Air Boat Hull with outboard motor

[Fastfred]

I need help on a air boat hull project, which we put a 20hp outboard motor on it instead, turn out okay, but traveling down canals with tight turns is like a pin ball game, how in the world can I modify the hull for better slow speed control? Help

 

[John Fenner]

You need a keel toward the front, airboat hulls are designed to slide in any direction, I can just picture your experience, not knowing what type of hull you are using, assuming that it is a fiberglass hull?
Flip hull upside-down, with a tape measure, find center of hull front to rear, with a string pulled tight along the centerline use a tire marking pen and run a mark full length of the hull.
Not knowing length of hull, measure from transom forward 2/3rds of the way, make a mark there, then where the rake of the hull begins go a foot past that.
Get measurement of that length, cut a piece of 1-1/4" pvc pipe to said length, cut a taper in the front to follow the rake a bit smooth transition to the pipe, same at rear of pipe, using stainless screws, fasten pipe to hull up the centerline, use a sanding disc, rough up the hull about 2" around the perimeter of the pipe, a fitted width of fiberglass cloth and resin to lock it all in place and seal it, maybe a couple layers for good measure.
Sand it all smooth, apply some resin to glaze it, "can add pigment to match bottom" and that should cure your navigation issues.

 

[Deano]

I need help on a air boat hull project, which we put a 20hp outboard motor on it instead, turn out okay, but traveling down canals with tight turns is like a pin ball game, how in the world can I modify the hull for better slow speed control? Help

As John pointed out an airboat hull by design is meant to slide, an as such, has no horizontal resistance to such an occurrence.
This is exactly what you lacking that you need to add. John's method is one such means of making that addition.
When you say you need to make "tight turns" it leads me to think you will ultimately want more 'horizontal resistance' that what the described means will afford you.

I have had better results by adding a pair of keels (for lack of a better word) or external stringers (sounds stupid, but is descriptive) to the bottom than adding a single one up the center line. Basically what I have done, is take a length of wood and rip it diagonally to end up with two stringers to be placed fore and aft on the bottom of the hull.


Place these stringers such that they have about 1/3 of the bottom width between them. Note that the flat side goes toward the center.
By positioning them in this fashion the inside of the turn will pull (or grab) on the flat side of the stringer, but the outside of the opposite stringer slides over the water and tilts the hull a little bit but does NOT grab. You want the grab and the tilt to favor the inside of the turn for obvious reasons. High siding a boat is not much more fun that high siding a motorcycle.

As far as the placement is concerned, because they are not on the center line and because the planing surface is all the way aft, the best results will be attained by having them start right at the transom going forward.

I would attach them as John describes with the exception being that a couple full stringer widths of fiberglass mat will allow you laminate the stringer directly to the bottom before glassing over to seal it.

 

[Fastfred]

As John pointed out an airboat hull by design is meant to slide, an as such, has no horizontal resistance to such an occurrence.
This is exactly what you lacking that you need to add. John's method is one such means of making that addition.
When you say you need to make "tight turns" it leads me to think you will ultimately want more 'horizontal resistance' that what the described means will afford you.

I have had better results by adding a pair of keels (for lack of a better word) or external stringers (sounds stupid, but is descriptive) to the bottom than adding a single one up the center line. Basically what I have done, is take a length of wood and rip it diagonally to end up with two stringers to be placed fore and aft on the bottom of the hull.

View attachment 97882
Place these stringers such that they have about 1/3 of the bottom width between them. Note that the flat side goes toward the center.
By positioning them in this fashion the inside of the turn will pull (or grab) on the flat side of the stringer, but the outside of the opposite stringer slides over the water and tilts the hull a little bit but does NOT grab. You want the grab and the tilt to favor the inside of the turn for obvious reasons. High siding a boat is not much more fun that high siding a motorcycle.

As far as the placement is concerned, because they are not on the center line and because the planing surface is all the way aft, the best results will be attained by having them start right at the transom going forward.

I would attach them as John describes with the exception being that a couple full stringer widths of fiberglass mat will allow you laminate the stringer directly to the bottom before glassing over to seal it.

Thanks, I will try it!