Bunk spacing on trailer

A

Anita Marie

New member
I just replaced my King Marine 2005 5700# trailer bunks. I went from four 2x6 bunks to two 4x6 bunks. I have them set at 54" apart measured from the outside of the bunk.
The question is: is this wide enough? I would like to set them right the first time I put the boat on the trailer.

Thanks
Fred
 
Fred,
sorry, but is this trailer for the 16 or the 25. I went back thru your threads, and it was not clear what boat this trailer was for?
 
Sorry Bob, I only have a 25 now. I have rebuilt most of the trailer. New hubs, breaks[electric/hydraulic], break line kit, tires and bunks.
I think 54" would support and keep the boat secure on the trailer. I just want to be sure before I drill the bunk lag bolts
 
Without seeing the setup, I can't say exactly. But there are two things about the change that I would be concerned about. The first is the possible reduction in overall surface area. How have you maintained the square inches of support when reducing the number of bunks? The second also relates to surface area support. A 2x6 has a flex to it to cradle the slight curvature of the hull. A 4x6 is going to flex a lot less. It is possible to have the same overall square inches of "bunk board" but reduce the actual support because of a stiff bunk that doesn't properly flex. Think of it this way. If you had 6x6 bunks, the weight of the boat would hardly flex them. It would be similar to setting the boat on the garage floor. You would end up with just a couple of point loads instead of spreading out the load over a flexed bunk surface.

To further complicate matters, the distance between the bunk supports is also going to play into whether the new bunks flex properly to conform to the hull.

I know, I can imagine lots of potential problems and no solutions. It's what I do best.

Mark
 
This setup is what King Marine rigs their trailers for C-Dory's. I totally rebuilt the trailer for a trip to Port McNeil in July and Powell in September. I was never very happy with the old setup on the trailer.
I appreciate the help. I think I will set it and retrieve the boat and see how it rides.

Fred
 
Fred, I am with Marco here. There are many factors. But a 5700# capacity trailer is really light for the 25. (and that was one of the reasons I was concerned about the boat.) I had a 6500 lb trailer for my 25, and I felt it was too light. If I had it to do over again, I would have gone with 8,000 lb trailer. The second factor, is that two 4" may not enough surface area to properly support that hull.

Some of the deep v boats are supported with bunks which are 4 x 8 (or wider) set on edge. Back when I had to buy a used trailer for my 22, one of the options I has was one with 4 x 8 bunks--I was not happy with the way that it would not fit the contour of the C Dory hull. Many of the thick bunks are cut to fit the contour of the boat on the top of the bunk.--and often these are put right under the stringers inside the boat--the C Dory does not have stringers. (at least to the extent many deep V's have them)

Back tracking--what was it about the original set up which was negative or unsatisfactory in handling?

A photo of the old and current bunk set up would be of great help. Also putting the boat on the trailer with the new bunks may show you if there is enough proper contour (for the hull) support) (That is does all of the bunk support the boat, or is there only one edge of the bunk, supporting the boat forward? How are you supporting the forward part of the hull?

I have always used 4 bunks for the trailers both on the 22 and 25. (I do have a set up of lift bunks which are four true dimensional 2 x 10's set on edge for my boat lift--but this is a static position--not bouncing down the road. Support of the transom (back of the boat's hull) is important. It is possible that the two bunks may be OK--but without seeing how they fit..

British Columbia has a trailer law which states: "where the weight of the trailer exceeds 2800 kg, (brake) must be capable of being applied by the driver of the motor vehicle towing the trailer from his or her normal seated position."

2800 Kg is equal to 6173 lbs...and there are few if any of the C Dory 25's on the trailer, with gear which weigh less than 6173 lbs. What this effectively means, is that to be legal, you have to have either electric over hydraulic (most practical), or pure electric or air brakes. (or one could have hydraulic from the truck--but most trucks and RV's don't have that as an option.

I am not aware of anyone who has been ticketed for a trailer without controllable brakes (surge brakes don't count in this case, unfortunately) But if there were an accident, or some other reasons a person was pulled over, it could be an issue--again, just for awareness.
 
I have "Rebunked" 2 Pacific trailers, The first because one of the bunks was getting soft, the second, because I wanted to lower the bunk height, effectively lowering the boat on the trailer. I have 4 bunks, the inside ones are 12.5 feet long the outside are about 5 feet long. All 4 come to the aft end directly under the transom. They are 2" by 6", and do have some flex, but the trailer cross members are about 3 feet apart.

I fail to see a good reason to go to 2 bunks. Not knowing your bunk plan, (short or long, and where) I would think, decreasing the bunks, decreases te support under the boat. That is not only just support, but the friction there provides some lateral stability as well as additional support under more of the hull, which, over time could develop unwanted curvature due to the total weight of the boat being only supported by that reduced amount of surface area.

What is the advantage of going to only 2 bunks?

Harvey
SleepyC :moon

IMGP6704.thumb.jpg
 
I have replaced all the bunk hardware. The trailer had the King Marine bunk hardware that was failing. I worked with the dealer/factory on the setup.
I will take some pictures but the new bunk hardware is very beefy and takes much more room on the frame.
If two bunks do not support the boat properly what about the folks with rollers. They have way less support square footage wise.
 
