View Full Version : Prop Performance

07-28-2005, 05:50 AM
I really need to buy a new prop, mine's got a couple of larger chips in it. My question is: Can I achieve a higher top cruising speed as well a smoother troll all with the same prop? Thanks for all the input!

HSU Steelie

07-28-2005, 06:46 AM
No. Unless you are already "over propped". By that I mean the pitch of your present prop is too high to allow the engine to rev high enough at WOT.

It's generally better to be somewhat "under propped". You want the engine to run in the upper half of the ideal rpm range at WOT with your normal load. *So if your engine is designed to operate at 5,000 - 6,000 rpm, you should use a prop that lets the engine turn between 5,500 and 6,000 rpm at WOT. That prop will also give the lowest load on the engine at trolling speed and should run smoother. For maximum top speed, you generally prop a boat to run right in the middle or slightly below the operational rpm range. However, the stress on the engine when carrying a heavy load is greatly increased.

07-28-2005, 07:13 AM
As far as I know it is a stock prop. Right now speed at WOT is a higher priority (I have an electric for trolling, put it is tough to use in the wind). If I interpreted the info right, to increase the speed at WOT, increase the pitch, but as a consequence the strain on the engine with a heavy load will be increased. Is that correct?

Is there any advantage to running a 4 blade over a 3 blade? It was my understanding that a 4 blade works better under a heavy load.

07-28-2005, 07:38 AM
You have it right.

A 4-bladed prop has more surface area and gives more "bite" on the water at any given rpm than a 3-blade with the same pitch. The result is a better hole shot with a slight penalty of a lower top speed because of increased drag.

I use a 15" 4-blade for heavy loads and higher elevations and a 17" 3-blade for a higher cruising speed at sea level. Lower pitch and extra surface area combine to give good load handling properties at moderate rpm.

BTW, they make LCD tachometers that attach to a sparkplug on smaller 2-cylinder engines and mount right on the engine. This might be a good investment if you want to tune your prop right.

07-28-2005, 07:56 AM
Doc, reading his signature line, he's running a 25 H.P., those prop dia's sound big for a small outboard. Are those dia's normal for 2 strokes?

07-28-2005, 08:37 AM
Hinrid -
Right now my prop is a 3 blade 9x10, so I might up it to a 9x11 at the largest.

Doc- how much do those tach's cost?

07-28-2005, 08:42 AM
I wasn't recommending any particular prop size for his engine. I just wanted to point out the difference between pitch, # of blades, and performance. For his engine something like a 10.5 X 11 prop would be more the norm.

I don't know what the tachs cost. I do know that Yamaha dealers sell them. You must have the non-through hub exhaust if you have a 9" diameter prop. 11" pitch is about as high as you would want to try. You don't want to overload you engine. That pitch should work on your 12' boat though.

07-28-2005, 05:51 PM
;D Hsu. You can get the service meters that the Dr is refering to from Cabela's I just saw them in there truck and rv catolog and I beleive they cost 39.99 I have used them on my other Mercs and they work well I will be buying one for my new 25 Merc. ;D