Help with prop recommendation

[seanmclean]

1) Performance issue you are trying to correct: Low WOT RPM, Low Max Speed, Poor Fuel Economy
2) Current prop manufacturer, model, aluminum or stainless as a minimum. Quicksilver Nemesis, Aluminum
3) Current prop diameter and pitch (required).14x22, 4-blade
4) Wide open throttle RPM and speed with an average load (very helpful) 4,000RPM, 35-7ish
5) Engine/drive make, model, year, and HP 2003 Mercruiser 4.3 Carb, 190HP, Good Compression
6) Boat make model, year, length and weight 2003 Searay 182 (18'6"), dry weight 2600lbs

Boat performs pretty well, gets up on plane quickly, but cannot get max RPM into this motor's range (44-4800), and can't get anywhere near the speed it should go (45-47mph). Consequently, I'm burning through fuel trying to keep this thing going a reasonable speed on plane, getting about 2.5-3mpg, when it should be doing closer to 5. I'm not super concerned about holeshot, we pull a tube occasionally, but not a primary focus for us.

 

[Sea Rider]

Welcome to Iboats,

Is it a new combo or bought it second hand ? Check if having water lodgged issues on hull, check if prop performs top doing a hub slip test to start with. If both are found ok and OB is 100% healthy, OB has an incorrect pitch prop installed.

At just 4K, OB is at the worse lugging side, is 600 wot revs off from middle range and 800 from max wot range. Will need to drop several pitches to pull wot revs way up if like running middle or max wot rpm range as usually loaded,

Happy Boating

 

[Maclin]

Welcome aboard!

It appears, to your engine anyway, that it is too high of a pitch with that prop. Going down in pitch will incrementally help holeshot and increase WOT RPM's. A 4 blade is for holeshot and low speed planing performance at expense of some RPM's and top speed, but your top RPM is about 400 rpm too low right now. Also the slip% seems a little high at 20%. Is the prop in good condition? How do you use the trim function?

If you are trimming the drive properly (down at takeoff then bumping up on cruise to the sweet spot just before porpoising) then I recommend a prop change. With your top RPM that low I think you would need to go down in pitch at least one step. So same brand of aluminum 4blade, really need to try a 20p or even an 18p. An 18p 4blade is more like a 19p in 3 blade as far as load on the engine. So your 22p is more like a 23p 3blade in my estimation, and that seems to be too high of a pitch for your combination.

Since you mention holeshot is less of a concern I think a 19p 3blade would be good all around performance.

Good luck.

 

[seanmclean]

Thanks both, boat is secondhand to me but had the outdrive serviced last season (with docs to support). Prop appears to be in good condition, some paint worn off but no dents or knicks. I trim the boat starting down, but once on plane I trim up occasionally until the bow starts to bounce, then trim back down a smidge.

 

[Maclin]

I recommend a 3 blade 19 pitch, or 4 blade 18 pitch in the same brand/series as you have now to keep the guess work down, and stick with aluminum for now.

 

[Sea Rider]

As don't have a Ownwer's Manual for I/O type boats compred to lots of OB ones as to check what's recommended when at hole shot, try testing the inverse. Trim lower leg to slight + trim, go full hammer down, bow will rise accordingly to trim, once on plane reduce trim to a neutral trim for combo to ride parallel to water level. Will achieve a well balanced combo gliding nicely on water. Perform that test on flat calm non windy, choppy, wavy water cond.

Report you findings if hole shot was much better that way as what you've ben doing previously. I'm not a porpoising lover myself, some boaters like running combo that way, In that case will need to reduce throttle some or combo will achieve a excessive & constant hull banging hard on water producing a uncomfy water ride to passengers, much worse on windy, choppy, wavy water cond.

Happy Boating

 

[Maclin]

Sea Rider The OP does not leave it in the porpoising stage. A pilot does that once or twice looking at the trim gauge if they have one, and notes the position that the bounce starts. They then only need to refer to the gauge to know where the sweet spot is. If no gauge then they bump up until they get the bounce, or experience prop ventilation, then back down as they mentioned in post#4. From that post I inferred that they are well aware of how trim affects their boat's behavior and how to use it to advantage in all conditions. Another way the pilot can tell is by feel and where the spray after planing comes out the sides, then moves farther aft as trim is raised. Most recreational bowrider hulls such as that model of Sea Ray behave this way.

 

[Sea Rider]

Sorry, must have been a senior moment LOL!! Know guys that likes boating like this, they don't understand proper deck weigh distribution nor trim settings as well. I like asking a (she) boating partner that sits up front to pass aft boat while on plane and sit next to me on a console seat installed way aft as it's a tiller driven OB.

What we really enjoy is when 1/3 hull is spraying sideways while the rest glides neatly on air at straight water courses on flat calm water cond, the only down issue is that need to throttle more in order for combo not to come off plane, and that consumes more fuel that sitting each one on their respective aft/bow seats.

Happy Boating

 

[seanmclean]

I recommend a 3 blade 19 pitch, or 4 blade 18 pitch in the same brand/series as you have now to keep the guess work down, and stick with aluminum for now.

Thanks for the recommendation, took it out again this weekend and confirmed no slippage. Best we can do with a fairly light load is getting up to about 4100 RPM. Going to order one of these props and see how we fare.

Which one would you advise?

 

[Maclin]

An aluminum 3 blade in 19 pitch.