Help..i think my prop is wrong

blazer159

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Apr 29, 2007
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5
let me start from the beginning. I bought this boat a couple weeks ago. It is a 15.9 Blazer bass boat 1984 model. the motor is a 70hp tohatsu ..model M70A2. I'm not sure what year model it is, thats one of my questions. it is a 2cyl. engine. it seems to run great. there is a manual jack plate on the boat that sits the engine back about 5-6 inches from the transom. I loaded up the kids today and went to the lake. 17 gallon fuel cell is 90% full (in the rear) two batteries in the rear. theres me, my wife, and 3 small kids the boat. the prop is a 11.75x17 SS. my RPMs were around 4200 at WOT. I trailered the boat so i could adjust the height at the Jack plate. The cav. plate "was" about 4-5 inches above the bottom of the boat. I moved the motor down as far as i could and that put the cav. plate at about 1 inch above the bottom of the boat. This slowed the boat down, without any real change in RPMs. I could gain RPMs by trimming the motor up, but started porpuising. I put the extra prop on which is a 12x15 aluminum. It kept the bow real low, and when i tried to trim up, to raise RPMs, it would ventalate. by the way, it has a whale tail on it, and i took them off to see it made a difference, but not anything notcable. another thing is that the hole shot is very slow, and when i am trying to get on plane, the motor does this weird thing where it feels like the prop is hitting something, but that goes away when the boat planes. any suggestions? I think my pitch is wrong. According to Tohatsu, that model of engine should be at 5500-5800 RPMs at WOT.
 

blazer159

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Apr 29, 2007
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Re: Help..i think my prop is wrong

i would like to add that at 4200 rpms i could run about 29-30 mph, and when i trimmed it up, just before porpusing, it would run about 32mph, but the rpms never got over about 4600.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Help..i think my prop is wrong

Not an expert but::
Be sure your motor is well tuned with good compression.
Now you're dragging too much motor through the water. I think you need to raise the motor back up.Maybe even a notch higher.The motor itself may have holes to adjust the height.Used together with the jack plate you may be able to do half notches.Using the SS prop Raise the motor until just before venting becomes a problem.Be sure it pumps water.
After a carefull set up come back and give us your final figures speed and rpm boat setup etc.The prop experts should be able to fine tune your prop set up.
As far as hole shot there's a lot of weight in the stern.Like your chubby uncle sitting on the transom.The jack plate moves the motor weight back as well.After all is set you may want to consider something like Smart Tabs folks get amazing results with the tabs worth every penny.
 

blazer159

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Apr 29, 2007
Messages
5
Re: Help..i think my prop is wrong

does anyone know what i mean when i say the prop sounds like its "hitting" something when I take off? that may not be a good choice of words, but thats what it feels like from inside the boat. it goes away when the boat planes.

Can anyone help with a year model?

Engine is a 70hp model M70A2, serial is 223210
Tohatsu
 

blazer159

Recruit
Joined
Apr 29, 2007
Messages
5
Re: Help..i think my prop is wrong

checked compression today..number one cylinder is 140psi, and number two is 145psi. The plugs were "wet" when i pulled them out, is this an obvious sign of something wrong?

thanks,-craig
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Help..i think my prop is wrong

Compression is good. If it starts good and seems to respond well to the throttle your probably ok."Hitting something when I take off" Could be a worn clutch dog or possibly needs a shift adjustment.Speaking generically unhook the cable and shift by hand if it solves the problem then adjust cable for a for a little longer throw.
 

blazer159

Recruit
Joined
Apr 29, 2007
Messages
5
Re: Help..i think my prop is wrong

I haven't been back on the water yet, but i pulled the shift cable today and It doesnt appear that the shifter can go any further forward that the calbe is currently pulling it, so i dont think it is a shift linkage problem.

anyone have an idea as to why i get the "hitting a log" symptoms (sometimes) while trying to get on plane, but it doesn't happen when I drive the boat up the trailer bunks? It seems like there would be more strain on the prop at that time.

thanks,-craig
 
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