Again with the transom bolts!?

[Techno]

Something that accured to me.
I always thought the lower bolts were lightly loaded if at all. Since the thrust of the prop is compressing these and tensioning the uppers.

BUT. The lowers go into tension anytime you turn or slow down.

[FCnLa]

Hi Techno,

I don't think there is enough drag to worry about. Some of the boats I worked on in the early '80's had aluminum lower bolts.

I really think the motor, if properly secured, relies some on clamping pressure to help hold it in one place.

I remember you having SS probems with gauling. I am fixing to rerig my boat and have been talking to the guy at Construction Bolts here locally. I asked him what I could do about the gauling. He said the guys at the paper mill use a combination of regular SS (don't know the numbers) and 316 SS. This is what I am going with. I will let you know if it works. Using some of that good ole antiseze with lead in it

I was amazed at how cheap the bolts were. Except for 4 SS shoulder bolts I special ordered......$25 ea...???

[Jeff_G]

Just use brass lock nuts with the SS bolts.

[Gordie Miller]

Don't forget, your assy. is only as strong as it's weakest link. I.E. Grade 8 bolts are great-but if you use off the shelf nuts, you are wasting your money. Gordie

[Techno]

Ahhhh. THe grade system.
for anyone interested this is SAE or Society of Automotive engineers.
2h nuts are hardened if you want to go nutso.
Anyway I don't think it matters much as long as they are stronger than the transom.
Ever wonder why they actually make a no grade? in the SAE? It must have some strength or they wouldn't have defined it.
And if you over torque your bolts then your eating into the strength of the bolt. Although the transom would probably crush at these levels.

I was just pointing out that the lower bolts are just as important as the uppers.
They wiggle and there goes the uppers.
I still think that the drag on the LU is huge otherwise the boat wouldn't slow, same as the hull. Even if the prop is freewheeling it's still drag, so is the LU. Not as much as the forward thrust (stress) but still considerable.
I mentioned in the past that SS galls without an antisieze I found out I have brass nusts so if'n I change them then that could might happen. I'm thinking of safety wiring the stuuf so I know it won't loosen.

[tombrown]

I'm using high grade SS. Someone (Greg Moss?) mentioned that they always use 60 ft*lbs torque on the clamp bolts. I would imagine that this number would have nothing to do with the strength of the bolt as any 1/2 inch fastener could clearly stand a lot more than that. What torque setting do you guys use?

By the way, I know it's prissy to use a torque wrench but it's the only way I can think of to tell if the bolts are stretching.


Thanks,

Tom Brown

[Jeff_G]

If you are using a 1/2 ss bolt you are not going to stretch it using it as a transom bolt! You would crush the transom long before that. Just good and tight without signifigantly crushing the transom. It also depends on the transom. You would not tighten it to the point of cracking the glass if a glass transom. A wood transom like a tunnel you could tighten the bolts even more. Mercury ships their motors with a ss bolt and brass locking nut and recommends it's use as the mounting bolt. My opinion is a grade bolt is not needed it this application. I the old days (70's) mercury even used a 3/8 carriage bolt for the lower and just the clamping screws for the top!

[tombrown]

Thanks, Jeff. I apologize for wording the question poorly.

When I put my jackplate on my plywood transom, I torqued the fasteners to 60 ft*lbs. The bolts are SS with SS nylock nuts. They needed re-torquing the first three or four times out but I knew the SS would stretch a bit.

My problem is that the port side lower bolt needs to be re-torqued after every few hours of boating. The wood looks solid. When I use a dental pick, it seems as firm as 25 year old plywood ought to be. I've changed bolts and nuts a couple of times but it made no difference. It doesn't get loose but it will always turn a bit before it hits 60 ft*lbs. On the inside, the bolts go through 2 inch aluminum angle (250 wall) to spread the load.

It sure seems good but it isn't. Any ideas?


Thanks again Jeff,

Tom Brown

[Techno]

My method.
Drill oversized holes so you can epoxy SS or brass pipe into the transom. If they are threaded even better. They should be rough or chewed up in someway. You epoxy the hole to seal the transom then epoxy the pipes in. The pipes should be exactly flush on both sides.
This is a method from the gougans brothers or west epoxy book for mounting multi ton Keels.

This does-
*seals the transom holes permanantly
*easier to remove -replace bolts
* speads the bolting force in a much larger area of the transom, The circumference of the 1/2" nipple is around 2" and is adding to the bolting thing. A through bolt only clamps and doesn't spread anything internal to the transom.
*still clamps the transom.
* enables as much clamping as you want
* won't loosen from transom crushing.

That last is what I think is happening in most transom bolts.
You can stretch a bolt but not stretched permanantly. Stretching the bolt is what torque is attempting to measure. You can use the thread pitch to "stretch" a bolt but it's really not worth the trouble

I think marking the flats on both the nut and head to see if the nut is backing off is the easiest check to see if the fastener is backing off. Thats one reason I want to go with safety wiring the things so I can mount and forget.

[BarryStrawn]

Hey Tom - When you tighten your transom bolts to 60lb/ft you are almost certainly compressing the transom. You may get them that tight but over time the transom will crush a bit under the pressure and the torque comes off. These aren't rod bolts and there is no need to attempt a stretch or preload - just enough to be secure - I would guess mine are around 20lb/ft. If the torque changes over time and the nut hasn't moved, then you are overtightening or the transom is too soft. Your transom plate appears to be working. If you only had washers, I think you would have a very visible compression in the glass by now.

Keep posting about your CVX project - my first boat was a CVX16 Deluxe I bought new in '78. Wanted to add a pad but just sold it and bought an Allison.

[Euroski]

When I purchased my CMC plate I called them to find the torque spec since it wasn't with the directions. Reply, with supplied bolts in kit 60ft/lbs! After a few runs and re-torques they have stayed tight. I do recheck the torque several times during the summer...

[H2Onut]

www.boltscience.com

[tombrown]

I really appreciate the help, guys.

Jeff, I appreciate your help too. When I read the flow of this thread it looks like I dismissed your comments. Let me assure you that isn't the case. I thought the SS fasteners would stretch a wee bit, even in 1/2 inch with a light load. It sounds like they won't.

Techno, I'm sorry for hijacking your thread.


Thanks again,

Tom Brown