Don't Understand??

[baitfish]

MY Motors wot. is 5500rpms. my motor runs wo at 5000 rps., and goes 32mph... hears my question... in order to bring my rpms. up I have to lower the pitch on the prop.right?? well won't lowering the pitch lower the mph.to?? Thank You......

 

[Dhadley]

Re: Don't Understand??

Yep, most likely.<br /><br />Thats why we work with the set up to gain rpm instead of simpliny lowering the pitch.<br /><br />Good luck!

 

[baitfish]

Re: Don't Understand??

I've got power tilt & trim..what other set up is there?? so what your saying is more RPMS. doesn't always mean more MPH.... right???

 

[catfish1]

Re: Don't Understand??

exactly! the rpm is all about the load on the engine.<br />a overpitched outboard is like a 3cyl car climbing a grade all the time, never hitting level ground, then imagine you cant downshift going up that grade. well it just wouldnt last as long as the 3cyl car that goes down a level road.<br />i dont know what pitch you have now! but example, if you are running a 17pitch you could go down to a 15p cupped and it should put you close to 5300rpm and thats not bad, if you lose speed it wont be more than 1-2mph.

 

[Dhadley]

Re: Don't Understand??

Rather than lower the pitch (which lowers theoretical speed) we try to reduce the drag then work on lift. <br /><br />If your motor is mounted all the way down (mounting bolts in the top holes) try raising it and see what happens. That should reduce the gearcase drag and therefore improve holeshot, top speed, top rpm and effeiency (fuel consumption).<br /><br />Next we work on reducing the wetted surface (increase lift) which reduces hull drag.<br /><br />Move your motor up (increase the X dimension) and let us know what happens.<br /><br />Good luck!

 

[baitfish]

Re: Don't Understand??

HI, GUYS, & THANKS 4 THE RESPONCE....IF I'M GONNA LOSE MPH.THAN I'LL JUST RUN WOT AT 5GRAND.INSTEAD OF 5500...MYBE SAVE A LITTLE ON GAS TO...THANKS AGAIN,....OH YEA, ONE MORE THING,THE CAV.PLATE IS JUST EVEN TO THE BOTTOM OF THE BOAT.& I'M RUNNING A 15" PITCH PROP........

 

[Dhadley]

Re: Don't Understand??

I think youre missing the point. If you work on your set up you can gain holeshot, top rpm, top speed, fuel mileage all at a better cruising speed. And without buying a new prop.<br /><br />Spend a little time, not money. Have fun doing it. Besides, if you change something like the X dimension, you have to go test it out, right? Ah, another excuse to go boating!<br /><br />Good luck!

 

[catfish1]

Re: Don't Understand??

crappycruncher i'm sorry, i thought you were thinking of changing props. follow Dhadley's advise! he can help you as he helped me, he knows more about this stuff than anyone on this board.

 

[Chinewalker]

Re: Don't Understand??

By lugging your motor down to 5000 top revs, you're putting added stress to the motor, and likely DECREASING efficiency. If you put the right prop on your boat you'll be able to plane off quicker and set a more economical cruising speed (.ie cruising at 3200 revs instead of 3600). If you tinker with the set-up a bit (jacking motor up) and try a different prop you can likely bring your revs up to the 5500 range and you probably won't lose any (noticeable) speed in the process. It's in your motor's best interest to bring the top end up to rated revs...<br />- Scott

 

[baitfish]

Re: Don't Understand??

HI GUYS,thanks for all the info but i'm an old Gar head that knows zip about set up.....i've got power trim & tilt. and a 55hp. outboard...with a 12&1/4"by 15" prop... how would i go about "setting it up"???? thanks again... hooora...

 

[catfish1]

Re: Don't Understand??

crappie cruncher, i'm hoping Dhadley steps in here. but i think what he is talking about is changing the set up meaning x dimension, this is done by raising the motor up or down on the transom(not tilting in or out). what he is saying is if your anti-cavitation plate on the motor is below the bottom of the transom, you can raise the motor up. the higher you can raise the motor (without cavitation) the faster the boat will go and the faster it will plane, and if it goes faster your rpms will increase also. so you may not need to get a prop if you can set it up right.

 

[DJ]

Re: Don't Understand??

