AB 9.5AL RIB-Yamaha 15MSH set up done

[Hocus-Pocus]

I’ve finally gotten our AB 9.5 AL RIB with a Yamaha 15 MSH rigged about as good as I can get and thought I’d share the information with any who might be interested.

The 9.5 AL is a aluminum hull RIB weighing in at about 50 kg. The Yamaha 15 MSH is a 2 stroke engine weighing in at about 37 kg. The St Croix “Eurohelm” helm station is about 37 kg. The remote controls for the engine are about 9 kg and 10 litres of fuel in tank is about 7 kg for a total weight of about 140 kg.

According to my owner’s manual for the engine it has a full throttle range between 4500 and 5500 RPM with maximum horsepower of 15 attained at 5000 RPM. The aluminum prop is OEM Yamaha for this engine and is marked 9 ¼ x 11 J. Factory recommended engine mounting height is to have the engine anti cavitations plate on the same plane as the bottom of the keel or as much as 1” below.

Information I have gotten from publications and the web, this site included, suggest that optimum mounting height should be as high as possible without resulting in ventilation. Information regarding prop shaft angle suggests that optimum position should be to have the prop shaft parallel to the bottom of the keel.

Information I have gotten from publications suggests that optimum Centre of Gravity should be approximately 1/3d of the distance from the transom to the bow. This doesn’t state if this distance is measured as the length overall, the length of the aluminum hull, or the length at the water line. I assumed aluminum hull length.

I had expected to be able to get at least some guidelines as to optimum set up from AB tech, but unfortunately this was not the case. They were no help at all, so I had to rely on the information I had gotten elsewhere and my own intuition.

AB has a bar welded on the inside of the transom plate to prevent the engine falling away if the clamps become loose. In order for the engine to be lost the clamp screws must be backed off at least ½”. This is a good feature, but it does limit the height at which you can raise the engine before having to modify the bar. The AB manual states that the engine can be raised as much as 7/8”. During my initial set up in the shop it became evident that even 7/8” would not bring the anti cavitation plate near the keel line, so I modified the bar to allow a total of 1 7/16” riser. This put the anti cavitation plate at or just above the keel line. Initial set up was at this distance.

Another thing that became evident was that the angle of the transom was such that it was impossible to get the desired shaft angle parallel to the keel with the existing angle adjusting holes. In order to get the parallel configuration I had to place a ¼” shim at the top of the clamp bracket. The adjustment position was on the second to last hole next to the transom.

I positioned the helm station so that with the operator seated the C of G was about 1/3d of the hull length forward of the transom. I placed wooden slats on the floor and screwed the helm station in place with wood screws so that I would be able to move it forward or aft if experimentation showed that the initial position might not be optimum.

Initial sea trials went extraordinarily well. With just myself aboard as operator I found no ventilation at all even in WOT hard turns. Perhaps I could benefit from an even higher engine height, but because this would mean some more metal work I decided to leave things as initial. Top speed was reached at 21 knots, 5550 RPM. Speed as reported by my Garmin E-Trex Venture hand held GPS and engine speed by a “Tiny Tach”. I tried moving my weight forward and aft to see if I could get any more full throttle speed, but anything I did only reduced speed.

As the boat is being set up as a two person runabout the next trials were done with myself and my wife aboard. With the two of us full throttle produced 19.5 knots at 5400 RPM. Again, no sign of ventilation. Shifting weight forward or aft only resulted in a decrease in speed. She cruises comfortably at about 15 knots, 4400 RPM. At this point it looks like between 12 and 13 knots is as slow as we can go before she wants to fall off plane.

The reason I went to considerable effort to try get the set up right was to avoid having to use trim tabs or “Dolfins” to correct a less than optimum set up. From the results of the trials I’ve concluded that if my initial set up isn’t optimum, it’s pretty close. I may be a bit under propped, but it does leave me wiggle room for more fuel or passengers before I have to be concerned about over loading the engine.

Hopefully this information might help anyone setting up a similar boat and engine combination.

As a side note in my quest for a tachometer for this engine I came across some information on the “Tiny Tach”. At the same time I found some Chinese knock offs on E-Bay for about 1/5th the price. As an experiment I bought a couple of the knock offs to test them. My initial tests on a 4 stroke multi cylinder auto engine proved the knock offs completely useless as a tachometer, but they may be fine as an hour meter. They also might work just fine in a single cylinder application. The genuine Tiny Tach worked as advertised right out of the box and seems to be accurate. I say “seems” because I haven’t done an actual test. I intend to do a side by side test on my 15 and compare it to readings from my own known to be accurate optical tach. I’ll post the results when this is done.