My Boat Must have the wrong prop

[vdubr928]

Hi all, I have a 92 Celebrity Cabin Cruiser with a Yamaha I/O. (454 likely 300hp) I got it really cheap, found several sources for parts, and have the boat running well and the drive working well. (note: the boat runs at about 5000' above sea level)

The problem is that I never see over 3800rpm. I should be getting about 4600. I took the prop off today and it is a 16x16. I am far from an expert but that seems huge for 300hp. Either way, it's a cruiser so I would like to see 1000 more rpm in the end. I wont be doing WOT runs and would like the extra punch. What do I do? Buy a 16x11? a 15x?

Thank you for any advice.

By the way, 3800 is just with me on the boat. I am lucky to see 3500 with people on board.

 

[jimmbo]

Well before you start changing props you find out what shape the engine is in. Compression test is a must. I will assume it has a carb, Holley or Q-jet? when you give it full throttle is the carburetor opening fully on the primary side? Is it clean inside? Is the timing set correctly, and is the centrifugal advance working correctly? What is the gear ratio of your unit?

 

[vdubr928]

I'll run compression. Haven't done it yet but the motor seems to run ok. Obviously at altitude I'm going to lose some ponies. I don't know the ratio either but I will do some research. I may have taken some pictures when I had the drive off that have the model# on them.

It's a Q-jet. I had it rebuilt. Both the primary and secondary are fully open at WOT. The idle circuit is still a little screwy (lean no matter what but not by much) but that shouldn't affect WOT. The plugs look good. (also new)

It's an old delco EST distributor. I replaced the module, which was kaput, with a new one and set the base timing correctly. I double checked full advance (around 2800 rpm with the EST setup) and it is dead on 30* right where it should be. A 2* advance gave me 200 RPM but I trimmed that back since it really isn't worth it.

So, I was thinking the motor is probably a little tired. It's got 660 hours on it. But is it common to have to re-prop for altitude or is there a reason not to do it? Even if it's tired, isn't it going to help me get out of the water with a boat full of people? I read that sometimes cruiser guys underprop just to haul the extra weight? I'm considering pulling the motor this coming winter.

 

[jimmbo]

The idle circuit is easy to richen, just a tiny drill bit. Good, the secondary throttle valves open, how much the air valve above opens we don't know. Hopefully not fully as a 454 @ 5000rpm will need about 525 cfm, and most Q-jets, the small ones, are rated 750 I always thought the big block liked about 38 - 42 BTDC max advance. Btw what is the initial timing set at? About repropping for altitude. I don't know what Yamaha does, but MerCruiser and Volvo offered drives with higher numerical gear ratios for high altitude use. Some swear that's better than a lower pitch prop. There might be some truth to that as higher pitch props do tend to be slightly more efficient than lower pitched ones. I just thought it was so dealers didn't need to carry an inventory of props with limited sales. Repropping with a lower pitch will restore rpm, but power will still be lower because of the less dense air. If the engine is indeed up to snuff, an 800 rpm jump is going to need a 4 in drop in pitch, so a 12 inch is in order. Make sure your tach is accurate

 

[Scott Danforth]

you loose 3% per 1000 feet of altitude.

is your tach accurate? you need to check with a good diagnostic tach. dash tachs are really inacurate.

 

[steelespike]

When dealing with elevation we want less fuel not more.
Rpm is important but without wot speed as well it doesn't mean much.

 

[Sea Rider]

Check with Yamaha if there's a high elevation carb jets for 5K elevation. Lower jets and less prop pitch usually impoves combo performance. With a 100% fully operational boat, power loss al altitude could be as much as 15% every 1 K/Meter or nearly 5% every 1K/Ft.

Happy Boating

 

[vdubr928]

All of your responses are most appreciated.

I will pull out my dwell meter and check the RPM gauge. Makes sense that I should have done that. I have heard about the drill bit trick on the quadrajunks and I will do some more research. As far as the jets go, my plugs look good so I suspect it was re-jetted for altitude. With your numbers I am 15-20% short on power at my altitude which is about what I figured. That alone would warrant a 13 or 12" prop. I am thinking about trying a 13 first. As far as timing, AC delco books say that base timing should be set to 10 before. With 20* of advance built into the module that leave me with 30* at full advance. I have heard of some setting to 12* at altitude. (less dense air so less likely to detonate) I may set mine back to 12 which will give me 32*. If I want more I would have to go to a mechanical dizzy.

Steelspike, I'm with you on the mixture but there is something wrong with my carb preventing me from getting as much fuel at idle as I should. This is a known issue on the q-junk.

Jimmbo, Parts are niot hard to find but are a little pricey. I will have to fix this with prop despite the added in-efficiency of a lower pitch prop. You are right that it would be better to change the gearing. At least that is my understanding as well. I'm currently seeing a top speed of 33MPH at 3800rpm so I think anything I do should improve that.

Again, thank you for all the responses. I will update after I make some changes and check the tach.