I measured a sponson to have 6 degree planing angle degree for 3-1/2" before curving upwards to sponson nose. That is measuring or referencing off of the flat sponson top which would also be parallel to the water line. But when the sponson is mounted on the tub, the top of the sponson is not parallel with the water line, it is angled down decreasing the planing angle from 6 degrees.

For example, the planing angle was decreased to 3 degree when installed on the hull. And the top of the sponson still angles downwards, not parallel with the hull. How does this design differ than having a parallel sponson top with 3 degree planing angle on and off the hull?

Questioning why the sponson tops should be parallel to water line or not?

Years of testing dictate that for a high speed rigger type model boat hull, the best tuning situation for a sponson's top surfaces, is to have them neither angled upward or downward at the hulls maximum running speed & without any dihedral on the top surface. The trailing edge of the sponson's top surface can be curved downward slightly to give a small amount of lift close the boats CG, but again without any dihedral.

Sponson bottom planning angles are may vary from 3* to 5* depending on such things as the hull's weight, area of the planning surface, dihedral amount used & surface finish. The photos show two types of hull drawings. The first two are oval type boats & the third one is a tether boat. NO HULL should ever be built without first completing a detailed drawing showing a side view & a top view.

Jim Allen

The design god has spoken thanks for your input Jim...I understand what you are saying. If you draw the sponsons parallel with 3* planing angle, how do you know that will stay 3* at full speed? How do you determine how much it will change? For example- Would you design the top of the sponson to be angled down 1-2 degrees, so when it is at full speed it will become parallel, adding 1-2 degrees to the planing angle? And therefore your planing angle should be 1-2 lower when drawn on paper? Hope that makes sense..

Getting the hull's attitude or tune to run as it was designed with whatever sponson planning angle is used will keep the tops of those sponsons level. If the front of the sponson's top surface is angled upward & the hull reaches a high enough speed, the boat will kite or fly. If the front of the sponson's top surface is angled downward it will push the boat down with increasing force as the speed increases & actually slow the boat down instead of allowing it to go faster.

The photo shows a rigger running at speed with the sponson's top surfaces parallel to the water's surface & therefore the top surface is not generating lift or any downward force. This makes the hull go up & down like an elevator at speed & makes the hull extremely stable at speed. Increasing the speed further does not make the hull less stable because only the sponson bottoms & the propeller are generating lift. The lift generated by the wide hull bottom is greatly reduce by a 3/16" high air dam placed at the front of the boat. Notice that this hull does not have a strut which would allow for depth & strut angle changes to compensate for a hull that has an incorrect balance point.

I hope this explains things better.

Jim Allen

Aaronhl,

Here is another photo of a boat being built that has the same features previously discussed. Both the hull's top surface & the sponson's top surfaces are designed to be parallel to the water at speed. Notice that the trailing edge of the sponson's top surface drops slightly. Also notice that the rear end of the very narrow hull rises slightly to keep the tail end from blowing out at speed. The sponson's planning angle has been reduced to 3.5* because the hull is very light in weight. The 3.5* angle is 4" long before curving upward.

I think it is very difficult to build a hull without the benefit of a print.

Jim Allen