Attempting to find the right Prop...

[poleclimber63]

I own a 17' Maxum bowrider with a Mercruiser 4.3L LX 175HP sterndrive. Currently, I have a 3 blade AL, 14.5" diam., 19 pitch prop. After running some tests last fall, I believe my WOT is slightly high. Trimmed, I max. out at 37 MPH. From Idle, with a steady increase in throttle, it took 20-30 sec to achieve plane. From plane with a gradual decrease in throttle, it looses plane at 2900RPM and speed drops off to 12MPH. It becomes much more difficult to maneuver and requires significant throttle to re-plane. Since late last season our family became excited about tubing and these performance characteristics make tubing quite difficult and unpredictable for the riders! I understand that going to a 4 blade will improve hole shot and low speed performance and control. I've researched various options: going to a AL 4 blade or a composite 4 blade prop. Piranha Propeller has an option with "easily replaceable blades, in the water" so replacing blades due to damage or for cruising performance would be simple. The problem is, the starting point to determining a good pitch maybe wrong with the prop that was on the boat, when I bought it. I understand when you go from a 3 blade to a 4 blade you should drop pitch by one point (19 to 18). However, with the above considered, I'm thinking I need more pitch, but I'm not totally sure where to start. I also have to consider cost: I live in Canada and for some reason, my shipped cost for a Piranha is $250!!! and a AL (from iBoats) is $158. If I screw up with pitch with the AL I'm done.... with the Piranha, I can swap blades..... Just a thought!! Any advice would be appreciated!

 

[Bondo]

Re: Attempting to find the right Prop...

I believe my WOT is slightly high. Trimmed, I max. out at 37 MPH.

Ayuh,... At What rpms,..??

 

[poleclimber63]

Re: Attempting to find the right Prop...

Sorry... my WOT should be 4400-4600RPM. I still had some throttle room at 4600RPM.... didn't want to go beyond that with my 1993 engine....
Thanks, for your interest!

 

[Tail_Gunner]

Re: Attempting to find the right Prop...

Hello welcome to i-boats, lets see if we can guide you though this...it's actually quite simple in the end. A few things are needed before a attempt is made to match a prop to your boat and engine combo. On your out drive there should be a stamped number such as 1.98....2.0...1.66. It is important to know this simply because some of the boats performance numbers are a bit off....such as 2900 rpm and 12 mph...not good It should be somewhere around 25 to 30 in the 2900 range.

 

[steelespike]

Re: Attempting to find the right Prop...

T-G He stated it falls off plane at 2900.that would reduce speed to about 12 and at that point
I think the rpm would be lower than 2900.
I would think that a 17 ft w a 4.3 might over rev with a 19" pitch. but it would seem it would be faster than 37 at 4600. My info indicates a 1.84 ratio in the 4.3 155hp or 175hp with a 4800 rpm.
max.
You likely no better than I how that ratio might work out.

 

[Tail_Gunner]

Re: Attempting to find the right Prop...

T-G He stated it falls off plane at 2900.that would reduce speed to about 12 and at that point
I think the rpm would be lower than 2900.
I would think that a 17 ft w a 4.3 might over rev with a 19" pitch. but it would seem it would be faster than 37 at 4600. My info indicates a 1.84 ratio in the 4.3 155hp or 175hp with a 4800 rpm.
max.
You likely no better than I how that ratio might work out.

Looks like i need to slow down a bit..It looks like a spun hub or a bad tach. Using 1.84 gearing and a 19p prop gets a slip factor of 21%. pretty dismal combined with falling off plane @ 2900 another hub issue.

It would be very easy to say a 21 pitch is in order...... but if you could verify the teach is reading correctly and the gearing is indeed a 1.84 ratio that would make a selection much easier and more accurate...

Checking for a spun hub...take a grease pen and make a mark on the hub nut and the barrel of the prop...if the line moves its bad...The only reason i am speaking to this is no boat like his should fall off plane @ 2900 rpm and if it is spun all of your info is not a good base line....BUt a 21 should be right with a 1.84....3 or 4 blade

Check the tach if you can... boat tachs often are miscalbrated....Umm are those speeds GPS or a boat speedo...

 

[steelespike]

Re: Attempting to find the right Prop...

Is your present prop in good condition?Does it have wear lilke it could be the original 1993 prop?

 

[cribber]

Re: Attempting to find the right Prop...

It really is pointing towards a spun hub with taking 20-30 seconds to plane out. At 2900 rpm you should still be on plane and cruising near 25 mph with that 4.3L.

 

[steelespike]

Re: Attempting to find the right Prop...

