What size prop?

[thrasher]

Okay Guys and Gals,

I asked this question before and you told me to go and get more data, so now I have the extra data!!

I have a 19 foot Glastron open bow boat with a Mercruiser V6 4.3L engine and an Alpha1 gen 2 outdrive. I am running a 13.5" prop with 23" pitch.
I was told to do a WOT and see what happened. Well triming it up as best as I could, I got 43mph at 4000RPM. I believe this engine should be running 4400 to 4800 RPM. I understood that going down 1" on pitch will give me 150RPM more, this sounds like I need to go down to a 19" pitch for this boat.. Does that sound correct? I also noticed that the original prop (was badly damaged) but had scalloped blades, does this give an advantage? Any recomendations on a good 14" to 14.5" prop with a 19" pitch?

Many thanks for any advise or comments.

Gary

 

[thrasher]

Re: What size prop?

Well if no-one can tell me a good make of propellor, or if scalloped blades make any difference. Can someone at least confirm my maths that I should drop the pitch by 4" to get the revs up to 4600RPM?

Thanks Gary

 

[Esox]

Re: What size prop?

I'm no expert but hopefully this helps. As long as you stick with the same brand/style prop the 150+/- rpm change per 1" pitch change should be close. As far as a good prop for your boat I can't say. What kind of prop is the 23 your using now? - Alum or SS, brand, style? Walleyhed has done a lot of prop testing and has posted some excellent data.​

 

[andy1canada]

Re: What size prop?

Howdy Thrash!

I'm going throught the same thing as you with the same engine/drive. You didn't state the weight of your boat; knowing the dry-weight is important (then you can figure and add the additional load, ie. fuel= 300/lbs, wife = 600/lbs, beer = 1.6/metric tons. etc.)

Once you've determined the average running weight of your rig then you have a starting point.

I'll repost these links for you. There's some interesting data there from Merc.

http://northamerica.mercurymarine.com/_media/pdfs/bhb/sterndrives/4_3MPIAlpha_Stingray210C_1006.pdf

http://northamerica.mercurymarine.com/_media/pdfs/bhb/sterndrives/4.3MPI_ChrisCraftLaunch22_0806.pdf

http://northamerica.mercurymarine.com/_media/pdfs/bhb/sterndrives/4.3MPI_Godfrey195SD_0906.pdf

(by the way, the Chris Craft/Launch weighed 4100/lbs empty)

Hope this helps.

I just go word from my mechanic about a boat shop/service outfit here on the Island that has a wall full of test props available for customers to try out.
I'm going to check it out.

Cheers,
Terry

 

[thrasher]

Re: What size prop?

Thanks Terry,

Can you tell me why we need to know the weight of the boat and the other data? I measuered my WOT with full fuel load and normal load in the boat (3 passengers + cooler), so I don't see how knowing the weight would change anything? As I understood I just need to raise the revs 600RPM from where they currently are at WOT with normal load? Sorry for the stupid questions, just trying to understand.

Thanks for the links, I will take a look.

Gary

 

[Esox]

Re: What size prop?

The actual weight of your rig is not much of a factor as long as you do your testing with the average load you will be using the boat with. You did right getting your current #'s with a normal load. What is the current prop your using now? Aluminum or SS? What brand/model?​

 

[Tail_Gunner]

Re: What size prop?

Okay Guys and Gals,

I asked this question before and you told me to go and get more data, so now I have the extra data!!

I have a 19 foot Glastron open bow boat with a Mercruiser V6 4.3L engine and an Alpha1 gen 2 outdrive. I am running a 13.5" prop with 23" pitch.
I was told to do a WOT and see what happened. Well triming it up as best as I could, I got 43mph at 4000RPM. I believe this engine should be running 4400 to 4800 RPM. I understood that going down 1" on pitch will give me 150RPM more, this sounds like I need to go down to a 19" pitch for this boat.. Does that sound correct? I also noticed that the original prop (was badly damaged) but had scalloped blades, does this give an advantage? Any recomendations on a good 14" to 14.5" prop with a 19" pitch?

Many thanks for any advise or comments.

