Gear Ratio vs acceleration/speed
 

For months now I've been having this conversation with a buddy of mine. He has a 42 Fountain with big procharged power. He has 1.36 Gears in his SCX with SC lower.

He keeps telling me that his problem with acceleration was due to the 1.36 30p prop combo. Because he said the boat accelerated so much better with the 1.5 gear 30P prop (6500RPM topped out). Well, I been trying to tell him you cant compare that scenario, and a more fair comparison would have been the 1.36 with a 26p prop.

Last season with the 1.36 and 30p prop, he turned 6000RPM all out. Which is where he wants to be. So he wants to install 1.5 gears, with 34p props, and he said the boat will hands down be faster and accelerate 10x better. We added about 80HP per side with some changes. So my guess is to stay at 6000RPM he may need to go up 2'' of pitch. I tried suggesting not changing anything since lets see how the boat responds with the engine changes. But he has been told that going to 1.5's will turn it into a MONSTER. I just cant see the gear change being that dramatic. Maybe im wrong.

My questions are for the guys who have ran both setups. I personally have never ran anything other than 1.5 gears in my stuff.

By going to 1.5 36p prop over a 1.36 32p prop setup...

Will the boat accelerate harder, and how much? He seems to think its gonna go from sluggish to neck snapping by making the change?

Will the top speed change??

Which setup would be easier on the drives??

Sluggish to neck snapping I doubt it, I believe the 1:36 with the 32 prop would be easier on the drive, how long wll the drive last if he slams the sticks with either set up? I don't know just sayin.

He might feel something with a 1.5 and a 32, doubt it will be neck snapping in a 42. What RPM will he see the extra 80 HP ?

i believe his imco scx drives are 1.3 ratio.

peak hp of 1195 was at 5900 rpm.

Actually I was wrong on the 80HP. He picked up like 100FT lbs from 3000-3800, and from 3800-5800 he was up 60-90FT lbs.

HP wise, up about 75HP from 3500-5800. Around 6000RPM he only up about 40HP and 40FT lbs. The goal was more midrange.

Also getting rid of his old carb setup, in favor of some new C&S blow thru's. Hopfully those will clean up his fuel curve, which was just not good for anything. Fat down low and lean up top.

fixx
 

theirs more to the story,,he is allso going from a -1 to a -3 shorty if im not mistakin...

The boat is going to get from point a to point b faster with the 1.50 then a 1.36 ratio..


I give up,let him figure it out..

Actually it looks like 1.34 according to this?

http://www.mkhammer.com/imco-sc-lowe...-p-302570.html

Nope. He's keepin the -1 now lol

Does the HP fall off or could push them to 6100 if need be and have a better hole shot.

it started dropping about 5 hp per hundred rpm,only pulled it to 6100 because were going to prop it to max at 6000.

Are you sure you have the right sheet? Mine shows HP climbing from 3000-5900. At 5900 and 6000 it stopped climbing but stayed the same number. No drop in HP on my sheet? Torque started falling off slightly after 5600

What speed was he running in the previous tests?

In general it is more efficient to turn a big prop slower than a smaller pitch prop faster. Typically you switch from 1.5 to 1.36 gears when you run out of props (pitch maxed out).

At the speeds I'm guestimating he's running a 40 hp on the top end will be negligible simply due to drag rise. It may come down to more of a handling issue at those speeds and with that pitch.

I don't think it will be a monster. If he wants a monster accelerator keep 1.5 gears and throw on some 28" wheels.:D

See from what everybody is telling me, they have gone from the 1.5 ratio to the 1.36 that they loose almost all acceleration and it takes forever to get to their top end. From what I have been learning and reading, as long as I dont go over a 36 pitch prop that the 1.5 ratio is more efficient, better accelleration and top end. Has anybody done this and proved this theory if so, please respond.

I agree with you being easier on the drives (strain wise) but one of the biggest factors for killing drives is generating heat as well. Since the 1.36 is spinning faster than the 1.5, what about the extra heat thats being made? Wont that or will that contribute to premature bearing wear?

Fixx
 

Where is Riverrat when you need him,,i bet he has it all figured out :party-smiley-004:


And so all the confusion begin's!!!:thankyouthankyou:

This ...:circle:...:daz:

fixx
 

That should cost him 3-5 mph..is he running the captivation plate extensions?

I'd be shooting for the -4 scx lower with #6 prop shafts!

I would have thought a 1.36 with lower pitch prop would accelerate marginally quicker than 1.5 with higher pitch. Due to the fact that the higher pitch prop is going to slip more and be less efficient until it reaches a certain speed. The lower pitch prop is more efficient at the lower speed (standing start and mid range). They may both even out at peak rpm as long as pitch selection is correct.
This is just my thoughts, I'm no prop guru.

fixx
 

i already mentioned that to him and a smaller diameter prop with your adaptors with the -3's...in one ear and out the other..Some people would rather have it look pirty then have it perform..me paint everything black and fine tune it and lets go boating..

I do! sorry to hurt feelings but sometimes things need to be said..

