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  Raise Engine-Use lag bolts for lower 2?

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Author Topic:   Raise Engine-Use lag bolts for lower 2?
Tim Ehlers posted 05-26-2004 12:38 PM ET (US)   Profile for Tim Ehlers   Send Email to Tim Ehlers  
Hello, I have a '72 Katama with a 120 hp Tohatsu. I replaced the stock aluminum 17 inch pitch aluminum prop with a 20 inch ss Laser 2 prop. The engine is currently mounted all the way down. I want to raise the engine. I currently have four bolts that go through the engine well. My engine has four adjustable holes on the top mounting holes which give around 1/2 inch of adjustment per hole. I can raise it about 2 1/4 inches total. The lower two holes have no adjustment.

The engine bracket tapers in and has two vertical slots that I can put two bolts into, but they are on a narrower pattern and lower so that I would have to use lag bolts since they are below the engine well. Have others had this problem of being too low for through bolts and is this how they have solved the problem? Can you lag into the transom? Any experienced recommendations on how much I should raise it? I was thinking two holes. Thanks, Tim

LHG posted 05-26-2004 01:27 PM ET (US)     Profile for LHG    
It sounds like your engine has the industry standard mounting hole pattern? You should determine if it does.
The top holes are 12 - 7/8" apart, and the bottom holes
10 - 7/8" apart. The vertical separation is 8" even.

Centerlines of top holes should be no closer than 1-1/4" from top of transom, with 1-7/8" the standard dimension.

All of this should allow you to install the engine with 1/2" through-bolts into the splashwell. Do not use lag bolts, ever.

With the Laser II prop, raise the engine 2 bolt holes, which is 1 1/2",or otherwise drill transom to accomplish this. This will put the TOP holes of the lower set (or top of slot), high enough that you can get through into the splashwell with them. This, in effect, means that the vertical distance between drilled bolt centers on your transom is now 6-1/2". This will clearly put the bottom bolts into the splashwell. If old holes exist in your transom, I would completely fill them in and refinish before this engine installation.

I know this works, as I had a Mercury 115, with current standard bolt pattern, installed this way on my 1971 Nauset.

All of above relates to direct engine mounting on transom. You can also gain mounting flexibility by first installing a jack plate, drilling it as necessary to bring the 4 bolts into the splashwell, and then mounting the engine to the plate. This will allow you to experiment with the correct engine mounting height, and even make adjustments based on use and expected conditions.

BQUICK posted 05-26-2004 01:33 PM ET (US)     Profile for BQUICK  Send Email to BQUICK     
Related to this....does a jackplate help reinforce the transom? Or, does it overall put more load on it?


Bigshot posted 05-26-2004 03:01 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Weight is not the problem with transom stress, HP is. A manual jackplate should do NOTHING tnegative to your transom if she is solid. Being a 120hp and still going...she seems solid to me. I would do the jackplate route personally, especially if performance is your concern. I have 5" lift by 6" setback on mine and my A/C plate is about 4+" above my keel.
Tim Ehlers posted 05-26-2004 03:58 PM ET (US)     Profile for Tim Ehlers  Send Email to Tim Ehlers     
Larry, Thanks for the quick reply and encouraging solution based on your experience. Yes, my motor has the standard hole dimensions you mentioned, plus the two lower bolt holes that are currently being used. As of now, my upper bolt hole centers roughly measure out to be 1-7/8" to 2" below the top of the transom. Top of transom to bottom of "splashwell" (thx for the proper term) measures 9". So your advice should work if I use the upper range of the slotted lower bolt holes. If I go to 1-1/4" CL below the transom for the top bolts, add 6-1/2" more to the lower bolts that brings my lower bolt hole centerline to 7-3/4" below the top of the transom. Add 7/16" (half the bolt head diameter) and at 8-15/16 below the top of the transom, it will just clear the bottom of the splash well. I would have to use a support washer with a side truncated and would have to insert the bolt in from the stern and nut it toward the bow.

Biggie, I see you have replied also. A jack plate is an option I am now considering more seriously. Cost deters me somewhat, but what the heck. What is the bolt hole pattern for the bracket? I would hate to redrill, fill old holes and then decide to go to a bracket and find out I need to redrill and fill again! My motor weighs 360 pounds. As I have a Katama, I have a 24 gallon Pate under the console which helps balance out things. Do you think a jack plate would set things too far back? All I ever read on this site is how much better they make the boats perform, but my engine is on the heavy end.
Do you have a jack plate size and maker you would recommend for my setup?

Oh yeah, I read somewhere here lately that using a high performance prop all the way down will make it perform as if it has two inches more pitch than it is designed to. I realize I need to raise my engine to reap the performance benifits of my new prop, but is this 2 inches thing generally recognized as true? BTW my old 17 inch pitch prop allowed me to hit 6000 to 6100 RPM. Again, thanks for any and all advice, Tim

Bigshot posted 05-26-2004 04:03 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
A standard 1 piece manual jackplate will be fine +/-$200. The plate use the standard pattern so your engine will bolt right up to it. The plate will bolt into the top holes and then you will need to drill 2 new holes in the alum plate for your bottom holes...easier then filling and drilling your hull, takes 30 seconds to drill alum. My engine weighs the same and I doubt you will notice any difference with a 5-6" setback. I have mine jacked up about 4+" from my keel so my draft is shallow and performance is great. If not happy, you can always remove it and sell it but I know you won't. Try E-bay for one used, etc.
Bigshot posted 05-26-2004 04:10 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Here you go!

Do NOT go more than 6" setback(standard) or you might run into cable length problems and a weight shifted too far back. 8,10,12" plates are great on bass boats etc, do not use one for this application.

Hobie1981 posted 05-27-2004 08:42 AM ET (US)     Profile for Hobie1981  Send Email to Hobie1981     

I'm looking at the TH Marine jackplate for my '69 Nauset. It has 4" setback. Available at West Marine and the like foe $90.00 or so.

I have a 1988 70 HP Yamaha 2S which I need to raise, as I'm mounting a Turbo-Lift (a brand of hydrofoil).

The motor is presently mounted on the transom using the top 2 mounting holes, and the 2 blind (threaded) holes.

I did have the T-L installed on the lower unit for a short while last season, but I realised that I needed to raise motor enough to get the T-L just out of the water at planing speed.


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