Gary

You probably would like to target 4900 rpm if your motor is a vortec or 4800 if it is not. Now it depend's on the qualities you are looking for in you prop selection, 1. overall all speed that is a 3 blade prop in all most all I/O's 2. Midrange tourqe, better hole shot, increased fuel economy that would be a 4 blade prop. Now the difference's are not night and day between the to but it is there and real.

Now the question is do you want a SS prop or alum, when you are testing have you added weight to reflect how your prop will perform in real world application's, do you know how or if your tach is accurate and is the speed your posting a GPS speed or taken off the boat's speedo there a little off.

 

[andy1canada]

Re: What size prop?

Howdy again Thrash.

Knowing the weight of your boat -with a typical load factored in - is important for you to make comparisons as to what sort of performance you should expect from any given prop/horsepower ratio.

If your engine is running up to snuff you can evaluate other boats - of a similar size/weight/hull configuration/powerplant - to establish a realistic benchmark for your own performance expectations.

Good luck!

Terry

 

[thrasher]

Re: What size prop?

Thanks guys,

in no particular order, I do not know how accurate the tach is. The speed was measured by both a GPS and the speedo (they matched very well!!).
I am currently running an aluminium prop and wanted to keep it aluminium as this is my first boat and I just know I will hit something with the prop and I would rather damage the prop than the transmission.. I am just cruising or pulling my 2 girls (7 & 9 years old) in a tube, so I guess I prefer a quicker hole shot rather than top speed, but I also have smart tabs so there is no hole to shoot!!.. I just wanted to get the complete system optimised. To minimimise stress on the engine.. I currently have a 13.5" prop with a 23" pitch, the engine is a mercruiser 4.3L V6, So I think a 19" pitch would be a good choice? tfo get the revs optimized with my normal load.. What do you think?

Gary

 

[athiker]

Re: What size prop?

You may want to try a Merc Alpha 4 in a 20p. It should be good for watersports, holeshot and plenty of pitch for a good top speed still. Its aluminum.

http://www.marine-products.com/props/mercury_props/alpha.jsp

http://sites.mercurymarine.com/portal/page?_pageid=126,49589,126_50268&_dad=portal&_schema=PORTAL

There is a chance it could be too much prop, but probably not by much. I run a 4 blade 20p or a 3 blade 21p on my 21 footer w/ a 5.7L. I have more engine but I'm pushing 2 more feet of boat. My 4 blade is a Piranha composite that gave me great performance #s but I have had 2 incidents of a blade shearing off while slalom skiing; once per season. My engine hp is at their upper range though, it would probably be fine for your 4.3L rig. I haven't tried the Alpha 4, as its rated to 250hp and mine is 260hp. My 3 blade is an alum Black Max (which actually has the same 250hp rating!).

Poke around the Mercury Marine site; there is a prop selector tool there. I tried to plug in your #s but it didn't like the 23p turning 4000 rpms and 43mph top speed combo so you may need to mess around with it. Do you have a high altitude drive ratio? I switched the ratio from 1.47 to 1.62 and it gave me 3 options. 20p Alpha 4 alum, 21p Enertia alloy 3 blade and the 19p Revolution 4 SS.

Good luck.

 

[tmh]

Re: What size prop?

Good advice above. I just wanted to say, I expect you'll be surprised just how much better your boat runs once you get the prop right! Your current 23" prop ruins your low-end and doesn't maxamize the high end either. Once you get the boat propped into the proper WOT range, all will be better, probably a LOT better.

The only advice I'll add is to emphasize propping for normal load. A few paople and some gear make a big difference. So, if your current 4000 rpm is with a less than normal load, you may well want to drop to a 19". If it was normal load, then somewhere between 19-20 I guess which means a typical prop compromise, which they all are anyways.

Let us know how it turns out!

 

[thrasher]

Re: What size prop?

Many thanks everyone. I do not know what ratio my drive is, How do I find out? All my lakes are at sea level, so I don't need a high altitude setup (I am not saying I don't have a high altitude setup, just that I don't need one!).
I think I will try a 19" pitch 4 blade as suggested. I am going to europe (leave tomorrow) for 3 weeks, so I will have to try the prop when I get back. I will definately let you know how it works out..
Many thanks.
Gary

 

[athiker]

Re: What size prop?

There is a tag, almost like a stamp or imprint, on the drive itself; unfortunately I can't recall from memory exactly where. It is definitely there though, right on the outdrive.