bunks on boat trailers
 

Why do boat trailers stop the keel bunks several feet from the end of the trailer, and only use bunks out near the strakes and chines to support the boat's transom?

btw, I stole this picture from another thread a while ago. This beautiful trailer is not mine, but another OSOers.

On that trailer the transom will be at the end of the rear bunks. The exta space and bar across the rear of the trailer is protection for the drives. The bar acts as a bumper in case of collision and swings clear when time to launch.

Trailer
 

The particular trailer in the picture is a beauty. If you look carefully at the rear bar with the Myco emblem, it is designed to be removed or swing out. The transom of the boat actually will sit at the rear of double bunks. The drives are protected by the additional removeable, swingable cross beam(bar) at the rear of the trailer.

Once the beam is in place it is like a rear bumper!

Most cat trailers are built in this fashion as well.

Thanks guys for your answers, but I'm not asking about the drive protector!!! The only picture I had of an empty trailer happened to be this one with a drive protector. Sorry about using a picture of a trailer that was confusing.

Look at the last cross member prior to the drive protector, and you'll see the bunks are at the outer edges of the boat.

Now look up three cross members, and you'll see bunks closer to the keel of the boat. That's what I'm asking about.

Express Cat, noticed you've got Pure Adrenalin and a Cig logo. Are you the new owner of a mint 35 cig of that name?

Good question.
I found a pic in the OSO Classifieds of an Apache belonging to Peter Pavkovich.
In the pic you can see where the rear bunks contact on this hull. Only thing I can say is that the contact area must be where the greatest strength is in the hull. Possibly due to where the stringers and engine beds are? Same for the contact points further forward?

Mr. Pavkovich, I hope you don't mind if I use one of your pics as an example.

Now that's a boat garage!!!

I wish I had one the size of an "aircraft hanger" too. :D

Directly above the chines are the stringers... stronger part of the hull... less chance of distorting the hull while it sits for a long time on the trailer.. That is why the bunks are put in those positions. I've seen boat trailers that didn't have good bunks.. and the hull sagged over the rollers along the bottom... ruined the hull... ( it was a junk hull anyway.)

So trailer mfg's don't put keel bunks at the transom to insure the outer bunks support the trailer at the stringers? That makes sense, but in the picture above Myco added a second bunk just inside of the outer bunk that lines up near the stringer, and the inner bunk would support the boat where there is no stringer or support at all. Whereas there is usually a stringer along the keel in the engine compartment, but not in front of the engine compartment due to the fuel tank. In addition, half the weight of the engines is supported by the keel stringer in the engine compartment, which is not supported by the trailer. I'm no naval architect, but this is not making sense to me.

Most twin engine boats have a "center stringer" that runs all the way from the stern to the front of the cockpit... and usually have two fuel tanks.... one on each side of the stringer.On the outer sides of each of the fuel tanks are smaller stringers that correspond to the outer chines. The large tall center stringer usually ends where the door into the cabin starts... there is a big bulkhead that goes from keel to deck with a "hole" to pass through... then two smaller stringers continue up toward the bow and are usually "boxed in" .. The seats inside the cabin are over the top of these... they usually end at a keel to deck bulkhead where the front bunk starts... does this make any sense to you? In the engine compartment you will usually see smaller stringers that run under the engines. Sometimes there are small bulkheads between these smaller stringers... and even smaller ribs running at a diagonal angle... these all stiffen the outer part of the hull. The pressure of the water being displaced is greatest against the area where most of the weight is... under the engines and transmissions.... and when entering the water from coming off a wave... this is the area that usually hits first... so builders tend to strengthen this area more. l If you look into the "cabin" area of a race boat... there are more stringers and bulkheads... and sometimes a balast tank for balance... but not much room for people.Does this explain anything to you ?Chart?:eureka:

One more thing.... the bunks don't always run directly under the keel... but close enough .. in the picture you posted .. the keel bunks look like they are only about 8 inches apart.. on a 40 foot boat the hull is about 1 1/2 inches thick along the keel... it's a pretty stiff area of the boat.. not only because of the thickness... but the shape of the hull... When the boat floats onto the trailer... it finds it's "center" between those inner bunks... then lays down on the outer ones when the trailer is pulled up the ramp... That might be something that you haven't considered Chart... the boat has to lower itself onto the trailer at an angle. The picture you posted is what is called a "float on.. float off" trailer... there are other types of trailers for race boats where the boat is lifted off with a crane and then lowered into the water.. the trailer is never submerged in the water.

Yes, but I had to go real slow and get help with the big words:o :p :D

You know much more about the design of stringer systems than I do, and I appreciate the lesson. It looks like the center stringer that runs up to the front cockpit bulkhead is actually supported by the keel bunks on most trailers under the cockpit/fuel tank area, but not under the engines.

