Prop Question ?

[cowboy_36618]

Hey fellows, I have a 1988 Evinrude 70 hp with a 13x17 prop off a 1978 Evinrude 70hp.The boat is a 1978 Tidecraft 16 foot "about" ,My question is, when im at WOT the rpms seem to be high, but not enuff prop, it feels like it could go faster by the sound of rpms. which prop would i need to go up to or down to, to get more top end ? I was told if I went up to a 13x19 I would lose around 200rpms and drop in speed. Im no boat tech or prop guy... how do i figure out which way to go on the prop . Im not worried about out of the hole or fast take off ,which ever way you want to put it, more than goin faster . I really want to more or less match rpm with the prop to get the best performance from the engine for the boat. please help me out to figure out which prop I need, up to a 13x19 or down to what ? thanks

 

[ob]

Re: Prop Question ?

This topic will likely be moved to the prop forum cowboy but while we're here I'll just say that the first detail we need to establish is the rpm that your current setup performs at wide open throttle with an average load aboard.You'll need to install a tach to determine this if you don't already have one.

That said.your engine is factory recommended to operate between 5500-6000 rpm at WOT.Nearer 6000 is better as it reduces load on engine at all throttle settings and will perform better out of the hole as well as provide the best fuel econmony and engine logevity.

Engine height setup on transom is also a critical factor it getting optimum performance results.As high as it can be mounted without experiencing prop ventilation when coming up on plane or operating in choppy conditions.The engines lower unit anti-ventilation plate (just above prop) being even with the boat pad when engine is trimmed level is a good starting point.Raising or lowering it hole at a time and water testing can be done from there to attain an optimal final setting.It may already be just fine where it is presently at.

I run a 96' model 70 hp evinrude on the back of a 15 ft whaler and use a 13 3/8" X 17" prop and get around 5900 rpm at WOT with an average load.I'm guessing that your setup will be comparable.

 

[steelespike]

Re: Prop Question ?

I would only add don't make any prop changes without knowing your present rpm.A tinytach is reasonable easy to install and will fit any similar engine
without regard to stator poles.Max rpm and speed are critical to determine
if your setup is working as it should.

 

[cowboy_36618]

Re: Prop Question ?

thanks guys...