Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/O

[colsoncj]

Ok, to help me and future searchers... I've thoroughly read this article (and recommend it for a read if you havent):

http://www.clubmarine.com.au/internet/clubmarine.nsf/docs/MG19-2+Technical

Now, to prevent confusion I term trim adjustment refering to my trim tabs, and tilt adjustment refering to my drives.

I know that using both they can really help you get on plane a lot faster, and really help control ride quality and speed. Also, understanding the operation, I figure its also possible to adjust them to a position where they are actually fighting each other, yet the boat seems to be performing fine, except for an expected decrease in speed and fuel consumption.

My question is, having the ability to see GPS speed, but not fuel flow, is there a proper method of operation to prevent fighting? The background is, I'm currently trying to get some good top speed runs, and I cant seem to break lower 60's, when I think the boat should be running in the lower to mid 70's. (I could be wrong though!)

 

[26aftcab454]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

what props do you run and at what WOT RPMs are you running???
has the boat ever hit 70MPH in the past???

 

[H20Rat]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

70? not many boats can hit 70! it takes a ton of power to go from 55 to 60, you can double horsepower and still might only gain 5 mph.

 

[cr2k]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

Trim refers to either trim tabs or trimming the angle of the propeller/drive.
In + tucked under Out = raised up with in the limit.

Tilt refers to raising the drive for trailering, beaching, etc.

They can be confusing. Never run an I/O tilted beyond the trim limit as it will drastically shorten the life of your U-Joints, Gimbal Bearing, etc.

 

[wca_tim]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

That's an awful lot of boat... and is probably goign to need a LOT more power to break into the 70's...

thanks for the link, good read.

 

[colsoncj]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

what props do you run and at what WOT RPMs are you running???
has the boat ever hit 70MPH in the past???

I've been told it has... I've never seen it. I have talked with 4 other owners of the same boat, and they say 70-75 range for thiers as well... but again, I've never witnessed any of it.

Currently running 25 pitch props at anywhere between 4100 all the way to 5000 (which is above the WOT operating range... so i back it back down to around 4600).

70? not many boats can hit 70! it takes a ton of power to go from 55 to 60, you can double horsepower and still might only gain 5 mph.

I think this boat should do at LEAST 65. I've seen only moderately modified versions hit over 90. (some engine internals, proper props, and 5lbs of boost)

Trim refers to either trim tabs or trimming the angle of the propeller/drive.
In + tucked under Out = raised up with in the limit.

Tilt refers to raising the drive for trailering, beaching, etc.

They can be confusing. Never run an I/O tilted beyond the trim limit as it will drastically shorten the life of your U-Joints, Gimbal Bearing, etc.

Thanks! and ya, I never trim the drives beyond the limit switches while the motors are running... I dont like changing u-joints.

That's an awful lot of boat... and is probably goign to need a LOT more power to break into the 70's...

thanks for the link, good read.

Its currently sitting somewhere around the 600HP to 750HP mark. I'm still working on getting accurate numbers for HP on the engines, then working on determining more accurate numbers after the cam swaps and exhaust work. Remember it has twins.

To the original question, how do you know you arent using your drives and tabs in a method thats fighting each other, short of experience, tinkering, and checking fuel flow?

 

[wca_tim]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

My understanding is that if you've got your hull and drive geometry, prop, weight distribution (center of mass) set up right, the tabs are only there to make minor adjustments at higher speeds... you shouldn't be using them more than to tweak the ride when you're running flat out... if you have to then something else isn't right.

 

[colsoncj]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

My understanding is that if you've got your hull and drive geometry, prop, weight distribution (center of mass) set up right, the tabs are only there to make minor adjustments at higher speeds... you shouldn't be using them more than to tweak the ride when you're running flat out... if you have to then something else isn't right.

I use them to get out of the hole faster, and make adjustments for bow position thru cruising to WOT.

I rarely use the drives to make adjustments.... primarily just for holeshots. but maybe I'm not doing it right!

I do know I plan to continue using tabs and drives to get it out of the hole, using both makes a HUGE difference.

So, if I understand your POV, then I should leave the tabs at neutral, and just use drives? Then when drives are set, use tabs to make minor adjustments? BTW, thanks thus far for helping increase understanding!

 

[seabob4]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

In '94, 7.4s in stock trim were putting out 330 HP. 7.5 Mags, 385. So that puts you around 660 to 770, depending on how tired the motors are. You should easily be seeing 70 MPH.

Don't mess with the tabs, except for holeshot or squirrelly conditions. Use the drive trim alone. If you were to put some serious motors in there, then I'd swap out for K-planes...

Have you checked out screamandfly.com?

 

[26aftcab454]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

I use them to get out of the hole faster, and make adjustments for bow position thru cruising to WOT.

I rarely use the drives to make adjustments.... primarily just for holeshots. but maybe I'm not doing it right!

