CONTINUOUSWAVE

My 1985 Johnson 70-HP engine is presently mounted on the transom of my 1985 SUPER SPORT 15 in the lowest possible position. The upper engine mounting holes are drilled in the transom about 1.75-inches below the top of the notched transom.



Q1: will raising the engine mounting height [from lowest possible to one-hole-up or two-holes up] improve [the top speed] performance of the boat?

Q2: when an engine is raised above the lowest possible engine mounting position, it is proper that some part of the engine mount [that appears to have been designed to rest on the top surface of transom] to be then above the transom top surface?

BACKSTORY
In the last two days I have spent hours reading many very helpful threads as I start a restoration of my 1985 SUPER SPORT 15 boat. I grew up with a classic 13-footer classic. The 15-footer is now a project boat for me to both have both fun boat and to keep me busy in my retirement; it is also a easily-controllable boat for my spouse.

I read in several threads about adjusting the height of the engine on the transom, targeting the distance between the bottom of the hull to the Anti-ventilation plate. The general advice was to move the engine up, which would--to a point--improve top end boat speed and reduce chine walking. I think I understand this based on the technical descriptions from the various people posting.

Currently recently acquired 1985 SUPER SPORT 15 boat has a 1985 Johnson 70-HP two-stroke-power-cycle engine. The top boat speed is not much over 30-MPH as measured by an OMC pitot tube speedometer. From what I have read I think the boat should be faster.

Many of the threads indicated the my boat's current engine mounting height [the lowest possible mounting height] might be too low. But, if the engine were raised, the over-transom-part of the engine mount would sit up substantially above the transom edge, depending how far I raised the engine.

davidB01 wrote:Q1: will raising the engine mounting height [from lowest possible to one-hole-up or two-holes up] improve the top speed performance of the boat?

Yes, up to the point that raising the engine any farther causes reduction in cooling water flow or pressure and also creates a tendency for the propeller to ventilate.

davidB01 wrote:Q2: when an engine is raised above the lowest possible engine mounting position, it is proper that some part of the engine mount [that appears to have been designed to rest on the top surface of transom] to be then above the transom top surface?

Yes. The only engines which are designed to sit on the transom with their mounting bracket resting on the upper surface of the transom are those smaller and lower horsepower engines which use a hand-operated clamps to secure the engine to the transom. Once the engine is secured to the transom by four large mounting bolts, there is no concern that the lip of the engine bracket is no longer resting on the transom--if it ever was when it was in the lowest engine height position.

Read the FAQ answer at

Q8: How does the engine mount ot the transom'
https://continuouswave.com/whaler/reference/FAQ/#Q8

Also read the Boston Whaler Owner's Manual. If you do not have a copy you can read my HTML version I published many years ago. See

Owner's Manual 9 - 17 foot Boats
https://continuouswave.com/whaler/refer ... stallation

Actually, as I recall for the 15-foot hull a special shim was needed for mounting certain clamp-on engines because the angle of the top of the transom was not exactly 90-degrees from the transom itself.

davidB01 wrote:I read in several threads about adjusting the height of the engine on the transom, targeting the distance between the bottom of the hull to the Anti-ventilation plate.

I don't know what "threads" you read but the inference you made is wrong. The purpose of raising the engine is to raise the engine so when the boat is on plane at normal cruising speeds or higher and the engine is properly trimmed so the propeller shaft is parallel to the water surface, then the Anti-ventilation plate should be located just at or just above the waterline for the flow of water that is passing across the engine gear case. To see examples of this relationship, read the article

A-V Plate Photographs
https://continuouswave.com/forum/viewto ... f=7&t=8605

Where the A-V Plate ends up relative to the bottom of the hull keel centerline is of no concern and is not a "target" for determining proper engine mounting height.

davidB01 wrote:The upper engine mounting holes are drilled in the transom about 1.75-inches below the top of the notched transom.

Your are in luck--that is the proper location. For more details of where the engine mounting holes should be drilled in the transom, read the article at

Transom Mounting Hole Layout for Shallow Splash Well
https://continuouswave.com/forum/viewto ... f=7&t=5808

IMPORTANT--the classic 15-foot hull has a shallow engine splash well. You should read the above article carefully to learn about options for where the engine mounting holes can be located on the transom in order to avoid the shallow engine splash well problem.

You illustration of the engine mounting of your SUPER SPORT 15 boat seen in Figure 1, above, does not show the lower engine mounting bolt location. You need to investigate where the lower engine mounting bolts are located and if they are visible in the engine splash well of the 15-footer.

jimh--The reference to the A-V pictures help considerably to know what my target [for assessing engine mounting height] should be.

