Help me set up my bunks

[ajgraz]

15' Trihull with 70hp OB, fully loaded (hull, motor, gas, batteries, gear), let's be generous to gravity and say maybe 1400 to 1500 lbs. Looking to redo the bunks on the trailer. This is a bunk-only, essentially float-off trailer. I am not looking to transform this into a roller or roller-and-bunk trailer.

In my diagrams:

• Black represents the trailer frame and crossmembers; frame is 5' wide.
• Light blue are keel rollers on fixed mounts on crossmembers, they don't support the boat and are mainly to prevent keel from banging into crossmembers.
• Orange is the bow roller on the winch post (that is adjustable).
• Purple are roller guide-ons (also adjustable).
• Red is the general outline of hull; about 66" wide at the chines, 76" altogether.
• Dark blue is centerline (keel).
• Green represents locations of stringers, and strakes are directly below stringers; these are about 12" and 24" from centerline.
• Gray are the bunks. These are on fully adjustable u-bolted swivel brackets (width, height, angle).

DIAGRAM 1: Here's what it is (was):

Bunks were carpeted 2x6 laid flat, about 4.5' long, rear bunks directly under outer strakes, front bunks directly under inner strakes. Four things I never liked about this setup:

1. On unload, when the bow reached the back of the front "half-length" bunks, it would "drop off" and the keel would bang the keel roller.
2. Conversely, on load, unless I really dunked the trailer far, the inner strakes near the bow would tend to hang up a bit on the rear edge of those same bunks.
3. Inner strakes are kinda pointy, and wore a hole through the carpet on the rear upper edge of those same bunks, probably because of that hangup. (Outer strakes are quite flat, never a problem).
4. Majority of weight (motor, gas, batteries) is at the transom, and I feel it could use more support back there.

DIAGRAM 2: So the first thing I want to do is make sure the inner bunks run all the way to the transom; I think this will solve all four of those problems:

But, do the bunks want to be directly under the strakes, as in Diagram 2 above?; or...

DIAGRAM 3: Should I move the inner bunks to just outside the strakes?:

If I'm doing that...

DIAGRAM 4: Should I move the all the bunks to just outside the strakes?:

For that matter...

DIAGRAM 5: If I'm putting the "inboard" bunks outside their strakes, do I even need the "outboard" bunks at all?:

Finally, given the weight of this boat and the length of the bunks, do they need to be 2x6, or would 2x4 be enough?

Thanks all for your opinions on this.

 

[jtexas]

Re: Help me set up my bunks

I also have a 15ft trihull, 70hp

My trailer has two bunks, just like your last picture, but they only reach to about halfway between the 2nd & 3rd crossmembers, as shown below. Except, move the bunks out just a bit, they're not right up against the strakes.

I have an additional roller in front, and the rollers are spaced evenly. You might think the front roller carries a pretty good load, but it really doesn't.

The keel roller at the very back, I took that off (forget why), and never got around to putting it back on.



2x4 bunks.

The roller nearest the bow is my marker for loading -- back down until it's sitting on the surface (like it's floating). Works every time for power loading.

Been like that since 1980.

 

[ajgraz]

Re: Help me set up my bunks

OK jtexas, I can see where that bunk-and-roller combo of yours could work; ain't no doubt the majority of the weight is in the rear 1/3 of the boat, my axle is just forward of the middle crossmember.

Hmmm, maybe if I put two 2x6 "half-bunks" in the back, just outside the inner strakes, then installed a good Stoltz roller on an adjustable bracket on the front-most crossmember (that keel roller now orange)...I can see where I could still "power-load" (at all of 3 knots ), make contact with the front roller and both bunks and/or the side guide-ons , and make the final winching even easier than it's been.

(I guess if I have a question about this, it would be, should that front-most keel roller be the traditional V style, or one of those self-centering types?)

DIAGRAM 6:

 

[jtexas]

Re: Help me set up my bunks

ah ok, I see something I missed before...my front crossmember is right where the trailer frames bend and start to converge, about where I drew that roller. I'm sure there's three rollers - I must have another crossmember. I have it stored off-site, but I'm fishing Thursday (if the weather holds), I can take a closer look, take some measurements if it'll help. If your front-most roller is too far aft, and has to carry much weight, the shaft might not hold up.

Few years ago I put one of those self-centering rollers like this:

on the front, but I screwed it up (mounted it too high & bent the shaft, it stopped rolling and wore a groove). It worked fine but I can't really tell any difference. Put one of these:

back on.

2x4's will reduce friction for sure (I drive mine right up to the bow stop, although I realize that's not allowed everywhere). And the bunks don't need to be under the strakes. As long as the bunks extend all the way to end of the boat, it's plenty of support.

 

[ajgraz]

Re: Help me set up my bunks

Thanks for the ideas jtexas, but after some deliberation, more measuring, looking at the boat's bottom, and realizing through some experimentation about where the center of balance of the boat is (red "x" in pic below), I've decided to keep it a bunk-only trailer, go like Diagram 5 above and do a single pair of full-length bunksnear the inner strakes:

If it had more crossmembers, maybe a bunk-and-roller could be done. Oh well. I think all-bunk is safer for my primarily solo launching technique, too.

Now just gotta decide whether to:

• go with 2x4 or 2x6 for the bunks.
• put the bunks just outside the strakes, or directly on them.