Another prop question

[Mr.Ladyfish]

Our 21' Wellcraft CC weighs about 3400 lbs. It has a 150 hp Johnson (carbs) and a 14 3/4 X 17 ss prop. When new it ran about 46 mph at 5500 rpm WOT. We have added a T-Top and now the boat runs about 38 MPH at 5100 rpm WOT. I knew I would have to change props when we added the T-Top and the boat is on the trailer for a month or so. My question is how do I know what prop to buy to get the 400 rpms back? I think I read here that I need to go with less pitch but how much?. Props ain't cheap & I want to get it right the first time.<br />Thanks for your help.

 

[muskyone]

Re: Another prop question

i belive it's 200 rpm per degree of pitch do a serch theres a ton of info on here about props

 

[Dhadley]

Re: Another prop question

Yes, its about 200 rpm per inch of pitch. Pitch is not measured in degrees. The 200 rpm only holds true when comparing the same make and model prop. IE -- if you have a 19" SSTII you cannot compare that to a 19" aluminum, a 19" SSTI or a 19" Turbo. <br /><br />Try playing with your X dimension (engine height) before you spend money on a prop. A very general rule of thumb -- If you can use 100% of your trim range without ventalating, then most likely you can raise the engine on the transom. This will gain back some rpm from reduced drag with out sacraficing top speed and overall efficiency.<br /><br />Good luck!

 

[AW]

Re: Another prop question

Second the motion for raising the engine. <br /><br />You could also try having the prop repitched, or get it thinned and blueprinted to make the boat faster again.<br /><br />The simple thing to do is get the tame type prop next size down in pitch. Then you have a spare prop, and if you do something to make the boat faster you don't have to get yet another prop. If you want to change the type of prop there is lots to think about.

 

[walleyehed]

Re: Another prop question

You might want to make darn sure the hull is 100% clean.....a very slight film build-up can cost more speed/RPM than you might think. A good fiberglass hull cleaner and a soap-n-water wash job helps more than most realize.<br />DO NOT wax the hull as this causes extra drag as well because of surface tension-causing the hull to be "sticky" in the water....

 

[AW]

Re: Another prop question

Originally posted by walleyehed:<br />DO NOT wax the hull as this causes extra drag as well because of surface tension-causing the hull to be "sticky" in the water....

See the thread "Waxing Hull" for other opinions.

 

[walleyehed]

Re: Another prop question

Opinions are like you know what.......everybody has one, and waxing the hull(not) is not my opinion, it is fact....Do your homework.