proper rpm at wot

[Zaxxon II]

I finally got my tach installed and took her for a test. 2000 Johnson 150 on a 17ft stratos. I'm running a 23 pitch aluminum prop. At WOT with two people and full load I reached only 5300 rpm trimmed out. I have a 17 pitch backup prop and was wondering if I should run that instead of the 23. I ran this prop (17) before installing the new tach and it was extremely fast out of the hole

 

[jim dozier]

Re: proper rpm at wot

You won't know for sure until you try but I'm guessing the 17" is too small and you'll overrev. Clearly the 23" is too much. I'd look for something around 19" to 21" pitch. I have a 17-foot Pacer with a 115hp and I have a 18" pitch Turbo Lightning Jr prop and it will do 5900-6000 rpm fully trimmed out. Mine came with a 15" pitch and had an impressive hole shot but would go past 6400rpm and I couldn't get max speed out of it. Sort of similar to your 17" I would guess. Get some props and try em out.

 

[Dhadley]

Re: proper rpm at wot

I would agree with Jimd and think youll end up with a 21. I a little surprised that you can only get 5300 on that hull. I'd think the 23 might be a bit steep but not that bad. <br /><br />However, if the motor is mounted all the way down I'd try raising it some. Most of theose boats have a jackplate so it shouldnt be too difficult. We may not be able to raise it much with an aluminum prop but it may give us a better idea of where to go with a different prop.<br /><br />BTW -- I bet the holeshot is indeed awesome with the 17!

 

[Zaxxon II]

Re: proper rpm at wot

I too was surprised at the low rpm since the factory advised they set it up with a 24 pitch. I don't have a jack plate, but the cav plate is already about 4 inches above the bottom of the keel.

 

[Dhadley]

Re: proper rpm at wot

That hull has a pad and a step, right?

 

[Zaxxon II]

Re: proper rpm at wot

Sorry Dhadley, I don't know what you mean by a pad and a step. It does have two indentations in the transome above the keel. The cav plate is about level with the second one and 4" above the very bottom.

 

[DJ]

Re: proper rpm at wot

A "pad" is usually a triangular shaped flat area at the rear of the hull just before, and continuing through, the end of the hull.<br /><br />Your boat may have a stepped up transom (rear end). The hull may actually end ahead of this step up.

 

[Zaxxon II]

Re: proper rpm at wot

That sounds like what I have. What does this mean in relation to the height of the motor? Where should the cav plate be for maximum efficiency? I could still go up two holes but that would put the cave plate about 6" above the very bottom.

 

[DJ]

Re: proper rpm at wot

With a pad, "normal" height restrictions do not apply. It's trial and error from here.<br /><br />You need to pick the prop that gives you:<br /><br />1. Max RPM at load.<br /><br />2. Allows good, to full, trim capability.<br /><br />3. Does not promote "chine walk'.<br /><br />My suggestion would be to try the 23" at one hole UP and see what happens.<br /><br />BTW, Dhadley is the expert in this area.

 

[Dhadley]

Re: proper rpm at wot

Optimum height (X dimension) has a lot to do with prop design. Some run higher than others. Some, a lot higher.<br /><br />We shouldnt expect too much from this aluminum prop but I'd try it a hole up to see how it reacts. <br /><br />It just sounds like its not stepping up on the pad. When it does it should take off like a rocket.

 

[Zaxxon II]

Re: proper rpm at wot

Thanks guys. DH - it already runs like a rocket, but I think I'll try the 17 prop now that I have a tach and if it's too high on rpm's I'll go back to the 23 and try raising the engine. by the way the 17 came with the boat when I bought it. What should the highest safe rpm I should reach, bearing in mind that I will run WOT only to test this thing.

 

[DJ]

Re: proper rpm at wot

I think you'll find that the 17" is way too small.

 

[Dhadley]

Re: proper rpm at wot

Ditto DJ. Double ditto.<br /><br />I run a 60* 150 (same motor as you ZII) on a 1976 20' Dusky. It weighs at least twice what your Stratos weighs and has no pad or step. And a lot more hook. I turn a 19" 4 blade Turbo 5800+.<br /><br />Theres no way a 17" prop is right for a 17' Stratos. No possible way.