1989 31' twin inline 4 prop suggestion

[Oshawapilot]

Hey everyone - I'm helping out a relative who owns a 1989 Sunrunner 302.

The boat is equipped with the twin in line four cylinder MerCruiser 470's and Mercruiser outdrives. Yes, there was plenty of discussion and thought about these engines before purchase based on their checkered history however like I said the boat was purchased two years ago and they are still both running well with proper maintenance. I had a thread here at the time of prospective purchase and lots of the usual "run away!" posts...but here we are 2 years later and they're both still running perfectly.

So, moving on... It's primarily used as a slow speed Lake cruiser spending probably 80% of its time barely above Idle cruising, and 20% of the time up on the plane but never above 3500 RPM.

My question is with regards to props – neither were in particularly good condition when the boat was purchased however suffice to say there was an incident a few weeks ago which now necessitates changing both of them.

We are unsure if what's on it is even the original designated props, or what would be best. It's hard to tell as the part numbers were damaged or hard to read but I believe the current props are 16" and 16 pitch. Does that sound right given a 31' (roughly 8-9000 pound) boat with 190-200 hp (but torquey as the 470's were known to be) engines?

Would there be better options for replacement? Again, this boat spends most of his time at low speed however some get up and go is appreciated at times so we do not want to prop it for strictly low-speed. With what's on it currently performance was decent and fuel economy was stellar so they wouldn't want to sacrifice the economy side of things.

Thanks!

Regards;
Mark

 

[Oshawapilot]

Anyone?

 

[steelespike]

What is the wot rating of the 470s? what is the actual wot rpm and gps speed.
​Do you know the gear ratio?
While you don't care much about speed knowing the wot rpm and gps speed can tell a lot about your setup.
That said the 16" props seem to work for you.
If the props are old and obviously worn new props may act completely differently.

 

[Oshawapilot]

What is the wot rating of the 470s? what is the actual wot rpm and gps speed.

Being old engines (and if you're familiar with the 470 you'd understand the reasoning even more) we've never taken them to WOT...always worried about something going boom, if you see where I'm coming from..

I understand redline is around 4400 RPM on these and I've had them up to about 3500 and they don't bog or hesitate whatsoever getting there...sure seems like if we were brave enough to go WOT it would quite readily fly through 3500 and well beyond. The boat pops up on plane (or at least it did before the prop damage) quite readily, but at around 3500 (again, before the prop damage) the boat was running at about 30KPH, or about 19 MPH.... The combination of readily popping out of the hole and a less than stellar speed at a reasonably high RPM (I can't imagine another 900 RPM will yield THAT much more speed?) is what's making me think it might be under propped.

I don't know what the gear ratio is. That's the challenge with this boat, being a 1989 and from a manufacturer that went out of business in the early 90's documentation is sparse...so that's why I'm wondering if the 16/16 prop combo is even what the boat called for from the factory or if the previous owner might have down-propped it. From what I could see online it seems like we should be getting about 50KPH (~30MPH) from this boat at around 4000 RPM with it's factory configuration - is that unreasonable for a boat that size/weight with roughly 380HP?

I'm contemplating suggesting going to a 17 or 18 pitch prop to sacrifice some of the "pop" for a slower build to a higher fast cruise speed - will the experts chime in on exactly how drastic (or not?) a change that is, and most importantly, will it affect low speed fuel economy? I've read online about people running up to 22-23 pitch although that's on speed/ski boats with the same engine...obviously this is a big tank comparatively.

 

[ttankmoran]

Your logic is flawed, you dont want to over-stress the engines by not going WOT, yet you want to put higher pitch props on to lower your cruising RPM's? You will be taxing your engines in the worst way possible, by lugging them! I see this far too often, people scared of a few RPM's because they think they should be lower like your cruising in a car...Not the case with a boat.

You need to prop your boat so that at WOT with a typical to light load you will turn as close to that 4400 RPM as possible. Then let the other RPM's fall where they may. You can typically cruise at slightly lower rpm and maintain slow plane better with 4 blade props but that doesn't change the fact you need size them appropriately to reach close to the peak allowable RPM. Doing this will introduce the LEAST amount a wear possible on your engines.

Seriously open that thing up.

 

[Oshawapilot]

You will be taxing your engines in the worst way possible, by lugging them!

I'm aware of that and the downsides of over propping and lugging, but like I said, I think it may be underpropped now based on it's stellar low end performance but lacklustre high end performance. Noted though, thanks.

If a WOT is the only way to determine things, it's not an option at this point as the props are shot and running them to WOT is going to be neither productive nor safe at this point. I guess in hindsight (knowing what I know now) a WOT run would have yielded some useful data, but it's too late now.

To be safe I guess sticking with the 16/16 is the best route then as the boat has been setup that way for several years and performance was satisfactory for the purposes the boat is being used for..but like I said, there is a temptation to opt for 16"/17 pitch just to pickup a little more speed on the high side of things, but maybe just picking up to WOT will yield that speed...but those little 4 bangers just seem like they're already screaming back there at 3500 RPM, 4400 just seems like tempting fate somehow. Yes, probably unfounded, I know 4 cylinders are safe to wind out and there's lots of people talking about running them up to 5000 without any issues.

Unless anyone suggests otherwise I think I'll recommend just sticking with the 16/16 combo now fitted to the drives and go from there. I suspect with 2 nice new crisp props and perhaps a willingness to push those throttles a little further forward we'll find that speed that seemed to be missing.

 

[ttankmoran]

After a very quick search the 470's typically got a 1.81 gear ratio. Using 16P props, 4400 RPM, and a probable 15+ slip figure puts you right in the low 30 MPH range. Sounds like that is expected out of a rig like this...

 

[Oshawapilot]

After a very quick search the 470's typically got a 1.81 gear ratio. Using 16P props, 4400 RPM, and a probable 15+ slip figure puts you right in the low 30 MPH range. Sounds like that is expected out of a rig like this...

Thank you...so the consensus is to stick with the 16"/16P prop combo then by the sounds of it.

 

[Oshawapilot]

For those interested, some follow up.

I ended up finding an irresistible deal on a set of Michigan Wheel 15x17p. After discussing with the marinas very knowledgable tech (who also shared tales of woe about the 470's, so It was clear he knew his stuff lol) we agreed that going up 1 pitch would makeup for the 1" in lost diameter.

Besides, with each prop being about $100 cheaper than many other options we were able to find locally, it was a worthwhile gamble.

They went on smoothly - thankfully no issues removing the old props, I was worried they'd have been seized on their shafts, but no such issues.

We did some single engine low rpm tests to ensure everything was ship shape with the reinstall (and since these props used the separate hub system I wanted to make sure all we well) and then went for 3000 rpm. The boat popped up on step faster than ever and we were already doing about 30kph (~19 MPH) on the GPS. Stepping up to 4000 RPM we cleared 40KPH (~25 MPH) and with a little bit of trim on the drives we hit 44 KPH (27.3MPH) while still maintaining only 4000 RPM. There engines were happy there and I actually felt them unload quite a bit as the hull got more and more up on plane than I think it had ever before - I had to throttle back a little actually to maintain 4000 for the purpose of trim tests.

Being very satisfied with that and there not being a lot of appetite for "pushing" the old 470's any further, we called it a great success. This turned out to be a good combination.

Thanks for everyone's advice.