Gap between motor and top of transom

[scoutabout]

Howling winter winds and swirling snow drive me to ponder all sorts of boating issues, large and small. Here's today's burning issue...

My 90 Yamaha 4 stroke rigged by the dealer is actually sitting up off the top of the transom as you can see in the pics below. The shot of the anti ventilation plate shows it seems about the right height, so I understand why they'd want it raised.

What I don't get is why there isn't a spacer or even a chunk of hard rubber in there to close the space and thus transfer some of the weight and torque loading to the transom where it belongs. Not such a big deal underway on an even keel I guess, but crank the helm hard over at speed and the prop is going to be trying to lever the engine sideways pretty good. I'm thinking that results in some considerable shearing forces on those two "little" bolts holding the engine on.

Thoughts? Anyone else rigged like this? do you think I should try to slide something in that space?

(scuse the ratty engine electrical conduit. Going to get to that in the spring... )

 

[TOHATSU GURU]

Re: Gap between motor and top of transom

It's not abnormal at all. The engine is secured by the 4 to 6 bolts used in the installation. If the engine was using the older style or small engine "hand tightening clamps" you would need a block to keep the engine from shifting.

 

[mickjetblue]

Re: Gap between motor and top of transom

I raised my 18hp up 1.5", and I used a piece of wood to fill the gap.
I also added some 1/4" steel plate to the front and back to beef up
the addition, and used 4 bolts to keep it all together. Works fine.
They did use a support bracket across the front to help out the transom
on yours.

It looks like they used two 1/2" bolts double nutted to mount your motor,
as I don't see any more than those two. How does it feel to you when
you're running? Are those heavy duty ( grade 5 or better ) bolts, or maybe
stainless steel? I had a 90hp, and that's a lot of power for two bolts.
You be 'da judge.
I'd check those nuts often to make sure they're not working loose.

 

[scoutabout]

Re: Gap between motor and top of transom

Thanks for the replies. I took those pics in the fall and haven't seen them in a while but the more I study them the less I like that setup. The boat leads a hard life what with frequent trips with max loads interspersed with lots of watersports. You can see Yamaha intended for the transom bracket to actually hook over the back of the boat to help counteract the forces of hard acceleration. I don't even have the benefit of that here.

And as for bolts, yep -- just those two. Feels fine while running, although so will an aircraft wing spar right up until it snaps in half...

I guess I'm starting to answer my own questions here. Scout makes a lot of their super-strong transom design but it's how the motor is attached that's got me worried. If I have to run at that height I think I want at least two more bolts and possibly something sandwiched in there as well. Drilling reinforcing rods down into the transom through the spacer is probably overkill but still...

 

[Lone Duck]

Re: Gap between motor and top of transom

Yup! I would block it up and add more bolts. And put the nuts on the INSIDE! My old90 merc. has (4 )1/2" SS bolts.

 

[TOHATSU GURU]

Re: Gap between motor and top of transom

There should be at least two more bolts to hold the engine in place on the transom. If there is not then the dealer that did the installation was negligent.

 

[ondarvr]

Re: Gap between motor and top of transom

Like T G said, there should be four bolts and possibly larger than the two in there now, use the largest size that will fit through the transom clamp. The gap means nothing, the tabs on the bracket are to keep it from falling off the rack when it's off the boat for service. Make sure you seal the bolts with a good product before sticking it back together

 

[tashasdaddy]

Re: Gap between motor and top of transom

agreed there should be 4 bolts see pic red arrow

 

[F_R]

Re: Gap between motor and top of transom

Just confirming everything. There must be 4 bolts, and they must be the CORRECT bolts and washers. The dealer was grossly negligent by leaving the bolts out. It is lucky you haven't lost the motor and he is lucky he hasn't gotten sued. It is entirely normal for it to be sitting above the transom like that, and not to worry. The hooks on the brackets are for hanging it temporarilly on a motor stand.

 

[Frank Acampora]

Re: Gap between motor and top of transom

Those are usually 13 mm or 1/2 inch bolts, fine thread series supplied with the engine. Yamaha is not going to supply crap on this critical part. They have a tensile strength of probably somewhere around 90,000 psi and a shear strength of probably 50,000. That's the low end. If they were equivalent to grade 8, it would be up around 120-150,000 psi tensile. With a little less than 1/4 square inch per bolt, at the low rate, it is going to take 10-12,000 pounds to shear it. NO outboard is going to generate that kind of force --even momentary impulse force due to collosion.

Now, in auto crashes I have seen V8 engines ripped out of the car and several hundred feet away. However, the engine mount bolts did not fail, rather, the rubber engine mount itself was torn apart.