I do know I plan to continue using tabs and drives to get it out of the hole, using both makes a HUGE difference.

So, if I understand your POV, then I should leave the tabs at neutral, and just use drives? Then when drives are set, use tabs to make minor adjustments? BTW, thanks thus far for helping increase understanding!

start with your tabs & drive down-pop up on plane raise both drives and tabs. the more of the boat you lift out of the water the faster you should go- there are limits to how far you can trim and excess rpms and no speed increase- then trim back down some. Adj tabs for a fast yet stable drive-if you get squirrelly then trim down some.also level out the boat.- practice until you are familiar with what does what before you go running balls out.

 

[colsoncj]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

In '94, 7.4s in stock trim were putting out 330 HP. 7.5 Mags, 385. So that puts you around 660 to 770, depending on how tired the motors are. You should easily be seeing 70 MPH.

Don't mess with the tabs, except for holeshot or squirrelly conditions. Use the drive trim alone. If you were to put some serious motors in there, then I'd swap out for K-planes...

Have you checked out screamandfly.com?

Agreed on the 70MPH. Motors are pretty solid, and have been cam'd. Also have a nice Corsa thru hull exhaust setup. Havent checked out the website you mention... I'll take a gander.

Also, any good method to certify exactly what HP motors I've got? I'm pretty sure they started as the 330's, but the previous owner swore they were the 385s. Soon as I get that, I'd like to get some stock engine specs: bore, stroke, comp ratio, cam grind specs, etc... so then I can swap out the original cam for the new cams on a dyno program and see what my new HP should be around.

Finally is that 330 at the crank or propshaft?

start with your tabs & drive down-pop up on plane raise both drives and tabs. the more of the boat you lift out of the water the faster you should go- there are limits to how far you can trim and excess rpms and no speed increase- then trim back down some. Adj tabs for a fast yet stable drive-if you get squirrelly then trim down some.also level out the boat.- practice until you are familiar with what does what before you go running balls out.

So would you run the tabs to level relative to the bottom of the boat, then trim drives to best setting, then use tabs to adjust boat stability?

Right now I've ran it several times at WOT and have a pretty good feel for the boat and how it acts. I've gotten good at adjusting the trim settings to were the steering gets real light (which I believe is where you want it). Normally I just raise the drives a bit, then adjust to max speed and stability with only the tabs... I may start trying both down, pop up on plane, run tabs to level, then adjust drives to max speed. From that point just use tabs for running adjustments....

 

[smokeonthewater]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

that's how it's done...... use as absolutely little tab as possible... NONE if you can.....

 

[hungupthespikes]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

try and stay away from the trim tabs at high speeds. use the drive only. Too much trim tab, high speed and a turn are no place you want to be. add in 3 footers and when your bow is goes down and the waves are up. not a good memory.

 

[Philster]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

PLEASE READ:

I have read none of the posts so far to give my unbiased and objective opinion.

I'd guess your top speed with nice/strong 454's to be around 70 and a touch more if you lean on it in smooth water. Reading hundreds of posts at Speedwake.com and Offshoreonly.com makes me EXTREMELY confident in this prediction. Guys with twin 496 Mag H.O. aren't doing much better. That is 850 H.P. to get into the mid- to upper-70's on modern/stepped hulls!! EIGHT HUNDRED AND FIFTY H.P. on similar-sized boats, with modern hulls and other modern goodies (like props).

Trim tabs are utterly overrated for boats with such power. Note: I did not say useless -- just overrated. They can provide stability in some seas/conditions and help you put the nose in a position that helps. They are, essentially, always producing drag. At times, you will take the drag for what they offer in control.

High-speed running: Tabs up/out and trim up/out. You are trying to lift the hull out of the water to reduce drag. To a point, this works... until so much hull is up/out that you get unstable and/or can't turn.

PROPS: The last five years in prop technology is worth 2 MPH. Going back to my comments about guys with twin 496 Mag H.O. engines and 850 horses... which are stuffed into stepped-hull boats with modern/advanced cup/rake props, and they hit mid 70's.... well, you need to consider your speed against their speed when all variables are controlled for. If you can record the specs at wide-open throttle, and make a ton of notes about what you WANT and what she does, you can get new props, or new labbed props, or maybe just get the old ones labbed.

It's a 70 MPH boat. Modern props, labbed and getting that puppy absolutely humming puts her into the 70's. Again, for the sanity check, think about all the advtanges modern/stepped hulls have with their twin 496's making 850 horsepower and delivering it through modern propellers. They do business in the 70's and lean on it to hit upper 70's.

Now I am gonna read all the other posts.

