Boat Speed/Best Prop

[haxsaw]

I have a 18' Nitro 180FS with an Evinrude 150 HP on it, I am trying to determine the speed of my boat, the prop is a three blade 14 ½ x 24, I went to this site, http://www.rbbi.com/folders/prop/propcalc.htm, but not sure I’m doing something right.<br /><br />1)What is "Propeller Slip"? How do you determine what it is?<br /><br />2)How can I find what the gear ration is on my motor, went to the Evinrude site and they show the new models with a gear ratio of .54, Does this make sense?<br /><br />3) What are the RPM’s on this motor when running full throttle?<br /> <br />4) Is this prop sufficient for this motor? I want to get a good whole shot and all the top end I can get?

 

[DMC]

Re: Boat Speed/Best Prop

Haxsaw:<br /> I am pretty sure your gear ratio is 1.86. There is a formula that will give you a baseline for what your motor and prop combination will be able to produce. But it doen't take into consideration what your slip percentage is. DIAMETER / RATIO X RPM / 1056 = Theoretical Speed. So lets say in your case you are turning the 18 pitch prop 5500 RPM. 18/1.86 = 9.6774194 X 5500 = 53225.807 / 1056 = 50.403226 MPH. If you achieve 5500 Rpm and are only making 42 MPH then your slip rate would be about 16.6 percent. <br /><br />I hope this has been some help, I too have been doing much prop research.<br /><br />Good Luck<br /><br />DMC

 

[MGuckin]

Re: Boat Speed/Best Prop

Formula Discussion<br />1) Diffence in maximum and actual power transmission. Will vary do to hull, mechanical condition, motor installation, prop configuration and numerous other factors.<br />2) 1.86<br />3) Runs ideal at 5800 when wot.<br />4) Insufficient info. If you post your loaded weight, max rpm @ wot, prop type and size and mph @ wot, might be able to assist more.

 

[kyricochet]

Re: Boat Speed/Best Prop

the wife bought me a gps and the speed questions are no longer an issue <br />these things are great will tell you how fast how far you will be able to determine mileage<br />best investment she ever made <br />you can pick one up at walmart for around $120 bucks

 

[haxsaw]

Re: Boat Speed/Best Prop

I have a GPS and a speedometer, I guess what I wanted to know is, the 14 1/2 x 24 prop, is it a good all round prop or should I do something different?<br /><br />My whole shot isn't the best in the world, my speed is fine.. Will a hydrofoil stabilizer help?

 

[Jack Shellac]

Re: Boat Speed/Best Prop

According to the OMC manual, the 150 operating range is 4500 to 5500RPM at full throttle. Of, course you want to be in the upper quadrant of this range for best overall results. If you're not getting there, I would say you need to go down in pitch some. Propellers can be a tricky subject since so much 'depends on'. I keep two, 2" in pitch apart, one for pulling skiers and one for running down to the Gulf. RPM's are 5100 and 5500 between the two SS props. Your gear ratio, by the way, is 14:26, so .54 or 1.86 is correct, depending on which way you look at it.Good luck.

 

[haxsaw]

Re: Boat Speed/Best Prop

MG,<br />this prop thing is all new to me, I take it the 14 1/2 is the size of the prop and the 24 is the pitch?<br /><br />My boat is a Nitro 180FS, weight is approx. 1,200lbs, slick hull.. Stainless 14 1/2 x 24 prop, 55mph @ 5500 wot.. is this enough info for you?

 

[Dhadley]

Re: Boat Speed/Best Prop

I just did a quick calculation and it looks like at 5500 and 55 mph you are just under 19% slip with a 24" prop. Thats not bad but its not great. <br /><br />If you have a jackplate try raising the motor slightly and record the data. Watch the water pressure.<br /><br />Let us know!

 

[MGuckin]

Re: Boat Speed/Best Prop

Think that hull is more like 2000 lbs. 1200 hull, 389 lb motor + 2 passengers, gear and fuel.<br />As Dhadley says try lifting motor a bit. Should increase your rpm and cause less drag, thereby getting you up in the powerband quicker.(better holeshot)<br />Watch for handling problems and water pressure.<br />Try this before playing with props. <br />The object is to get the smallest amount of boat and motor in the water possible and still control it. A short burst over revving won't hurt it.<br />You can always pull it back with the throttle or drop a size pitch.<br />(edit)<br />I have a 150 rude hung on a 18' Starcraft cuddy (aluminum hull), spinning a 21" prop. I can get about 53 mph @ 5650. Thats a loaded weight of about 2075 lb. I can spin it up to 5900 with 1 less passenger. <br />You may have too much prop.<br />Keep us posted.

 

[kyricochet]

Re: Boat Speed/Best Prop

haxsaw <br />i know what you are going through because im currently going through the same thing <br />although my set up is a little different being im running a 150 merc on a 18ft hydra-sports<br />ive found alot of info on helping me along at this website im running a 23 pitch prop and a hydra-foil i get good hole shot and decsent top end but its never enough lol <br />the hydra foil has helped the low speed cavitation but i could use a 22 pitch prop to get the rpms up i did raise my motor a notch and it improved fuel economy<br />im currently saving to get a jackplate and my ideal prop well good luck and good boating

 

[haxsaw]

Re: Boat Speed/Best Prop

are there any good web sites I can go to to better understand the prop thing?

 

[Spidybot]

Re: Boat Speed/Best Prop

There is a book to browse and download here. <br />It's a Quicksilver publication and deals with motor/prop/hull and more problems.

 

[kyricochet]

Re: Boat Speed/Best Prop

hello haxsaw<br /> im no expert but from what ive gathered from other people is to find out what your optimum WIDE OPEN THROTTLE (WOT) rpms are for your engine. after getting that info go out and run your boat wide open with your current prop. now if forsay that you are running 400 less rpms than your recomended WOT RPMS then you need to drop down 2 inchs in pitch <br />each inch of pitch is worth around 200 rpms either way, up or down <br />like i said before im still learning but this formula seems to be right<br />goodluck

 

[haxsaw]

Re: Boat Speed/Best Prop

thanks for the info hydro, I am going by memory, still working on my transom.. hope to be over this weekend to check my speed and rpms..<br /><br />UU, Looks like you have a lot of time involved in this prop file, I will have to read and hopefully get a better understanding.. thanks..