Low Rpm's

[bcribb52]

I have just purchased a 2004 90 HP yamaha 2 stroke. At wot I can only get 4200 rpm's.The prop is a SS 17 inch . What is a recommended size prop for this engine. My boat is only 17 feet long and weighs approximately 600 lbs.

 

[rodbolt]

Re: Low Rpm's

hello<br /> first ya need to find a dealer with a test wheel to insure the engine makes power. have you ever seen it run correctly with that prop? you need to see if its an engine/tach problem or a hull/prop problem.

 

[bcribb52]

Re: Low Rpm's

The dealer set up the tach for me and I have only ran the boat 1 time. I had a 60 hp mariner that would push the boat 40 mph and the 90 yamaha will only run 32 mph. I ask the dealer about a test prop and his reply was that they don't carry test props.Sure hate to keep buying props till I get the right one. My main concern is speed more than getting out of the hole. I don't understand how by increasing RPM'S how I would lose speed,thats what I was told by the dealer.

 

[walleyehed]

Re: Low Rpm's

Does he understand your 60 was faster???<br />#1, your not in the rpm range you really should be, and #2, a 90 with a 17" on a 600lb, 17ft'r should haul right along....In fact, I would assume an over-rev with a 17, not knowing any better.

 

[chuckz]

Re: Low Rpm's

Here's how it's mathematicaly possible to raise the RPM and lower the speed. If you have a 17 pitch prop it means the prop will travel 17" forward with each revolution (assuming zero slip).<br /><br />Say the engine is turning 100 rpms. That means you would travel 100rpm X 17 inches/rpm= 1700 inches in one minute.<br /><br />Decrease the prop to 15 and repeat the experiment. The engine now turns 110 rpms. Wow a 10% increase. Wait. 110 rpm X 15 inches per rpm = 1650 inches traveled in one minute. You lost speed.<br /><br />The reason to increase the rpm is to keep the engine operating in it's power band. Therefore if you increase the rpm into the powerband you will go faster because the engine is putting out more horsepower not because it is spinning faster. This is most applicable to two stroke engines.

 

[rodbolt]

Re: Low Rpm's

ok<br /> mathematical BS aside<br /> YOU MUST make sure your engine is making power. YOU MUST use a test whell or a dymo. ONLY then can you attempt to prop it.<br /> if your dealer does not have the proper test equipment find a real dealer. your going to a shade tree.<br /> test wheel part #YB1620 and a new engine should spin it 5100-5300 RPM. if it wont the engine has a problem. if it will you have a hull/propeller problem<br /> I once watched some snakearses play with a 24 mako for 3 weeks and tried 7 different props. finally my boss had them bring it in on a bet. took me 22 min exactly to find and repair the problem had it been anyother motor than a suzuki it would have toasted itself. dont try to prop for RPM until your sure the motor makes correct power out.

 

[Dhadley]

Re: Low Rpm's

Sharkba852, what Rodbolt is refering to by a "test wheel" is a tool. It doesnt tell you what the correct pitch is, but applies a known resistance to a given motor in the water. The motor will then, if running correctly, rev to a given rpm at wide open. That rpm may or may not be what youll set up for later. But it does tell you the motor is putting out what it should or not.<br /><br />If it doesnt reach the specified 5100-5300 then you look at a motor problem, not a prop or set up issue. You will need to verify the test with a shop tach. Again a tool thats a known good instrument.<br /><br />Who knows? Your tach could be set wrong. I would have thought that a 17 would zing that baby. <br /><br />Stick with Rodbolt, he'll get ya goin.

 

[rodbolt]

Re: Low Rpm's

Thanks for the tech assist Dhadley .<br /> sometimes I forget not all know what I am talking about sometimes I aint even sure after a day spent on a 28ft CC with a pair of 8.1s.<br /> that was a good explanation. wish I had thought of it

 

[bcribb52]

Re: Low Rpm's

Thanks for all of the info. I am kinda dissapointed that I don't have the speed that I thought I would have.The motor is a 2004 but it is brand new ,never ran till on my boat,so I expected it to be right. I've never had to deal with this kind of situation before, should the dealer I purchased this motor from take care of getting it right? I have ran it 2 times and that was to test the performance.I'm really dissapointed knowing that I could have went with a smaller HP 4 stroke and would have probably had the same speed as what I have now.I thought that with a 90 hp motor I would be running close to 45-50 mph on my size boat..

 

[rodbolt]

Re: Low Rpm's

well at 4200 at WOT you have a problem.<br /> as long as you have a problem you will have reduced performance. I am trying to explain the steps that must be taken to correct the problem.<br /> wont be the first motor out of the box that had problems. all makes have them.<br /> several years ago I had a 70 on a McKee craft that another dealer mounted. the guy had constant problems. one of his church members reccomened me so he brought it over. the timing was set 12 degrees retarded and had been that way since it left japan. reset all the lync and sync and added 4" of pitch and it ran like a scared monkey.<br /> 90% of all dealerships gundeck the PDI sheet. 100% of do it your self installers gundeck the PDI. Pre Delivery Inspection.<br /> Tohatsu used to be famous for nothing being correct out of the box other than it would in fact run.<br /> but until you run it up on a dyno or a test wheel with a known good, preferably inductive pick-up, tach to verify engine performance and the hull tach your chasing ghosts.<br /> if the dealer your using cant or wont check it, find one that can.

 

[bcribb52]

Re: Low Rpm's

Thanks again. I'll check with the dealer here to see if they have the equipment to check it with. If not I might have to find out where your'e at and bring it to you. Hopefully someone here can check it out.I live in Wilmington NC, know of any good techs around here?

 

[bcribb52]

Re: Low Rpm's

My boat is in the shop for the power test.I will let you know the results when I find out some information.

 

[bcribb52]

Re: Low Rpm's

I had my motor dyno tested and it was ok. I changed the prop from a 17 ss generic prop to a 17 ss yamaha prop and my rpm's are now at 5200-5300 .Thanks for all of the help and maybe what I have found out will help someone else in the future.

 

[31900]

Re: Low Rpm's

Now that you have the RPMs sorted, how is your speed?

 

[bcribb52]

Re: Low Rpm's

I'm getting close to 45 mph on the gps. It might do a little better when I get back on straight gas. Right now my tank is mixed with a little extra oil.

 

[31900]

Re: Low Rpm's

Great stuff! You've had a win.