The Wall

[AndyBrown]

I don't think it's the prop at all.

We have run low 70's on our rigger with the H-4 and High 70's with the H-7.

I think it's mostly lower unit drag and a little boat design. Throw a little pipe work in there for good measure.

 

[topfuel443]

things that make you go hmmm......

Have you run your outboard yet Andy?

~James

 

[GTR]

Hi Guys

If I had to guess I'd say it's a pipe problem. Problem is you need a longer pipe to pull big pitch when running up to speed - counter to this is you need a shorter pipe when speed is getting close to where you want to be to pull the RPM needed for high speeds.

I'm not sure about the drag issue - other than for the skeg hanging down as there is no more driveline in the water than a rigger. Now the amount of hull in the water is a completely different issue. Actually if you think about it the frontal area of a tunnel is way larger than a rigger and this has got to be hurting the top speed - I would expect at least 8-10MpH. On top of that there is probably a lot more driveline drag because of the extra direction change the flex goes through - gotta cost at least a few MpH more. I guess theoretically I think a 21 tunnel will max out around 68 MpH. Unless we can find more powerful motors that will rev an extra 5k RPM and a way to reduce driveline drag.

Hey these are just my thoughts on the 'wall' - more power to you guys and I hope you get a chance to reset the records. With AB now playing who knows what may be in the pipeline.

 

[Slideblues]

Yes Im VERY glad Andy is on board too, One of us is bound to find out what it takes.

I might have the lower unit drag covered, Im using a Design Graphics lower with side mounted removable skeg, tunable like a rigger fin, also has the setback like a Lawless only one piece construction, shaft log has a nice round over too (not blunted off) and no cav plate. Frontal area, well that is the question isnt it Since I will be running a Rinker stlye cowl I dont know how much more you could reduce frontal area, Maybe center section leading edge profiles........ then there is sponson ride surfaces.....Dont know

Gene

 

[topfuel443]

~James

 

[Slideblues]

Ahh now what tha heck are you smile'n about!!!!!! :lol: :lol: :lol:

NeNerNeNer

Gener

 

[topfuel443]

that was "baffled" smile.... after what i just read under Kev's thread on the general forum about props...

:blink:

I DONT HAVE A CLUE...... Theories yes, BUT NOT A CLUE!!!

we will figure it out though...

~James

 

[Slideblues]

Correcto Buddy!!! We will get er done

Kevs talkin props? I'll go check it out,

Gene

 

[Bob Kensill]

Andy's right about lower unit drag. I cut the length of my skeg by half and picked up 5mph. Longer pipes are not the answer. I run a boat twice as heavy for SAW as I do for heat racing, my pipe is as short as it will go,and I'm running a prop way too big for a heat racing boat but my boat still jumps right on the pipe and accelerates hard and winds high.

 

[AndyBrown]

So what is the current 21 OB record (SAW) ?

 

[Slideblues]

IMPBA 64.212

APBA 61.307

NAMBA 58.29

 

[user 115]

65ish?

So what is the current 21 OB record (SAW) ?

73245[/snapback]

​

 

[Slideblues]

Charley split the difference between all three and its 61.5 ish :lol:

Interesting the 3 mph difference between the current records too.......

So gang are we back to Pipe Short/Prop Big....... Er what ?

Im a soooo confused :wacko: :lol:

Gener

 

[Bob Kensill]

What i've found on my 7.5 is a "big belly" pipe set as short as possible,and a large prop running deep.

 

[Slideblues]

Ok..... In tha past I have run low volume cut way short, with dia reduced, backcut, and cupped but with as much pitch as possible and surfaced.

Whos next ?

 

[Bob Kensill]

Well I went 74.95 mph last Saturday,so that makes it pretty up to date!

 

[Slideblues]

Bob, Yes thats true,

What I am doin is compairing setups to see if we can figure this all out,

Gene

 

[GTR]

Hi Guys

some further thoughts - prop set deep has to be more efficient as long as engine can pull the pitch. non-lifting props would have to be the go as they will not attempt to lift the rear of the hull (making for great instability on a tunnel). We really need to see all the effort going in to making the hull go forward and let the tunnel do the flying thing. Maybe 3 or 4 blade props????

Other thing I thought of that makes it really impossible to compare rigger speeds and tunnels is hull stability. Think about it for a moment a rigger has the front sponsons and the prop as stability points - now compare the rear of tunnel sponsons and prop position - the triangle is much smaller making them inherantly less stable. Really makes the challenge of a tunnel going that fast a very fine balancing act. This may well make an OS/Lawless length leg, with no transom inset the desireable set-up to maximise the stability triangle. Now I won't even discuss turning ability as it nots particularly relevant and you could slow way down for the turnaround for the reverse pass.

Just some further food for thought.

 

[topfuel443]

I agree with you last paragraph Gary, Im running 7.5 inches of distance from the drive dog to the sponson rears/transom.... for the same reason you mentioned. sounded reasonable to me.

~James

 

[Slideblues]

The triangle of stability....This is getting interesting, 1 3/4 " inset with a extended mount so transom to drive dog is at 6 1/2" for me, believe the most I have went out was 6 3/4"

Gene