Crappie Cruncher,<br /><br />What Dhadley is trying to tell you is that even though your cavitation plate is even with the bottom of the boat, that doesn't mean it is at it's most efficient setting.<br /><br />Most boats can easily handle a setting with the cav. plate 1/2" to 1" above the bottom. <br /><br />Unbolt the engine and mount it in the next hole down, that will lift the engine one inch.<br /><br />Running that engine at 5,000 is NOT good for it. It likes and needs more to stay healthy.

 

[MajBach]

Re: Don't Understand??

Did I miss something in high school physics? Where does everyone get the idea that reducing pitch results in lower top end? Yes, it will result in higher rpms which thus raises the prop rpm which should even out in top speed. An engine "max's" out wehn the power to accelerate is even with the drag incurred at that speed. All things being equal, a boat should go the same top speed with any prop. In reality, you are limited by two three main things: Engine red-line, prop efficiency and probably most important, peak horsepower, i.e a torque curve. <br />My boat:<br />top speed with a 19" prop = 39m.p.h. @ 5400 rpm.<br />top speed with a 17" prop = 38m.p.h. @ 5700 rpm.<br />top speed with a 15" prop = 42m.p.h @ 6100 rpm.<br /><br />WOT = 5000-6000 rpm. Redline 6500rpm.<br />Peak HP is supposed to be 5400 rpm with this motor. Go figure.

 

[pine island fred]

Re: Don't Understand??

thought i would add my experience. hung a new merc 50 on my old boat. dealer put the prop on, boat flew (to fast) but the motor sounded strained. was lucky enough to damage the prop which size although new could not be determined. got a prop out of port charolette that matched the recomendation in the service manual.now the boat is slower but that motor is happy (it actually smiles). that noise was the motor detonating(pinging) would have surely ruined it. cant give rpm changes as i dont have a tach. my recomendation, give up some speed and make that motor happy.

 

[catfish1]

Re: Don't Understand??

crappycruncher, one more thing to look at. <br />the recommended rpm info you gave: 5500 rpm<br />is that the max rating, or is the range 4500 to 5500rpm? the reason i ask is, on my 50hp evinrude it states the range being 4500 to 5500rpm and it makes 50hp at 5000rpm and it also states the wot rpm should be as close to the midpoint as possible. so in which case i am where i need to be at 5000 rpm at wot, and you may be also. check it out! <br />the x dimension can give dramatic results if it is not set up properly to begin with.

 

[baitfish]

Re: Don't Understand??

THANKS AGAIN GUYS,....I'M gonna raise the motor one set of holes & see what that does....I'll let you know what happens..... hoooooora!!!!!!

 

[Dhadley]

Re: Don't Understand??

Now ya got it!!<br /><br />Lets go testing!!!

 

[baitfish]

Re: Don't Understand??

HI, GUYS, well i raised the motor., it's just above the bottom of the boat...it seems to plain out quicker,and goes one mile faster, but still runs at 5000.rpms????it sounds faster??? maybe i should go down in the pitch??? the most important thing to me is NOT HURT THE MOTOR!, ,Thanks for all the advice. ....hoooora!

 

[BillP]

Re: Don't Understand??

There is no exact science when dealing with props. Rule of thumb is...<br />Speed = diameter<br />Pitch = power<br /><br />I've personally never reduced prop pitch and had it run a hull faster. It increases rpms and make the boat perform better with heavy loads. Reducing prop diameter is what I normally do to increase rpms and get speed up. The downside of decreasing diameter is longer hole shots and more sensitivity to heavy loads. Top end speed can then suffer greatly with a heavily loaded boat vs lightly loaded one. A change of both diameter and pitch may be the answer too. Any good prop rebuilder will be able to steer you real close to the right numbers.<br /><br />I suspect the post from MajBach is prop diameter reductions with the same pitch and not pitch reductions with the same diameter.<br /><br />top speed with a 19" prop = 39m.p.h. @ 5400 rpm.<br />top speed with a 17" prop = 38m.p.h. @ 5700 rpm.<br />top speed with a 15" prop = 42m.p.h @ 6100 rpm.

 

[Dhadley]

Re: Don't Understand??

Great! You made a small adjustment and your motor/hull responded. That what we were hoping for. <br /><br />Lets address the tach question. You should have only picked up about 100 rpm or so. Are you sure its showing the exact same reading? What brand is it?<br /><br />Having fun yet? Raise it a tad more.<br /><br />Im gonna go play with the X dimension on my Dusky tomorrow. The new hydraulic jackplate makes it real easy!