I believe the 1.36 spins appox 400 rpm faster at 6k then the 1.50, I won't worry about the heat, I run 1.36 with showers no discoloration, This along with Prop Labbing has and always will be trial and error situation. Good luck Keep us posted on the results.

i stand corrected.:party-smiley-004:

my deleted post was a wise crack about riverrat ,and it is now just like him,gone!:lolhit:

In all the ones that I have swapped gears in, the 1.34-1.36 does lose a little acceleration as compared to the 1.5. I only change the gears when we just flat out run out of prop. If you can run 1.5's, then I would run them.

I'm in agreement with Joe that an SCX/SCX-4 (depending on X dim.) with a #6 prop shaft would be the way to go. We have done it on a few 42's and they really seem to like it.(again, depending on X dim.) I'm in the middle of dialing one in now. It has an SCX -4 running 17 X 36 5 blades. It was a little to high, along with a few other issues. We've tried a few different spacers to get the X correct. Now that the X is where it needs to be, we are playing with props. If it ever stops raining, we may get to run it.
Eddie

Hey Eddie , you've done a lot with the 42 beakers; what have you found to be a good shaft height relative to the bottom??

Joe

You mean prop shaft height. :party-smiley-004:

With a full stagger and #6 props, about 2" - 2.5" above the bottom seems to be the spot to be.

THe one that I have here now was quite a bit higher than that. The owner installed a -4, so it was ike 4.5" above. The boat got on plane just fine, but wouldn't carry at all. The slip was near 30%. He installed a 1" spacer with little change. I took it out with the 1" and the boat would make a SHARP right turn at 105-110 because it was unloading one of the props. Talk about scare the crap out of you. We almost got pitched out the first time it happened. I then removed the 1"spacer and installled a 2". That put it at about 2.5" above the bottom. It was a totally different boat then. I'm waiting to try another set of props with less rake. He has 18* rake props and it doesn't seem to like them. I found a set that are the same size with only 16*, so I want to try them.
Eddie

I know a lot of time the 1.36 is used to cure the "running out of prop" issue but doesn't the 1.36 also help reduce the stress on the drive and make it live longer behind bigger HP? I have a 1.36 drive behind my 22 Velocity and its not really needed but I had it and it pulls pretty good with a 24 Bravo. I still have the 1.5 drive that came on the boat and after hearing what everyone is saying I am thinking about trying it this summer with a different pitch prop to see how it affects my top speed. So far with the 1.36 drive at 5270 rpms I have run 77 GPS mph. If I can run a 1.5 drive and get the same style prop to get me to the 52-5300 rpm range will I pick up more speed? My boat gets on plane easy now with 450 hp and the 1.36 drive so planing is not a issue with the 1.36 for me.

I need to measure Joe's. His hull is a 96. I told him 1,2,3,4'' shorties don't really mean anything, as its all about where the shafts end up relative to the bottom. I'd imagine his drive height was rigged more conservative than say a 2006 42.

I don't really like the idea of going to high with the bravo style props. With a -1 lower, if the numbers he gave me are true, then his slip is good at 10% with a 4 blade bravo. Almost too good to believe. We'll see come spring.

That's scary aeriating a prop at 110mph! Do you think the bullet was skating on top at that speed? :eekdrop:

If the boat is that old, then you can count on the drive being fairly low. The one I have here is a 99, but it was re-rigged in 09 to the current specs at that time. So, it has a fairly high X. That's why the -4 was too high.

If he is truly getting 10% slip with 4 blade Bravo prop, then it must be pretty deep. These boats seem to respond very well to higher X dim., at least with the #6 props. He obviously won't be able to get that 4 blade to close to the surface. It he wants to get the most out of it, then he needs to go to the 6 props and raise them up.

It was a very scary ride when the boat hooked at over 100. I would have bet any amount that we were going swimming. I came out of the seat enough to pull the kill switches. That's why I always wear them.:lolhit:

Scary man! I know he tried the -3 lowers, and he said it planed no problem at all. I need to measure his shaft height....wait that sounds bad. :party-smiley-004:

Now if you said legnth i would have been like hmmmm..
sounds like the nose of the boat dug in and started to spin out,,we all saw that video at loto..

This just recently happened in a 42 Fountain. Actually, one of the props broke loose. The tach shot up and the boat immediately hooked. Besides the props shafts being to high, there was a few other things that contributed to it. We have it fixed now. It's just a matter of finding the best prop for it.
Eddie

in the 07 38 fountains with 600 sci's , ITS bravo drives, they came with 1.36 and 1.50 gear ratio, same top end speed and rpm's. I don't know why but they did. They where not running out of prop with the 1.50. Some had merc props some had herrings.
Mine had the 1.36 and 32 6 blade herrings and I did an whipple upgrade and can't believe how it pulls thru midrange, I go easy on it to save the gears.
I would think if the 1.36 is propped for a certain rpm, and the 1.50 is propped for the same rpm the acceleration should be pretty close to the same .

You want me to come down there and drive that boat for you pumpkin? :bunnydance: