Hull Speed

K_willo · #1 12-04-2011, 05:22 AM

Hey guys.

Looking at re-proping my 5m plate ali bream boat, as the prop has seen better days. I wouldn't mind a bit more speed also. At the moment with a Yamaha 70 on the back im getting about 30knots, after doing some calculations with different pitchs it seems that's about as fast as i can expect to go. But after reading about some of your boats it seems so of you guys are getting around 30knots with much smaller motors.
Example: I read Brad Yates in his tracker got clocked at 34 knots by DPI (summer night series a few years ago if i remember correctly?) with a 50HP mercury and a 13P prop, according to some calculations thats simply not possible, I'm not disagreeing here, but what am i missing??

haha sorry for the long post guys, im just a little confused.
cheers Kurt

Brad Y · #2 12-04-2011, 08:27 AM

Hi mate. DPI said thier radar was 33 knots. Boat would only go 29 knots on the GPS with that motor so thier radar must have been out. Didnt matter though, got DQ'd from a comp that I would have won as I had the best bag by a bit from memory. With the 90 etec it just gets 40 with one person and minimal gear and the 19p viper prop.

Main thing with propping is that calculations are only ever a guide. You will need to get on the water with different designs and pitches to get the most from your setup. Find your max recommended RPM and try to hit that as close as possible. You may need to raise your motor so the cav plate is on or near the surface at WOT, but be certain you are getting plenty of water through the motor as you dont want to burn out an impeller. However when your trying to improve your performance there is no substitute for more horsepower.

Just propellers in Osborne Park is the place you want to go.

K_willo · #3 12-04-2011, 09:16 AM

Hey mate, cheers for the reply.
actually went to just propellers, tried out a few props and nothing has been as good as the old busted up one that came with the boat. haha it would seem that i may actually have the best propeller for the boat which is weird because apparently they dont make this prop anymore (11" D, 19" P).
Yeh running at best rpm, cav plate looks to be the right height, i guess if i want to go faster more horsepower is the only answer.

yeh shame about the comp, i remember that.

Justin Boyle · #4 12-04-2011, 09:42 PM

The plate boats do need a fair bit of extra HP. I also have a tracker with a 40hp. 5.2m and has 27kn. On sunday was doing 24 2 up with 80l live well full. I'm pretty sure that is the speeds people are getting out of their quinny hornets etc with similar hp but only 4.2m.

As Brad has said you need to look at your rev's. If they are maxing out you could play around and you might get an extra knot for your trouble. If you aren't maxing out you could drop a pitch for every 200rpm you are away from redline. You might find some extra speed doing that but the biggest advantage will be the acceleration and would certainly be better with a full boat.

A mechanic told me that the solas props aren't as good as the factory(Merc) ones but at less than half the price who gives a !@#$. You could try a solas SS prop for the price of a factory aluminium one if you still want to play around.

The gear ratio on your yamaha must be completely different to Mercs and that is probably why the 13 pitch calculations you did for Brads boat didn't make sense. 19p at max revs on my merc if it could turn it would be 40kn. At your 2.33 gear ratio 34 seems just right

Brad Y · #5 12-04-2011, 09:45 PM

Do a search overseas mate, if you find the prop you currently have, buy two of them.

Justin Boyle · #6 12-04-2011, 09:55 PM

http://www.seamedia.com.au/_tmp/seamedia-kS8Z4h.pdf

70hp yamaha 4 stroke review. I'm guessing yours is 2 stroke but 70hp is 70hp. The biggest difference is arguably a 2 stroke will get to speed quicker but the top speed should be similar. Smaller plate boat but probably comparable and their top speed is lower.

K_willo · #7 13-04-2011, 08:37 AM

Cheers for the info guys.
My hull would be pretty similar shape to a tracker but a fair bit heavier i reckon.
tried out a 17" Pitch prop today. Did about 32knots which isnt too bad, the motor is revving a bit too high (5700-5800) but i was by myself with no gear, so should be fine, the next size up (19" P)i top out at about 4400pm. hmmm maybe an 18".

ahhh im pretty sure i stuffed up the gear ratio in the calculations, completely different to the yammys.

marco_r89 · #8 24-05-2011, 05:13 AM

I max out at 27 knots with a 60 yammie 4 stroke with the stock standard prop on a 4.5m tinnie. I find that depending on the tilt of the engine i get different speeds. My cousin has a smaller glass boat thats probably heavier with a 40 four stroke that goes almost as fast. 30 knots doesnt sound too bad for a 5m plate boat or maybe mines just slow lol.