First off i would suggest that you read the
'sticky' at the top of this forum & educate yourself on prop selection. Its not rocket science and most of the links in the sticky can give you a basic understanding of selecting a prop for your boat.
Second, yours is a pretty common question here. Bottom line is that your engine has a recommended WOT range of 4400-4800 and its clear that you are below that range. As a result your engine is working much harder than it should to get your boat moving & to maintain speed. So the solution is fairly simple & you have identified that you need to reduce the pitch of your prop to get better performance. As a general guidance, for every 1" drop in pitch you will get a corresponding 200 (give or take up to50) increase in your WOT rpms. You need to get yours up into the 4600 - 4800 range where the engine & prop combination operates most efficiently. A 19" 3 blade prop should have you right around 4950rpms and a 21" around 4600. Being
slightly over your recommended range wont hurt as long as you dont run the boat at WOT for any length of time. Running at WOT for any length of time causes excessive wear & tear on your mechanicals and is a fuel waster.
Most members here run a 19" 3 blade. Some of us run a 20" - 4 blade (extra blade = extra bite), there are a few here that run a 21" 3 blade (used mostly for cruising) and if i remember right one person here runs a 23" (but he's a bit odd
). I would suggest that if you use your boat for pulling tubes/wakeboarders/skiiers or generally have 4 or more adults in it, that you consider using a 19" - 3 blade or a 20" - 4 blade.
The following are general rules of thumb:
Lower pitch = faster holeshot, better planing ability, increased rpms, lower top speed.
Higher pitch = slower holeshot, slower planing ability, lower rpms, faster top speed.
3 blades are generally the most efficient props. 4 blades provide extra bite but cause extra drag which usually results in a slightly lower top speed when compared with the same pitch 3 blade.
Cupped blades provide extra bite & reduce ventilation as well as allowing you to trim up just a bit more. They also reduce the effect of pitch change.
Aluminum is the least expensive prop material for general use and provides good performance.
SS props costs more but usually provides slightly better performance.
Composite props (IMHO) are good for spare "get me home" props only.
There is also another option which helps with planing ability and improved low speed control and that involves the installation of a pair of Smart Tabs. Many of our members here with 18-22' boats have installed them (me included) and are very happy with the additional benefits they provide. Cost is usually just under $150, installation takes about 30 - 45 mins and its DIY. You can do a site search for Smart Tabs or read about them/watch the video
here at the Nauticus website. If interested, your boat will use the Smart Tabs SX version with the 60# actuator arms.
prop for merc 4.3 Alpha 1
