rpm problem

[wiskerchaser]

I have recently bought a 1980 15' astroglass v bottom boat that weighs about 890 lb. plus motor and fuel with a 1979 70 hp johnson motor. The problem i am having is that the engine is turning almost 7000 rpms at full throtle, running close to 23mph. I know this rpm is to high, I also know the boat should run faster than this.currently running a 13 1/4x17 pitch prop. Was curious if some one might have a set up like this that could help me out, so i dont have to go through 2 or 3 props trying to find the right one. ANY help would be appreciated. Oh also speed was the same with 2 people as it was with three.

 

[waterinthefuel]

Re: rpm problem

Oh god, don't run it like that. You are going to throw something out of that block.

 

[Hooty]

Re: rpm problem

First thing I'd check is the prop hub. It might be spun. Put a mark on the prop that aligns with a mark on the prop nut, then run the boat but don't spin it 7,000. Keep it about 5.5-6. Now, examine the marks on the prop and prop nut. Are they still aligned? If they aren't, you've got a spun or slipping hub. Your prop shop can fix it. If they are, check the tach. I might be wrong but I don't think that engine will spin that prop 7,000rpm on that boat. Something's hokey. Is the pulse counter switch on the back of the tach set on "6"?<br /><br />c/6<br />Hooty

 

[walleyehed]

Re: rpm problem

And some of these guys are worried about 200RPM over max????????<br />I turn a 13.25X17 on my 115 Johnny @6000 WFO on an aluminum 18ft'r....I'm about 400lbs heavier, but I don't think that 70 would spin'r that fast...I'm with Hooty on the hub AND the tach....

 

[rickdb1boat]

Re: rpm problem

Yeah,<br /><br />23 MPH with a 17" pitch prop( Which is real close for your set-up), 70 HP Motor and 7000 RPMs on a 15' boat, is definitly a problem. You should be running around 5800 RPM's and around 40 MPH on that rig. As the others have said, the hub is spun...

 

[R.Johnson]

Re: rpm problem

KennyB: I knew that 7000rpm would perk you up. I was hoping he could tell us how he did it.

 

[wiskerchaser]

Re: rpm problem

it was actually 6800 for about 5 sec. befor i relized the tach was banging.got off throttle and started praying.went to shop and had tach checked, they sayed it should be right.they didnt believe me either, but it was 6800rpms promise!

 

[Hooty]

Re: rpm problem

Maybe the prop was just ventilating. <br /><br />c/6<br />Hooty

 

[OBJ]

Re: rpm problem

Somethings "hokey"???? Geez Hooty, and ya' think I talk funny.......<br /><br />Anyway....Maybe the props venting cause the engines set to high on transom or adjusted/trimmed out to far? Just a thought.

 

[Javelin 353]

Re: rpm problem

wisker, I have a 1991 70 Johnson on 90 javelin 15.5 ft 13x18 ss prop . Wot @5700 rpm speed @37-39mph my boat doesn't have a very wide range with the trim or the prop is out of the water good luck

 

[rickdb1boat]

Re: rpm problem

wiskerchaser<br /><br />Where is your cavitation plate in relation to the bottom of your hull?

 

[wiskerchaser]

Re: rpm problem

The best way i know to tell you is there are 2 fins on lower unit not sure which is cavitation plate. The top fin is dead even with the bottom of the boat in first and second trim hole. I have a 13 1/4x21 cupped prop thought about trying it today. hope this helps you with the cavitation plate question . do you think my blades might be flattened causing it to run free at top speed? by the hub is not spun.

 

[rickdb1boat]

Re: rpm problem

If the hub is not spun, then I think your prop is ventilating at high speed. Try trimming it in a hole or two. I assume you do not power T&T? Your cavitation plate should be even with the bottom of the hull. Here,s a reference:<br /><br />

 

[wiskerchaser]

Re: rpm problem

That is exactly how it is my boat. Thanks for the diagram.What does ventalating at high speed mean ? That must be my prob. How can i fix this?

 

[rickdb1boat]

Re: rpm problem

Trim the motor in a hole or two or get a real good prop, with alot of cupping to it. You may even take yours in and have some extra cupping added so that it will "Bite" the water better at high speed. You may also try a fin on the cavitation plate to cut down on the ventilation.<br /><br />some reading:<br /><br /> web page <br /><br /> web page 2<br /><br />Or:<br /><br />Ventilation occurs when air from the water's surface or exhaust gasses from the exhaust outlet are drawn into the propeller blades. The normal water load is reduced and the propeller over-revs, losing much of its thrust. This action most often occurs in turns, particularly when trying to plane in a sharp turn or with an excessively trimmed our engine or drive unit. Ventilation can also be caused by aerated water from the step bottom hulls.

 

[wiskerchaser]

Re: rpm problem

thanks for all your assistance. really helped me understand more about the prob. Do you think it is worth trying the 12 3/4 x 21 prop i have or stay with my 13 1/4 x 17 and if so can i have my 17 cupped because i know its not, or do you suggest a totally differant blade all together? Do you think a fin will make a noticable differance? <br />LAST QUESTION I PROMISE. really do appreciate all your help. Thanks chase

 

[OBJ]

Re: rpm problem

ATTABOY Rick! Love them diagrams!<br /><br />Whisker...trim the engine in a couple holes and try the 17. Pay close attention to the rpms and see if they come down. Then try the 21 and keep track again of rpm and performance. Let us know what the outcome is.

 

[rickdb1boat]

Re: rpm problem

IF you know for a fact that your 17" prop has NO cupping at all, then that IS a problem. Do as OBJ suggested and if it does well with the cupped prop, have your 17" cupped by a prop shop. It will depend on the prop how much they can do with it. They tend to work more with SS props, when it comes to cupping.

 

[Hooty]

Re: rpm problem

WhiskerC,<br />I'm over here in Coppell, not far from you. Drop me an email with your fone nr. and I'll give you a call and see if we can get you fixed.<br /><br />email: jehart1@comcast.net<br /><br />c/6<br />Hooty

 

[wiskerchaser]

Re: rpm problem

Jehart,<br /> I will take you up on that. Sent you an email, if you dont get it let me know on this post. I'm a beginer at comp.