How do I chose right prop?

[Rookster]

Hey Guys, I have a 2,200lb 18ft Bowrider, which has a 3 blade 19pitch prop driven by a Merc 3.0L I/O. It's my only prop and I'm we're supposed to have spares. So, I'd like to upgrade and keep current one as the spare.

I'm told that aluminum is too soft, but stainless steel is so strong that if it hits anything underwater, the prop will survive but internal parts will break.

I'm also told that a 17 pitch give better hole shot but I'd lose top end, and the opposite for a 21 pitch.

But then I heard that if I get a four blade prop with two blades at lower pitch and two blades at higher pitch, I will get the best of both worlds - good hole shot and good top end.

Thoughts?

 

[ondarvr]

Throw out about everything you've heard.

​Go with whatever material you want, aluminum is cheaper, SS will get you slightly (maybe) better performance, yes SS is stronger, Aluminum is cheap to repair.

​The pitch of the prop is determined by RPM's at full throttle, you may want to go with slightly less pitch if you do water sports or carry a heavy load, but you don't go up in pitch, that's a no no. So you need to know what the current RPMs are at WOT, don't guess, you need a working tach and the speed by GPS.

​A 4 blade prop "might" be better at some things, but are rarely better at everything, all of your choices will be a compromise, if you want more of this, you get a little less of that.

All of these things are dialed in by you doing the work, you need to test each prop to see how it works for you on your boat, what it does on somebody elses boat is irrelevant. And just because a prop says the pitch is 18" doesn't mean it will perform the same as an 18" of another brand or material.

 

[jimmbo]

And just because a prop says the pitch is 18" doesn't mean it will perform the same as an 18" of another brand or material.

That is far more relevant than most people understand.

There is a lot of truth about the use of SS props doing damage to gears and propshafts.
Rubber slip hubs are there more to cushion the load imposed when the prop goes from 0 to 200 - 450 rpm in less than a blink of an eye, than they are to save the prop from impacts. Most replaceable hub systems have solid hub system that may shatter on an impact, and then leaves you sitting with no forward and/or reverse until prop is pulled and new prop and hub are installed

 

[Rookster]

[QUOTE: The pitch of the prop is determined by RPM's at full throttle, you may want to go with slightly less pitch if you do water sports or carry a heavy load, but you don't go up in pitch, that's a no no. So you need to know what the current RPMs are at WOT, don't guess, you need a working tach and the speed by GPS.[/QUOTE]

At roughly 4,000 RPM, my boat will do about 30mph with two adults and 2 kids onboard. Does that help to know?

Sounds like either way, I should try going down in pitch - that seems to be the bottom line point in your message?

I'm not sure though, why going up in pitch is never a good idea - doesn't that generally give more top end speed?

There must be a few basic rules of thumb, as I don't have the capacity to spend hundreds of dollars trying numerous props unlit I get the perfect one.

What about trim tabs or smart tabs?

 

[ondarvr]

You need to target the upper end of the RPM range for your motor at WOT, this typically gives you good all round performance (or at least as good as possible with your boat/motor combo) and keeping it in this range will extend the life of your motor. If you go with more pitch and reduce the RPMs it put's more load on the motor and it starts to lug, which is very bad for the motor. Going with less pitch is OK if you watch the RPMs and don't over rev the motor, this helps when pulling skiers, tubers, or carrying a heavy load.

​What you need to do to start the process of buying a new prop is find out what the peak RPM range is for your exact motor, find out the exact RPMs and speed with your current prop, go up or down in pitch to get the RPMs in the correct range.

​Whether you go with aluminum or SS depends on your exact needs and what makes you feel better, the difference in performance might be worth the difference in cost, but that's up to you. Just because you go with SS doesn't mean you will get a noticeable difference in performance, and you will probably need to try many different SS props get the best performance possible.

​There is far more to choosing a prop than this, there are books written on the subject, but this is a start, others will add to this.

 

[wahlejim]

With your engine, you should be between 4400-4600 RPM for a max WOT RPM range.

It sound like in your case, with the information provided, you have maxed out your pitch. Try a run with just you on board, maybe the Admiral. If it runs at 4400, you have really maxed your pitch out and it is dropping your RPMs with the added weight. It doesn't have the bite you need for the additional weight.

If you do drop your pitch, your RPMs will rise, but you will have a better hole shot and similar top end speed while the boat is loaded. However, you run the risk of exceeding your max RPM range when the boat is not fully loaded, so you need to be careful with that.

As you can see, there are many variables in the equation. To really start dialing in, we can't work with "About" or "approximately". We need verified speed with a GPS, Exact RPMs (do not rely on your boats Tachometer, use a shop tach), how the boat was loaded, etc. Try a couple of different runs with different loads. The more data we get, the better.

Oh, and I forgot to mention that all of this needs to be done after you make sure that your engine is functioning correctly.

Hunting down the "best" prop for your boat is a fools errand. Find the one that you like, that performs well in your conditions, and don't look back.

 

[Sea Rider]

The "right prop" can only be dialed on wot water tests as usually loaded preferably on calm water cond. Running middle wot revs will achieve slight better top end speed while losing top hole shot. If running max wot revs, hole shot will be improved, but will lose slight top end speed. Which one do you prefer ?

Can prop for max wot range with say 2 up, when with 4, rpm should drop towards middle wot range, which still it's perfect.

Anyway, near impossible to dial a perfect prop on first try, usually on second one, so some $$ investment has to be made.

Happy Boating

 

[Rookster]

Thanks for everyone's advice - time to go testing and shopping!