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 Post subject: Bunk vs Scissors Trailers
PostPosted: Wed Feb 04, 2009 8:58 pm 
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yahoo, i finally found the stability discussion i posted over at OPDB back in July 2005. So I am reposting it here so we have something else to argue about. Let the games begin!


07/20/2005 02:13:29 PM

I don't think a scissors trailer initiates a rollover much before a bunk trailer, but I think the problem is that the scissors trailer allows it to continue to a full rollover more easily.

The severity of turn goes as speed squared divided by turn radius. This is termed the centripetal acceleration. Ignoring tire tuck, for the inside tire to lift the condition is b/h equals twice the centripetal acceleration divided by the acceleration of gravity, where b is the track (side-to-side distance between tires) and h is the height of the boat+trailer center of gravity above the ground. In other words, a trailer's tendency to start tipping depends on the ratio of track to CG height. Considering the motor, deck, toons and gear, I would estimate that a pontoon boat's CG is pretty close to the deck height. The trailer's mass will lower the CG of the total package somewhat. My scissors trailer puts the deck at about 3 feet high or so, so the package CG is probably near 2.5 feet. The track is something like 4.5 feet. So the ratio is 4.5/2.5 = 1.8. A friend of mine has a bunk trailer, and by memory I will estimate the deck at 5 feet, so the CG around 4.5 feet, and the track around 9 feet. So the ratio is 9/4.5 = 2. These are similar ratios. We could measure the track widths more accurately, but it's hard to measure the package CG.

The point is this: the scissors trailers have a much reduced track width, but that is somewhat offset by the lowered center of gravity.

I think the real issue is what happens once the inside tire leaves the pavement. For the rollover to complete, ignoring the hitch loads, the CG of the boat+trailer package needs to pass over the contact of the outside tire. This requires lifting the CG. The bunk trailer has a much wider track, so the CG must be lifted considerably higher than with a scissors trailer. You can get a bunk trailer up on one side and have it come back down at higher speeds than a scissors trailer.

I think this is an issue mainly for high speed evasive maneuvers, which are when the centripetal acceleration is most likely to get large. Like everyone else, I take it very easy through turns and have never had a problem. I deal with the high speed maneuver issue by leaving a lot of space between me and the car ahead of me. I pull the trailer with the mindset that any deer that runs out in front of me will see in-line braking only and may be history.

So anyway, I don't think the concern is rolling the boat over in a turn on the road (unless it is driven by a maniac). I think the concern is rolling it during an evasive maneuver.

But there may be a price for having that boat up higher--that head-on collision thing, where a boat on a scissors trailer may hit the vehicle frame and stop, but the boat on a bunk trailer may cruise through the vehicle and chop your whole family in half. We've beat that discussion to death in the past, and it's all just conjecture, because we have no data.

I suppose the bottom line is "be careful out there".

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Thu Feb 05, 2009 1:29 pm 
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geese.. there was math in there and everything.. :lala

Personally, bunk here.
I just dont like how close the toon is too the ground on a scissor, and of course, the narrower width between the tires is a negative.
Folks have said that scissor trailers are for short distances and that makes sense by just looking at them and their functionality.

That said, some people have pulled their toon thousands of miles with a scissor and had no problems.

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Fri Feb 06, 2009 5:12 am 
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Location: Rochester Hills/Six Lakes, MI
I have a scissor jack trailer and indeed you have to decide what the prime use will be.. I have hauled it over highways and secondary roads without any problem.... Just take it easy..
It for sure beats a regular bunk type trailer whilst loading and unloading the pontoon.. I have no problems doing all of this by myself, and most importantly I don't need much water to take her in or out of the water..hence you don't need to back up that deep on the ramp.
Where I keep my toon the boat ramp has a very small ramp angle, using a bunk type trailer forces you to go very deep down the ramp with the risk of having you tires to jump off the end of the ramp.. :hammer

Be sure you buy a good brand name.. I have a heavy galvanized Genesis (build in Michigan) and love the quality and robustness..

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Fri Feb 06, 2009 8:46 am 
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I'd be willing to be that this is a crank up trailer contributed to this flip.


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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Sat Feb 07, 2009 10:53 pm 
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based on one photo? i look forward to hearing your testimony.


what about the pontoon i saw lying upside down on a two lane highway?
that had a bunk trailer.


what about this deckboat? a pontoon on a bunk trailer would behave like
this or worse. on a scissors it probably hits the tow vehicle frame.

Image

Image


how about these?

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strapping issues? maybe.


how about backing in too far on a shallow angle ramp?

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i guess i'll make you guys happy and get one of these trailers.

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and i'm gonna buy a really good hitch.

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related to towing, has anyone seen a u.s. source for these orange prop bags?
i'm just finding them on u.k sites.

Image


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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Mon Feb 09, 2009 3:23 pm 
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I was right behind my brother in law's pontoon on a scissor trailer when it flipped. He missed the turn and slowed to about 5mph as he pulled onto the shoulder of a 2 lane highway. Just as he started to turn left to make a U the thing flipped right off the trailer into the ditch.

The shoulder had a good slope to it. A tennis ball would have rolled to the ditch if you placed it on the ground.

I bought a bunk trailer.

Dunbar

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Mon Feb 09, 2009 10:28 pm 
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Location: SW MO
They make training wheels for the narrow trailers or you can make your own. For a scissors trailer they need to be removable which is easily done.

