Rear Hydro Sponsons

[Joe Warren]

Long extended rear sponsons (afterplane) Make a Hydro appear effectly longer on the water. But how many believe it would be best to extend the tub the same length as the extended sponson length and bob tail the rears off. It is a lot less to drag thru the water and easier to turn the boat if you dont have the long extended (pass transom) rear sponsons on the boat.

 

[dwilfong]

Is there not a relationship between the prop and rudder length behind the boat that changes how a boat turns?

This is all changed with the plaining surface moved to the rear with extended rear sponsons as thy should only come into play when you turn?

So thy become the back of the boat changing this relationship.

David

 

[Joe Warren]

Yes moving the prop dog back (extended strut hardware) does make the boat think it is longer. But the boat dont know if the strut became longer or if you moved the rears forward? same effect & the extended hardware is moving the strut weight futher away from the trailing edge of the fronts so it is adding running strut weight.

 

[Greg Hahn]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

 

[dwilfong]

So you can make the boat think it is longer by extending the drive dog rearward by what ever means.

then extend the rear planing surface to keep it on top and not dig in the corners.

 

[Dave Wardle]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA) Next step, deeper ski !

 

[Tom Foley]

The rear sponsons come into play on the whole course as both an aerodynamic lifting surface and a hydrodynamic surface in an intermittent manner in race water . On pure calm water by yourself there still is ground effect from the sponsons even if they are not touching .

 

[Greg Hahn]

The rear sponsons come into play on the whole course as both an aerodynamic lifting surface and a hydrodynamic surface in an intermittent manner in race water . On pure calm water by yourself there still is ground effect from the sponsons even if they are not touching .

totally agree. I was commenting on the "why" extending the strut way past the rears could cost speed in the corners.

 

[Greg Hahn]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA)

Dave, simply put (and I know I'm going to get hammered on this one) the larger a JAE gets, the more important length aft of CG is. for your gas JAE to get running good you need more "wheel base" aft for stability and weight.

 

[Tom Foley]

The rear sponsons come into play on the whole course as both an aerodynamic lifting surface and a hydrodynamic surface in an intermittent manner in race water . On pure calm water by yourself there still is ground effect from the sponsons even if they are not touching .

totally agree. I was commenting on the "why" extending the strut way past the rears could cost speed in the corners.

Yes , I was just making a general comment . I think rears really help when you have to chop throttle and then wide open again as well as a stabilizing effect in rough , wide open cornering that might send a rear sponson-less boat for a loop or spin !

I think those running those long extended struts should change them every year and really balance their props well . broken struts on a twin cost a heap of money !! Been there !

 

[Greg Hahn]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA) Next step, deeper ski !

Dave, before you deepen anymore try moving the CG back (add weight in the rear) and if needed raise the sponson angle of attack. sounds like there's not enough weight in the back to keep the prop hooked up. The boat running a neg. AOA will inherently make the boat light in the rear end. good handling is the product of good balance.

 

[David Murany]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA) Next step, deeper ski !

Dave, before you deepen anymore try moving the CG back (add weight in the rear) and if needed raise the sponson angle of attack. sounds like there's not enough weight in the back to keep the prop hooked up. The boat running a neg. AOA will inherently make the boat light in the rear end. good handling is the product of good balance.

Also with the JAE you want to take all of the lift out of the propeller. Our 45 JAE's love to hop the transom unless the strut has 0 angle and the lift is taken out of the prop. Makes a big difference in the attitude of the boat. I know of people putting weight at the transom also for the gas boats.

Dave

 

[Joe Warren]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA) Next step, deeper ski !

Dave I have seen some JAE hydro with the 3 1/2 of the ramp removed. Still launches just like always & you can move the strut down more.

 

[Dave Wardle]

To all the replies many thanks and apologies for the semi thread-hijack..

Will certainly try the rearward CG suggestions. To be honest to date Ive been trying to move it forward, lift the rear with forward CG / lower strut and get the boat to ride level. In the early days it was flying off the water nose first at relatively low speed ... hence why the pitch down changes or AOA reductions have been applied.

We have not yet even looked at its cornering ability (or lack of) although a little anti trip application last weekend looked promising .

Will keep all posted with further testing.

 

[Albert Araujo]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA) Next step, deeper ski !

