Throttle control

mister p · 17 January 2012, 22:13

I recently spent some time on the water with the Spanish Border Control. Cool guys. They were lending a hand on a shoot by allowing us to shoot from their RIB.
By "shoot" I mean photos, though they are very well catered for in the other shoot scenario.
We were tracking a HUSTLER which was touching 85 knots while we nudged 65.
This is a very physical photographic environment I hasten to add! "Haste" being a very operative word here.
Both helms were throttling off when airborne to prevent over rev and engine damage.......one job that on a two man race team the throttle-man obviously takes charge of.
Needless to say I experienced weightlessness for a very large proportion of the job........and have to say that in this environment the weightless aspect is very promptly greeted with lots of weight to the non fleshy elements of the human body. I was rapidly beginning to resemble a carcass.
I digress.
How many helms out there employ throttle control when airborne?
I have a good reason for asking as am developing a throttle system and it would be good to have some real experience feedback across the board, as it were.

HUMBER P4VWL · 17 January 2012, 22:35

Only when I'm expecting it, ie wave jumping, or able to. When in rough, a) sometimes it occurs when u least expect, or when you are trying to avoid being airborne b) it's hard when being thrown about if skippering alone.

85 knots!!!!!!

http://www.hustlerpowerboats.com/ WOW!!

mister p · 17 January 2012, 22:55

These things cruise at 80mph in luxury cabin and heads trim, topping out at high 90's.
This was the 355 Slingshot in race trim. And thank the Lord for the foot braces.
A skiing black run doesn't come close to the work-out, honestly.
Funny thing is, you crave swell and chop.........it's not the speed, it's the air!!!
Out of interest, this was on the Gib Straits where cigarette boats earn their name as they do just that, still. Smugglers simply build the fastest boats they can to outrun Customs to drop cheap fags in Spain.
I didn't realize it still happened!
Despite the technical advances in their hulls and speed, they still launch from the same spots in Africa and aim for the same places in Spain.
For the border control it's mostly night-time interception using 65 knot armed RIBS, but it does appear that it's like shooting fish in a barrel. The smugglers don't employ any evasion techniques other than night and speed.........keeping to the same route.

Nathan k · 17 January 2012, 23:14

this may seem like a stupid question but how come there is no rev limiter to prevent engines over revving in the first place?
also how much do you throttle back?

mister p · 17 January 2012, 23:29

Quote:

Originally Posted by Nathan k

this may seem like a stupid question but how come there is no rev limiter to prevent engines over revving in the first place?
also how much do you throttle back?

On almost all engines that will get you out of the water today there is a rev limiter.
The thing is that you don't want the engines screaming on the redline without load.
WOT with the props (not screws!) in the drink is the design brief in most cases for RIB mounted engines. Only when they get airborne "over rev" is catastrophic for bearings and friction surfaces. Valve bounce and all the other terminalities associated with heat, metal, balance and overstressed lubricant come into play. Momentum will also play a part in allowing an engine to spin longer than is necessary when without load even when rev-limited. This will happen when you're WOT and airborne. Piston rings are designed to operate under load, not freely spinning.
A few "over revs" every now and then are designed in but half a second every three or four for a sustained WOT over a minute are usually terminal.

wavelength · 17 January 2012, 23:30

Quote:

how much do you throttle back?

not at all....it hurts less when the boat lands and skips forward... rather than slamming down like a ton of bricks and breaking bones if you've throttled back.

mister p · 17 January 2012, 23:31

also how much do you throttle back?[/QUOTE]

You throttle back enough to unload the engine.........with the props in the air ideally throttle back to the stop, but be back on full gas as you enter the water.

mister p · 17 January 2012, 23:51

What you should hear when correctly operating the throttles whilst planing is a drop in engine note when airborne as apposed to the readilly heard "whaaa.....whaaa.....whaaa" of engines redlining as they skip the waves.

Cookee · 18 January 2012, 09:18

As a throttleman in a race boat it's something that I do all of the time - engines will rev past a limiter with the inertia of the engine making most limiters ineffective.