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Mon Feb 09, 2009 10:59 pm 
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> Just as he started to turn left to make a U the thing flipped right off the trailer into the ditch.

he didn't have it strapped to the trailer? ouch.

question: would you pull your pontoon through a ditch, whatever trailer it was on?
i wouldn't.

btw, i know a guy who flipped his pontoon boat off his bunk trailer.
that one didn't prove anything either, other than you shoud strap your boat down.


> I bought a bunk trailer

most people do. i work on my own boat--like to be able to drop it whenever i want.
pulled it thousands of miles. i don't understand the panic and fear.


some pilots hate high wings. "unstable on windy taxiways."
others hate low wings. "death traps in zero zero conditions."
this is just another one of those ongoing arguments.



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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Tue Feb 10, 2009 12:33 pm 
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Location: Colorado Springs, Co
Quote:
some pilots hate high wings. "unstable on windy taxiways."
others hate low wings. "death traps in zero zero conditions."


Well ive got a buttload of hours in both..
The high wing, while i preferred it when teaching as its much more forgiving, does indeed suck more in high winds....
The wind, once on the ground, can get under the wing and flip ya easier...
it makes common sense on why this is the case...

The low wings are a much faster design and on long trips, for the advanced pilot, gets ya there much faster on less fuel...

Not sure what 00 has to do with hating low wings...
in 00 yea cant see past the wings anyways... :)

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Tue Feb 10, 2009 1:39 pm 
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Location: Orlando Fla
All I have is high wing time and yes you have to stay on top of it, not as much as a tail dragger but you have to watch the winds till you get it tied up.

In Florida you get to like that high wing as you have some shade anyway under the wing.

I flew in a Cherokee in Civil Air Patrol in July and it was hot in there every time we hit the ground we popped the door open to get some cool air till we started the take off run.

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Wed Feb 11, 2009 11:19 pm 
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So ya got some flying time Herb? Cool!
Ive got my ATP, and am a CFII MEI. :)

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Thu Feb 12, 2009 5:00 pm 
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It was strapped with 2 ratchet straps on the rear. Both broke. The trailer went over too but the boat was off the trailer before the combo hit the ground.

No one pulled anything through a ditch. The toon landed in one but the truck and trailer never left the shoulder of the road, until the trailer went over.

I strap my toon down to my trailer every time.

Dunbar



FloterBoter wrote:
> Just as he started to turn left to make a U the thing flipped right off the trailer into the ditch.

he didn't have it strapped to the trailer? ouch.

question: would you pull your pontoon through a ditch, whatever trailer it was on?
i wouldn't.

btw, i know a guy who flipped his pontoon boat off his bunk trailer.
that one didn't prove anything either, other than you shoud strap your boat down.


> I bought a bunk trailer

most people do. i work on my own boat--like to be able to drop it whenever i want.
pulled it thousands of miles. i don't understand the panic and fear.


some pilots hate high wings. "unstable on windy taxiways."
others hate low wings. "death traps in zero zero conditions."
this is just another one of those ongoing arguments.



.

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Thu Feb 12, 2009 5:20 pm 
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WaltF wrote:
So ya got some flying time Herb? Cool!
Ive got my ATP, and am a CFII MEI. :)


I'm a Cessna driver 172 ,172RG, wanted to do more but 9/11 and down turns at work sort stopped my flying for a while, I keep saying if I had the money I would do the ATP thing.

But I got to take Mom and Dad up a few times and one time we flew up to Cedar Key for lunch and they had a art fair in the little town so we had a flew fun hours and came away with a Story.

Due the art fair it was crowded with planes so we parked in the grass off the runway and got stuck like a bug on fly paper, but we weren't along so we figured we have help when we got back.

Had some great groper and took the taxi back to the field and we were in time four more people stuck wanted help out and we said we need help to so we all got each other out of the sand and then we hit the air.

That was the last trip Mom got to take with me as she died a few years later.

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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Fri Feb 13, 2009 8:29 am 
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Posts: 75
I have a scissor.

First, bunk trailer have your boat 2-3' higher in the air. Wind and turbulence are way worse in these bunk trailers. I have driven both types.

As for the straps breaking, sounds like you used too small straps. Straps really only have to hold the weight of the trailer, but as a rule of thumb double the weight to be safe and take into account the shock of suddenly keeping the trailer suspended in the air.

I have driven thousands of miles with my scissor trailer. I have replaced axles, re-welded it, and put about a hundred boats on the ground with no damage. I know I could not say that with a bunk.

I even use a roller type bunk trailer. That is the most unsafe of all. Not a lot of sideways friction for side to side resistance from sliding. They look cool. After about three years, you are replacing rollers at $30 each.


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 Post subject: Re: Bunk vs Scissors Trailers
PostPosted: Fri Feb 13, 2009 2:12 pm 
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walt> Not sure what 00 has to do with hating low wings...

i agree with you, but some pilots have argued to me that early low wings didn't have the
right dihedral to give them the roll stability of high wings. they may be right, i flew mostly
bonanzas and cherokees. too busy with kids to fly much these days.

i recall from aircraft design theory that you have to trade off some roll stability to get a
fast slippery plane. so the fast ones will in theory enter a hands free spiral more quickly
than a slow one. as you said, the low wings are generally faster, so maybe some pilots
are equating the low wing with roll instability, when it's really a tradeoff for speed.
jfk jr should've studied this more and gotten a lot more time under the hood before he
let is wife and sis-in-law talk him into that flight.

anyway my point was that pilots will never give up the hi-low argument, and our fellow
tooners will never give up the bunk-scissors argument.

fb


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