Prop Pitch

[02-310]

I've tried this request once before on another board, but really didn't have any responses, so I thought I'd try it here.

Anyways, I have a 310 with 5.0L MPI BII drives...  I love this combination and the hole shot with my wife and I on board is about 5-6 seconds, I can't even tell you what the top speed/max RPM are because I haven't had it at WOT.   The props that are installed are alumimum and 21p.  What is everyone elses input regarding the type and pitch of props you're running on your 310/312/320 with BII drives, and are you happy with your performance.

Thanks

[merwin10]

We hear this all the time 5 seconds is plenty fast to come on plane however remember every time you make that hole shot you are putting undue stress on the drive train and engine - Thus reduceing its life and incurring expensive repiars -

I guess the answer to your question depends allot on the WOT RPM if you are achieving WOT RPM then a pitch change will definately throw that off! If the WOT RPM is less than spec'ed then you are over prop'ed and need to reduce the pitch - likewise if you exceed WOT RPM spec then you are under pitch and need to increase the pitch - Now as for increasing the bite you might consider stainless steel cupped props of the same pitch  - Just remember that the SS version don't flex as much a their aluminum counter part - Hence if you hit something you are more likely to damage something else besides the prop -

Mike -

[02-310]

We hear this all the time 5 seconds is plenty fast to come on plane however remember every time you make that hole shot you are putting undue stress on the drive train and engine - Thus reduceing its life and incurring expensive repiars -

Mike,
I don't think I'm trying to do something with my boat that anyone else doesn't want to do (get on plane).  The time I gave was average, and it is not getting there at WOT either.  The RPMs probably don't even get over 3200 to get the boat on plane in that time period.  I'm looking for a lower cruising RPM at the same speed or faster nothing else.  I cruise with my boat between 3200-3400 RPM and that gets me to 27-32mph.  Again, I have NEVER been at WOT with my boat.

Are you indicating that we should cruise around below 2000 RPM so we don't plane and so our motor(s) last longer and don't need expensive repair?

I guess the answer to your question depends allot on the WOT RPM if you are achieving WOT RPM then a pitch change will definately throw that off! If the WOT RPM is less than spec'ed then you are over prop'ed and need to reduce the pitch - likewise if you exceed WOT RPM spec then you are under pitch and need to increase the pitch - Now as for increasing the bite you might consider stainless steel cupped props of the same pitch  - Just remember that the SS version don't flex as much a their aluminum counter part - Hence if you hit something you are more likely to damage something else besides the prop

Thanks, maybe that is why I haven't figured it out yet, because I haven't been at WOT.  But, what is everyone else with the same motor/drive configuration using (I guess with the responses to my thread, no one else has the same setup as myself).

I've check the Mercruiser website and when I input all the data from my boat it says that I should have 25p props for midrange cruising and even for overall performance , so I wonder why they installed 21p on my 310...  hmm

BTW, this is the link to the site I'm referring to:  http://appcenter.mercurymarine.com/propSelector

Thanks!

[merwin10]

No I am not indicating that you should plane at 2000 rpm - what I am saying is the indication of the correct prop is that the engine can achieve the manufacturers WOT rpm - Let just say for example (I am not sure what your WOT RPM is) that the manufacturer states that WOT RPM is 4400 RPM for your boat trimmed up - Now lets say when you try it with half tanks water and fuel with three or four people on board clean hull and prop - trim properly - WOT is 5200 RPM - that tells me you are under prop'ed you need more pitch - or - WOT is 3800 RPM your over Prop'ed reduce pitch - In either case it is the achievement of WOT RPM that will tell the story -

As you stated you have never been to WOT RPM so it is difficult to give any statment or recommendation of what prop - In general I could make a recommendation but you may be worst or better off - it would be just a guess - You see there is this spec called slip and it is different for every boat even within the same models it is a variable that can only be for your boat and your load at the time of the WOT test - The more load the harder it is for the prop to push the boat thru the water hence the engine needs to work harder - If you look at any of the boat test they even figure in the temperature and humidity at the time of the test - for the most part they figure sea level but on some of the high western lakes you even need to consider altitude - engines run differently at 6000' than 0'. You will notice that the Mercury calculater does NOT consider the slip factor (read loading) it generalizes the load - Slip is the amount of forward motion the prop makes in one revolution in your case 21" of forward distance - but that is in a perfect world with no resistance - you have a load to push so maybe your boat only moves forward 15" instead of 21" the difference between the two numbers is the slip factor - The slip factor can be figured out by knowing a few things WOT RPM - Transmission or drive ratio - Speed over ground - and of course your current pitch - By comparing the perfect world number against the real number you can come up with a percentage of slip - You take four runs at WOT speed one north - east - south - west add them up and divide by four this takes the current and wind into effect - and compare that against the perfect world number - hence a slip percentage - or better resistance to forward motion which has many parameters to it!

If you want to get the best accuracy I would also recommend a digital tach instrument on the engine - If you have twins go by the engine with the digital tach and use the synchornizer to bring the other up to the same speed the sync will tell you if one engine is a few hundered rpm greater on either engine - Why you ask? Boat tachs are problematic with errors and will have you make errors in the calculations -

We have run into that to many times to mention - Just remember this is a realative measurment to your boat on the day of the test with the load you currently have and it will most likely never be the same twice there are to many variables - so the best way to run the test is on a average of how you normally use (read load) the boat -

Mike -  

[02-310]

Mike,
Thanks for your detailed reply, and I may end up using that to determine what I actually require.  But I was just looking for feedback on what others were using with the same power train and drive setup that I have.  This way it could be used as reference.

Cheers...

[mrraff]

I own a 92 260 with a 350 merc. 260hp engine coupled to a bravo 2 outdrive.  I had a badly worn 17.75x21p prop on the boat.  I went to a 17.50x23p Power Tech stainless, and gained performance in several ways.  My fuel effenciency was improved drastically, the acceleration at every part of throttle improved, and I gained a top speed of 41mph from 34mph before.  I guess what I'm saying is that on these heavy boats with the bravo 2 outdrive you benefit dramatically from going with a stainless prop, due to the larger diameter. There's more surface area to flex, or not flex with a stainless prop.  I know this is an old post, but I figured I would give my 2 cents......

Todd