WOT 3500 rpm HELP PLEASE

[planerboardman]

I just put a honda 115hp on a 21 ft starcraft cc and borrowed a prop to see what pitch i would need,this is what i get. At WOT 3500 rpms 33mph with a 15 pitch prop on a 21 ft aluminum vee hull with a new tach.I know the rpms should be between 5000 to 6000 what size do i need?Thanks for a reply.

 

[Navigator]

Re: WOT 3500 rpm HELP PLEASE

planerboardman,<br /><br />Wow, that must be a heavy boat <br /><br />Typically, a 1 inch drop in pitch will raise your RPMs around 200. But as you get to lower pitches the RPM variance will rise to 300 or maybe even 400. It seems your really lugging on a 15 inch prop, you may want to a 12 inch or less pitch. <br /><br />See if you can "borrow a 12" and see how it runs. ALthough the numbers may not put your RPM in the proper range, you may find that the 12 will allow your engine to reach WOT.<br /><br />Something else, make sure the boat is properly balanced. Too much weight in the stern will cause it to plow to some degree even when on plane. This will rob speed and make the engine work harder. Try shifting a bit more of your weight up front.<br /><br />Let us know what happens.<br /><br />Nav

 

[Salmon Tub]

Re: WOT 3500 rpm HELP PLEASE

Don't do that, your numbers are way off, so the info is wrong. Think about it, if you take a 15" pitch and calc. it out, at 3500 rpm, with even a 1.8:1 gear ration, you would only get about 27 mph with no prop slip, so, your boat has either broken a few of the known laws of physics, or more likely, your tach has several pole settings, and yours is set on the wrong one. More than likely you are really getting aprox. 4800 if you have a gear ratio of 2:1, OR 4300 if 1.8:1. I have no idea of what your ratio is, but that should be easy enough to find out. A 12" pitch would be way too low, and you would be at or past red line if your tach is on the wrong setting with such a prop. Good luck.

 

[Salmon Tub]

Re: WOT 3500 rpm HELP PLEASE

Just checked, the 115 has a gear ratio of 2:1. That means every time the engine does 2 rotations, the prop will do one. So at 3500 rpm, your prop is spinning 1750 times a minute. At 15", it is theoretically, pushing the boat forward 26250 inches per minute, or 2187.5 ft. In an hour you would go 131250 ft., or aprox. 25 miles. If you are getting 33 mph, then either your speedo is way off, or your tach. Check the instruments first.

 

[planerboardman]

Re: WOT 3500 rpm HELP PLEASE

Checked tach it has No pole settings on the back it is a honda tach also.The speed was checked by gps

 

[hondon]

Re: WOT 3500 rpm HELP PLEASE

You're tach is wrong.Honda makes a tach with no adjustment for their triples but farms out the tachs for the quads and above.Have that rigging dealer water test this thing with their shop tach and ditch that triple tach.

 

[planerboardman]

Re: WOT 3500 rpm HELP PLEASE

Thanks I will do that next week.

 

[planerboardman]

Re: WOT 3500 rpm HELP PLEASE

Can you use a teleflex tach gauge or is honda the only gauge that will work?Also does the tilt and trim gauge have to be honda also?thanks for a reply.

 

[hondon]

Re: WOT 3500 rpm HELP PLEASE

Teleflex will work fine,in fact you can order Teleflex tachs from Honda with Honda branded on them.Follow the instuctions that come with the tach as far as pole settings,and if you have trouble they have a pretty responsive tech line.The trim guage is Honda all the way. I hav'nt seen or heard of any other guages compatable with that sender .That do'es not mean they are not out there.

 

[planerboardman]

Re: WOT 3500 rpm HELP PLEASE

Thanks Hondon I have a teleflex that i took out, i will put it back in and use it.