Strut & Rudder relationship ?

Tony Jacuzzi · Jun 29, 2015

OK so a couple of questions on how the strut/rudder offset affects performance on monos. Before I buy a bunch of shafts, pushrods etc for testing I would like to know the likely outcome. What I mean is the actual distance fron the transom, etc so in no particular order:

Increasing the prop distance from the transom on the strut will it actually increase the driveability ? Seems like all the gas boats run extended stinger drives that put the prop much further back from the transom.. WHY what are they gaining (mechanical advantage?)
Extending the distance for the rudder blade from the transom affects the boat how and why? Same issue wth all the gas monos running the 4 or 6 inch standoffs on the rudders? Is there a physical advantage to doing this on these purpose built (large) hulls?
Does the rudder need to be in front or behind the prop and why?

Thanks for the input.. Tony J

John Finch · Jun 29, 2015

Tony,

Having the rudder back from the transom does give an advantage in leverage in relation to the turn fin. The further back from the turn fin the more mechanical advantage to tuning the boat since the turnfin is more or less the pivot point. I have experimented with this greatly for SAW, and it does apply to heat racing as well. I have set up an electric mono with the rudder close to the transom and the boat would not turn left at all with full left rudder. Moving the rudder back a few inches the boat turns left just fine. Leverage!

I like the rudder either beside or rearward of the prop for the same reason. I had a twin nitro mono that would not turn at all with the rudder in front of the prop and very near the distance from the transom as the turn fin. No leverage. On my gas boats the prop is further back than the turn fin and then the rudder further back than the prop and everything comes into balance with good leverage for all parts concerned. I like the strut rather than the stinger because it allows for easier and more infinate adjustment. Set the driveshaft a little right of centerline (maybe 1/4 inch or so)where it comes out of the transom and the prop lift on the right side will help counteract torque roll for a smooth chine walk free ride.

RonShaw · Jun 29, 2015

John Finch said:
Tony,

Having the rudder back from the transom does give an advantage in leverage in relation to the turn fin. The further back from the turn fin the more mechanical advantage to tuning the boat since the turnfin is more or less the pivot point. I have experimented with this greatly for SAW, and it does apply to heat racing as well. I have set up an electric mono with the rudder close to the transom and the boat would not turn left at all with full left rudder. Moving the rudder back a few inches the boat turns left just fine. Leverage!

I like the rudder either beside or rearward of the prop for the same reason. I had a twin nitro mono that would not turn at all with the rudder in front of the prop and very near the distance from the transom as the turn fin. No leverage. On my gas boats the prop is further back than the turn fin and then the rudder further back than the prop and everything comes into balance with good leverage for all parts concerned. I like the strut rather than the stinger because it allows for easier and more infinate adjustment. Set the driveshaft a little right of centerline (maybe 1/4 inch or so)where it comes out of the transom and the prop lift on the right side will help counteract torque roll for a smooth chine walk free ride.

Great advice John. X2 as this fulcrum leverage point is the same challenge on tunnels. The further back the skeg is from the end of the sponsons, the better your steering. Only problem with tunnels, you are also moving a lot of weight as you are moving the engine as well, so then becomes the challenge to find the best placement for both.

Patrick Mulvany · Jun 29, 2015

I have always believed the theory of the prop farther from transom fools boat into thinking it is longer. I prefer the rudder behind or along side prop so in the turn it isn't disturbing the water for the prop. Some things you get told and they work so you don't question why especially to Dad???

Later!!

Pat

Kez · Jul 2, 2015

John Finch said:
Tony,

Having the rudder back from the transom does give an advantage in leverage in relation to the turn fin. The further back from the turn fin the more mechanical advantage to tuning the boat since the turnfin is more or less the pivot point. I have experimented with this greatly for SAW, and it does apply to heat racing as well. I have set up an electric mono with the rudder close to the transom and the boat would not turn left at all with full left rudder. Moving the rudder back a few inches the boat turns left just fine. Leverage!

I like the rudder either beside or rearward of the prop for the same reason. I had a twin nitro mono that would not turn at all with the rudder in front of the prop and very near the distance from the transom as the turn fin. No leverage. On my gas boats the prop is further back than the turn fin and then the rudder further back than the prop and everything comes into balance with good leverage for all parts concerned. I like the strut rather than the stinger because it allows for easier and more infinate adjustment. Set the driveshaft a little right of centerline (maybe 1/4 inch or so)where it comes out of the transom and the prop lift on the right side will help counteract torque roll for a smooth chine walk free ride.

Hi John,

Is there an optimal distance from the transom to the prop for a surface drive set up? Will the Speedmaster mono strut work on a 40 size boat as a surface drive?

Thanks,

kez