Dave, before you deepen anymore try moving the CG back (add weight in the rear) and if needed raise the sponson angle of attack. sounds like there's not enough weight in the back to keep the prop hooked up. The boat running a neg. AOA will inherently make the boat light in the rear end. good handling is the product of good balance.

Add Weight??? NEVER. My JAE Gas ran like crap. It was hunting for support on the back ski. I was told to add weight but instead I removed the ski and added a pair of Roadrunner rear sponsons. This gave me twice the support area at the rear and the boat ran great after that.

Albert

 

[Michael Costanzo]

I look at a boat hull as an Airplane that skimms the water, problem comes when cornering the water becomes a "limiting surface" unlike in the air we can't change the angle of lift or the center of pressure by "banking" our boats in the corner. since we must make lateral turns against a limiting surface, the prop's (which is a rotating hydrolic wing) angle of attack changes in the turn and the decending blade becomes a shovel pulling the rear of the boat down burying the prop. The rear sponsons are there to counter this effect and keep the prop on the surface where it belongs while cornering. The closer the back of the rear sponsons (or shoe) is to the leading edge of the prop the less downward pressure (drag) there is on that surface.

But as the prop load changes in and out the the corner (due vector feed) its lift and hence hull and turn fin AOA must change slightly too ..

Very interested in all this as a JAE gas Ive built is a serious PIA to get running well .... just cannot physically get the strut deep enough with the stock ski for it to run happy ...... (bottomming it to the point of bending the lower ski skin really improves everything.. its almost as if the the sponson AOA is too steep and the boat hull really wants to run a negative AOA) Next step, deeper ski !

Dave, before you deepen anymore try moving the CG back (add weight in the rear) and if needed raise the sponson angle of attack. sounds like there's not enough weight in the back to keep the prop hooked up. The boat running a neg. AOA will inherently make the boat light in the rear end. good handling is the product of good balance.

Add Weight??? NEVER. My JAE Gas ran like crap. It was hunting for support on the back ski. I was told to add weight but instead I removed the ski and added a pair of Roadrunner rear sponsons. This gave me twice the support area at the rear and the boat ran great after that.

Albert

OK, since i do not race much but have a great running jae the way to fix the problem is to make the hull 5" longer,just the after plane and you will have a boat that will do anything you want it to do..including making left turns.also,if it is a nitro version,use the GAS turnfin from zipkitts and the boat will stay planted and will handle ruff stuff better.

 

[Dave Wardle]

So we buy and meticulously build to the letter, a hot design .... and then just throw on an extra 5 inches of hull for the crack ??

Now this is really sense of humour testing stuff !! ...... :wacko:

 

[Jerry Muro]

My first thought would be to contact the JAE team and tell them what you are experiencing. I know Martin has helped me with some issues I was going thru with my JAE gas boat.

 

[Michael Costanzo]

So we buy and meticulously build to the letter, a hot design .... and then just throw on an extra 5 inches of hull for the crack ??

Now this is really sense of humour testing stuff !! ...... :wacko:

there is no testing! the reason the japan racers never had good luck with the bigger boats and the always hopped and had all sorts of problems was the tub length. i spoke to a well know boat builder and we disscussed this for many days before making the hull longer. the "hot design" as you call it only works for the smaller boats and was never tested on the bigger boats. i have been in model boat racing over 30 years, do not race much any more but if boats are running good,i will run with anyone out there. if you need proof, i will post some pics from early namba nats with the gold in hand.

 

[Joe Warren]

So we buy and meticulously build to the letter, a hot design .... and then just throw on an extra 5 inches of hull for the crack ??

Now this is really sense of humour testing stuff !! ...... :wacko:

there is no testing! the reason the japan racers never had good luck with the bigger boats and the always hopped and had all sorts of problems was the tub length. i spoke to a well know boat builder and we disscussed this for many days before making the hull longer. the "hot design" as you call it only works for the smaller boats and was never tested on the bigger boats. i have been in model boat racing over 30 years, do not race much any more but if boats are running good,i will run with anyone out there. if you need proof, i will post some pics from early namba nats with the gold in hand.

Mike you cut the last 3 1/2 inches off that ramp and you will be surprised how well it works too. if you add Anti trip to the front sponsons Skin the top of the sponson over onto the add on anti trip. the JAE works well in the water by itself. But the SKI ramp ( its prop plaining surfae) it the 1st thing it bumps its ass on in heat racing water & unhooks the prop.