Re: porpoising
Red, that's a tougher question than you might think. I just emailed a friend some stuff about this so thanks to the magic of cut and paste you can have a bunch of advice for free (just remember what you paid for it).<br /><br />Every motor and boat combination is unique and you are too far away from what I think is the right prop for me to tell you for sure even if I knew the prop details you have now. You're probably going to have to use a little trial and error. In my view, you ultimayely want to be able to run the boat at 5800 rpm or higher to be letting the engine usually run in a comfortable range. My guess is that the same prop as you have at 2 or 3 inches lower pitch would get you there. However, the change in performance for a prop change that big is hard to predict, especially if the diameter increases as the pitch drops on your prop model. You also may not want to keep that prop design if you are dropping pitch because 2-3 inches of pitch drop is going to cost you top speed you won't recover in the increased operating rpm. You accordingly may want to switch to a faster 3 blade prop and only drop an inch or two in rpm. <br /><br />Either way, I'd start with what you know and work away from that. So take your existing pitch diameter and prop model. Record its speed and performance at a few different rpm intervals and then follow one of the two options below.<br /><br />Option one - ask the prop dealer<br />The path of least resistance is to make it the dealers problem to advise you what prop to try, so it's his fault not yours if he's wrong. There are half a dozen companies selling props on the web. Call one up or a local dealer and ask them. Be careful of dealers who don't offer you the option of trying out more than one prop. No matter what they tell you I doubt they will know for sure what will work best for you the first time in this case.<br /><br />If you want to figure it out more precisely yourself, or you want to switch from a 4 blade prop to another deisgn, then consider the following variables: <br /><br />-every dereased inch of prop pitch will gain you btw 150-300 rpm at w-o-t or vice versa.<br /><br />-every increase of 1/4" in prop diameter costs you about 100 rpm and vice versa.<br /><br />-if you switch from a 4 blade to a 3 blade, you will gain w-o-t rpm on the same diameter and pitch prop, but the exact amount will depend on the prop design. 4 blades give better hole shot and control but usually cost you high speed (the best 4 blades apparently perform almost as well as three blades at speed, but I can't afford them).<br /><br />-going to stainless steel from aluminum/composite will cost you 150-300 rpm or vice versa. A s/s prop will be faster than an equivalent aluminum at the same rpm, and may perform better overall.<br /><br />-manufacturers tend to decrease the diameter on a prop as the pitch increases to increase the range of props that can be used on the same boat/motor. This tends to reduce control and increase slip for higher pitch props. Stay with the manufacturues recommendation, but remember that a bigger diameter gives you more control.<br /><br />-Slip is the measure of the inefficiency with which your prop pushes you boat through the water). Normally a small power boat will have less than .2 slip (or 80% efficiency) but it varies greatly by design. You can calculate your slip in two steps. First multiply the pitch of your prop in inches by w-o-t rpm x 60, then divide it by the gear ratio on your motor leg, and then convert the result from inches to miles (divide by 63360) and you have the speed in mph for your boat if it was perfectly efficient. Then compare it your actual speed at the same rpm and you know your slip. A bad slip rating may point to a poor prop chose for your boat/motor.<br /><br />-lower pitch props are slower, but generally offer better hole shot, and, since they are bigger diameter, more control.<br /><br />whew, I'm going home
