WOT question

[mikemdd]

I have a Boston Whaler 160 Dauntless with a Mercury 115 HP Four Stroke. The WOT range is 5800 to 6400. With a Vengeance 15P, I was turning 6300 at WOT, but it was slow to plane. I changed to a Trophy Plus 4 blade 17P and performance improved dramatically. However, the rpm at WOT is now around 5700-5800...right at the bottom of the WOT specification. I have the following questions:

1) How exact is the rpm WOT range, and how much below the specification can you go without engine damage? I know Mercury will say ZERO tolerance, but what are your opinions or experience?

2) I am somewhat confused by the determination of WOT rpm. I assume the standard determination is done by reaching full throttle, then trimming out for maxiumum efficiency to get the number. However, this seems to have a flaw. If the WOT determination assumes no lugging of the engine, WHEN FULLY TRIMMED OUT, what is happening at anything below WOT when the engine is not trimmed? Is it always lugging at lower rpms? It would seem a better measurement would be to reach WOT without any trim, which would be a worse case. On my boat with the Trophy Plus 17P, without any trim, it reaches 5200 rpm, which is clearly lugging badly.

Anyway, I am concerned that I am too close to the WOT spec and do not want to hurt the engine if running at midrange rpms without trimming exactly right. Hope this makes sense. All opinions appreciated.

Mike

 

[steelespike]

Re: WOT question

Not an expert but increasing the pitch and blades made it a sure thing to lower wot rpm considerably.As I understand it they like to run these 4 strokes near the top of their rpm.I would think a 15" pitch 4 blade would put you there and keep your improved performance plus.
Your much better under propped than over.you can always throttle back a little if your uncomfortable with the rpm.

 

[mikemdd]

Re: WOT question

Thanks for the comments. Unfortunately, for my engine setup, there are few SS 4 blade options at pitches less than 17 or 18. Also, the characteristics of the Trophy Plus are perfect for what I am looking for. If the consensus is that 5700-5800 rpm is too much of a risk for my engine, I guess I could take the prop to a shop and have the pitch reduced to 16 or even 15 to gain some rpm and still maintain the excellent characteristics of the Trophy Plus prop. I could also raise the engine to gain a little bit of rpm, but the boat already has a tendency to porpoise and I don't want to make it worse.
Mike

 

[Woodnaut]

Re: WOT question

Mike - I have a 90 HP Yamaha 4 stroke, which is extremely close to a Yamaha 115 HP 4 stroke, which MIGHT be very similar to your engine. (I heard that Yamaha made Merc powerheads for a while, but I don't know exactly which models.) I put on a prop that had too much pitch and that, in turn, lowered my WOT RPM from 5,800 to about 5,200. The biggest problem that resulted was that my engine started "making oil", which means increased oil level in the crankcase due to blowby of unburnt fuel past the piston rings. I decarboned the engine, changed the oil and went back to the old prop. Life is good again. Bottom line is overpropping (too much pitch) on a 4 stroke is really not a good thing. Hope this helps - Woodnaut

 

[mikemdd]

Re: WOT question

Good information. My concerns are valid, I'm afraid. As reference, what is the specification for your engine, relative to WOT range? Is 5200 below the recommended range, and if so, by how much? Just trying to determine where problems start to develop.

Thanks again.

Mike

 

[Woodnaut]

Re: WOT question

According to my owners manual (Yamaha F90), it appears that the WOT range is 5,000 - 6,000. (The Yamaha web sites gives this info for both the F90 and F115.) But as I mentioned, I got into a "making oil" problem when I setup a prop that had WOT at 5,200. My setup now has WOT at 5,800 RPM and the problem went away.

A WOT range of 5,800 - 6,400 seems a little high to me, but I honestly am not familiar with that particular engine. If that's what the owner's manual says, then I would trust the engineers at the factory.

If you search through the Yamaha & Suzuki engine forum you can find more info on propping to the high end of the range and the "making oil" scenario. (Rodbolt is the expert - he's covered this topic a few times.) Also, you might try a post on the Mercury engine forum. The general consensus seems to be to prop so that you come close to the higher end of the WOT range, and if possible, spend 20 - 30 minutes at or near WOT on the way back in.

 

[Irv964]

Re: WOT question

Assuming your motor is a 2001 or newer model, the max rpm rating is 5000-6000rpm as per the Merc prop selector link above.......Irv

 

[mikemdd]

Re: WOT question

My engine is a Mercury 2007 Four Stroke, rated at 5800-6400 according to the owners manual.