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Thread: Going from a 9 pitch to a 10 pitch, for a 15HP 4stroke Tohatsu on a Freighter?

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    Question Going from a 9 pitch to a 10 pitch, for a 15HP 4stroke Tohatsu on a Freighter?

    The Tohatsu site measure loads by weight alone. Since I'm skinnier than most loads, I'm thinking I need to qualify myself as a low-load and go for the 10 pitch, instead of the stock 9 pitch.

    15HP 4 stroke shorty, raised another 6 inches, on a Scott Albany 19' with 4' beam.

    Advice?

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    Logic seems sound. I have always thought of props on planes or boats as power or speed related....do you want more speed...go to a flatter prop to allow the engine to run faster sooner. A prop that takes a bigger bite will give more power sooner. If you get too much pitch and don't have the power to turn it at the proper speed, you loose effectiveness. Just the way I think of it.

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    Default prop tuning

    Quote Originally Posted by Lowrider View Post
    Logic seems sound. I have always thought of props on planes or boats as power or speed related....do you want more speed...go to a flatter prop to allow the engine to run faster sooner. A prop that takes a bigger bite will give more power sooner. If you get too much pitch and don't have the power to turn it at the proper speed, you loose effectiveness. Just the way I think of it.
    I lose "prop" (meaning, any difference in speed) at half throttle, so this seems like the right course. Agreed?

    The opposite is bad, to lower one's pitch to where the engine will over-RPM. I don't have that problem.

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    Your either running up to hull speed at 1/2 throttle or the engine is all ready running full rpm at 1/2 throttle. If at full rpm at 1/2 throttle you definately need more pitch. If the motor will rpm more without gaining mph you are at hull speed and more pitch will only drop rpm which may not be a bad thing. If you are at hull speed at half throttle you have more motor than you can use. My guess is that 15hp on that canoe is more motor than it needs. If at hullspeed and it will fit, an inch more in diameter may be more affective as it should get you moving quicker.

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    Hull speed is the fastest speed the hull will go without diminishing return. If you exceed the hull speed it takes significantly more power to gain just a little MPH. Efficiency is lost above hull speed....for example if you burn 5 gph @40% power to go 15 knots and crank up the throttle to 75% power and burn 12 gph to get 18 knots....not efficient.

    I think going from a 9 to a 10 increases prop output at a given RPM. I would use the GPS to determine the best power setting for each load in the boat. You should reach a point of deminishing return where more throttle only gives a little increase in speed.

    ALso, I think more load will provide more wetted surface and cause more drag which will make the boat go slower for a given power setting on that hull.

    Thoroughly confused yet??

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    Prop pitch actually means little to a canoe, they'll all bring yah home at a low speed. With canoes, the hull will only allow so much speed anyways. With the 15 horse tohatsu having so much torque and the prop lifted, I'd go for a prop that had one size up from the stock prop regarding pitch. The stock prop does fine with or without a load. With only a minor $40 dollar change needed to adjust the wide open throttle of a tohatsu 15 hp; that is literally the only difference between the 15hp and 20hp models. The torque of the 20 is still there with the 15hp tohatsu is my point, you want more aggressive pitch when your prop is lifted high because your rpm's go up substantially and the motor is working less.

    The more aggressive prop will make the motor work harder and bring the rpm's down to a more appropriate figure that was more inline with the rpm's the motor would run with a stock pitch with the motor placed deeply in the water. Prop pitch selection will be different for most motors. When I ran a 2001 Suzuki DT6, the slower pitch prop ran the motor too high of RPM's no matter what the load. That slower pitch would have been great for a kicker motor on a large boat, but was a waste of potential power. I realized this when I was pulling a broken down boat back to his camp (large 90 horse powered fiberglass boat) and there was no change in rpm's. Canoes with lifted motors usually do best with a prop that is slighty more aggressive than stock, while saving the stock prop as the load hauler prop in my experience.

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    I'll report back what my findings are, this spring. I'm not in the know, but I suspect I was at my rpm limit, and its the right move. Stock is a 9 pitch. Tohatsu recommends 10 pitch for a "light load" which I have to assume my skinny boat is.

    TohatsuOutboardParts.com had the prop for 69 bucks (vs. $102. in town), so I tossed in a new impeller and a new prop hardware kit too, just for insurance against ever breaking that hardware.

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    Default likely a good move

    Quote Originally Posted by mainer_in_ak View Post
    I'd go for a prop that had one size up from the stock prop regarding pitch. ...

    Canoes with lifted motors usually do best with a prop that is slighty more aggressive than stock, while saving the stock prop as the load hauler prop in my experience.
    Both of these notes would lead me to believe that I'm right, moving from a 9 pitch to a 10 pitch.

    The reason I thought so before, is that a few months ago, I "polished" a prop to less than its stock size, and saw a 25 percent speed reduction, though I believe I heard the same RPMs (no tach). Weight was close to the same each way, since I poured out all my extra water before heading back, and had eaten some food that I'd carried in, which would mostly make up for the caribou that I took back.

    My fastest speed ever: 18.4 mph (no load except for just me, and my standard safety gear)
    My canoe's "happy speed": 14
    My top speed heading in, a few months ago (quite heavily laden): 12.? mph
    My top speed heading home, a day or two later: 9.? mph

    At all those speeds, I hear about the same RPMs. That's what makes me think its a prop-thing.

