Faster

[flipbro]

Be careful using the 'on-line' calculators. I have checked a few and none of them allow for cupped blades. When calculating for cupped blades, you MUST add 1" to the pitch for the numbers to be right... (I have written that into my calculator )

Chris........

Ah good to know.

 

[Fieldsken1]

I brought a digital rpm gauge. I will get it tomorrow I will check it against the tach and see that way
I don't know how the shop checked it

 

[Fieldsken1]

Here's another calculation.... 3800 RPM on a tacho connected to a 6 cylinder engine but set to 8 will be....5066rpm.... And if the plane out time is good, I'd be checking to make sure the tacho switch is set right... Just one more possibility....

And those numbers look even better!



Mr Fieldsken1, nobody is implying you're stupid. We are all just offering advise. We can't see or hear the engine or boat. We rely solely on your eyes and ears to give us input. Things we suggest or ask you to check may seem, trivial, but we don't have the benefit of looking at it ourselves. I once spent 4 days working with a guy in Norway to find a very interesting problem on an outboartd engine. It turned out to be a missing ground wire. Had I been there to look at the engine I would have spotted it in a few seconds, but I wasn't, he was my eyes. We went thorugh everything with a multimeter and eventually found the problem... That went well, and we got the problem fixed. Hopefully we can with your problem too. Please be patient and work with us... It will take time and effort on your part. The alternative it to take it to a shop, and pay for it to be done. I'm guess you'd rather save the money for fuel, that's why you're here. Am I right?....

Thanks.....

So I checked it today and the tach was set for a v8. I put it on v6 so according to the chart above every should be right now???

 

[stonyloam]

When you are properly propped, at WOT you should be near the WOT spec for your engine, IN YOUR CASE 4800 rpm. So what you need to do is go out and do a WOT run with your digital tachometer and gps, with what you consider to be a normal load. Note rpm and speed. If your outdrive is the lower ratio (1.91 vs 1.84) your WOT may be higher than recommended. You can compensate by changing the prop. As a general rule you will lose about 200 rpm per inch of pitch increase with the same diameter prop. Now comes the complicated part. You have about a million prop choices, and picking the right one is not easy. In general a 4 blade prop will give you a better hole shot, but a 3 blade will give a better top end. The folks in the prop section can help you out. Don't expect to pick up a ton of speed by repropping. Going from a underpropped 17 to a correct 19 might give you 2-3 mph, and you will lose a bit on the hole shot. When you increase pitch you are losing rpm. Find a good prop calculator and run the numbers. Assuming a 10% slip is not unreasonable. Good luck.

 

[achris]

So I checked it today and the tach was set for a v8. I put it on v6 so according to the chart above every should be right now???

:thumb: And the shop didn't check that? :facepalm:

 

[Fieldsken1]

:thumb: And the shop didn't check that? :facepalm:

They said they did??? Oh well it's at the v6 setting now and it's according to the stamping 1.84.1 gear I am going to try it on Wednesday
Does this sound better?? Can you tell where I should be according to those numbers or do you need the rpm?? I should have that on Wednesday

 

[stonyloam]

You need the rpm, but for your setup we can do a rough calculation, with a 17" prop, 1.84 ratio, multiply rpm x 0.008749 will give you the 0% slip mph. Multiply by 0.9 for 10% slip. Example 4800 rpm x 0.008749= 42.0 mph, 10% slip = 37.8 mph. So record your actual mph and rpm. Calculate 0% slip speed using actual rpm. Devide the actual speed by calculated speed, subtract that number from 1.0, multiply by. 100 and you have % slip. I think I did that right.

 

[Fieldsken1]

Today went to try it out and I guess it's working right. My next question is. Can I change props and how much faster could I go top end and how would it change the hole shot?? Or should I leave it alone??

Here's the results today

14.8/17 prop 41mph @ 4900 RPMs planes out fairly fast

 

[stonyloam]

That calculates out to 43 mph with 0 slip not going to get much better than that. If your recommended WOT is 4800 rpm that sounds pretty good to me. You MIGHT gain a little top end by going to a higher pitch prop, maybe 1-3 mph, but remember you will drop your WOT rpm significantly, and will pay for it with a longer plane out time. I say leave it as is.

 

[achris]

... I say leave it as is.

+1....