bayliner ciera 25' prop tradeoff questions

[hammerhead_77]

Gurus of aquatic propulsion...

I have a 1987 Bayliner ciera 25' with penta 225D (350 GM V8) and 280 outdrive. the PO did some rock boating and the current prop has some bend in it. It is a 15x17 elephant ear.

The boat runs nicely...33mph @ 3800; 40mph @ 4500...which is as high as that engine should rev...load was two 225+ guys. I had a lot (LOT) of throttle left...

So, I was thinking that a larger prop might lug down the engine a bit and put that extra torque and HP to work, below 4500rpm. (i'll never run it above 4000) for more than a few seconds... BUT, this thing is already tough to handle at low speed...as in, it ain't got a low speed. 780rpm idle is still way too fast when I'm pulling into the dock... in-gear, out, in-gear, out....tough to steer that way. pushes about 2.8 knots at idle...

My concerns are primarily around fuel economy and getting the most out of that tiny little 56 gallon tank. need to get up on plane and stay there with the lowest throttle setting possible, while having plenty of power to handle the frequent 3-4ft wind chop that we get offshore.

Ideas? Stick with what I got or change to something that will bog the engine down a little?

 

[Georgesalmon]

So you want more pitch? And you are already unhappy with the boat speed at idle? Doesn't make sense to me. Your current setup seems about just right already.

 

[Bondo]

I had a lot (LOT) of throttle left...

Ayuh,.... Until ya try full throttle, you'll never know the rpms at Wot,....

The remainin' throttle could give ya no more rpms,....

 

[steelespike]

I agree you can't select a prop based on your incomplete wot run.lightly loaded Wot and gps speed.
Motor needs to rev freely within its wot rpm rating.
It's possible you need to increase pitch but too much will actually make the motor work harder.
And may turn the boat into a slug.
As a guess I would suggest a 17" 4 blade prop.this should help the boat to plane easier, stay
on plane easier at reduced throttle,resist ventilation, and respond better around the dock.
It will cost a little top end.
Without your actual performance numbers you might as well put some props in a barrel turn the lights off
and pick one.

 

[Maclin]

Might be worth refurbishing your current prop then do the WOT tests. It may be slipping a llittle or a lot now, and if in good repair might give your more bite and keep the RPM's in line and as a by-product give a speed increase.

 

[hammerhead_77]

Georgesalmon Well, George...you're not wrong. It may be that there is no better solution than what I have right now. I'd be happy to lose some top end speed in exchange for improved cruise economy...but being unable to go slow at idle is a pain and not something I want to make worse.

Bondo I don't disagree, but I don't want to rev a GM 350 v8 to 5000 RPM...too much chance of sucking a valve or having a lifter stick. Broke too many of them back in my high school days! Top rated RPM for that motor is listed as being 4400. I took it to 4500. I'd be real happy if it maxed out at 4350...since I don't intend to run it up to 4k very often.
steelespike normal load for me is 2 or 3 people (normally 2) and a full bait tank. I did have the full bait tank, but I will throw some sandbags in the front to simulate the 120# of ice I normally have in the big cooler, but that isn't going to radically shift things much... I'm interested in your 4-blade suggestion. responsive around the dock, good cruise and lower top speed sounds fine to me! Days that you can run 30+ in a deep-v out on the salt are few and far between. Besides, this thing is a complete barge when it comes to backing up! any better response there would be great!

Maclin I'm new enough to this that I just don't know how banged up a prop can be before it is TOO banged up. Ideal is pristine, but how much degredation do you get from some erosion and a slightly bent blade tip? I can feel the vibration from it, but it isn't bad... I'd rather get a new prop, so its more of a decision to get another 17x17 or something different.

If I were to look into steelespike's suggestion and look at a 17" 4-blade, would I stay with the same 17 pitch?

 

[Bondo]

Top rated RPM for that motor is listed as being 4400.

Ayuh,.... That's because ya got a 2bbl. carb that runs outa breath at that point,....

A SBC will safely run up to 5000 rpms, without a problem,.....

Unless there's already problems with the motor, 5500 in short bursts won't hurt the motor,....

 

[Maclin]

My opinion.....Feeling the vibration means it is severe enough to affect the prop shaft bearings and seals eventually. Prop blades have a very specific curve and cup on each blade, especially those Volvo props for that drive. No way to asses visually, but if you can see one of the blades is bent then I am sure the others were hit at the same time as the event that did the bending. That plus being able to feel the vibration mean it is time to get it fixed before lower unit sealing is affected, my advice.

 

[hammerhead_77]

Maclin You are not wrong, and I acted on your advice today. Bought a new $200 replacement 15x17. Prop when on without a hitch. Bad news is that I noticed a little leakage from the shifter seal and...YEP! you guessed it...water in the oil. The drive was overhauled, but then spent alot of time sitting unused. I think the seal dried out and when I took it out with the kids there was a ton of fwd-n-r-n-fwd-n-r-n-fwd in and out of gear action while waiting in the queue for the launch...

Gonna replace the shifter seal, flush the leg, oil it up, hang the new wheel and go see what it does.

 

[hammerhead_77]

Found an after market prop that was a fraction of the Volvo price. It is a 15.5x17...so 1/2" bigger but slightly less "elephant ear" on the blades. I think the brank is Black Diamond? Anyway, switching them out was a breeze... I replaced the shifter seal and the zincs while I was at it - although breaking one of the transom shield screws slowed me down a bit.

I'll get it on the water this weekend and see how it runs. Bondo I stand corrected. You are right that that SBC will run above 4500 without damage....I forgot that it is under load and it was the no-load revs that tend to float lifters and such.