Prop Confusion

[BigBoatinOkie]

Howdy. I have an '84 wellcraft 18ft. bowrider with a Merc 228 chevy 305 and alpha 1 outdrive. When I got the boat it had a SS 14.5x22 3-blade prop an it would run about 50mph but only turn about 3400 rpm. So I happened to have a SS 14.5x20 3-blade and it helped on the rpm and acceleration a bit. It would run 50mph at 4000 rpm and was still a little sluggish out of the hole. Pretty much all I do with the boat is pull BIG 250lb+ skiers. So I wanted some more pulling power. I put on an alum. 14.25x18 4-blade and it helped pretty good with holeshot, but now it only turns like 3600-3800 rpm at wot. Shouldn't I be able to turn more rpm now? Or at least the same as the 20" prop? I did lose top end speed down to about 42-44 and lost rpm. Did I miss something? Sorry my numbers aren't exact but I haven't had it in the water in a couple months now. I'm a bit bumfuzzled.

 

[Bondo]

Re: Prop Confusion

Ayuh,... Dropping the Pitch should Raise the rpms....

Hard to say Why it didn't happen that way... What else has Changed,..??

 

[CharlieB]

Re: Prop Confusion

The 18 pitch should have made a positive difference

EXCEPT

the added blade area of the 4th blade seriously increased the load on the motor.


Try another THREE blade 18 or 17 pitch prop.

 

[steelespike]

Re: Prop Confusion

Were these props all the same brand?,model? SS?
Any prop change is an educated crap shoot.Many times a 4bld. will cause a little loss of top end your seems a little extreme.Sometimes you just need to run the 4 blade for water activities and go back to trhe other for running around.

 

[Tail_Gunner]

Re: Prop Confusion

Howdy. I have an '84 wellcraft 18ft. bowrider with a Merc 228 chevy 305 and alpha 1 outdrive. When I got the boat it had a SS 14.5x22 3-blade prop an it would run about 50mph but only turn about 3400 rpm. So I happened to have a SS 14.5x20 3-blade and it helped on the rpm and acceleration a bit. It would run 50mph at 4000 rpm and was still a little sluggish out of the hole. Pretty much all I do with the boat is pull BIG 250lb+ skiers. So I wanted some more pulling power. I put on an alum. 14.25x18 4-blade and it helped pretty good with holeshot, but now it only turns like 3600-3800 rpm at wot. Shouldn't I be able to turn more rpm now? Or at least the same as the 20" prop? I did lose top end speed down to about 42-44 and lost rpm. Did I miss something? Sorry my numbers aren't exact but I haven't had it in the water in a couple months now. I'm a bit bumfuzzled.

BumFuzzled... me too.....

You need to do several things here..1. check the accuracy of your tach. 2. Use Gps for your speed ..

50mph @ 3400 rpm is just to far out of the norm...If you get those readings tightned up things will be a lot eaiser..

 

[BigBoatinOkie]

Re: Prop Confusion

I'm not 100% positive that the tach is correct, but when i went from the 22 to the 20 my rpms changed by about 400. That's exactly what I should expect right? 200 rpm per 1 inch of pitch? So I was expecting to run about 4200 with the 18.

The speed was not measured by gps, but i did compare my speedo to my dad's by running side by side at a couple different speeds and they where spot on with each other. So it's probably not too far off if any.

The 22 and 20 where stainless steel from precision props. The 18 4-blade is aluminum from Solas. I assumed there would be a bit more drag from the extra blade, but figured it would be made up for by increased rpm due to the pitch change, if that makes sense. I should really have started this convo when I can get in the water i guess. I thought I'd make sure that others agree that somethin ain't right though.

To be honest, I'm not too concerned with the top speed because I primarily use it for towing. My main goal was to get the rpm in the 4200-4600 range as recommended.

 

[walleyehed]

Re: Prop Confusion

I test props on an exact match to this rig....I run 49mph with an Apollo 19P at about 5200rpm...
Your tach is quite a bit off, not uncommon though.
The 18P 4-bld should be way above top rpm, unless you've got 6-8 people in the boat, fuel, coolers, several anchors, etc...
The best 2 props I've found on this rig I test with is a 19P 3-bld Large diameter Apollo and a 20P Bay Pro II 4-bld. One would think the numbers would be the other way for the number of blades, but that's the difference in blade design.
The 19 Apollo is actually a better load prop than than the 20P 4-bld and I believe it is a result of hull design in this case.
I need about 3500rpm to plane out smoothly in a timely matter, so I know the 3400 you speak of is about 1000 off, maybe a few hundred more.

How far are you (lindsay) from Grandlake???

 

[BigBoatinOkie]

Re: Prop Confusion

I test props on an exact match to this rig....I run 49mph with an Apollo 19P at about 5200rpm...
Your tach is quite a bit off, not uncommon though.
The 18P 4-bld should be way above top rpm, unless you've got 6-8 people in the boat, fuel, coolers, several anchors, etc...
The best 2 props I've found on this rig I test with is a 19P 3-bld Large diameter Apollo and a 20P Bay Pro II 4-bld. One would think the numbers would be the other way for the number of blades, but that's the difference in blade design.
The 19 Apollo is actually a better load prop than than the 20P 4-bld and I believe it is a result of hull design in this case.
I need about 3500rpm to plane out smoothly in a timely matter, so I know the 3400 you speak of is about 1000 off, maybe a few hundred more.

How far are you (lindsay) from Grandlake???

Thanks for the advice walleyehed. Sounds like my tach sucks. I'll probably be replacing all my gauges anyway before spring, along with the deck and flotation foam. I ran that prop one time before putting the boat up for the winter and I believe I had a couple people in the boat with me(250lb and 160lb, and I'm 275lb) about 20 gal of fuel and very little gear. Once again, I'd be willing to bet that there is quite a bit of wet foam under the deck. It took for freaking ever to plane when I had the 22" on it. slightly better with the 20and better still with the 4-blade 18". When I hit the water again, I'll do some more thorough testing. I'll make sure the engine is tuned up good as well, and maybe shed a few pounds. I was a little premature with this thread since I can't go to the lake for 4 months or so to do any testing.

Lindsay is about 3 hrs from grand lake 40 miles south of Oklahoma City. We do most of our boating at Lake Eufaula though, since my parents live there. We have talked about spending some time at Grand next summer. My wife has some relatives that live right on the water. It's such a beautiful lake, from what I've seen it looks like the nicest lake in OK. Do you go there?

 

[walleyehed]

Re: Prop Confusion

I spend quite a bit of my time on Grand, that's why I wondered about your location. If it was close enough I'd be glad to give some hands-on testing, so if you decide to come to Grand, PM me and I'll bring some props and we'll figure it out, but I do believe the tach is bad....that boat will barely stay on plane at 3200-3300 so 3400 (for real) probably won't get it up to start with.

 

[BigBoatinOkie]

Re: Prop Confusion

I spend quite a bit of my time on Grand, that's why I wondered about your location. If it was close enough I'd be glad to give some hands-on testing, so if you decide to come to Grand, PM me and I'll bring some props and we'll figure it out, but I do believe the tach is bad....that boat will barely stay on plane at 3200-3300 so 3400 (for real) probably won't get it up to start with.

I'd make a trip to Grand just to get some help if you really didn't mind. I definately need to try some different props, but that can get expensive pretty quick if I buy them. As soon as it gets warm enough to de-winterize I'll PM ya and I'll buy you a steak for your troubles. Actually, I'll be coming through your hometown on my way to Branson next week.