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Putting a Yamaha 70 on a 15 Sport CC and the mounting choices are top bolt with blind plug, third hole from top by drilling bracket below blind plug or acquire a jack plate. I know the top bolt puts the motor too low and concerned the third puts it too high but would rather avoid the jack plate if possible. Opinions appreciated.

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Take a look at this article:

http://www.whalercentral.com/articles...icle_id=82

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Thanks Phil. Lag bolts may be my best, most flexible option.

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NO, No, no.

You do not need to use lag bolts.

You do not need to use a jackplate.

You merely fill the two lower "blind holes" and drill two new one to allow the motor to be set one, two or three "holes up". The motor should NOT be set all the way down on the transom.

Please study the article Phil has pointed out.

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Looks too close to the bottom of the splash well for me. If the drill was perfectly square to the transom it would be close. I don't trust myself to be that perfect with a hand drill. The bolts would also have to be inserted from the rear with nuts in the splash well which is not a problem but it is that close.

I have pulled the motor back off, and in process of filling the old lower holes. I have the yamaha bracket drawn in cad from their rigging guide. I will plot at full size and match from both sides to see how it fits before using the lags.

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Why?

If the lower bolt holes are only 6-1/2" below the upper bolts holes, they will clear by a mile. The motor will "two holes up" and you will still have the option of raising it yet another set of bolt holes.

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Tom,
There is a 6.5" gap between the third hole down and the top of the slotted track. There is 7.25 inches between the second hole and the top of the slotted track. From the second hole there is 1/4" clearance to the splash well from a 1/2" bolt shaft. You are right if I knew I was going to use only the third hole down it will fit fine. The third hole puts the cavitation plate 2" above the bottom of the hull. The water intake is right at the bottom of the hull. Looks to me that the second hole is the most conservative location and best trade off.

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Yes, your measuring is all correct.

If you only want the motor to be "one hole up" then drill the holes with a 7-1/4" vertical spacing and cut a flat on the washers for the lower mounting bolts, per Boston Whaler rigging advice (c. 1984)

If you want to use a modern stainless steel propeller, then use the third top hole for a "two holes up" mounting position.

This will allow you to drill the lower holes and use a full washer and it will also result in less lower unit drag, less steering torque, less bow rise upon acceleration and better top speed and fuel economy.

Having the cavitation plate sited 2" above the keel does NOT mean that it will be 2" above the surface of the water. This is an important, and all too commonly misunderstood point. The surface of the water will "rise" (as viewed from the boat) as soon as it comes out from under the hull.

The cavitation plate should never being dragging through the water while the boat is planing; it only aids the boat and motor while climbing on plane. "Two holes up" will be fine.

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Thanks Tom! Great advice!

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Just out of curiosity Tom, for $225 Bob's Machine manual jack plate, do you feel there are any drawbacks to a plate?

I not only respect your opinion but appreciate your knowledge, willingness to assist, ability to explain on all levels, AND selling me the RIGHT prop at a price I could not beat. Thank you for all you do, here and on other sites, as well.

zotcha

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There are advantages and disadvantages, costs and benefits of anything.

In this situation where wpruitt is just trying to mount his Yamaha 70, a $225 Bob's Machine manual jack plate represents an extra $200 he doesn't need to spend. That is almost enough to pay for a new stainless steel propeller.

I you want to use a jack plate to milk a little more top speed out of your boat, or minimize its draft, that is fine. But to spend hundreds of dollars on a jackplate simply to get an unattractive bolt hole adapter is silly.

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I'm in the same position as the OP. I just mounted my 2 Stroke Yamaha 70 using the 3rd hole down (in other words, "two holes up"). My cavitation plate is about level with the bottom of the boat. Measuring from the top bolts down 6-1/2" I should have room for the lower bolts. My bolt washers will need to be bent to accommodate the curvature of the hull.

I tend to agree with Tom on the Jack plate. Jack plates serve a good purpose if you are fishing the flats or shallow water most of the time.

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Fellas, we have a very large discrepancy between those two descriptions. Where is the cavitation plate (A/V plate) relative to the keel?

wpruitt -- Are you sure you are not confusing the spray shield with the cavitation plate?

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May be confusing the two. When I said cav plate I was meaning the larger horizontal structure on which the trim tab is mounted.

I, perhaps wrongly, measured the height above the extension of the line of the keel. I have no doubt you are correct on the proper mounting height.

I have to replace the prop as it has a 15p now and have a line on a 13.25x17 stilleto. I don't care much about top end but want good all around ability including pulling a skier. I am most concerned about over-revving with the 17p prop and so a 19 may be better as long as it pulls well.

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yes, that is correct. 2 holes up on the top set of holes. If I use a straight edge. My plate is a half inch higher than the keel.

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Surfncnow,
Are you talking an older 15'? I pulled the engine to plug the blind holes and I measure 16" from the second hole up to the cav plate on the engine and 17.5" from the bottom of the keel to the top mounting holes on the transom. So the cav plate would be about 1.5" above the bottom of the keel on mine. I had eyeballed it before and measured 2" above the keel but that could have been off a bit.

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The height of the transom, or any point on it, is measured vertically, not along the sloped angle of the transom.

It is the difference between the length of a triangle's altitude and it's hypotenuse.

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I love it when you talk like that.

That would put my eyeball measurement about right then as it would be around 2" above rather than the 1.5". I will mount it higher to start...lot easier to change down than to change up.

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No, not quite. If you measure 17-1/2" along the transom, then the top hole is about 16-3/4" *above* the bottom of the keel. That would place the cavitation plate about 3/4" above the bottom of the keel, much closer to what Surfncnow reports, and that 1/4" discrepancy may be explained by whether the measurements were taken to the edge of the 1/2" diameter bolt holes or to their centers.