With 4 bolts positioned properly, the transom or engine mounting clamps would most likely fail before the bolts. With 2 bolts, yes the engine MAY wiggle a bit IF they are not properly tightened, but again, other parts will fail before the bolts. For example, I was shown an engine that was put into the air and came down wrong at full throttle out on the ocean. One engine mount clamp was torn in two but all the bolts were OK.

 

[scoutabout]

Re: Gap between motor and top of transom

Thanks for all the feedback guys and the modified pic with arrow, TD.

This thing was rigged for the original owner in North Carolina. I'll ask my local Scout/Yamaha dealer who is doing some gelcoat work over the winter what he thinks and probably ask for another two bolts regardless.

 

[steelespike]

Re: Gap between motor and top of transom

I wonder what might happen if emergency full power reverse was applied.
Probably wouldn't rip out but I could see some movement would be there.

 

[BillP]

Re: Gap between motor and top of transom

Nothing wrong with the motor sitting high but using only two bolts is a blunder. Sure the bolts are strong enough but they don't let the load be shared on the transom and could cause stress and flexing way beyond normal. In the short run it may not show but in the long run you may have a broken boat or motor bracket. The bottom two may have been left off because they interfere with the splashwell or floor...it's hard to tell from the pics but that's my guess. No matter what, I wouldn't be comfortable with just two and I doubt Yamaha or Scout would put their seal of approval on it. Get rid of that dealer and find one who knows better (which should be REAL easy to do in this case).

 

[Dhadley]

Re: Gap between motor and top of transom

Yep, nothing wrong with the motor being up off the transom but you need 4 bolts. In fact that motor is too low anyway. But then again that prop isn't the greatest so it may not work correctly any higher.

The "lip" on the stern brackets does not have to be touching the transom. The front (down) side of the lip probably wouldn't touch the transom even if the motor was all the way down.

Besides, if the motor was on a jackplate the lip wouldn't be anywhere near the transom.

 

[scoutabout]

Re: Gap between motor and top of transom

Ugh -- this is going to be awkward. I was rooting around looking for more photos of the engine to see how two more bolts would sit relative to the water line...and found this...

Top left arrow indicates one of the existing bolts as shown in first post. Bottom two arrows represent bolts that run through the transom in the other direction. Not sure how they are fastened as they must emerge under the floor on the inside. Because the heads sit flush with the mount they didn't show in other photos.

So...there are indeed four bolts as you all have indicated there should be and having the engine up off the transom is ok. Sorry to have cried wolf on this...

 

[Nos4r2]

Re: Gap between motor and top of transom

My yam is a full 1.5" raised off the transom and secured with 4 (proper yam) bolts. No movement at all-even after some really hard landings from 5-6' up. I wouldn't worry about it at all-I put mine through far more than yours will ever go through.

 

[Frank Acampora]

Re: Gap between motor and top of transom

Don't know what type of boat it is but most of the bass boats with a step in the hull like yours (that I have seen) have had the anti ventilation/cavitation plate pretty much even with the flat step. Reason is that water coming off the Vee rises. That's why you can run an engine on a set-back jackplate higher. Amount of rise varies from hull to hull and amount of setback of the engine from the Vee. Setback also provides more leverage to get the bow up. The step also reduces buttom area at speed for less wetted surface---more speed.

From the angle of the photo and from the amount the engine is already sitting up off the transom, it looks like you will not be physically able to get that high. Maybe the hull was designed to have the engine sitting low to avoid a known or supposed ventilation problem in the hull design.

OHHHH! I just went back and looked at the original photos. What the hell were those idiot Yamaha designers thinking? That piece of crap can't be run higher because what appears to be the water intakes are above the cavitation plate. I don't see anything else on the lower unit that looks like water intakes. Apparently that engine is meant to be run low in the water.

Sorry! Got carried away. You engine is not a piece of crap, just not designed the way I would have done.

 

[mickjetblue]

Re: Gap between motor and top of transom

I am glad you posted the extra pic showing the bottom 2 engine mounting bolts,
although I don't think it will help me sleep any better tonight.

We are all in various stages of planing, or learning.

 

[scoutabout]

Re: Gap between motor and top of transom

...What the hell were those idiot Yamaha designers thinking? That piece of crap can't be run higher because what appears to be the water intakes are above the cavitation plate. I don't see anything else on the lower unit that looks like water intakes....

...
Sorry! Got carried away. You engine is not a piece of crap, just not designed the way I would have done.

My baby forgives you...this time

 

[BillP]

Re: Gap between motor and top of transom

Whew, I'm glad to know there wasn't a dealer out there doing only two bolts. To check for proper height of cavitation plate (anti-ventillation plate for the p-correct crowd)...run the boat at high speed and look over the stern. If the plate is above the water it's ok. If below the water it's too low.