Could that be because the transom, one of the strongest parts of the boat, supports (holds) the keel stringer and inside engine weight more than adequately, when the trailer bunks support the outer stringers? This prevents the boat from deforming the "running surface" if the trailer is not set perfect for the boat? But forward of the engine compartment, the transom is too far away to offer strength to the center stringer, and so the trailer has have a keel bunk?

Reed: You posted #12 while I was writing #13. You may be on to something there about aligning the boat during launching and loading. Without keel bunks in the front of the trailer, the increasing vee of the front of the boat might hit cross members in some conditions

That is correct Chart.. on some trailers there are rollers... but if the boat doesn't hit the roller dead on... the frame can scratch the keel .. I've done it on my boat. That is the reason for the bunks near the front.

Hers a pic of my old trailer for a step bottom boat. The outer bunks were right near were the chine flattens out and the inner bunks were inside a set of strakes which also lined up with the hull stringers. The bunks are split for the step and at different heights and they ran right ti the edge of the transom. Worked good for loading and didnt put any hook in the bottom after 6 years.

One more post for you Chart... in looking more at the picture you posted... the reason the center bunks for the keel don't run as far back as the bunks for the chines at the outer part of the trailer is this... the boat has a notched transom.

Reed: That's a good point. Once again, you could be on to something.

DanB: I would think your trailer offers better overall support at the transom, but there must be a reason Myco, Manning, Eagle, etc elect not to do it that way.

We've got members that mfg trailers (Manning, ??? from West coast of FLA sorry I don't recall the name) and Myco advertises here. It'd be nice if one of them weighed in here also.

What is your concern Chart? The thing to watch out for is the trailer not supporting the hull... over time the hull will deform and then have a "hook" in the bottom.. Hence... the trailer has to be quite rigid from stem to stern. But steel tends to bend ... and since it is supported just at the axles.. they tend to bend right there.

It's not concern, but curiosity. I've wondered about this for years, and finally asked. Heck, we had a hotly debated post years ago about which freezes faster, hot or cold water. People weighed in with all kind of scientific and engineering insights as to which was right. And that was not even boating related, unless you count that it was water, and that ice is needed for the coolers.

Occasionally a picture will appear with the boat overhanging the bunks by 2-4 feet, and we know that is hard on the hull. It simply strikes me as odd that the heaviest part of the boat has the greatest span between the bunks.

Reed, you've contributed the most to this discussion, and thank you for that.

You may be right about that Chart.... sometimes why things are made a certain way doesn't make sense.. until you factor dollars in... today's manufacturers factor cost more than anything. If they can get away from putting in another row of bunks... they will do it. Boats on trailers spend more of their life laying on the bunks than they do in water... even here in California where it doesn't freeze. Only blowboaters in the marina.. or really large boats don't have a trailer.. then you have to deal with barnacles and underwater care. There are some offshore boats in Marina Del Rey that are on hydro hoists... and those are just floating trailers. :p

Maybe it is a holdover from long ago. My 88 Myco is designed the same way, except the strake bunks are 4x6 cypress boards on edge, and difficult to align the boat evenly on. That's why the bunks are now angled along the hull under the chines, I'm sure. But, the keel bunks are 2x4's 12 feet long. Maybe, back when all bunks were one solid board, 12' was the longest reasonable length, an needed to be further up on the hull than placing at transom would allow. And that placement is still used today. Either that, or it's a vast conspiracy started by space aliens in cahoots with the government of Atlantis for purposses we don't yet know. Elvis is somehow involved in it;.

Ahem.... I have it on good authority that Elvis is running a liquor store in Arkansas....

Here is a Myco with Drive guards good trailers will be supported passed the back off the boat this I imagine avoids developing a hook in your bottom that can rob of a lot of speed.

Funny thing is i have yet to see a rack storage place that has supports that goes all the way passed the rear bulkhead, most of them are way short submitting the hulls to the same stress.

I think that if there were 6 bunks, they would have to be all dead nuts on location and height in order to prevent any uneven weight supporting distribution.
It is easier to mount 2 outer bunks in location than it is to mount 1 outer bunk and 1 keel bunk. Plus any twisting of the trailer while towing will be less likely applied to the hull. Also, the notched bottom sounds really legite. :)

Maybe 'Boatme' can chime in since he works for Manning Trailers.

I am not sure if this has been said yet but is it possible that the boat that was sitting on that trailer had a Notched transom? Kind of like a Superboat Tempest.

Jon

See post 17 :cool:

US1: You also might be on to something. Hull distortion near the transom from too high a keel bunk might cause worse handling than no keel support at all. Too high a keel bunk could reduce the deadrise, and even cause the chines to distort in extreme cases.

