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Hoping for clarification on engine mounting height, as well as mounting holes on a 90hp Yamaha. I've seen suggestions for both mounting the cavitation plate equal with the keel, and mounting so the cavitation plate 1 1/2" up from keel. Does this differ depending on boat/engine setup as well as weight distrabution for each boat?
I have a 1968 16' Currituck with a 1991 Yamaha 90hp Yamaha, its currently mounted such that the motor bracket is sitting virtually on the top of the transom. Although I cant be sure. but I believe the caviation plate is roughly even with the keel (I'll check this weekend). My boat porpoises a fair bit when planning, and is made worse by trimming the engine up. Could this be caused by a motor is mounted incorrectly?
Next mounting issue. It seems that changing the mounting height with my current setup would require drilling new bolt holes. Currently the lower two bolts are threaded into the motor bracket in blind holes. Does this sound right? What are other people doing with similar setups?
Any suggestions on height or correct mounting procedure would be wonderful. Although she's totally blocked by snow now, I want to be ready when I dig her out.... as if I already dont have my hands full!
Twenty or thirty years ago, it was perfectly normal to mount a motor on the 16' 7" Whalers with the motor mounted as low as possible and the AV (cavitation) plate below the keel.
With much improved propeller design in recent decades, the motors can be mounted higher with the AV plate above the bottom of the hull and running above the surface of the water.
Different motors will have their AV pates at different elevations relative to the motor's mounting bracket so describing a "one hole up" or "two hole up" doesn't necessarily put the AV plate in the same spot between different manufacturers of outboards but the goal remains the same: Get the AV plate above the water when the boat is running.
Now in your case the motor is mounted all the way down. I am sure that the reason it is mounted that way, even though it is a 1991 model, is that your hull was originally rigged with an OMC motor. For the old OMC motors to be mounted all the way down, the blind holes had to be used to get the lower bolts to clear the inside of the splashwell.
When the boat was repowered, these blind holes do not allow any vertical adjustment so the Yamaha went on in the same place. (Yamaha copied OMC's blind hole bracket design.) This is very common, but unfortunate.
What should happen in your case is the lower bolt holes should be filled and new holes drilled in the transom to allow the motor to be adjusted up, bolt hole by bolt hole.
Once new lower holes are drilled in your transom, any motor can be mounted in the future without any new holes and the motor can be adjusted vertically to get the best performance.
Without even examining your particular boat, I can tell you that it would benefit form raising the motor one bolt hole (3/4"). It is entirely possible that it could go up two holes depending on the propeller used.
Yes, raising the motor may help alleviate porpoising. In conjunction with a good stainless steel propeller, it most certainly will.
Thanks for the explanation, the more I read it the more it makes sense. Now I just need to go look at it.
It sounds like I can continue using my upper mounting holes in the transom, fill the holes that were used with the blind holes, and drill new holes that will allow for adjustment. Does this sound correct?
When I get to this point I assume I should not be faced with splashwell issues?
Of course seeing it should answer some of my questions.
What effects on speed and performance out of the hole does raising the AV have? I just looked mine and it appears that it is about 3 1/2 inches above the keel on my 15' Sport.
Good Question. Back when I was busy with mine too with my new 1990 Yamaha 2-stroke, I had the same problem of the mounting issue.
I'll check mine tomorrow with it full down where it would sit with the AV plate at the same height as the hull or not.
My engine bracket on my 15 sits a couple of mm's above it and it runs great. I can't wish for anything else. I have friends down here with it about 5 to 6 cm above it and I think that the boat stays in a bouncing mood, causing it to loose speed in my opinion.
So I guess the professional help I got installing it was worth it even though I didn't drill new holes. Check my photobucket link in personal page.
If you look at the excellent drawing Joe has referred to, you will see that the now standard B.I.A. bolt pattern has an 8" vertical centerline dimension. If I remember correctly from the 16' Nauset I used to own, the splashwell is not deep enough to accomodate the 8" dimension. Therefore, use the next dimension, the 7 1/4" vertical spacing for the height of the bottom holes. This should bring the bolts into the splashwell where they can receive the washers and nuts (bottom bolts always go in from the outside, to allow proper bolt shank sealant application before the engine is pressed up agasint the transom).
When a transom is drilled for the 7 1/4" vertical spacing of the bolt holes, this automatically raises the engine 3/4" on the transom, since the top bolts will be forced to go through the 2nd set of holes, while the bottom bolts go through the uppermost of the bottom set. You can still go higher, but for a Yamaha 90 on this hull, with top speed around 43 MPH, no more height than that is recommended, at least by the Mercury Engine Rigging Manual. (they recommend 1/2" up for each 5 mph of speed over 40 MPH, plus 1/2" up for each 6" of jackplate or transom bracket setback).
