Prop change needed?

bendellee

Cadet
Joined
Aug 31, 2009
Messages
8
Lightweight 14' Duracraft.
1970 Evinrude 6hp.
2 blade, 8 x 7/14 prop (maybe 8 x 7; i'll verify after work).
I don't know the wide-open throttle RPM.

Last year, top speed with me and fishing gear was 12-13 mph, but I replaced my 6 gal tank with a 3 gal, moved the gas tank towards the bow, went with a smaller ice chest, and moved my trolling motor battery back just a hair.

Now, the boat will almost plane with just me and fishing gear (it feels like if I just take off my shoes and throw them to the front, the boat will level out). I go from zero to my "almost planing" top speed in no time flat. So, it seems like its got plenty of thrust, just not quite enough top end for the thrust to do anything with.

If I switch to a 3 blade prop, with the same diameter, and a little less pitch, should I be able to get the necessary thrust without giving up too much speed?

I'm concerned that by changing props to get a little more thrust I may lose enough speed that the "need to plane" dilemma is moot. I often fish a 27k acre lake, so I don't want to give up too much speed.

There are too many variables for me to wade through alone (i.e. should I reduce pitch but add a blade; increase pitch with two blades so that top speed overcomes thrust deficit; etc.). What do you prop wizards suggest?
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,771
Re: Prop change needed?

Shucks -- nobody can tell you since you left out the most important piece of information. Any engine has a limited amount of power that it can use to twist a given prop. That's why engine manufacturer issue a recommended wide open throttle rpm range. Small engines like yours generally come from the factory with a specific prop whereas engines above 40 HP do not since the manufacturer has no idea what boat (work boat, barge, planing hull, heavy, light, etc) the engine will be installed on. Each of those takes a very different prop. So until you determine how fast the engine is reving with the current prop at wide open throttle nobody can tell you what to do. Yes - lowering pitch will cost you some speed UNLESS the engine is already lugging (over propped) and can't reach its recommened RPM. If the engine is over reving already (too little pitch) then decreasing pitch will make it over rev more which is obviously not a good thing. The point here is you need a tachometer such as the simple little "Tiny Tach". Siimple to install. Then with what you consider an average load, make some wide open throttle runs and note what the maximum RPM you observed is. If the rpm is close to or at the upper end of the manufacturers recommended rpm then there is nothing else you can do. If under the recommended rpm then decrease pitch, or if over the recommended RPM increase pitch. Forget the diameter issue. It is a function of prop design.
 
Top