Hermco/Suzuki question
 

Merry Christmas!! I'm re-powering my 23' with Suzuki 300 and am asking a question/opinion directed at Don and others here......Suzuki is recommending two bolts on top engine mount, six total. Don, is your XL bracket good to go for another hole in that area? Any issues drilling another hole below the top set? Are others here with Suzuki 300 mounting them with six fasteners either transom or bracket?

4 quality bolts on mine with no issues. I have a Hermco twin in the shed waiting for the winter.

4 bolts on mine.

I would not have any qualms with adding the extra bolts although I am not sure it is necessary, Denny/Bushwacker may have some input on the engineering aspects, I would say just be sure you have the engine height dialed in before doing so or u may be drilling more holes than u need.
:)

Let's see, 1/2" fine thread 316 SS bolt X 4 of them. Hummm.....I would Google the tensile stregth, shear numbers, etc., you will be shocked. It's not 7Marine 627, jeeeez

Don's right, focus on getting the engine height correct first! Although the 23 is a different animal, my experience on a 20 is that AV plate will need to be almost 6" above keel to get AV plate above clean water, so you may need a jack plate to get it high enough.

Regarding bolt strength, 6 bolts is rather serious overkill! Although I would defer to the opinion of Capt. Terry, who was much more of a "hard metal" mechanical engineer than me (I specialized in the "hot air" heat transfer/thermodynamics end of the business!), my take on it is that 4 bolts are more than adequate. Per the Marx Mechanical Engineering Handbook, 4 Grade 5 1/2-13 UNC bolts are very conservatively good for a working stress of at least 85,000 PSI at a proof load and 120,000 PSI ultimate stress. This translates to a proof load of about 4800 lbs for 4 bolts; any higher than that and they'd start to stretch, but they'd go up to about 6800 lbs before they'd break. That means you could lift your entire boat with just those 4 bolts! If you wanted even more strength, Grade 8 bolts are good for 150,00 psi ultimate and high carbon Cro-moly steels are much stronger than that, but you may have trouble finding stainless Grade 8 bolts.

The main thing is to just make sure the bolts are torqued evenly and correctly with an accurate torque wrench. (I don't trust the fancy click-stop type because you have no way of knowing if they're out of calibration! With the simple direct reading beam-type, if it doesn't read 0 with no-load, it's real obvious if it's been over-torqued! Mine reads about 12 ft-lbs with no load on it, but it's real easy to compensate for that!)

Thanks to all. The engine height would need to be correct first for sure.

So Don, If I decide to add two additional is that area of the bracket the same configuration as where the other holes are? Your bracket came pre-drilled so I never saw chips.

On mine I plan to test fit the bracket and motor on the current transom prior to the rebuild to help determine the optimal mounting for each. This will allow me to run the boat with the bracket mounted to Don's specs and see if I can get the motor at the right height and test it under load with my standard crew and gear. My goal is to not need a jack plate for the 30" Zuke considering I got the double bracket.

Considering the Zukes like to run a little deeper due to the large prop size I am optimistic the specified bracket mounting location will put me in the zone where I don't need to mount the bracket higher than specified. Is there any serious consequences if it has to be mounted an inch or so higher?

Very smart approach.

You'll lose some buoyancy, and since the whole point of running the twin bracket is increased buoyancy, I'd think you'd want to avoid raising it. Generally you want to mount the flotation tub as low as possible to maximize flotation/self bailing, and I think Don has that dialed in very well, so I'd try to mount it per Don's recommendations. Motor height is a separate issue and best handled with a minimum setback manual jack plate, or some fixed brackets once you have the height dialed in.

Drill baby drill. :cool:

Wrong election.

I know of a scarab 36 running trip Rude 4.0 V8s done by Monty racing.
430 hp per.

I would not do this.

.

Bolt strength & capality
 

[QUOTE=Bushwacker;
Regarding bolt strength, 6 bolts is rather serious overkill! Although I would defer to the opinion of Capt. Terry, who was much more of a "hard metal" mechanical engineer than me (I specialized in the "hot air" heat transfer/thermodynamics end of the business!), my take on it is that 4 bolts are more than adequate.
The main thing is to just make sure the bolts are torqued evenly and correctly with an accurate torque wrench. QUOTE]

Happy New Year-
Bolts are strongest in pure tension, next in shear and weakest in bending. Properly designed bolted joints avoid bending. If only one bolt at the top of the transom was supporting the outboard, it would be in bending due to engine weight and the prying action of the torque on the engine bracket due to the thrust at the prop. However, with a “good foot print”: between the engine mount and the 4 properly preloaded bolts to the transom, this can be assumed primarily as tension and shear load, but minimal bending in the bolts

In an ideal bolted joint the bolts are torqued to elastically stretch the bolt to nearly the yield limit of the bolt material (exceeding the elastic limit would permanently stretch it). The stack under the bolt is typically aluminum, steel, titanium, etc. which have much higher modulus of elasticity and therefore stiffer stack than one consisting of the engine and fiberglass transom. The objective is the initial or preloaded bolt stress should be the highest stress the bolt incurs. If a bolt were lightly torqued, operating stresses could exceed preload stress resulting in premature failure due to fatigue of the bolt material. Not so if the bolt preload stress exceeds the operating stress. And strange as it may seem the operating stress is not additive to preload stress and is not detrimental unless operating stress exceeds the initial preload stress. Bolt preload is dependent on the applied torque and the thread coefficient of friction. For instance dirty bolt threads will absorb some of the initial torque resulting in lower bolt preload than desired. Hence the reason for cleaning & lubricating head gasket bolts with oil before torqueing.

About a year ago I raised my ETEC. I found a SST torque table online which listed 45 ft lbs for a .500” pitch diameter stainless bolt (assuming some coefficient of friction for the adhesive I was using). I measured 7 – 8 ft lbs to overcome the run-on torque of the nylock self locking nut. Therefore I torqued the bolts to 50-55 ft.lbs. Later I think I saw the ETEC owner’s site recommended 40 ft. lbs. I noticed afterward a bit of compression/deflection in the transom (a bit soft or compression from the initial engine installation?). Thereafter, I added a 2’ long aluminium right angle extruded bar between the lower engine bolts to spread out the load on the transom.

A marine 304 SST .500” diameter bolt, torqued to 90% of the 31,200 psi room temperature yield strength, would have a tensile preload of about 6000#. Any shear or bending would detract from this capability, but 4 bolts should be enough to support the Suzuki. However, if Suzuki recommends six, why depart from that and provide them with anything to void a warranty claim? Perhaps they know of some flexing and cracking of the engine where it attaches to the transom, similar to what I had on a Chevy Astro where the bracket on the front of the block which supported all the accessories cracked due to vibration from an improperly torqued bolt.

Thanks Capt Terry, that's a lot of typing on your part and I appreciate the input.

I am running 1/2" NAS bolts in my race car four-link brackets and they are rated at 180,000 psi each. There are only four bolts on each side holding the rear axle under the car with 20" wide slicks that see tremendous strain. My car is only 800 hp for now but the 2000-3000 HP promod cars run the same set up. Many guys run just grade 8 bolts. If 4 1/2" bolts can hold up to that your motor should be no problem. You can go on ebay and find some real nice NAS or AN stainless steel bolts for pennies on the dollar if you want something super strong and are worried. Most of them are in the 150,000 psi range for stainless.

Thanks. Suzuki supplies fasteners and I'll use those.