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Posts: 3518
Location: north central wisconsin | I'm posting this for Jeff Sarver, who doesn't have access to the site. He called me today because he is having trouble with his new rig. He bought a brand new Esox Mag, with a Yamaha 60 HP tiller 4 stroke, purchasing the boat and motor separately. He said his motor is cavitating badly when he tries to run at full speed. his prop is a 11x15 G(I asked and it isn't the semi cleavered design). His marina helped him lower the motor a hole which didn't help, and he labors a bit more coming out of the hole. I thought he might be over propped, but the cavitation is something I couldn't help him with. It is a long shaft and an aluminum prop.
Feel free to email or call me if you'd wish with reply if you can think of anything. I'll pass the info along to him. |
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Posts: 32835
Location: Rhinelander, Wisconsin | Definitely the prop. He should talk to a Tuffy rep, perhaps. 7156809265. |
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Posts: 3478
Location: Elk River, Minnesota | Hiya,
Any chance of getting some pictures of the motor mounting and the stern of the boat?
Looking at the weight of the boat (900 pounds), and the gear ratio of the motor at 1.85:1 maxed out at 60hp, I would bet that the motor is over propped by about 2 inches or so.
I'm running a boat that weighs just a bit more than the esox magnum at 961 pounds in a console version and maxed out at 90 hp with a 2.1 ratio. I'm running a 17 pitch prop and am just a touch under max rpm, and a tad slow out of the hole.
With a gear ratio turning a motor 1.85 turns with one turn of the prop, I have a pretty solid feeling that the prop is just too much to turn.
In the normal mounting position for the motor (basically standard on most boats), the motor is mounted one hole up off the transom. This puts the cavitation plate even, or sometimes just below the lowest point on the transom, which allows for good overall handling and performance. What I am betting is happening in this case, is that when the boat gets on plane, the motor is too weak, thus the prop just doesn't bite the water. This can happen with a higher pitch prop when the motor does not reach an RPM level that can handle the prop. When a motor is over pitched, getting out of the water is slow..almost painfully slow. Then, when on plane, the motor is not turning enough rpms, so the prop cannot bite the water and still hold the bow up.. so....the bow stays down, and the prop gets too much exhaust since it is not being thrust far enough away fast enough, drawing it into into the prop, or the prop is too close to the surface and pulls air in. if the latter is the case, it can be offset a bit with a cupped prop, but going to a cupped prop reduces rpms more, and a lower pitch is needed to offset the reduced rpm. In many cases, one can raise the motor if they go to a cupped prop.
If you lower the pitch, the higher rpms will help the motor keep the bow up and not allow air or exhaust to enter...better grab, better out of the hole performance, and overall a much better handling boat too. He'll be able to trim the boat and it will respond.
I'd say see if he can get an 11 x 13 semi-cleaver yamaha prop and go from there for a test run...I think he will be pleasantly surprised, and he can potentially move the motor back up to it's original position.
Steve
Edited by VMS 9/30/2010 7:53 PM
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Location: Des Moines IA | I'm running a 1700 Osprey (1100 lbs) with a Suzi 70 4 stroke. It's running a 13.25X15 aluminum prop and has good hole shot and top end. Is a 11 inch diameter prop too small for that motor? My Suzi has a 2.59 : 1 gear ratio though. How much difference does that make? |
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Posts: 3518
Location: north central wisconsin | Thanks guys. VMS, that makes sense to me. I also run a Yamaha 4 stroke and those motors love the semi cleavered props. I tried a number of them and always come back to it. I run a 15P 13.25(I also thought his 11 diameter seemed small). as well, and I could almost stand to go down a tad, or get mine vented. That said, my boat actually gets better hole shot, top end, and grab, with my 17p aluminum so gonna stick with that for now.
For Jeff I recomended he look at a lower pitch for hole shot, and your explanation as to the cavitation seems to be on the correct path. I'll have him call the number provided for the Tuffy rep. He has an appt. set up with the place that sold the boat to him but it is almost 3 hours away and he wanted to see if he could get it figured out close to home before making the run. Thanks much. |
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Posts: 3478
Location: Elk River, Minnesota | Hiya,
The F60 has the smaller hub, so the 13 inch diameter is out like what we have.
I went to the yamaha website and there are two props in 13 pitch to select from...a 10-3/8 x 13 and a 11-1/8 x 13. If he can try both it would be good, but I would bet he would do well with the 11-1/8 to keep the blade size up.
The suzi with a 2.59:1 would be a heck of a motor coming out of the hole!! Lots of good power build up, and it also allows for a higher pitch prop. It really does make quite a bit of difference. That is a bunch more turning of the motor as compared to one turn of the prop. My old motor (johnson 50hp) had a 2.42:1 ratio and was able to turn a 17 on that with a 700 pound boat. Most boats like mine that had yamahas or Mercury (1.85) could get a 15 pitch to spin... Top speed was the same, but the power out of the hole was a bit slower... I miss that motor...
Steve |
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