He is applying a steady increase of the throttle I think that would likely slow the holeshot
compared to a quick firm but not forcefull application.
If he doesn't trim down/in it would tend to drop off plane easier.It seems that little 17 footer
would be pretty nimble with a 4.3 I wonder about wet foam weighing it down?

 

[Tail_Gunner]

Re: Attempting to find the right Prop...

He is applying a steady increase of the throttle I think that would likely slow the holeshot
compared to a quick firm but not forcefull application.
If he doesn't trim down/in it would tend to drop off plane easier.It seems that little 17 footer
would be pretty nimble with a 4.3 I wonder about wet foam weighing it down?

....... check you PM

 

[poleclimber63]

Re: Attempting to find the right Prop...

Wow! all great ideas! I checked the hub tonight and it appears tight. No movement between the hub nut and the barrel of the prop..... I indeed have 1.84 and it is very possible it has the original 19 prop (some minor damage, hence a new prop). This boat hasn't spent alot of time in the water as the previous (and original) owner lived in Vancouver and only came up to the Okanagan for a few weeks in the summer, the rest of the time it was stored at a local boat shop. As for my numbers, they're off the boat tach, which I'm embarassed to say, may not be totally accurate....as for the wet foam... I'll investigate all of the above before this, as wet foam would probably require a survey??? at a boat shop....possibly?

 

[poleclimber63]

Re: Attempting to find the right Prop...

"I would think that a 17 ft w a 4.3 might over rev with a 19" pitch. but it would seem it would be faster than 37 at 4600. My info indicates a 1.84 ratio in the 4.3 155hp or 175hp with a 4800 rpm." ----- Even though I'm now doubting some of my numbers reading the posts here, I'm convinced my RPMs are high. My top end maybe a couple of MPHs higher, I believe. With the comments here, I'll get out the tuneup tach this spring, when I pull her out of the garage and check the tach and borrow my neighbors' gps... Thanks guys!!

 

[poleclimber63]

Re: Attempting to find the right Prop...

Thought I should post my results after I got my Maxum back in the lake... I purchased a 4-blade 20" pitch Piranha prop. Huge difference! My maneuverability, time to plane (improved from 20s or 30s to 15s) and RPM to plane (improved from 2900 to 2400) ...are much improved. Top speed has marginally improved (39mph at 4200rpm), and WOT is now right in range. I will be honest, as far as performance is concerned, you can't really compare a 3 blade to a 4 blade. Performance wise, a fair comparison would be AL 4 blade, SS 4 blade and my new Piranha composite 4 blade. I can't really say which would be the best, but for about $190, I'm pleased with the results... AND I can change blades and/or pitch for about $18 a blade.

 

[steelespike]

Re: Attempting to find the right Prop...

Nice to see you had some positive results.Usually suggested to prop near the top of rpm range
Normally you would decrease pitch when going to a 4 blade.Probably a 19 would have put you about right I think an 18 would put you right at 4600.
Running your motor near the bottom of the rpm range isn't giving it a break its more like going uphill in overdrive in a car.Doesn't leave much room when adding a load of people and gear.
Any 4 blade will improve low to mid range performance and handling.
Likely best with ss,and composite and al pretty even.
I still wonder where your speed is. A typical healthy 17' 3.0 will run in the low 40s
a 4.3 approach 50 or more.Unless your Maxum is really heavy or extreme DeepV.
You might want to do a compression check,be sure it is timed right and the throttle opens all the way.

 

[poleclimber63]

Re: Attempting to find the right Prop...

Thanks for your reply! I took my boat out on the lake yesterday and briefly hit WOT, trimmed, I was able to hit 43mph. This boat is actually an 1993 1800SR and it is quite a heavy hull. I understand what you're saying about the performance, therefore, I plan on taking it out for a maintenance run, hopefully next weekend. When I change the plugs(original plugs!) and tune, I'll check the compression. However, I have connected a vacuum gauge, and see nothing suspicious... Thanks! I'll post my findings!

 

[poleclimber63]

Re: Attempting to find the right Prop...

The weather finally warmed up enough to get out on the lake and have some fun! I found a minor leak at the intake manifold (re-torqued), found a spark plug wire was faulty and retuned (timing was out). My 4.3 runs like a top, now! Approaching WOT I achieved 48MPH with my new Piranha 20" 4-blade. I'm pleased with the results, especially the prop upgrade. Now I'm heading south this weekend, to pickup my new Bimini top and tow toy---- ready to have some fun! Thanks again to all the posts!

 

[steelespike]

Re: Attempting to find the right Prop...

A vast improvement Does it make the 4800 max rating?