prop-shop says diameter dosn't matter..........?????

[RIDEPATE]

heres my set-up- 14'8 v-hull ski boat 650 merc-prop is 13/19, michigan reccomends 11 1/2 to 11 3/4- 15????prop shop guy says 13 is fine and dia don't matter??i fool around with airplanes and dia sure does matter so why would'nt it on a boat?? my problem is with the 13/19 i'm only at 3900 wot@29mph should i look for a 13/17 or something in the 11+/- dia size????thanks jim

 

[12Footer]

Re: prop-shop says diameter dosn't matter..........?????

Sure it matters. I'm no prop whizard, and I can tell you that

<br />Maybee he was refering to props of the same diameter or somethin...Hmm..Strange.<br />A smaller dia will raise the RPM, as will a higher pitch.<br />Given this little understood fact (at least it is little understood to me), I would go with Michigan Wheel's recommendation.

 

[RIDEPATE]

Re: prop-shop says diameter dosn't matter..........?????

hey 12 ftr-would'nt a higher pitch lower rpm???

 

[Bob_VT]

Re: prop-shop says diameter dosn't matter..........?????

Just the opposite - A lower pitch will turn faster and a higher pitch will turn slower. Of course this is on a curve based upon horsepower, hull design, and overall weight (load). You need to find a prop that will permit your motor to turn the optimum RPM's under load at wide open throttle. Right now I would tend to say you are "over propped" and it can damage your motor.

 

[RIDEPATE]

Re: prop-shop says diameter dosn't matter..........?????

yes i was thinking i am over-propped, when 12 footer said "a smaller dia will raise RPM,as will a higher pitch" i know the smaller dia. will raise rpm,but also a lower pitch will raise rpm too right??maybe that's what he ment to say, anyway my question still is should i go with a smaller dia????

 

[wvmarine]

Re: prop-shop says diameter dosn't matter..........?????

Hi Gary, based on your having a merc 650 and 13 X 19 prop that fits it with no special addaptations, I have to assume your 650 is a two cylinder model, 1971 or older. What the orop guy was telling you was enerally true. In most cases the diameter of the prop decreases as pitch increases. Just take it for granted as the prop engineers know they perform better that way. There are exceptions to this, especially on older motors where the design of the motor retricts the diameter of the prop that will clear. If I have you motor pegged right(your serial# will verify this-2997339 or below) then all your Michigan Wheel aluminum props are 13" diameter. Since your motor should ideally reach closer to 5000 rpm you probably need to go down to a 13 X 15 to approach your desired rpm. You hole shot will certainly improve. Good luck.

 

[RIDEPATE]

Re: prop-shop says diameter dosn't matter..........?????

HEY WV, my motor is a 72 4-cly.model, not a two, what you are saying makes sense, same goes for aircraft props (more dia=less pitch)and i should have looked at it that way, i guess i'll have to try a 15-17 pitch on her i just hate to buy one and it not work,i don't have any sources to borrow one so i'll have to take that chance.thanks

 

[kansasmike]

Re: prop-shop says diameter dosn't matter..........?????

Rideplate,<br /><br />Take a look at the Hustler brand prop to see if it is available for your merc. You can find them on the iboats home page. I just bought a 17 and I couldn't be happier with it. It's a high performance aluminum prop at a low cost. I've read that others are equally impressed with the quality and performance for the buck.<br /><br />I got a 19 and a 21 with my boat but even the 19 was a bit "tall" for it. The 17 gives me a great hole shot and the same top end speed at 400 rpm higher.<br /><br />Mike

 

[RIDEPATE]

Re: prop-shop says diameter dosn't matter..........?????

hey mike i looked at the hustler, but didnt find anything in the 13 size for my boat maybe i missed it and need to look again,but yea they look like a good prop for the money if anybody knows the hustler pn.# for a 13/17 please post......thanks

 

[River Rat 01]

Re: prop-shop says diameter dosn't matter..........?????

Ridepate, if you've got a 4-1/4" gearcase, the part number is HU1315 or HU1317 depending on what pitch you want. I've got a Merc 650 on a 14ftr, I can pull a tube with the 17, but can't ski behind it. The 15 is perfect for my setup. Oops, the hub should be a 1A or S. Also go to turningpointpropellers.com, they have a dealer locator, so you can find the dealer nearest to you. I did this and it saved me 10 bucks per housing over iboats. Good luck and please let me know how things turn out for you.<br /><br /> RR01

 

[RIDEPATE]

Re: prop-shop says diameter dosn't matter..........?????

RIVER you kinda lost me on the 4 1/4 gearcase thing, please explain...thanks jim

 

[RIDEPATE]

Re: prop-shop says diameter dosn't matter..........?????

RIVER, after some thought mine is a 4 1/2 i believe, its the longer of the two lowers (although i wish it was the short one)but i still wonder where that measurement comes from??jim

 

[Big Four]

Re: prop-shop says diameter dosn't matter..........?????

Take the prop off and measure accross the opening at the rear of the gearcase or measure the hub dia of the prop where it fits against the gearcase.<br /><br /> If she's 72 then she uses the same gearcase as 6 cylinder black stacks(6 inline)up to 150hp, which will be 13" dia props. You'll need to drop back to a 15". You figure 200rpms change for every 1" pitch change in the prop. If you drop back 4" in pitch to a 15" you'll see 8-900 rpms more and should bring her close to the 5000 she rated at. You might see 1000rpms more because you'll be moving faster and there will be less boat in the water causing drag.<br /><br /> The Michigan Catalog your looking at is probably giving you props for the newer 50-60hp mercs that use the smaller gearcase. Check the prop dia for Merc Big Foot gearcases.<br /><br /> She won't live long overpropped like that. Lugging causes all kinds of nasty things to happen in a 2 stroke. Combustion chamber temp's can climb to where she'll melt pistons. You want to get this engine propped right soon. Dunk