Prop for upped HP on rebuilt Xflow

[interalian]

Hi - I just finished building a 140HP powerhead as a replacement for the 90 I had on my old boat. The rebuild was done using a '79 140 block/heads/bubble exhaust/carbs, with rotating parts, new 0.030" os pistons/rings, and electricals from the original '82 90 motor. From the pan down, it's an '82 90 which is a complete match to the '79 140, so I expect what I have now is essentially an '82 140, but without the newer carbs that have midrange jets.

The boat has always used a standard aluminum 13-1/4x17 3-blade prop and that's what I'm turning now. When the rig was new in the '80s, it would hit 5200rpm max with one person. I'm still breaking it in and haven't run WOT for more than a few seconds but I'm going over 5500rpm now.

Boat is a 900 pound aluminum Springbok 16' closed-deck sport boat with padded seats, windshield etc. Weight with helmsman and fuel is just over 1500 pounds.

My holeshot can have minor blowout, but not excessive. Here's a clip with some launches and fast running, not WOT. https://www.youtube.com/watch?v=Kt4AFg92Cc0

Prop suggestion that won't break the bank?

 

[SkiDad]

You didn't state your goal ? If you are looking to maintain your hole shot and lower rpm some look at a 17p turning point hustler. Or maybe a 19p Turing point if you are looking to go faster still.

 

[interalian]

My main concern is preventing over-rev at WOT. With such a light rig, holeshot might not suffer much with a 19" pitch. Based on the video, would you say it pops quick? I can do a GPS speed of about 45 at 5500 now - pretty quick for the tin hull.

 

[Sea Rider]

What's the max wot rpm range for that OB.
With current prop, assume a 17 what's your max wot rpm range you alone ?
On that vid, were you alone ? deck weight seems not well distributed, combo balances up-down a bit.
OB has a manual or electric trim ? If manual assume you were trimmed to achieve OB sitting at 90?, right ?

Happy Boating

 

[ondarvr]

My main concern is preventing over-rev at WOT. With such a light rig, holeshot might not suffer much with a 19" pitch. Based on the video, would you say it pops quick? I can do a GPS speed of about 45 at 5500 now - pretty quick for the tin hull.

Sounds like it's about right.

 

[interalian]

Sounds like it's about right.

It would if it was at WOT, but if I left the hammer down I'd be well over 5500 - just don't know how much right now.

 

[interalian]

What's the max wot rpm range for that OB.
With current prop, assume a 17 what's your max wot rpm range you alone ?
On that vid, were you alone ? deck weight seems not well distributed, combo balances up-down a bit.
OB has a manual or electric trim ? If manual assume you were trimmed to achieve OB sitting at 90?, right ?

Happy Boating

Max RPM is supposed to be 5500, but since re-powering, I'm sure it'll go well beyond 5500.
Yes, I was solo on the run, but not WOT. Being a light boat, it rides high and soft.
Power trim - I tend to run it up until it bounces a bit, then back down. I'll be going to play with it again this week.

 

[jimmbo]

The motor will hold together at speeds above 5500 rpm, but it will be producing less HP. The sweet spot for your powerhead is between 5300 -5400 rpm. That usually gives the best top speed when propped for that RPM range. Propping for higher rpm will improve holeshot but with a slight loss of top speed, propping for lower rpm results in poorer holeshot and lower top speed. The further from the sweet spot, in either direction, the lower the top speed

 

[ondarvr]

It would if it was at WOT, but if I left the hammer down I'd be well over 5500 - just don't know how much right now.

You left that part out.

Let it spin, see what it will do, you can go over 6,000 without a problem, you just need to get an idea so you get close on the correct prop. You only need to do it for a of couple seconds.

 

[interalian]

Thanks, I'll see what it does then - let 'er spin. My other thought at this time is that I have only about 10 hours on the rebuild, so it may not even be making full power yet. And I retarded the timing a bit when assembling, and it shows about 25-26 degrees at around 4500 - sticker says 28. I'm running 91 octane premium.

 

[Sea Rider]

If 5500 rpm is the factory stated max wot rpm, need to go full hammer down as usually loaded on flat calm no wind water cond and check max rpm achieved, not an issue if over revs, it's just few second at hammer down. Then will know if prop is spot on for current load or in need to go for a prop maximization to achieve max wot rpm factory stated.

OB must run with advance timing and carb sync as factory stated before going for a wot rpm test or latter for a prop max. That's if wanting to get the full horse herd out..

Happy Boating