GLH: That's some gaudy looking barge you pictured there. (Can I have a ride?!! Pretty please?!! Huh, huh?!:D ) Very interesting point you brought up about rack storage bunks. Most of the ones I've seen were designed for shorter boats than they actually store.

Reed: Go easy on AudioFN, he's been breathing too much fiberglass dust lately, and it's affecting his judgment. He was actually talking about adding a mast to his boat....:eek:

Just when we figured this out, here comes Trooper Trout messing with our little world.

Okay, Troutly, we just determined that keel bunks at the transom cause your boat to be worthless. You'd be best off dumping your now distorted boat for any price you could get. (GET BACK BOYS, I'S HERE FIRST) PM me, and I'll help you out of this mess before Mrs. Trout figures out what a mistake you made.:D


Hey, anyone want to ride in my new 382?

Actually Troutly, thanks for posting that picture. Do the middle bunks at the transom line up with any stingers or supports in the engine compartment?

Did you notice the notched transom on Trouty's boat?

You know, I missed that.

I recall that grid stringer setup from the factory tour a few years ago. Pretty amazing that it is actually suspended 1/4 inch above the hull, and then fiberglassed to it. That must have caused Scott Adams and the engineers many long hours to design and test.

I've also met the family from IL that owns Prestige Trailers, and they are proud of their product, and work hard to build high quality, yet affordable trailers. But, from your picture, they do their bunks differently at the transom than most other high quality builders for offshore boats. Prestige builds more trailers for runabouts, and I THINK the trailers for big offshore boats is a new direction for them. It could be they come at trailer design from a different perspective, without the "baggage" of the Mannings, Mycos, etc. who built trailers for offshore boats from decades ago. Or it could be there is a good reason not to use keel bunks at the transom. I'd really like some trailer builders to start weighing in here.

BTW: Guys, it looks like it might be a while before you can ride in my new 382, but I'm still working on Troutly and his deformed blob of resin that wouldn't even be good enough for Cuban refuges:eek: :D

I don't think you are missing anything here. My comments about your "disformed boat" are in jest.... jest cause I envy your boat.;) But, you do have one of one boat trailers I've seen sofar for a big offshore with keel bunks at the transom. If you look closley at the picture in post one, you'll see the keel bunks stop several feet in front of the transom, and the outer bunks are all that support the back of the boat. Most trailer mnf do it that way, and I wondered why. Prestigue does not, and I wonder why they don't. There have been interesting ideas posted, but not from any trailer mfg's yet.

If you guys have any questions on manufacturers bunk placement for specific boats, please give me a call at Myco Trailers.

941-748-2397

Thanks
Bill Tweedie

Bill:

Thank you for your offer. You just got a phone msg.

I feel honored that you made Dude's first post on this thread.

Just got off the phone with Bill Tweedie of Myco Trailers. He was very generous with his time, and offered to help any OSOer with trailer or bunk questions. I hope he knows what he just offered!:eek:

Disclaimers:
- I'm paraphrasing our conversation
- and speaking in general about deep vee trailers for offshore boats. Myco builds custom trailers, and works closely with many boat manufactures' engineering departments for ideal placement of bunks on each model and year.
- Any mistakes or inaccuracies are probably my fault


Myco does not use keel bunks at the transom because pickups, speedo foots, etc. tend to be in this area, and bunks can tear them off. Most boats are incredibly strong around the transom, and are well supported by double bunks near the strakes. If a boat mfg specifies keel bunks to the transom, then Myco would do it that way, but few require it.

Now, don't tell Troutly it was that simple, because I'm still working on his 382.

Original Post
 

CHART, Sounds like you quoted Bill Correctly. IN the original post photo, the bunks are under the engine / mounts. Cigarette, for example uses a pitot tube pick up on the transom that has to be considered for clearance.

As Bill told you, the manufacturers are very helpful in providing hull specs to ensure proper hull support. Specs include hull step, balance and sling points.

There are many ways to set up the bunks to the hulls. Generally, most trailer manufatcurers will work with the factory to verify and ensure proper support / balance whether it is a fixed bunk system or an adjustable bunk system.
since there are no aluminum trailer photo's to reference with adjustable bunks - this is an adjustable bunk system for a 40 outlaw which can be adjusted to also fit a non-step 38 Top Gun.

Ttt

If you ever see an old boat with a "hooked" bottom it was probably stored and towed on a badly built trailer or one with those rollers. Worst way in the world to ruin a nice boat is not having a trailer engineered for a specific hull...

Is there a concern about the strakes sitting on bunks. I am trying to tweak out a trailer on 31' boat that the bank had just sat on a new aluminum trailer without proper adjustment. The rep from the tailer distributor stated that the keel support bunks should not carry any weight and are meant to be guides. I think i will have to move the main beams in in order to raise the boat so the keel will clear the cross supports near the tongue. Im trying to balance the weight across the 3 axles. Thanks for any feedback.