.
Hopping, Bouncing, Propoising,, I have witnessed it from full size to models. I live near "Devils Lake" where full size boats come to set speed records and when they're bouncing something is obviously not set right
but they usually find it before the event is over.
Our boats are no different with this aspect anyway and a solution can found. Most model tunnels that are
set-up correctly will bounce if they'er engines are not procucing they'er normal full power and speed.
During the test running phase of any boat race boaters are finding the setup for that particular pond and
weather conditions, so don't think it "strange" that you have to do it too.
Honestly, if your engine is developing it's max HP, with the right setup and prop on there, flat water isn't
going to affect things that much. Is a slight "breeze ripple" gunna make'm faster? You bet! but it's not
essential with the current top tunnels.
Many of you (without help) will have to take on the learning curve of this area of the hobby, you can't
escape it , you have to go thru it like all of us did.
Some of the things I have seen is that many run too much prop on their motors, never letting the engine
develope enough power to do anything fast. A 40mm prop is too **** big for allot of these O/B engines,
use a 637 , 438 or 40X53 to let them spin up to work thru the motor adjustments.
Blueprinting of the hull running surfaces is an area that most leave undone and a bunch of speed
and handling can be lost right there. Get next to someone that knows how to do this and let him show you
so you don't mess your only boat up like I did once.
Balancing lead weights in or on the boat, your gonna have to use lead, just get over it right now and
save yourself the frustration. Very few have no balancing weight in or on they're tunnels, you find one,
shoot'em and do us all a favor.
In 21 O/B engines heat is a huge factor, getting them hot enough will help the engine develope its full
power allowing you to use more fuel and lube in the process. Allot has be said about this right in this topic,
don't ignore it, you won't believe the speed and rpm that can be gained by just getting them hot enough.
I'm done
JW
all very true. listen to him! i'm a noobie to .21 tunnels, but have been around full size hi performance boats of all types all my life. i can remember in the late 50's and early 60's, tunnel hulls/ cats were a new concept in racing. everything i have learned in close to 50 years of full size boats applys to everything we do. motor heat, rpm and prop can affect hopping almost more than balance. too big a prop will produce more lift, but load the motor. as the bow rises, the prop angle and depth change. deeper prop, less rpm to sustain the power needed to carry the nose. nose drops, prop frees up, rpm and power rise, repeat as needed. i only run k&b's (supposedly more torque than os?), and i've run into this. sometimes raising the motor can help, but usually a smaller prop is the cure. if i don't remove the water from my motors, they are just a few degrees above the pond temp! pond 74 last weekend, 88 ambient air, motor 83, after 5 minutes of all out running. with no water, i can only see about 143, at the base of the glow plug, with a ir thermemeter. on byron's 40%, immediately after coming off the water, 5 or 10 seconds. but i can tell a differance, have to run richer needle. running a tongue cut x440, thinned, reduced to 38.5 mm dia., tips cupped/repitched. on a great planes top speed, 3/32" above sopnson, 1.5 degree + trim. have about 3 1/2 oz. in the nose, another 2 0z. lead in front of the radio box. balances at 7", directions call for 7 1/4".red ucing prop diameter and cupping the tips to help direct the prop thrust helped hopping/rise and fall more than anything. about lead- i have 7 1/4 oz. on the front/outside of the radio compartment in my dumas hot shot sprint, to hit 8 1/2"
! it can be tuned out in some hulls, others will require help, like strakes that grim suggested. seen similar solutions on magnum missles in the later 60's ( an early production tunnel by don aranow/later of cigarette boat fame). listen to these guys, it's good info, but no substitue for testing and tuning. EVERY boat is different, you have to learn what it wants, and not be afraid to try it. but, when you hit the set up, the nose lifts high enough to worry you, the whole boat rises another 1/2",. the motor is turning higher than ever. at the end of the straight, you turn the wheel, the nose settles, the sponsons bite, and the boat cuts a nice tight arc, with no hopping and very little sliding. straighten out, the nose pops back up, the motor screams, and i've got a huge grin on my face
. that's what i'm talkin' about!
