Latest thinking on break location

[Mike Bontoft]

I'm working on a new project and trying to get my head around where is the best placement for the bottom tunnel break. My full size electric had the break very far back in the rear 5th of the boat. I though it took a lot of speed to get the tunnel fully cleared out, It was quite heavy for the tunnel wing area. The gas scale hydro's which can also be heavier for there size and having a lower scale speed seem to like the break some forward of center. Bringing the break forward of course does increase the angle of attack in that frontal area. With that in mind would you say it's location is determined some by center of gravity, final design speed and weight? Your experiences and thoughts on this would be very much appreciated. Wishing everyone well

 

[Hydro Junkie]

I'm no expert on R/C boat design but, with that said, I do know that most of the full sized unlimiteds that are running today have the break in the mid 80s to 90s from the transom, those numbers being in inches. If you look at the 1973 Pay'N Pak, where the airtrap stops is where the break is. The break has an angle of 5 degrees(and yes, I measured it) but you need to remember that this particular boat was designed to carry the weight of a Merlin and was still competitive with a heavier Turbo Allison for several years after being sold to the Miss Madison camp prior to the 1978 season

For those wondering, this was shot in Ron Jones shop prior to the boat's delivery to the Pay'N Pak team
On the 0706 Madison hull, the bottom, aft of the break, is 88" long. That does include a 2" overhang beyond the transon as shown in the picture below. The picture also shows the break on the bottom just forward of the shaft log(AKA stuffing tube) at the far left of the picture

 

[LohringMiller]

I wrote an article on breaks.

 

[Mark Anderson]

Mike
Good to see that you are still playing with model boats. We were talking about you at a race on Sunday wondering what you were up to. In my opinion the break and the high point of the deck will end up behind the C.G. as you stated how far depends on the weight of the boat and the speed. The boats intended purpose also comes into play. A circle race boat will be different than a dedicated straight line boat. In Lohring's article you can see the break location on the Mutt. The C.G. should be around 3/4 of an inch behind the sponsons. When it was designed, that combination worked very well for heat racing. At 8 pounds it is stable up to about 70mph at sea level with the air dam. For Straight line, it was a bit too far forward and held the speed attainable down. With todays nitro engines, it would be better a little further back, and with electric needs to be back even further. A more subtle change that might get the wanted results for heat racing would be to move the high point of the deck back and leave the bottom alone. That would not effect the launch ability but would put more lift in the back as the boat accelerates.

 

[Mike Bontoft]

Thank you so much for the the input guys. Lots to ponder over. Very early on in the project at the moment. I'll let you know how it goes.
Been heavy into the kart engines since the move. Karting is big over here.
.
Maybe I'll show up at a race one of these day. Miss you guys. Cheers

 

[Mike Hughes]

Hi Mikey,
Did you get back in carting? Give Bridge a hug for me. MIss ya buddy.
Mike

 

[Mike Bontoft]

Brigitte sends hugs back. Been doing the Karting thing for 7 years now. Hard to believe its been that long since the move. Retiring is sounding better all the time though. Hope you and Family are well. Cheers Mike