J.A.E .21 FE rear ski ?

[Andris Golts]

Hi all glued ski under center tub looking at this assembly a ? came to mind . When this rigger is on plane how many inches of the ski near the stern would be used as a planing surface ? Looking at the side profile of the sponsons they look as the riding surface is at about 50 % of the sponson length would this be correct ? The J.A.E. looks to be a fairly wild design ! Regards Andris Golts.

[NativePaul]

It depends on several factors. The weight of the boat and the speed that it is going, the lighter it is and faster it travels the less will be in the water. Accelerating out of the hole it will all be a planing surface. Even with a given weight at a given speed it will constantly be varying from the ripples in the water.

From memory (I haven't had the ski on mine for years) I think it had less angle of attack than the front sponsons, so I would expect it to have a longer wetted area.

[785boats]

The fastest one at our club is one without the Ski. I've just built a mockup of the Keps 21 without the ski. I won't be putting one on that either. It rides well enough without one.

[nobbiworld]

Have tried both. Found that the skis are usually faster. The ski allows a bigger prop to be run. With the ski you aren't relying on the prop to do the lifting. Hence more prop can be used for more thrust. Paul, my new hull that I ran at the last meet is considerably quicker than any others. You missed the action.

[785boats]

Sorry bud. Must have been busy fixing my boats when yours was racing. So I'll correct that statement & say that Damiens' used to be quicker.
Where were you today?

[nobbiworld]

All good mate. Had mother-in-law duties today.

[Brushless55]

The fastest one at our club is one without the Ski. I've just built a mockup of the Keps 21 without the ski. I won't be putting one on that either. It rides well enough without one.

What prop is on that rigger ?

Have tried both. Found that the skis are usually faster. The ski allows a bigger prop to be run. With the ski you aren't relying on the prop to do the lifting. Hence more prop can be used for more thrust. Paul, my new hull that I ran at the last meet is considerably quicker than any others. You missed the action.

What prop are you running ?

thanks

[nobbiworld]

I am running abc h5. Cupped. Not much. 2800 kv

[Fluid]

My JAE 21FE runs 12-second laps on the 1/6th mile oval, topping out in the high 70s on the straights. Runs a Castle 1415/2400Kv, a cupped H-7, and the ski. IMO a ski, or some kind of rear sponsons are needed for the best speeds especially in the turns.

FWIW, the fastest P-rigger in NAMBA (120 mph) runs rear sponsons.




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[785boats]

What prop is on that rigger ?



What prop are you running ?

thanks

Todd.
I'm not sure what prop Damien was running on his JAE 21, But if you were asking about my Kep's hybrid, I was using a Prather s225 in the last test run before I hit a buoy & broke a spar. I haven't had a chance to try my ABC H series props yet. The motor is a 3674 2430kv TP motor.
New spars have arrived & I'll have it ready for our next test day.
The beauty of the Kep's drawings is that the Skis are designed to be attached with double sided tape, like the different length ride pads. So I guess I can knock one up at some stage & see what difference it makes.

[JohnZ]

Fluid, are you using a 4S pack or have you been able to stuff a larger one in the hull?

[Fluid]

A single 4S/5000 mAh pack.


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[wuzofoz]

i think there are only 2 running a ski in our club. i'm personaly not a belever in them but each to his own.
the course we run on is short with dam tight turns (200m a lap at a guess) so there's no time for bigger props to wind out so a ski would help in the tight turns but i'm stubborn and persisting without one. i'm running the same motor as 785boats and starting to get some good pace so lets see if i can take it to the larger can motors with there ski's

[Fluid]

I think Wus is right, it is in the turns and exits where the ski really helps. I have raced riggers on and off for over 20 years and only ran one SAW rigger with any rear ride surfaces...until the JAE. Without rear sponsons or skis the prop has to do two things - push the boat forward and hold up the transom. With a ski the prop only needs to push so a more efficient prop can be used with no wasted power going to lift the rear. The ski also shrouds the prop so it doesn't run too deep and increase the load.

Over the past thirty years riggers have gone through phases, sometimes two rear sponsons are used, sometimes a ski, sometimes nothing. If one type had a major advantage over the others then all riggers would be the same. The fact that they are not proves that there is more than one way to skin a cat.

That said, I probably won't go back to no rear support. Some complain about extra drag on the straights, but it is lap time which wins races, not top speed. I use less than 3000 mAh during a one-mile heat plus mill, topping out at ~78 mph so any drag can't be too bad.





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[785boats]

Jay.
You certainly put forward a good well balanced case for the ski. As do others. I've always said that I will knock one up for mine & give it a try sooner or later. Looks like it might be sooner. Now, where's my scrap ply box & knife......