Not sure about right prop size

[BigBabyLou]

I searched this forum and found some links to websites which calculate the right prop size for a boat. Mostly, the result for my 26' minicruiser w/ 460 OMC King Cobra was 15x15. That's what I have right now (it came with the boat).<br /><br />Still, the boat does not reach the WOT RPM's when going at full speed, I hardly get up to 3000 (the WOT range is 4400-4800).<br /><br />I tried to troubleshoot the engine and other components and everything seems to be all right.<br /><br />I would like to get another prop with less pitch or smaller diameter and see what effect it has on the RPM's. But what do you think, guys, if I can't find a suitable prop with lower pitch, should I try smaller diameter? (BTW: I will try the two piece prop - separate hub and separate housing, it's probably the right way to go)<br /><br />Thanks in advance for any responses<br />Lou

 

[Spidybot]

Re: Not sure about right prop size

As a rule of thumb one inch pitch down equals 200 rpm up.<br /><br />When using calculators one must know gear ratio etc. - I take it you have the exact figures for yours?<br /><br />Have you checked if lever full forward actually opens WOT, your boat running weight and if the hull is clean?<br /><br />How much does removing or redistributing weight affect the problem?<br /><br />If you turn to a good prop shop they'll help you testing props so that you'll end up with one only.<br /><br />If you like there's a free book on props to download here

 

[BigBabyLou]

Re: Not sure about right prop size

^^

 

[BigBabyLou]

Re: Not sure about right prop size

Thanks for the reply, UU.<br />Is there a rule of thumb for the prop diameter too? That's probably what I'll end up doing, reducing the prop diameter, since I can't find any prop for my outdrive with pitch under 15".<br /><br />Yes, I have the exact gear ratios and other numbers right from the user's manual.<br /><br />The running weight is around 8500# and the bottom is clean (boat trailered and maintained).<br /><br />Redistributing weight nor trim tabs (down, up, whatever) help.<br /><br />Thanks for the link to the prop book but I'll have to wait to download it since I'm on a pretty slow connection right now.<br /><br />Lou

 

[Mikedg]

Re: Not sure about right prop size

Looking at the low RPM you are pulling and the fact that you can't find a smaller pitch prop I would look to another problem besides the prop. My 24 foot express ran a 15 x 17 and pulled 4700. A boating pal just had one of his 5.7l only pulling 3,300 while the other was on spec. Turned out it was a carb problem.

 

[BigBabyLou]

Re: Not sure about right prop size

Darn, a carb problem? That would not be good since I don't know much about carbs. How do I troubleshoot this? How can I make sure it is / is not the carb? What can I do?<br /><br />Thanks for the carb tip, Mike.<br /><br />Lou

 

[Boatin Bob]

Re: Not sure about right prop size

BigBabyLou.....I have to agree with UU and Mike, I would look elsewhere for the problem, there is just too much gap between 3000 and 4600 to be a prop problem. I have a 35 ft express cruiser with a pair of 460's (OMC) and they have 15x17 props (one is counter rotating) The boat weighs 12,500 (dry) and I have no problem getting the engines up to 4600 with a top speed of 44 mph. I would really suspect fuel or ignition problems. Has 3000 always been the top since you have had it?

 

[BigBabyLou]

Re: Not sure about right prop size

I agree, the RPM gap is way too big to be caused by a wrong prop size.<br /><br />To tell the truth, my tach is just a junky gauge and I don't trust it at all. I've had the boat for over a year and never paid attention to the tach much but it usually showed somewhere between 3000 and 4000, so I figured it was just an inaccurate gauge.<br /><br />I will try to get a good tach and hook it up and see what's up with the RPM's.<br /><br />Thanks, guys!<br />Lou