Prop advice?

[TMALEGA]

So i have what i was told is a 13x19 prop on my 85 hp force motor, the boat is a 1974 starcraft america v bottom. Unsure on total wieght. However i was out last night and after messing with the trim i got it up to around 27 mph and still dont have a tach working so no idea on rpms. It didnt sound like it was even near max on them though. The prop is damaged and bent at the ends. I dont feel any vibrations yet so i had been putting it off to fix some other small things. I plan on testing a few different props this weekend. My question is what pitch should i test? I do plan on replacing the tach before the testing. Will the prop being damaged cause that much of a lack of power? I was using my phone gps to get the speed not a speedo on the dash.

 

[Silvertip]

Re: Prop advice?

With zero performance data we have no idea what you need. Fix the tach. Make sure the engine is tuned properly and making full power. Then make some WOT throttle runs with what you consider an average load noting the RPM. Only after this can you compare "observed" WOT rpm with the manufacturers "recomended" WOT rpm which tells you which way you need to go with props. If the engine doesn't make full power a prop change won't compensate for that. If the boat is full of water soaked foam that also affects performance. Speed alone is of no help in determining what prop is right. This info by the way was requested in the READ THIS FIRST sticky at the top of the page.

 

[TMALEGA]

Re: Prop advice?

I did read the read this first part. I thought i had enough useful information compared to what i had read on other posts.

 

[PROPer]

Re: Prop advice?

Once, you get the tach going this weekend you will get a much better idea of what direction you need to go with your prop.
Yes, a damaged prop will really rob your boat of performance. You should be a happy camper once you get a new prop that is the right prop.

 

[Silvertip]

Re: Prop advice?

Without a tach you have no idea whether the engine is running within the manufacturers WOT rpm range. The proper prop is what makes that happen. If the engine is lugging (can't reach the recommended range) then you need a prop with less pitch. If the engine tends to overrev then the prop has too little pitch and you need to increase it. In fact right now you have no idea whether you have the right prop or not because you have no idea what the current WOT rpm is. Top speed will be whatever the correct prop provides. You select a prop for WOT rpm -- not maximum speed.

 

[TMALEGA]

Re: Prop advice?

Ok thank you the prop shop i called told me that if i had a speed then the rpms wouldnt be so important. They could figure it out from all the info i had. However now that i asked and got some explinations on it i will not put a different prop on my boat till the tach is fixed this weekend. And most likely find a different prop shop since it sounds like they are just blowing smoke. To try and make a sell no matter what it does to my engine. Thanks for the explinations all.

 

[Silvertip]

Re: Prop advice?

Your prop shop can't be accurate without tach readings either. They can certainly plug a speed into a prop calculator knowing the lower unit ratio and "receommended" rpm but that still leaves too much data on the table. They have no idea what prop slip numbers are and whether or nor how the hull design and weight affect performance. Fix the tach and THEN report back. Remember, we need WOT RPM with what YOU consider and average load.