More speed!??

Report the performance of your boat, supply as much information as possible (weight, HP, prop, speed, etc.).
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dabren
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Location: Vancouver, BC

More speed!??

Postby dabren » Thu Sep 08, 2011 12:54 am

My TNT is running really smooth with the motor trimmed down but with that and two adults by gps I'm only getting 33mph. RPMs are only maxing out at 4200, want to get that up to 5500 which is the max recommended for my engine (30hp 2-stroke Tohatsu). I haven't removed the prop to check for sure but I think it is stock 9.6 x 13. I'm thinking I need another prop...if so what should I be looking at? Any suggestions would be great.

Thanks a lot,

Dan

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DaveLott
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Re: More speed!??

Postby DaveLott » Thu Sep 08, 2011 4:57 am

dabren wrote:My TNT is running really smooth with the motor trimmed down but with that and two adults by gps I'm only getting 33mph. RPMs are only maxing out at 4200, want to get that up to 5500 which is the max recommended for my engine (30hp 2-stroke Tohatsu). I haven't removed the prop to check for sure but I think it is stock 9.6 x 13. I'm thinking I need another prop...if so what should I be looking at? Any suggestions would be great.

Thanks a lot,

Dan



Dan - If you engine is running good but not able to reach peak RPM then is appears to be bogging down from too big of a prop. I am not familiar with outboards at all but I would start by trying a prop in the 9x12 range and see what happens. At this point trial and error is a course of action. Hopefully someone on board has experience with your engine and can point you more directly
Dave

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Re: More speed!??

Postby upspirate » Thu Sep 08, 2011 5:50 am

I also think you stated that your motor was below the bottom of the boat.

I'd try raising the engine with a 1x2 under the clamp bracket.I ran my TNT this way but I drilled the holes so I could raise if necessary.

Even if you have to run without the mounting bolts,you can do it for a quick pass to see what you get(don't stay out all day unbolted, and keep looking at the motor to see if it torques up)

Usually raising the engine gets you some extra RPM and speed.You might get some cavitation, but that can be addressed next.

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thudpucker
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Re: More speed!??

Postby thudpucker » Sat Sep 10, 2011 12:28 pm

You might try Tohatsu with that question.

wright6627
Posts: 53
Joined: Fri Sep 30, 2011 9:42 am
Location: Sacramento River Delta

Re: More speed!??

Postby wright6627 » Fri Oct 21, 2011 9:35 am

Sounds like you need quite a bit less pitch and some work on trimming.

Trim out as far as possible and still have the bow hold steady. Like Pirate said, jack the motor up til the cav plate is a little above the transom chine edge.

Now take a tach reading with just you and a half a tank of gas in the boat. Sounds like you will buy a prop to me. Each inch of pitch less than your current prop will increase your RPM by 200-300 rpm

When you're trimmed for speed (up and out as far as possible) your prop will probably cavitate (suck air) a couple of times as you punch it from idle. This is normal.

Alot of guys think more pitch = more speed, and it does if the motor is in the upper end of its operating range. If it's running in the 4,000-5,000 range, more pitch = less speed as the motor can't make its rated power at the low wide open throttle rpm. When you get your rpm up to the 5,500 area, your top speed and your acceleration will be dramatically improved.

Over the years, I've found far more boats are over propped than under. Probably a good job of selling at the marine dealership: "You want to go faster? Here's a speed (more pitch) prop for your motor" Many small runabouts have no speedo or tachometer and their owners have no clue that their prop is killing the boat's performance.

Randy


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