slipping prop or something worse

[spacerust]

I just bought a new prop a little over a month ago. I have a Nissan 90HP and put a 13 pitch prop to get my WOT at 5300 with a full load (3 people, 15 foot shallow sport, and gear, 3/4 tank of gas). I did this so when I have less load I would be close to 5500rpms max. Well the last few trips coming in I've noticed that rpms would spike a bit close to 6000 rpms and I would lose speed. I though maybe I needed to redistribute weight and getting air underneath the prop and went out the following week and had more weight in back. It did the same thing. I fish in real shallow water, sometimes as shallow as about a foot. At first I didn't know how to get a good holeshot, but figured out that if i have my passenger(s) lean right on the boat (tunnel hull) that I was able to get out circling pretty quick. But the first few weeks I just tried to drive straight out. This past weekend when I got to about 5200-5300 rpms i would kind of feel like it was slipping. When I'm just at half throttle or less, everything is ok. It is when I try to go full throttle i spike and lose speed. Now...

I live pretty far from water so I read that if you put the boat in gear and try turning the prop if it turns you may have some slippage. I put it in gear, grabbed the prop and tried turning it. Is the prop suppose to turn at all? I am able to turn the prop but it feels like I'm turning a gear and I hear like a whoosh, like when you are turning a cylinder and hear the compression. Is this normal or do I have something other than hub slippage?

 

[Chris1956]

Re: slipping prop or something worse

Sounds like your prop is ventilating. I am not an expert on tunnel hulls, but someone who is should come along, with some suggestions.

 

[Texasmark]

Re: slipping prop or something worse

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I live pretty far from water so I read that if you put the boat in gear and try turning the prop if it turns you may have some slippage. I put it in gear, grabbed the prop and tried turning it. Is the prop suppose to turn at all? I am able to turn the prop but it feels like I'm turning a gear and I hear like a whoosh, like when you are turning a cylinder and hear the compression. Is this normal or do I have something other than hub slippage?

The prop should resist your standing on the left side of the engine (on the prop) when in F gear. It should either stay where it was, or it should yield, but ensure that the crankshaft is rotating and producing the yielding, not the prop only....prop only is spun hub. While in F it should ratchet when turned the opposite direction. That is your overrun clutch.

There is a lot of turbulence in a tunnel hull and how much depends upon several things including the design of the tunnel. The height of the engine with respect to the water and the tunnel matters too. When you are doing your hole shot the water that feeds the prop has to come through the tunnel till you get up. If the prop spins faster than the water can replace what the prop pushed away it will ventilate. Once up and running it depends upon how much prop is in the water as to whether or not it will ventilate.

You didn't specify the heights nor what kind of prop you are using. A high rake, SS prop with good cupping will help you to hold onto the water. Cupping is most important. Next would be lowering the engine in the transom mount bracket to get more prop in the water.

When you turn as you are hole shooting, you are allowing more water in the tunnel which supports my theory. You shouldn't have to do that and you should be able to hole shoot easily in 1 ft of water.

Mark