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    Don't take this the wrong way, but just because you are "skinny" that does not mean you are "light". The more weight, the lower you sit in the water and the more "wetted area" you have to push thru the water...meaning more drag or resistance to the work the motor is putting out. So, I think you need more "bite" or an increased pitch on the prop to be more efficient if the RPMs stay the same at different speeds.

    I have more experience with plane props than boats so I may be "all wet" in my assessment...so listen to Mainer.

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    Default Re: "listen to Mainer"

    Quote Originally Posted by Lowrider View Post
    I have more experience with plane props than boats so I may be "all wet" in my assessment...so listen to Mainer.
    I usually do. And most of the times I don't, I turn out to be wrong. (OK, maybe all of the times I don't......) Not just about canoes, either. He nailed the scope I needed, and I didn't listen and bought one radically different from his advice, so now I had to order the one he picked out originally for me..... well, because he was right.

    I think he suspects that the 10 pitch will work better for me, but I pulled the trigger on the purchase before I heard back from him, as is my way sometimes.

    I certainly listened to him about the Princeton Apex headlamp, plus I found a cheaper source for it than he had. It just arrived a few days ago, and I love it.

    I'll report back on the 10 pitch after I get it wet this coming spring. If its too much prop, I'll just use it as my shallow water rock-hitter prop and after an hour or two it will merely be a shadow of its former self and should be great then.

    But my suspicion is that the 10 pitch will be my new open-water prop, then I can switch to my already well "polished" prop when I pull over and raise my motor (using my Mainer-lift - I really like how that works) to head down the skinny waters.

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    Default Different props, and different top speeds

    The three props below are, left to right:

    1 - And old 9 pitch, well polished by many river rocks
    2 - A brand new 9 pitch
    3 - A brand new 10 pitch







    Prop 2 gives me a top speed of 18.4 mph, empty, and about 15 mph heavily loaded.

    Prop 1 gives me a top speed of 9.9 mph, heavily loaded.

    I'm hoping that prop 3 gives me 17-18 mph heavily loaded.

    Notice how much of the actual surface of two identical props (1 and 2) is gone; the only difference is lots of polishing to prop 1:



    Personally, I think that prop 3 looks pretty sn***y too mounted on my black motor:



    Prop 3 is Tohatsu brand name, and I got it online at TohatsuOutboardParts.com for about 70 percent of the OTC price in Anchorage (for a no-name brand prop).
    Last edited by FamilyMan; 10-31-2011 at 23:11. Reason: LOL, you can't say S N A Z Z Y on here. ;-)

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    Did your "polishing" take place with or without the rock hopper? Also, any idea how much you loose speed wise with the skid plate installed. I've been thinking about making one for my 9.9 but I don't want to loose too much in the way of cruise speed since I'm usually in deep water anyway, but it sure does look like it has saveg your prop from "rock polishing" a bunch of times!!

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    Default rock hopper

    Quote Originally Posted by Lowrider View Post
    Did your "polishing" take place with or without the rock hopper? Also, any idea how much you loose speed wise with the skid plate installed. I've been thinking about making one for my 9.9 but I don't want to loose too much in the way of cruise speed since I'm usually in deep water anyway, but it sure does look like it has saveg your prop from "rock polishing" a bunch of times!!
    No, the rock hopper has been permanent since day one. This last year bent it down; must be from getting a rock between the prop and it. Previous years I've bent it UP, from hitting the bottom of course.

    I can't say if a speed loss occurred from installing the rock hopper, but I wouldn't do with out it.

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    there is a different stile rock hopper [not the true bran name] that goes from the plate down the leading edge of the lower unit that will help when you hit a rock with the gear box

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    The guard on FamilyMan's motor is not a Rockhopper, but some other brand of guard. The true Rockhopper motor guard is much more rugged and heavy duty.

    http://www.rockhoppermotorguard.com/

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    It's a Mac's River Runner.

    The Rock Hopper would impede/prevent my installing a doel-fin. Sorry for the mis-name.

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    FamilyMan ...

    You're incorrect. The Rockhopper mounts on the underside of the anti-cav plate. You can mounts the dole-fin on the top of the anti-cav plate, as I did on my 15 hp Yamaha w/ Rockhopper/StingRay Jr.



    Sorry the picture isn't a close up. Here are a Rockhopper mounted under the anti-cav plate with a StingRay Jr mounted on top. Note: the Rockhopper has been modified by the removal of the under-prop wing, which collected vast amounts of grass and often got stuck in mud/silt. The wing, though, did well protect the prop from rocks, as I learned on just one trip with a new prop and no wing.

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    Are any of you guys running four bladed props? With the bigger boats you gain stern lift and you can run the motor up higher. Hole shot is also much better.
    Spending my kids' inheritance with them, one adventure at a time.

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    Quote Originally Posted by Rick View Post
    FamilyMan ...You're incorrect.
    I stand corrected.... again. Good pic; good info.

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    OK....so....everyone agrees...the REAL Rockhopper is best???

    That design looks like it would cause less drag than the Mac's FamilyMan has on his motor....but the flat piece on the bottom of the Mac's does seem to make more sense from a gravel standpoint.

    Opinions????

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