prop question

[jdcook]

I have a 1992 glasstream 21' with a 4.3L v-6<br />mercruiser 175hp. I cannot get the rpms over <br />@4100. The numbers on my prop are<br />48 78118 a40 17p. I assume the 17p is the pitch, but I don't know what the other numbers mean. The boat runs great, no problems, do you think that the prop that I have is about right, or would a change help.<br />Any advice would be great advice.

 

[FatBack]

Re: prop question

How fast do you think you are going a that speed?

 

[Boatist]

Re: prop question

Jdcook<br />I think also a 17 pitch but do not know what the number mean. Has RPM always been that low or is this something new?? I do not know the spec for the 92 but todays motor is 4400-4800. Assumeing your motor is not down on power and the bottom is clean then I think you should go to a lower pitch prop. Every inch change in same type prop makes about 200 RPM difference. So you could safely go to a 15 inch pitch and turn about 4500 rpms. This would give you a better hole shot but may not change speed much.

 

[zzzzz]

Re: prop question

the 48...... number is a Mercury part number

 

[Bondo]

Re: prop question

The prop you have is a 17"... The other #s are The Part #s.....<br />I agree with boatist on this one... Either drop to a 15", or Find out Why you're Down on Power....

 

[jdcook]

Re: prop question

The boat has a decent hole shot and top speed is about 32 mph, it s a big boat, and I'm not trying to win any races, but I think I could get a little more top speed. <br />The boat had a problem with the rpms dropping off at wot, but cleaned fuel filter, checked gas vent hose, and replaced plugs and that problem has seemed to stop, but still can't get the rpms up to where I would like.<br />Can running below the rpm range do any damage to the motor?<br />Thanks everyone fo their advice

 

[Boatist]

Re: prop question

Jdcook<br />Yes lugging the motor can problems like burnt pistons. Your motor is not far below the normal range. Has top rpm always been 4100. Motor down on power, dirty hull, or extra weight can all decrease top RPMS<br /><br />I not sure lower pitch prop would give you any more speed but would give you more power. My boat for example is a 21 foot Runabout with a 1980 OMC 140 hp. My range should be 4200 to 4600. I have two props that I use both Stainless steel. I have a 17 pitch that turns 4600 RPMs with a speed of 35 mph. I also have a 19 pitch that turn 4200 RPMs with a speed of 38 mph. I use the 19 pitch in ocean waters where never run over 25 mph as it gets better gas mileage and we tend to make long runs, up to 2 hours. I use the 17 pitch in river and lakes where tend to start and stop more for other boats or marineas and also usually never run over about 20 minutes.

 

[jdcook]

Re: prop question

Boatist,<br />Thanks for the info.<br />I bought the boat just recently, and 4100 rpms is the max that I have had it. I have never had a diff size prop on it. The motor seems to be running very well, no missing or loss of power, that I can tell. What things cause an engine to be down on power? The hull is clean, and there is no more than your basic weight on board. I think that I will try a 15 pitch prop just to see what happens. Do you think that I would loose mutch speed, and does that boat/motor/prop set up sound right?<br />Thanks for your help.<br /><br />jdcook

 

[Boatist]

Re: prop question

JDcook<br />Things that can cause loss of power are many but I will list a few and other I am sure will add.<br />Bad spark plugs, motor running cold or over temp, should be 150 to 170, Plugged up flame aresster, timing not set right, Valves out of adjustment, weak spark.<br /><br />A 15 pitch prop will give you more power but I am not sure if you speed will go up or down. One thing that can help is a stainless Steel prop. When I changed to Stainless steel prop it was 3 mph faster than same pitch Aluminum. It never hurts to have a spare prop once you get over the price.<br />Good luck and let us know how you make out.