Gear Ratio

[MrWick]

If the book says the gear ratio of my merc outboard is 2:1 does that mean one prop rev for every 2 engine rev or vice versa.I was trying to figure out if I went from a 19P to a 17P prop to increase rpm closer to 5500 WOT would I gain/loose on top end.I figured I would lose about .7mph........

[MrWick]

If the book says the gear ratio of my merc outboard is 2:1 does that mean one prop rev for every 2 engine rev or vice versa.I was trying to figure out if I went from a 19P to a 17P prop to increase rpm closer to 5500 WOT would I gain/loose on top end.I figured I would lose about .7mph........

[ncst8er]

One prop rev for every two engine revs for 2:1. Divide engine revs by gear ratio to get prop revs.

[MrWick]

So with a 19P prop at 5000rpm I will run 44.9mph if you say a prop is 100% effective. I know it isn't but just to keep it simple.If I put on a 17P prop and turn 5500rpm's I will run 44.2mph with a similiar prop.Is it worth getting that extra 500rpm's(book says WOT should be 5000-5500) or just leave it alone and stay near the lower rpm's.....

[ncst8er]

You need to base prop changes either on actual data from your boat or using info. from a similar combination. Unless you have a low horsepower motor, my suspicion is that you will have a tendency to overrev once slip is factored in, given the pitch/mph nos. you're using. However if the scenario you describe were accurate, you would probably notice the increased acceleration more than the lost speed. I want to stress again not to change props without real data. You may find that your speed will be rpm limited to much less than you'd like once slip is accounted for since slip in a heavier vee hull like a Checkmate will usually be 10% or more. Leave it alone for now since a 10% slip would put you at the upper end of your operating range.

[vinny p.]

Here is the formula used for calculations regarding speed , rpm, pitch....


rpm X pitch / gear ratio / 1056 = speed

plus don't forget to add in your slip factor

[175checkmate]

Here is a great prop calculator. Just enter the figures and a way you go.

http://www.rbbi.com/folders/prop/propcalc.htm

http://www.checkmate-boats.com/images/avatars/GusAvatar2.jpg


1975 Checkmate Tri-mate 2, 2.4 200+ / Sky jacker 6" plate / 25p Laser 2 prop

[Mark]

Mr, wick
Your boat would probably benefit by raising the RPM. An engine produces it's maximum horsepower at the top of the range. You will probably pick up top end as well as acceleration. Plus better cruising power when you are lugging a load of people. What is just as important as pitch is finding the right prop. Try to borrow a few to test. Try a 18" high performance. Or a 19" Ballistic, they work well on the Merc inline 6's since they rev higher than other hp props and therefore you can go with a higher pitch.

[MrWick]

My boat 85 Predictor with me in it and 1/4 tank of fuel runs at 44mph(speedometer)at 5000rpm's with the engine trimmed up.It has lower speed and rpm's at WOT with slight trim.I am guessing at the momemt that the prop I have is a 19P but can't find any markings on prop to confirm that.With a couple of passengers and a full tank I am pretty sure the boat would not reach the rpm's suggested by mercury for WOT.I will have to take the prop to a merc dealer and maybe get some assistance on figuring out what pitch it really is.It maybe a 17P already or it could be a 21P so I am guessing right now.