 

[colsoncj]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

Thanks guys for all the info!!!! Next time out I'll lay off the tabs and work on just trimming the drives. Keep any info coming. Looks like I should look into fixing my startboard drive indicator now...

hung... that doesnt sound like fun at all! So far no scary experiences, but I really respect the boat and if it gets squirrely or starts doin something I'm not comfortable with, I back her off pretty quick.

philster... great post.

I'll also be starting a thread in the prop section to work with some of the prop guys in getting that end lined out. I found out one of the gurus that frequents the prop section only lives about 35 miles from my house and he's willing to come out and do a 1 on 1 with the boat!

 

[Philster]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

If you can over rev the engines, you are 'under propped' and need bigger props, which will let you realize higher top speed and bring engine RPM down a bit.

Get over to the prop section.

When you are set up properly, you will be doing this:

> With a typical amount of people/fuel/gear... As you lean into full throttle (after getting on plane which you did with drives down/in) you will then have the drives coming out/up to their max position (determined by you by sensing the control and safety they offer at whatever limit you feel is right by experimenting). You've got your big V hull out of the water now, which is essential> You will then watch engine RPM rise. Speed is building. >If all is perfecto, the engines hit a wall a smidgen below max recommended RPM (redline) and this will be your top speed. You are pretty far up/out with the drives, and you might have a little more to come up, but it might add instability/purpoising, so you don't. This is the sweet spot. This is all she has to offer for all practical purposes.

NOTE: About people bragging about their GPS readings:

In rivers and areas that are tidal, to boost GPS-measured top speed, head down river/current (or thru the intercoastal waterway with the tide going out) and you can add 3,4,5,6,7,8,9 or more MPH to your GPS reading.

If your boat is going 65 MPH as the speed over the land (yes, land) and you head down current or out with the tide, you might hit 70! Is it 'really' 70? Well... not really. If you turn around and head against the current, you might only hit 60! You average it out and presto: 65. Most guys skip the part about turning around and just say, "Hey, I hit 70."

 

[RL Gman]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

Most guys skip the part about turning around and just say, "Hey, I hit 70."

Hey, thats what I do...except that first number is a 5...

 

[colsoncj]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

If you can over rev the engines, you are 'under propped' and need bigger props, which will let you realize higher top speed and bring engine RPM down a bit.

Get over to the prop section.

When you are set up properly, you will be doing this:

> With a typical amount of people/fuel/gear... As you lean into full throttle (after getting on plane which you did with drives down/in) you will then have the drives coming out/up to their max position (determined by you by sensing the control and safety they offer at whatever limit you feel is right by experimenting). You've got your big V hull out of the water now, which is essential> You will then watch engine RPM rise. Speed is building. >If all is perfecto, the engines hit a wall a smidgen below max recommended RPM (redline) and this will be your top speed. You are pretty far up/out with the drives, and you might have a little more to come up, but it might add instability/purpoising, so you don't. This is the sweet spot. This is all she has to offer for all practical purposes.

NOTE: About people bragging about their GPS readings:

In rivers and areas that are tidal, to boost GPS-measured top speed, head down river/current (or thru the intercoastal waterway with the tide going out) and you can add 3,4,5,6,7,8,9 or more MPH to your GPS reading.

If your boat is going 65 MPH as the speed over the land (yes, land) and you head down current or out with the tide, you might hit 70! Is it 'really' 70? Well... not really. If you turn around and head against the current, you might only hit 60! You average it out and presto: 65. Most guys skip the part about turning around and just say, "Hey, I hit 70."

Again, great post.

Concerning the over-rev, I've only managed to pull that off once. It was mild chop which seemend to help the boat's top speed quite a bit. All the other times the water has been smooth as silk. If I get another chop day I'll try again and see what I get. I figure some work with the prop guys should take care of all that.

I know its an experience thing to hit the sweet spot, I just really want to thank everyone for helping get the methodology down of how to do it in a boat like mine.

Concerning people and bragging... I knew about the current thing, but around here we all boat on lakes... there may be some current, but it is extremely marginal at best. Windspeed would have more of an effect. I also am sure that quoted GPS speed is about like the length of the fish someone pulled out of the water...

 

[seabob4]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

A slight chop will definitely help your speed as it aerates the hull. Stepped bottoms in flat conditions will typically outrun a straight, non-stepped, deep V, but the non-stepped will catch up in the chop.

As far as your motors, see if you can pull the S/Ns off them and check whether they were the standard 454s or the 454 MAGS...

Phil has some excellent posts, and he owns my favorite line of go-fasts...a Scarab! Hey, I only worked there for 6 years...

 

[Philster]

Re: Keeping Trim Tabs and Power Tilt from Fighting - Proper Trim/Tilt Operation on I/

Hey... Got any Scarab stuff that ''fell of the loading dock'' before you left that might be up for sale now?

One thing to be noted in all this ''go-fast talk'' is that one need gobs of power and/or technology to squeeze out another 5 MPH. Oh, and a boat load of money.

The puns are free.