Johnson 55. Only 4500 rpm at WOT.

[steviecops]

Hi<br />I ran my boat for the first time since fitting a tach and discovered that I'm only pulling 4500 rpm at WOT.<br /><br />It's a 17 ft cathederal hull boat, 1980 Johnson 55, 11 3/4 X 17 prop. Today it was fully loaded for fishing, ie, two adults, 2x anchors, full fuel tank and spare fuel, fishing gear, auxillary engine, safety gear, etc. At the 4500 rpm we were doing about 21 knots according to the GPS. I think that equates to around 35 mph.<br /><br />My manual says this engine should run between 5000 and 6000 rpm at WOT. The engine doesn't seem to be bogging down or losing power. Even if I dropped to a 15 pitch prop, this would only give about another 400 rpm which would still be under the 5000 mark. <br /><br />I thought maybe it could be the WOT timing needs looking at?<br /><br />Any ideas?<br /><br />Thanks in advance<br />Steve

 

[VistaMan]

Re: Johnson 55. Only 4500 rpm at WOT.

How much lower is the prop shaft from the bottom of the hull? For a 50 HP I would think 2-3 " for the prop saft below the bottom. Sometimes raising the motor will help as you reduce the drag from the gearcase and allow the motor to speedup a bit but you have to be carefull not to go too high or you will loose water pressure.

 

[steviecops]

Re: Johnson 55. Only 4500 rpm at WOT.

Hi Mark<br /><br />I don't know the exact measurement, I'll measure it tomorrow, but it's definately more than 2 to 3 inches. See picture.<br /><br />picture <br /><br />Thanks<br />Steve

 

[Dhadley]

Re: Johnson 55. Only 4500 rpm at WOT.

You will want the AV plate even or just slightly above the bottom. If you go to a better style prop you will be able to get a bit higher depending on which design you go with. <br /><br />Thats quite a bit of weight also. Of course the obvious thing is to make sure the motor is running as it should too. All linkages going full travel etc. Yes, the wot timing should be verified as part of the sync-n-link process.<br /><br />21 knots should be a touch over 24 mph.

 

[steviecops]

Re: Johnson 55. Only 4500 rpm at WOT.

Dhadley<br />I didn't know that there were different design props! I thougt they were just different pitches and materials. Would any particular design be preferable to the others, and how much of an increase could I expect to achieve?<br /><br />All the linkages have full travel, but I haven't been able to do the WOT timing as I don't have a test wheel. I might try Joe Reeves' method at cranking speed. Would that work on my engine? The timing arm does seem to hit the stop a long way before the throttle plates are wide open.<br /><br />You are right about the weight. It's a heavy boat and I carry a lot of gear. At the end of the day, it's a fishing boat. I'd be quite happy to keep running it as it is, but I don't want to damage my engine. Is that a possibility?<br /><br />Also, looks like my speed calculations were well off! ;o)<br /><br />Thanks again<br />Steve

 

[ddaigle]

Re: Johnson 55. Only 4500 rpm at WOT.

Stevie, I have a 60 horse and I run a 15 pitch prop and it does very well, but my boat is light and a flatbottom. You need a smaller pitch prop for that much weight. Dhadley was probably talking about varying amounts of cup and rake in the design of the prop. These things let you run the motor higher by giving the prop more bite with less of the prop in the water. More cup also means you have to run less pitch generally because of the increased bite.

 

[steviecops]

Re: Johnson 55. Only 4500 rpm at WOT.

Thanks ddaigle<br /><br />I'll drop down a couple in pitch, that should improve things a bit. I don't think I'll get away with raising the engine because the top of the mounting bracket is already about 2 inches above the top of the transom.<br /><br />Ideally, I could probably do with a 70hp on the back, but I'm loathe to change engines after I've got to know this engine, and almost gotten it to the stage where it is easy to maintain ;o)<br /><br />Thanks again<br />Steve

 

[Bass Runner]

Re: Johnson 55. Only 4500 rpm at WOT.

Looking at your pic it looks like it's mounted right, try going to a smaller dia. and lower pitch prop, from the looks of your boat and all the gear you said you had in it you just don't have enough motor for the boat you have if you are trying to go faster you would need about a 90hp on it, or deprop to get the rpm up and just cruse around

 

[ob]

Re: Johnson 55. Only 4500 rpm at WOT.

From my view it looks like that engine needs to be raised at least 2 inches,and my initial recommendation is to go with a 15" pitch prop.Only a water test will confirm final placement of engine and prop change results.That's a big rig with a comparatively small engine.

 

[steviecops]

Re: Johnson 55. Only 4500 rpm at WOT.

Thanks for the input<br />I'm not after more speed, condition on the sea where I fish are rarely good enough to go very fast. All I want is to maybe increase the revs a bit so that I don't damage my engine. I've read that lugging the engine by using too much pitch isn't good for it and wondered if the same applied to my engine as it only pulls 4500 rpm.<br /><br />The engine doesn't seem to struggle and because of the hull design, the boat gets up on the plane really quickly. I'll experiment with prop first, then maybe look at raising the engine a bit.<br /><br />Thanks again<br />Steve