Final drive ratio and prop speeds/suggestions

[Skinnywater]

Typically speaking, if an outboard has a redline of 6000 rpm, what is the prop rpm?<br />Specifically, if an outboard has a redline of 6000 rpm and a final drive ratio of 2:1, what is the prop speed?<br />Again specifically, my outboard is equiped with a 1:1 drive ratio for the jetpump and it is geared by the factory 2.09:1 ratio if it was propeller.<br />My outboard redlines at 6 with 5700 being max power. <br />My boat is a 16'aluminum shallow V, and is 1500lbs. loaded and wet.<br /><br />I'm looking for prop selections and why, or why not? <br />Any good links to the above questions?<br />"Cannot be done", would be a bad answer but useful if it were a qualified one.<br /><br />This is what is available for a Honda BF50. <br />12-1/2 Dia. 8 Pitch <br />12 Dia. 10-1/2 Pitch <br />10-7/8 Dia. 11 Pitch <br />10-3/4 Dia. 12 Pitch <br />10-1/2 Dia. 13 Pitch <br />10-3/8 Dia. 14 Pitch <br />10-1/8 Dia. 15 Pitch <br /><br />Thank you.

 

[Spidybot]

Re: Final drive ratio and prop speeds/suggestions

Interesting project.<br /><br />Prop & more stuff here: http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=014920 <br /><br />There are several aspects involved and as always prop selection comprises a number of facts pulling in different directions. You'll end up with a compromise between the props' properties.<br /><br />Also remember to make a budget total as some of the involved parts aren't cheap.

 

[Skinnywater]

Re: Final drive ratio and prop speeds/suggestions

Thanks UU,<br />I'm fully expecting to spend $15-1600. <br />Compromise is always acceptable to me but gambleing more then a roll of quarters isn't. <br />I'm waiting for the magical, mathematical formula<br />to make it a go or no go.<br />I'll read the link you've provided and continue my search.

 

[sho305]

Re: Final drive ratio and prop speeds/suggestions

$1500 for a prop? No way. What are you doing? Changing it over to a prop from a jet? <br /><br />Correct me if wrong, but a 2:1 ratio would be the prop going half the engine speed, 1:1 the same speed. Mercury has a calculator for their motors on their site that you can use and put your ratio into it. It is close. A light boat has less slip. Or get a prop from a place you can exchange until you like it. Depending on your use you go for max rpm at WOT in an empty boat. Less pitch if pulling skiers and you have a hard time but dont overrev when empty. Or get 2 props. Maunufacturer can tell you too. <br /><br />A prop is more efficient, and will go faster/take less fuel than a jet any day. It can get full of weeds too, or hit rocks and trash your prop. It can hurt somebody if you let them play on it while in use as well. Jets are easy to make, but that is why most uses like prop drives better.

 

[Skinnywater]

Re: Final drive ratio and prop speeds/suggestions

sho305, <br />I'm probably a little conservative in my cost estimates yet. <br />That figure is more then a prop. It would be a jackplate,complete lower leg with extra water pump assy, shift shafts, hardware and incidentals.<br /><br />I'm not doing away with my jetpump. I'm cruising in water less then 18 inches deep everytime I hit the river. They cut the flow last week and it was "pucker" time in one 600 yard stretch that had me at WOT in 8-10 inches of water. Caught a limit that day (Sat.)with a 28 and a 37 pound King Salmon. Hooyaa!<br /><br />The conversion to prop is for the reasons you've mentioned when I take to larger water. I want to beat that 9lb Brown trout I caught in Lake Tahoe a few years back. And scuba diving off the Mendocino coast keeps life interesting.<br /><br />I'm itching for the versatility the swap back and <br />forth would provide. Kind of like 2 boats in 1, so it's worth the expense to me. <br /><br />The Mercury site you provided says a 12.5X 9 in pitch would fit my ratio/profile. So it seems it is possible while retaining my ratio. I'll have to pin down an expert face to face yet.<br /><br />The big problem is my Honda is a factory equiped jet. The primary gearset is different from a prop unit. To change these gears is labor intensive enough to make the swap back and forth impractical.

 

[ivar]

Re: Final drive ratio and prop speeds/suggestions

Here's a useful link for prop calculations:<br /><br /> http://go-fast.com/Prop_Slip_Calculator.htm

 

[Skinnywater]

Re: Final drive ratio and prop speeds/suggestions

Thanks ivar for that one to.<br /><br />Do most other outboards have a gearset also or is the ratio done at the gears in the final drive gears at the prop shaft. <br />In the case of Honda is the 2.09:1 ratio the combination of the gearset ratio AND the final drive gears??<br /><br />If the prop RPM is less then engine RPM. Is that because gearing the prop to do double engine RPM causes it to cavitate or otherwise be less efficient?<br /><br />Gotta tell me if I'm getting annoying here.

 

[ob]

Re: Final drive ratio and prop speeds/suggestions

If prop rpm is double that of engine rpm it would over load engine and run like a turtle out of the hole

 

[ivar]

Re: Final drive ratio and prop speeds/suggestions

As far as i know , most outboards only have a reduction in the lower unit , with the crank directly attached to the driveshaft.<br />(Suzuki 90/115/140 have an offset driveshaft(gear driven?) to make the engine more compact)<br /><br />with a 2.0 ratio , the prop will turn at half the engine rpm. <br />With a "conventional" setup , with a fully submerged prop at normal boat speeds , efficiency decreases as prop speed increases. A slower rotating big diam. prop is more efficient than a smaller prop at higher speed. <br />Don't know too much about jet pumps but if I'm not wrong , they get more efficient as rpms increases (as long as it dont cavitate)