only an inch or two help get me up a little faster? What are the consequences of moving it too far back? Will it porpoise or something?

There was a topic I believe that mentioned moving the motor back did generate more bow rise. Is this correct? When setting the engines on our jon boats we raise em until they start to porpoise then back off a little for the right setting. Do you think I can apply this same logic on the airboat?

 

[cntry141iq]

its probably a little more complex than what i am going to say but if I am off some one will say so, hopefully LOL

I would suggest setting your motor attitude dead level, if you can leave a little room for tilt to be added or taken away. If it is barging already I would try moving it back but you need room there depending on your prop to allow for flexing of it. so without knowing your prop type it is a tough question but you can always call the prop maker and ask them how much room you need to allow for flex. You can always call some airboat manufacturers which are listed here on this site under airboat directory at the top and they will probably help you. You might try the one who made your hull.

 

[Anonymous]

The first thing I would do is shim the FRONT of your engine UP about 1/4"

This should help the boat plane out

 

[Larry]

I usually set the motor level with the hull. You can usually use the top of the runner to level the hull out or even the trailer beam if it is straight. If you still have a plowing problem I would raise the rear of the motor or drop the front(which ever is easiest), so the motor pushed down on the rear of the boat and picks the front up. You would do the opposite if you had a slight porpoise problem to put more of a load on the nose. The more the hull is out of the water on plane the easier it will slide on top of the water and takes less power to push it. Also I try to run the motor as far back as it can go and put the fuel tank and battery as far back as well. When running mud or dry ground the boat is always trying to lift the rear and it will push easier when it has most of the boats weight in the rear.

Larry

 

[Airhead]

Guys I've been out of town for a week or so and wanted to go over one more issue on shimming the motor up or down. First off I'm gonna try and run the motor, gas tank battery etc as close to the rear as possible.

Secondly the shimming issue still has me a bit confused. It seems that Red Dwarf and Larry's post seem to contradict each other. To raise the bow Larry's post seems more logical suggesting to raise the back or lower the front whichever is easiest. If as Red Dwarf said you raise the front it would only push the bow lower. Which is right?

 

[Anonymous]

My post is logical if you think of the propeller as a screw (which, by simple laws of physics, it is) which cuts through the air in order to push the boat. The air coming off of the prop no longer has any connection to the prop, and is not what pushes the boat forwards.

Do a few searches for motor or engine angle. You'll find this topic has been covered several times.

 

[c chardt]

I always set mine up level with the stringers and then if I need to I can shim the rear of the engine up or take a washer out I usually put about 4 washers under the back of my engine stands for that reason so there is room to move

 

[Waterthunder]

Raising and lowering your motor is a fine tune adjustment if your boat is rigged wrong or the hull doesn’t work it doesn’t have enough adjustment to over come a major issue how ever if your boat runs OK jacking the motor will make some changes. Raising the back of the motor up will raise the bow up or get your hull off the noose. Lowering your motor will make it ride on the noose it’s the opposite of what some people think but remember your prop always seeks to be level. I always make my rear motor mount jacks out of threaded rod so I can adjust my motor height easily. The more power you have the more you need to jack your motor.(to run free in the water) to make it fast on the hill is something else.