marc wrote:I don't have a tach, I only have a GPS for the speed checking, everytime when I need to know what's the RPM of my engine, I'd enter some data to this calculator to find out, like how accurate, I don't know, it has been there for many years.
props4u.com/prop_chart.htm

ken0276 wrote:
considering you dont know how much prop slip there actually is... that wont be very accurate.

INEEDSPOTS wrote:The calculator only says the prop slip for a mono, hydro and rigger, you're right, don't know the prop slip for a cat, they should provide more info.

they cant provide more info because there is no possible way to know how much slip every prop, with different mods, on every boat has.

Thats why if you want true RPM readings you need a tach. And even then if the prop unloads out of the water for a split second, the tach will not show what the actual load RPM was.. unless you have something like the EagleTree Datalogger which will show you RPM throughout entire run (which my SAW boat will have).

I can see how someone starting from scratch with a new boat and no props could use that calculator to get in the ballpark. Can't be much worse than just guessing it. I may have been a little bit too harsh on the calculator. I think we all agree though, that by far the best method to get the best out of a boat is testing, testing, testing and using onboard tachs and GPS (or some guys will use radar guns...that's fine too if they're accurate). :drinks:

Personally I do so much testing just be sight and sound, I would get badly criticized if I should that here!!! Tell you what though, in most cases my boats end up being very competitive when I go to the races...most of the time! ha ha!

Oh Will, 75mph is not unrealistic for a Sprint cat. Some heat race near that speed.

i know Marc. It was a joke since Ken went around saying i was lying about the speeds i ran with my old insane cat (even though an entire group of people were there and saw the back to back runs and gps readouts )

INEEDSPOTS wrote:Sorry about that Will, at that time I might have been jealous the other boaters who could run his boat over 70 and I did not, but since my cats went over 70, I believe everthing now.

ken0276 wrote:
Ken i gotta hand it to you m8 it takes a big man to admit when he was wrong :drinks: .
Mart

marc wrote:Lol Marc thats how ive alwlays done it,i now have a gps but it seems everytime i use it something goes wrong lol.First time i got my t boat over 65 mph the video camera battery was flat ,last time out i managed to get a short video while retuning the rebuilt carb and i flipped her at 61 mph :dash: .Wouldnt have minded if my mate had caught the flip on camera as it was spectacular(dam those zippkits are strong)oh well maybe next time :cry: .
Mart

martno1fan wrote:Glad to know your engine is still running well, well, before I donated it to you on about March last year, it was running around 57-58 mph on my yellow Stryker cat with a mod 6518 prop but without complete tweaking, as I told you before, it was M6 than I sent it to SS to get it redone, but the funny thing was SS did not put a SS logo on the pull starter, there was still a WHH logo but it was not important to me. To reach more speed, same oldthing that I advided you over RCU last year, you might send your 6717 back to round the tipes and pitch it up a bid or get a mod 6719, also put a hot pipe on it, surely your GPS reading will change.