The current engine uses the blind holes, and that's why they don't appear in Figure 1. The new Tohatsu [does not have a blind hole boss incorporated in the engine mount].

I know I need to carefully fill the holes [in the transom] from the blind mounting bolts. I can properly drill new holes [in the transom] for the [lower] bolts on the Tohatsu engine [mounting bracket.]

I will put the Tohatsu engine on the transom in a fit-up [perhaps means using just the upper mounting hole in a two-holes-up mounting height]. Then I plan to drill the lower holes in the transom so they are in the center of the slot on the engine mounting bracket, which will then permit the engine to be lowered to one-hole up or further raised to three-holes up if later desired. I will see how this looks with respect to good clearance in the engine [splash] well before [actually] drilling [the lower mounting holes in the boat transom].

Also, [before committing to drilling the lower engine holes in the transom], I will also compare [the engine mounting height] to Tohatsu’s recommendation, which says the A-V plate for this engine should be 10-30 mm [sentence not completed, perhaps means 10 to 30-mm above the boat keel].

Thanks for confirming the engine does not need to have a spacer between engine and transom i[when the engine mounting height is raised above lowest-possible position].

You need to read the Boston Whaler service bulletin that was linked in the FAQ before you decide where to drill the bottom holes in the transom for the lower engine mounting bolts. See

Boston Whaler Dealer Bulletin 10-84
https://continuouswave.com/whaler/refer ... 84-10.html

What that bulletin says: if you want to be able to use a one-hole-up engine mounting, the lower mounting bolt hole in the transom will most likely have to be drilled on an angle going upward from transom to splash well in order to be able to be certain that hole emerges in the splash well above the bottom surface of the splash well. This is also mentioned in the FAQ at the answer to Q8: https://continuouswave.com/whaler/reference/FAQ/#Q8

As for insisting that the lower engine mounting bolt hole in the transom must be located in the center of the slot on the engine mounting bracket, I don't see how it could be possible to do that and have that lower hole in the transom emerge on the inboard side above to splash well--unless for some reason the mold was changed on the 15-foot hull liner by 1985--which I tend to doubt. Also, unless the Tohatsu has FIVE upper mounting holes, the center of the slot will not be in a standard bolt hole location, because with four holes of range, the "center" of the slot will be between standard hole positions.

ASIDE: I think the use of a slot for the lower engine mounting bolts is to make it easier for the installer. With a fixed hole position for both the upper and lower mounting bolt location on the engine bracket, the installer must use very good accuracy when drilling the bottom holes at the proper spacing. With a slot there can be a lot of slop in the lower hole position. It was (and maybe still is) NORMAL that when drilling the engine mounting holes in the transom a metal template was used so the installed couldn't make a mistake in where the holes were to be drilled and also on the proper right-angle (square to the transom). Using a template was the norm, at least at an OMC dealership that knew what they were doing.

ASIDE: as I recall, on my 1976 SPORT 15 the lower engine bolt mounting holes were drilled on an upward angle, and there was an interference fit of the bolt head to engine mounting bracket and also for the retaining nut at the transom where is was just above the bottom of the splash well. Also, the diameter of that hole was smaller than the usual size employed for mounting modern engines, as it was just a 1/4-inch bolt as I recall.

davidB01 wrote:Also, [before committing to drilling the lower engine holes in the transom], I will also compare [the engine mounting height] to Tohatsu’s recommendation, which says the A-V plate for this engine should be 10-30 mm [sentence not completed, perhaps means 10 to 30-mm above the boat keel].

Outboard engine manufacturers tend to be very conservative when suggesting an engine mounting height. If the dimension you stated was in reference to the boat bottom or keel at hull centerline, that is not a good benchmark for determining where the A-V Plate will wind up when the boat is on plane.

If the A-V Plate is positioned so it is some fixed distance above the hull bottom, the relationship of the A-V Plate to the waterline is going to depend entirely on the boat's draft. There is nothing intrinsic or axiomatic that ALL boats will have a certain distance between the the top of the transom notch and the bottom of the hull below the transom, as the design is just to fit either a 20-inch shaft length on smaller boats, or a 25-inch shaft length on larger boats, or even on some boats a 30-inch shaft length, and will then have a fixed draft when on plane. Typically the draft at static trim is always greater than the draft when on plane, as the hull is raised out of the water when on plane

The SPORT 15 hull was designed for a nominal 20-inch-shaft engine, but that does not mean that the absolute best engine mounting height is determined by some certain distance for the A-V Plate to be above the hull bottom.