Tohatsu USA sent me a lower pitch prop to try on the boat free of charge in their efforts to try and help me with the performance issues I have been having. I got out yesterday and put the new prop on and got some preliminary static thrust measurements.
The prop on the engine goes on and off super easy. Pull the pin on the nut and it just turns off with two fingers easy as pie. The prop then comes right off and the new one slid right on. We just turned the boat around so with the engine up I could sit on the edge of the dock and reach the prop with the engine in the up position. Maybe 2 minutes to change it out.
It was nasty and rainy out when we did this and I had some friends with me so I was not so hard core that we took the boat out yesterday to test it on the water.
However I did put the scale on the boat and tested the static thrust and rpms for the old prop and the new prop at the dock. The results were surprising.
The stock 8 pitch prop as I have noted before gets 3800 rpm at WOT at the dock. At 3800 rpm tied to the dock it exerts 145 lbs of force. This is actually 15 to 20 lbs more than the old engine did in the same test. I really didn’t think this would be the case but on reflection It does make sense. I have already noted that at lower rpms this engine seems to pull stronger than the old one such as in reverse of just pulling from the dock. The boat performance issues have to be tied to the torque curve in the higher rpm range.
The 6 pitch prop was a real eye opener. Static thrust jumped to 175 lbs of thrust and the RPM jumped to 4700 rpm. I was expecting the thrust to go up some but totally not expecting to see a 900 rpm increase in prop speed. According to tohatsu I should have seen a 300 rpm prop speed increase.
I am really looking forward to doing the on the water tests and seeing if there are any surprises there. Based on the 900 rpm increase at the dock I should see a roughly 1400 rpm or better increase in motion on the water based on the difference that was seen between tied to the dock vs on the water in motion rpms with the 8 pitch prop. If this holds true then I should see 5300 rpms or there abouts on the water. It will probably be Saturday before I can do the on the water test. Results as they come will be posted.
As a side note here is some video of me coming into the inlet with the engine at WOT (wide open throttle) doing 4300 rpm and 5.3 knots over the water speed. This was taken this past sunday with the new Canon A640 Digital camera. The video from the camera looks even better than what I put up on the web. Unfortunately it was about 100 megs in size. After rendering it down to a webusable size this is what you get (still pretty good)
Engine running on water video
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