Bunk vs transom relationship

[boatman37]

Probably asked on here 1000 times but on another forum I am on a member said Sea Ray recommends having the bunks stop about 4" short of the transom? Doesn't make sense. What brought up this topic is he just bought a brand new Venture trailer identical to mine and I noticed in his pics his winch post is in front of where the I-beams make a V and he said his bunks stick out past his transom. My winch post is behind the 'V'. I can use the extra set of holes to bring it forward about 3" but my bunks would still be short or I could move the winch post in front of the V like his but then my bunks would stick out about 15" past the transom and likely have to adjust my axles to redistribute the weight.

 

[alldodge]

Every "bunk" trailer manufacture I've discussed this issue with is you want the transom to sit directly over the rear frame of the trailer. Moving it further forward can cause cracks in the hull.

 

[GA_Boater]

You adjust the winch post and axle for safe towing tongue weight. You adjust the bunks to fit the trailer to the boat.

As AD said - The transom should be about even with the rear frame and the bunk ends should be slightly uncovered by the transom to prevent a hooked hull. There is a lot of weight supported by the bunks and short/misadjusted bunks concentrate the weight on one of the vertically weaker areas of the hull forward of the transom. A few more inches of bunk places the ends under the transom which is about the strongest vertical part of a hull.

Why don't you have a pic of the area you;re concerned about - The transom and bunks?

 

[ahicks]

When it comes to I/O's and outboards, the easy way to think about it is you would always go for the best support possible for the transom. Can't think of a single reason why you would not.

 

[Scott Danforth]

float boat, move bunks back 5", put boat back on trailer

 

[boatman37]

I agree with you guys on the bunks being at least even with the transom but figured I'd ask since a member on the other site said SR told him not to. My plan was to move my bunks back in the spring after I put the boat back in. I can gain about 3" by moving my winch post forward then move the bunks back 3" and that should put them about even with the transom. I don't think doing that will make much difference in the towing but will try to get the tongue weight or at least see how it tows at highway speeds.

Those were pics I already had. I don't have any of the bunks but probably don't need any to know it's too far back. I don't think I have enough to move the bunks back 6" by themselves as the front bracket is only about 6" from the end so the new holes would be right on the edge. The only other thing to do would be to buy new longer 2x's but then I need to recover them as the material on the current ones would be too short. My outer bunks are 2x6's upright and side by side so actually about 3" wide.

 

[jimmbo]

Having the transom area unsupported over the winter(or any other long period of time) is just asking for a deformation of the hull. The only time a hull is properly/fully supported is when it is in the water.

 

[alldodge]

Moving the bunks back without having support under the bunks just makes the bunks bow. If you cannot get the cross bar frame under the transom, you have to short a trailer.

 

[jimmbo]

Moving the bunks back without having support under the bunks just makes the bunks bow. If you cannot get the cross bar frame under the transom, you have to short a trailer.

If the bunks are doubled 2X6s on end, I doubt there would be much flexing if they extended past the rear frame a foot or so.

 

[boatman37]

Correct. They are 2x6 (possibly 2x8 but haven't looked that close). I will try to get a look tomorrow and get pics

 

[harringtondav]

Double 2x6 edgewise bunks make a stout cantilever/overhung beam past the support. Two such bunks, very strong. Per all advice above, you need to get your transom moved forward, or the bunks moved rearward so your transom is supported. If you move the bunks rearward, you may need to shim the top of the bunks under the transom to preload the overhung beam to get even support along your hull. A few feet of outdoor carpet and SS staples will hide the hack.

 

[DouglasW]

Hang on for a minute. My Mariah boat has a slight downward slope (like a mini trim tab effect) starting about 10" in front of the transom. A member on that forum asked about it, thinking it was damaged. Well, I found a photo showing it at the factory, so it was designed in. Then another member said his friend's Searay had the same feature. Perhaps that is why it's recommended to not have the bunks go all the way back? I would examine the boat for a similar feature first.

Whoa, I just checked photos of my boat on the trailer and the bunks stop about 5" from the rear edge of the transom, and the trailer is custom/factory built for the boat.

 

[alldodge]

Edge wise bunks, got it. I read bunks and saw flt 2x6 with metal supports.

as a slight downward slope

Its called a hook, and my Formula also has this right in front of the trim tabs. This is an issue because the bunks need to lower towards the stern, the keel is straight. My previous trailer (I still have it) used 2x6 for the keel on edge, and 4x4 on the outside edge.

Noticed the hook so I had boat sitting just past the hook. After so many years and when sanding the hull down I found a crack just in front of the hook. So my hull cracked and I also had support on the keel, the think that may keep the Searay from cracking is because they are not as heavy as Formula

 

[ahicks]

Hang on for a minute. My Mariah boat has a slight downward slope (like a mini trim tab effect) starting about 10" in front of the transom. A member on that forum asked about it, thinking it was damaged. Well, I found a photo showing it at the factory, so it was designed in. Then another member said his friend's Searay had the same feature. Perhaps that is why it's recommended to not have the bunks go all the way back? I would examine the boat for a similar feature first.

Whoa, I just checked photos of my boat on the trailer and the bunks stop about 5" from the rear edge of the transom, and the trailer is custom/factory built for the boat.

Your Mariah is a straight inboard, no? If that's the case, they don't need to follow the same rules that apply to outboards and I/O's, as the weight of those engines are supported entirely by the boat's stringers.

 

[DouglasW]

Your Mariah is a straight inboard, no? If that's the case, they don't need to follow the same rules that apply to outboards and I/O's, as the weight of those engines are supported entirely by the boat's stringers.

No, as stated in my signature, It is a Mercruiser Bravo 3. I was concerned about this built-in hook until I saw the photo of a boat being built at the factory and it showed the same slope even though it was not mentioned. The keel is straight.

 

[dingbat]

Moving the boat forward will increase the tongue weight. Might need to move the axles as well.

 

[M2HB]

If the boat is well balanced on the trailer, I would move or replace the bunks to make sure they were under the transom.
The pictures aren’t great, but the boat looks like it is located properly on the trailer, assuming the tongue weight is correct.

 

[DouglasW]

This is what I am talking about being part of the design. Notice how the bottom turns down about 10" from the transom. This is a factory photo showing a hull coming out of the mold. Sorry for the small image but this site will not let me delete it and post a larger one.

 

[jimmbo]

That molded-in Hook is along the chine. Many hulls have that, it helps plane boat quicker. As the boat speed rises the hull rises out of the water and the hook is no longer in the water, except when the hull banks on turns. If you put a straight edge lengthwise against the hull along the Keel and where the bunks rest the hull should be flat, no downturn(hook).

My hull also has the same type of hook, but at speed, it is clearly out of the water

Edit: I should have said many hulls have a hook molded-in between the Chine and outer Strake

 

[DouglasW]

That molded in Hook is along the chine. Many hulls have that, it helps plane boat quicker. As the boat speed rises the hull rises out of the water and the hook is no longer in the water, except when the hull banks on turns. If you put a straight edge lengthwise against the hull along the Keel and where the bunks rest the hull should be flat, no downturn(hook).

My hull also has the same type of hook, but at speed, it is clearly out of the water

Not true on my boat. It is in two places if you look at the photo. The lower one is certainly not out of the water at any speed. Also, that is where my bunks are in 3 places on each side and why they do not go all the way to the transom since the bunks are straight..