Overpropped???

[Mangas]

I am new to boating and have a few basic questions. I have recently purchased a 18' Baja with a 200hp Black max outboard. I have tried a 21" pitch aluminum prop that worked well for waterskiing, it feels very torquey and the boat planes very quickly. Incidently, top speed is close to 60mph with this prop. I also tried a 25" SS prop it did not have nearly as much torque and top speed was over 70mph. <br />My question is: With the 25" SS prop, it struggled to plane, and almost felt like it was going to stall although it didn't. Now I proceeded to built up speed slowly when it did this, as I gave it more gas it almost felt as if I was standing still. After about 30 sec the boat planed however.<br /><br />Am I overpropped with the 25" SS prop for this type of boat..and if so what type/size prop would be best for all around performance<br /><br />Thank you in advance

 

[DJ]

Re: Overpropped???

Mangas,<br /><br />More important than feel is the top rpm acheived with each prop.<br /><br />As an example, if the 21" gives you 5800 rpm at WOT-a 21" is probably best. Conversely, if the 25" only gives you 5000 rpm at WOT then you are lugging the engine.<br /><br />Outboards cannot stand to be lugged.<br /><br />Let us know what your top rpm readings were with each.

 

[Spidybot]

Re: Overpropped???

FYI: The standard range of props that was offered for this motor was 11" - 25".<br /><br />Sounds about right that 25" on your boat is slightly overpropped and will not run WOT within the stated range (5300-5800).

 

[Mangas]

Re: Overpropped???

With the 21" prop my rpm at WOT was 5500 and with the 25" it was 5000rpm. However, in both circumstances I believe I could have raised the trim a little to gain more rpm/speed. I have been told that the redline on the motor is 5800, should I aim for a prop that is close to that at WOT. If so, for how long should I run it at that rpm before possible damage.As well, when checking for rpm at WOT how far up should the motor/trim be adjusted to get max rpm/speed? Is it that I just play with the trim adjusting upwards until I don't get any increase in rpm/speed? <br />A second question I have is that when I tried planing with the 25" SS prop it really felt like it was struggling in the beginning. Is this what you call lugging?<br />Thanks for the timely response

 

[Spidybot]

Re: Overpropped???

Propellers always are a compromise. You are just now able to decide, what characteristics of a prop, you'd like.<br /><br />If you want high pitch (like high gear on a car), you get less accelleration but better top end and cruising mpg. Low pitch works opposite.<br /><br />Trimming won't solve the struggle getting on plane but sure will affect WOT.<br /><br />The 25 even has a higher speed when idling which may be considered regarding manouvering marinas, ramps etc.<br /><br />Within the WOT range mentioned, you are free to choose. If you like the 21 it's OK as it WOT's at 5500. When carrying more load it might drop a little yet stay within the range. A 23 would WOT at 5250 and be OK also, performing between the two others (compromise changes). The specified upper rpm limit is where the manufacturer tested the motor to last for extended running periods. At the speed you acchieve it is not likely to be hours in your case and the fuel consumption is huge. Your setup will allow cruising at reasonable rpm, good mpg and relatively low noise while maintaining good speed.<br /><br />Otherwise I remember having seen recommended: 75% of max rpm is safe as long as you want.

 

[Mangas]

Re: Overpropped???

I appreciate your response. I have one more question if you don't mind. I have seen this question before and I am confused with the response. I am currently running 87 octane in this motor. Is there any advantage or disadvantage to running a 89 or 92 or even a 94 octane unleaded fuel?Will a higher octane fuel hurt the engine at all or will it be better for my outboard( 1986 200 hp Black max)in terms of performance?

 

[Spidybot]

Re: Overpropped???

Questions always welcome.<br /><br />Octane is an expression that tells how good it withstands detonating ("knocking"). In cars the demand for this relates to compression and a number of other factors.<br /><br />Higher octane means better ability to withstand - acchieved by more additives. <br /><br />There is no more power in it. Regarding performance the best advice is to keep your gear clean and well adjusted (boat, motor), to avoid carrying unneeded load and to use good quality lubes (which I believe you do already).<br /><br />So when your manufacturer states a certain octane that is the lowest acceptable value. Nothing gained from using higher than that - you might hear someone mention more deposits in the engine due to the extra additives but I can't say I've ever seen it.<br /><br />Where I live we can't get lower octane than 92 unleaded. So my Merc has been run on this since new (1991) and never missed a beat or fouled a plug. On certain occasions (needed fuel where there was no choise) I've had to use 98 and there wasn't any difference - apart from the price.

 

[ivar]

Re: Overpropped???

Re: the prop's, <br />there can be a big difference between 2 props with the same pitch. Straight pitch vs progressive pitch , cupped vs uncupped etc.<br /> You just have to try different props for yourself to see how they work on your boat/engine combination. <br />If you want one all-round prop for skiing/cruising, maybe you should try a 21" ss , and a 23" ss for cruising/top end. <br /><br />Have you played with your mounting height?

 

[DJ]

Re: Overpropped???

Mangas,<br /><br />You do NOT need more than 87 octane-save your money.<br /><br />Props.<br /><br />If you go with a 21" Stainless, your RPM may drop a bit. Stainless props tend to bite better and hold better.<br /><br />When you are trimming your engine for max. speed, watch the speedometer, not the tach. Your highest RPM may NOT be your highest speed. Every rig has a spot that the prop. will bite best and provide the best overall top speed. As said before, that may not be top RPM.<br /><br />A 21" sounds just about right for you, even if you lose a few hundred RPM's you are still in your range at 5300 to 5800.