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Thanks to all. SurfNC was right. Two holes up or 3 holes down the cav plate is 1/2" above the keel. Looks good there. Thanks

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So, did you drill the GREEN holes in this diagram?
http://www.whalercentral.com/articles...icle_id=82

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Yellow because I wanted to have the ability to drop a hole if necessary. The whaler article about cutting or bending the washers is correct.

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There are all types of washers.

My prior Montauk had square washers on the bottom holes as does my Outrage 18' as opposed to round washers.

Glad it worked out.

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Mine had square on the bottom as well. The hole is too close to the curve of the splashwell and the bolt head is hitting the curve of the well. I am modding a piece of starboard to use as a base for the washer.

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Check out my personal page. My motor is only mounted with the top holes 2 holes up. I did this to get an idea on where to mark the lower holes. Like you, it's gonna be real close drilling the lower holes. Before the motor was temporary mounted. I filled in the lower holes and re-gel coated the lower holes.

This is a real PIA. What prop are you gonna try? I just purchased a Yamaha Performance Series 13 1/4 X 16 stainless steel prop.

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surfncnow and wpruitt,
Can both of you post some close-up pictures of your engine installation? It will help all of us that will be re-powering our 15 footers.
Thanks,
BobL

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Happy to post pictures...probably late tomorrow.

Going to try a used 13.25x17 stilleto. More interested in pulling than top speed but worried about over revving. Looks like it will be real close. IDK about a 16". Maybe others with experience can weigh in.

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See link below for photos on my mounting solution.

http://www.whalercentral.com/infusion...e_id=12317

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Boy, looking at that last photo it sure looks like the motor should go UP another set of bolt holes.

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I'm not trying to hijack the OP thread. But I'm trying to mount the same motor. I'm 2 bolt holes up with about half inch to 3/4 inch above the keel. So, if I go another set of bolt holes up. That should put me at or above an inch above the keel.

Also Tom, what do you think about this prop setup? Yamaha Performance Series 13 1/4 X 16 stainless steel prop.

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If the cavitation plate is 1/2" to 3/4" above the keel in the "two holes up" position, then it will be 1-1/4" to 1-1/2" above in the "three holes up" position. The bolt hole spacing on all large outboards is 3/4".

Looking at wpruitt's photos, his cavitation plate looks like it is barely 1/2" above the keel now.

Remembering that the water surface of the water will rise after it comes out form under the hull and before it gets to the motor, I suspect the cavitation plate will still be dragging in the "two holes up" position.

No, the 13-1/4" x 16" Yamaha three blade Performance Series is not the right propeller. You want the 13-/4" x 18" Performance Series or the 13-1/4" x 17" Stiletto Advantage or Turbo Turbo 1.

They are all the same propeller (same design, all made by Precision Propeller Industries, Inc. which is owned by Yamaha), so you may as well get the Stiletto as the Stiletto will be the least expensive.

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Tom,
I have one more hole to give. Should I start where I am or raise it up?
Warren

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I think you should try it both ways and let us know how it does.

You may as well start where it is now, especially if you are going to use an aluminum propeller.

I was just surprised to see the A/V plate still so low.

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It appears to me that there is a lot of difference between engine manufacturers as to how high the engine is compared to where it is actually mounted on the engine bracket.

Meaning they all don't seem to be the same height if mounted in the same holes.

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There is some variation among the manufacturers and even among the different models from a single manufacturer, but the difference is not great, probably no more than one bolt hole, if that.

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I removed a 2001 Johnson 50 and replaced with the Yamaha. In the stand I built, the yamaha had to be three holes down or the skeg would drag the pavement. The Johnson was mounted in the top hole and cleared. So there is a diffence in bracket to lower unit legnth.

I have a used 13.25x17 stilleto I will be installing. The aluminum prop is a 15" pitch so it is worthless on this boat unless I need to pull some stumps.

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Yamaha's will not do as well with the higher mounting location as say Johnson or Evinrude. The Yamaha will tend to suck air through the cooling intake, resulting in burnt out impeller and overheat, if mounted more than 1/2"-1" above the bottom. I would mount it so the cavitation plate is flush to 1" max above the bottom, that is to their spec I believe.

The mounting height has nothing to do with the skeg depth.

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No, that's not true. The propeller will give up and ventilate long before the motor looses cooling water.

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Have you tested out the new Yamaha 70HP?

I built new interior for mine. Last night I took it out and couldn't be any happier. I still need to get a tach but the motor height seems about right. The peanut crowed watching my speed passes called the side rails "Oh Crap Rails" and I must agree with that statement. IMO, the Yamaha 2 Stroke is the perfect weight and HP for this hull.

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Thank you for all the great information on this thread! Sorry to drag up an old post.

I am in the process of mounting a Yamaha T90 on the back of a 15 cc right now, and this was a very helpful article.

Right now I have a fully filled in transom. And I was trying to figure out how I was going to get the "blind holes" drilled. From this thread it looks like I will be able to get the other slots lined up.

Thank you!

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Wpruitt, I would like to know how that starboard washer/spacer works out. It seems to me that however slight, there is some flexibility inherent in starboard. I would want that motor to be absolutely and positively secure without any movement at all and seems that teeny tiny movement allowed by the starboard would eventually get larger and larger. I would either replace the starboard with aluminum or stainless steel or very often check the torque of the mounting bolts.

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This is a very helpful thread! I'm working on rigging a used 70hp 2 stroke on my 1977 15 sport and this is what I needed to know.

I'm a little worried about drilling new holes by hand in my transom, but, the old holes must have been from a smaller engine, the hole drilling guide was very helpful too.