Model Boat Mayhem

Dry Dock / Shipyard: Builds & Questions => Pleasure boats, Sports, Race, Power and Leisure Boats: => Topic started by: Paul R on January 29, 2012, 11:03:13 PM

Title: Robbe Sae-Jet Evolution (prop size?)
Post by: Paul R on January 29, 2012, 11:03:13 PM
Has anyone got one of these?, if so what size props are you using. The supplied 33mm contra rotating props are ok but at high speed both brushless motors seem to get out of step with each other once the boat hits a ripple or two in the water. Once this has happened you have to throttle down, close the throttle to reset both speedo's and start again. Iv'e changed both speedo's but still get the same effect at 75% up to full throttle. Would fitting larger props help to keep both motors same speed. Could the problem stem from the fact that at high speed the small props lose grip with the surface of the water therefore allowing both motors to temporarily run at different speeds?, untill throttled right back?

Any advise on this would be great..  The props are of the service pierceing type specified for the model.   
Title: Re: Robbe Sae-Jet Evolution (prop size?)
Post by: gregk9 on January 29, 2012, 11:46:00 PM
It could be that the motors are revving too high and the props are "cavitating".
be wary of fitting larger props as you could increase the drag effect as the props run too close to each other and yet again, cause too much water disturbance and drop your speed.

I tend to find that these boats are "speed dependent" on the water conditions, if its like a mill pond, they go like stink, if its slightly choppy or you go over someone elses wake, the boat looses all grip and you have to throttle back and start off again to get it back up on the plane again.

what batteries are you using by the way? as the power rating could also affect the motors.
Title: Re: Robbe Sae-Jet Evolution (prop size?)
Post by: Paul R on January 30, 2012, 09:47:16 AM
Hi Gregg, Thanks for your reply,

 I'm using a 3S Li-Po 5000mah and two 40amp speedo's, I think your'e right though. As soon as it hits its own wake, at high speed, thats it,  the motors are suddenly out of step and reducing the power doesn't seem to make much difference. You have to throttle right down and then start over again and its fine until it hits some ripples, could be too much power, perhaps a 2S Li-po would siut it better, as i think fitting larger props would just increase the current consumption of the motors which is something I dont really want to do, although they are water cooled. Talking of props being close to each other, i remember the original sea-jet had quite large props 40mm something like that which you had to set up correctly to avoid then breaking up when you pulled left or right rudder, i remember that setup worked quite well ...

Title: Re: Robbe Sae-Jet Evolution (prop size?)
Post by: gregk9 on January 30, 2012, 03:52:35 PM
Hi Paul.

40 amp esc's sounds a bit on the "light" side, you could be running them on their top line. It may benefit from 60 amp units, better to have over capacity on the controller, rather than at max all the time.

I have one of the older types, and yes its fine when its running right, but once one of the grub screws slips and the gearing goes out of sync, you can kiss goodbye to 2 props in an instant [ha ha].

i do prefer it to the newer one, but have driven the "evo" and it still misbehaves same as my older one, once it hits bow wash or a bit of choppy water.

once you get the hang of avoiding such trivial things in the water you sail on, you can still get plenty of fun out of it.

Weight is always a big thing with these craft, too much and it wont get up on the plane and allow the props to do their stuff, same goes for the actual "balance" of the boat too, you could always try a bit of experimenting with battery position, even a few millimeters fore/aft, could make a difference, its a lot of trial and error,but worth it in the end, as i found that for choppy water, battery further forward was better, it kept the bows down but move it back 10mm for "mill pond" and it was on the plane as soon as it left the bank.
Title: Re: Robbe Sae-Jet Evolution (prop size?)
Post by: Paul R on January 30, 2012, 07:11:10 PM
Hi Gregg, In fact I have tried two 60amp speedo's and still the same thing happens but even worse, and iv'e just put the 40amp speedo's back in. I'll do some experimenting with the batt ballance but I think I'll try a 2S Li-Po instead and see how i get on.
Thanks for your help,