Help, my 90 TLRD is a dog

[Ken]

Steve,
Yeah, when up on plane and WOT and trimmed out, take a look at the cavitation plate. On my Pathfinder, there is a recessed area at the bottom of the transom, a type of pocket area that allows the water to "swell up" when up on plane. With the motor trimmed so that the cavitation plate is parallel to the keel, the plate is actually about two inches higher than the keel. But because the water "swells" up from the effect of the pocket, the cavitation plate rides just on top of the water surface when on plane and trimmed out and WOT. This is what you are looking for. You may well indeed need to raise your motor one notch up.
I've never had to move my outboard up or down - but I think what Whitedog suggests might work - except that I would try to support some of the weight with the cavitation plate also, not just the skeg. An engine hoist would be ideal.
Yeah, "post Rita" is still going on - have been rebuilding the wooden fence now for the last two days - if the wind lays tomorrow, I'm gonna sneak off and go fishing [img]smile.gif[/img] .

Whitedog,
The performance data that you quoted is interesting - but I believe 5600 rpm for the 90 Yamaha is 100 rpm over max - does it make a difference? I don't know - but personally, I prefer to keep my rpm max at the mid-range for what Yamaha recommends - 4500 to 5500.
On Boating Life's test boat, were they running a full tank of gas with two persons and a full load of gear? That extra weight could make a big difference.
Good luck [img]smile.gif[/img] ,
Ken K

[Whitedog]

Ken,
After talking to a number of reputable outboard nut's I think 5600 rpms are in a safe operating range. The 70 horsepower model, while being of smaller displacement, is the same design and it's operating range is 5-6000 rpms. There is an article from Bass & Walleye boats magazine that tested a BRP 90 and a Yamaha 90. Interesting thing was the test found the BRP engine developed 91 hp @ 5000 rpm. The Yamaha developed 81 hp @ the same rpm. When spun up to 6000 rpm the Yamaha developed 91 hp. In view of it's diminutive displacement, I think the Yamaha needs to spin a little faster to make the power.

p.s. I went back and looked up the performance report. Weight was 370 pounds for people and 15 gals of fuel (which I believe is about full for that hull). My mistake, they got 40 mph @ 5400. It was the Yamaha bulletin that showed 42 @ 5600.

[Ken]

Whitedog,
Thanks for the info - since that performance report at 5600 came from Yamaha, then they must feel it's ok to run the 90 that high.
Steve may have a motor mounting height issue, and a prop pitch issue - but he is pretty close already with 37 mph @ 5000 rpm and a full load.
Good luck [img]smile.gif[/img] ,
Ken K

[SteveE]

Ken & Whitedog: Thanks to both of you for the helpful input. With the nice afternoon I decided to tackle the motor move which seemed to be the "most available" experiment. It was quite easy using Whitedog's method and no harm done that I can see. I raised the motor up one hole which is 3/4". Ken, your "pocket area" sounds exactly like mine. I measured the height of the cavitation plate above the line of the keel and was also about 2" above it. Now about 2 3/4" above, but still approx. 4" below the top of the pocket. I'm anxious to get it on the water to see if there is any improvement, either back splash and/or rpms. If this nice weather holds, maybe head over to Amistad later this week. I'll report back on the results.

If a 16p prop ever becomes available to borrow or a bargain to buy, I might experiment some more with the rpms along that line.

Thanks again!

[Bazooka Joe]

Whitedog,
Glad to hear you are getting some progress on the rig.

Steve,
Yes, run the Yam 90 every bit of 5500 and beyond. That engine LOVES to turn some revs and pulls smoooooothly from mid-lower 5K right thru to upper 5k.

For props, I currently have a 16p SS and a 17p SS. I use the 16p mostly for water skiing during the summer, while the 17p is my spring & fall fishing prop. The top speed is not a whole lot diff (41 vs 43 mph), but holeshot is substantially better.

However, keep in mind, that I can turn 5800 rpm with the 17p and the motor can hit 6000 with the 16p, although, I don't like running it that high rpm b/c it just doesn't feel like the motor is being worked hard enough at full throttle with the 16p.

Good luck with the rigs, guys. Mine is winter storage, so all I can do is talk about boats until April.