For example rev limit 7,500, you're going along at 5,000 as it's bumpy and all of a sudden the boat and prop leave the water and the engine is at WOT, the engine accelerates to 7,500 and the rev limiter cuts in but as the engine is accelerating hard it runs past the limit and hits 9 or 10,000! (V24's seem to be fairly bullet proof and the rule doesn't apply to them!)

As a turbo diesel throttleman the job is even harder as there is turbo lag involved, luckily the Yanmar BY260's a re great in this department and you have to be throttled back for quite some time to lose boost - the trick is not to push so hard that you're airborne for too long - it's slower anyway!

chewy · 18 January 2012, 15:13

I throttle back and then back on again for the props entering the water, we get told to keep the power on but I'd rather be sympathetic towards the engine than screwing the nuts off em!

9D280 · 18 January 2012, 15:51

I've yet to get the Merc prop out the water.....

On the Suz / SR4 I pretty much did what chewy said, although the ergonomics of your throttle lever doplay an important part in the "ease of doing so" stakes.....

wavelength · 20 January 2012, 14:51

Quote:

we get told to keep the power on

exactly, with good reason. Tis ok with a high powered boat and responsive engines doing x amount of knots to be doing all this throttle juggling and that is a skill pwerboat racers no doubt have in spades -- but Nathan has a "moderate" engine on a sr4, as does the o.p. So when you come off the back of a wave with that you aint gonna be airborne for long and ******* about with the throttle will likely see you come down dead in the water with no forward drive to cushion the blow and it will hurt-Been to recover a guy who fractured three vertebra landing like that in a 5.85 ribcraft/75 o/b. Its easier to mend the o/b (and I have never broken one or seen a leisure o/b broken thru over-revving when airborne) than it was his back. I also like to have positive exhaust pressure in the leg on landing thank you.

Cookee · 21 January 2012, 09:55

Quote:

Originally Posted by wavelength

exactly, with good reason. Tis ok with a high powered boat and responsive engines doing x amount of knots to be doing all this throttle juggling and that is a skill pwerboat racers no doubt have in spades -- but Nathan has a "moderate" engine on a sr4, as does the o.p. So when you come off the back of a wave with that you aint gonna be airborne for long and ******* about with the throttle will likely see you come down dead in the water with no forward drive to cushion the blow and it will hurt-Been to recover a guy who fractured three vertebra landing like that in a 5.85 ribcraft/75 o/b. Its easier to mend the o/b (and I have never broken one or seen a leisure o/b broken thru over-revving when airborne) than it was his back. I also like to have positive exhaust pressure in the leg on landing thank you.

I have driven quite a few smaller engined boats as well, and I would say that whilst the likelihood of damage is reduced it will still reduce the life of any engine to over rev it. The guy who "fractured three vertebra" would have done so on landing and no amount of throttle would have prevented that and to suggest anything else would IMHO be wrong. The exhaust in an outboard faces backwards, and despite coming back down with the boat almost vertical after a particularly big jump in extreme conditions I have never yet managed to stall an engine.

Having said that if we talking about tiny little jumps for a fraction of a second then I can see how it would be less of a problem, but if you're going out in big waves I really believe you need to learn some throttle control - it's not that hard!

Rokraider · 22 January 2012, 11:26

Does the same technique apply for jetboats? Too much time in the air will empty the jet and presumably lead to periods of no drive whilst it re primes. I imagine the delay in picking up the drive will be longer than with a prop?

On my project I have had to build a throttle lever setup that has a fly by wire sender unit that controls the throttle. there is very little resistance on the lever, so I have put a one way hydraulic damper on the mechanism that allows throttle to be taken off easily, but doesn't allow it to be opened up too quickly.

biffer · 22 January 2012, 13:31

I would reckon being airborne at full throttle is likely to do some drivetrain damage eventually.
When you've got a thousand hp swinging large bronze propellers through vee drives and gearboxes there is likely to be some kind of snatch when the prop bites again

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