Anita Marie":13b2uj6w said:
I have replaced all the bunk hardware. The trailer had the King Marine bunk hardware that was failing. I worked with the dealer/factory on the setup.
I will take some pictures but the new bunk hardware is very beefy and takes much more room on the frame.
If two bunks do not support the boat properly what about the folks with rollers. They have way less support square footage wise.
Click to expand...

Certainly understand the hardware, and I have also had to replace soft bunks on old trailers. Try and use pressure coated wood--the real salt water stuff.

As for the rollers--but they spread out the support over many parts of the boat. You find many rollers. Ref. my comment on the stringers--this is where you see the thick and high bunks--on the deep v which have stringers inside of the hull.
It may be fine, but I would personally want to be sure that the support is well distributed. We rely on the "I" beam type of construction which the glass/balsa/glass gives, for the stiffness. Some others use thinner glass, no core, but then use stringers or grids to give the stiffness.

Take some photos, and see what the hull contact is like when you put the boat on the trailer. It is always possible to add more bunks later if you need to--I adjust bunks and replace bunks with the boat on the trailer, using several jacks and supports.
 
Pictures of the trailer modification are now available in the Anita Marie photo album.

Fred,

I think there will be a problem. The 4x6 bunks have 4 supports under each one. That will stiffen the already stiff bunk boards so that it can't flex in the middle to form a cradle. The bottom of my little C-Dory looks flat, but there is actually some rocker to it. If it sat on your bunks, the hull rocker would make it rock back and forth like a rocking chair. It could be strapped in place to stop the rocking, but the hull will only be making contact in a small area.

I went through this a little when I put "roller bunks" on top of my regular bunks. I fit two 4 foot lengths with 5 pairs of rollers on each 8 foot bunk board. They were like these:

http://www.veveinc.com/shop/Roller-Bunk ... fgodunEKlA

The effect was that the two sections of roller bunk didn't flex and that kept the bunk board from flexing properly. I ended up sitting on only about 2/3 of the rollers. The bunk no longer flexed properly even though it did flex some in the middle where the roller bunk sections didn't overlap. I was already spooked a little bit from seeing incidents of roller bunks pushing in the hull because of lack of support, and I trailer over some rough roads, so I took the roller sections off and went back to regular bunks (although I did put on the plastic slicks like you have instead of the old carpet).

The old carpet actually make it difficult to see if my bunks are providing full support. Mainly at the front of the bunks, the carpet was touching the hull, but it wasn't compressed and the bunk wasn't actually providing support for it's full length. When I went to the plastic bunk cover, it was obvious that the front 18" of the bunk wasn't actually providing much support because I didn't have enough belly in the bunk. I could slide a business card back quite a ways before it got pinched. I moved my bunk supports 6" further apart and that made quite a bit of difference. The mid section flexed down forcing the cantilevered front of the bunk up. Now I only have about 6" of area in the front that doesn't touch the hull.

With the number of supports under your new 2x6 bunks, I don't think the boards will form a belly to support the rocker of the hull.

Mark
 
Hi Mark, I see what you are saying. I wonder if I took off the one support second from the back. The brackets are beefy enough I think they would have no problem supporting the boat. Then it might be able to cradle the boat.
 
Mark, Thanks for the info about the photos, I did not see any several days ago.

There are several issues. Not only is there "Rocker", which is for and aft as Mark comments on, but there is variable "Deadrise" (the degree of the bottom of the transom. The trailer frame may account for this...but as you go forward in the boat, the "deadrise" continues. to increase--at the transom in the C Dory it is a couple of degrees--forward it may be 12 to 15 degrees, and near the bow over 60 degrees. The bunks have to also follow this contour --or twist. If not there is not full engagement of the bunks to the surface of the bunk material.

Generally I have the bunk brackets set so they can swivel to allow the plank to twist to follow the contour of the hull.

Also many times the bunks are set in narrower forward on the trailer.

Setting the bunks on the trailer, I put the boat on the trailer with the aft bunks adjusted for angle of deadrise and to give support to the transom (and that is the reason for the 4 bunks on most of our trailers. If the bunks don't give full contact, I loosen the middle support of the bunks, and let the boat come down to give the twist and rocker support. If necessary, support the keel on rollers on the trailer or blocks. Use a jack under the bunks to get the best contact--and then tighten the bolts to keep that bunk in place--move forward as necessary.

The way your trailer is setup--it would be fine for a flat bottom boat--but even the C Dory is not flat bottom--as is a jon boat or scow.

I like to have support under the keel near the bow--there a single broad roller will suffice--and that might be indicated in your case. You don't want the boat forward of the bunk bouncing up and down on the trailer.
 
I have decided that I will be going with 4 bunks and change the hardware from the solid mounts to bolster mounts with swivels. I will be into it for another $150 but I think it is the best option.

Thanks Mark and Bob for your help it is much appreciated.

Fred
 
I saw Fred's trailer yesterday. He's doing a nice job [especially on the Hydraulic over Electric (or is it the other way around?)].....can't be easy.

From the looks of my trailer these days, I won't be far behind him. No rush yet since I only have a 4Runner to pull that 8000 pound boat, so I only go 15 MPH and less than a mile :D :wink:
 
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