Stainless prop selection/questions

[dmanmtl]

Hi - I have a 2007 Princecraft 18.6ft fishing boat - with an 2007 150 Optimax. Boat runs great, seems to perform well, top speed is about 42MPH and hole shot seems find. I find it a bit squirelly at low speed but that might just be my ability.

I would like to have a spare prop - currently it has the original size 14.25X21 so I don;t think I need to change anything with the pitch but I would like to get a stainless prop for daily use and keep the aluminum as a spare.

SS is about 4 times the price so I am wondering if it is really worth it...what are the main advantages of a SS prop? I know they are harder and less flexible so if you hit something you can do more damage, but what are the advantages?

Thanks in advance.
D

 

[Sea Rider]

If you haven't tached OB at wot as usually loaded on flat calm water cond won't know if current prop performs right or not. For than need to know factory stated mim-max wot rpm range and compare it against the wot test results, then can go for a prop maximization with 3 or 4 blades props, not before.

Happy Boating

 

[dingbat]

High horsepower, high thrust applications is where stainless props excel. Anything else is for the bling factor

Stainlees props are heavy and generate a good “bump” when put in gear, but I’ve ever bought into the strike argument given the wide spread use of torque hubs

 

[b.gagnon]

My guess is that you are slightly over propped but without WOT #'s there is no way to tell.
I have used stainless prop's on every boat that I have ever owned and never for looks! They have always improved every aspect of the boats handling for me. I even have one on my 13' Whaler and it was well worth the upgrade..... With the power that a V6 Opti has, a stainless should be mandatory! Now with that being said not all Stainless prop's are created equal and finding the proper one for your setup and needs is not always easy!
My guess would be a 19p Mercury Enertia.

 

[HT32BSX115]

squirelly at low speed

Howdy,
You'll have to define that. But if you mean slow speed directional "wander", It's not prop-related.

I would like to have a spare prop - currently it has the original size 14.25X21 so I don;t think I need to change anything with the pitch but I would like to get a stainless prop for daily use and keep the aluminum as a spare.

It's true. Stainless steel will absolutely excel in high HP engines and you may need to use a slightly lower pitch because there's far less blade flex with stainless, where an aluminum will start out at a particular pitch and flatten out slightly with high HP engine (flex to a lower pitch)

In any case, you need to run the boat with your current prop to WOT using a known tach and GPS for speed until you know your speedometer is accurate.

Once you know where your performance is, you can make an educated guess on a stainless.

 

[dmanmtl]

Thanks for all your replies - I get 5500-5600 RPM at about 42MPH (GPS) on flat water.

When I say the boat is squirrely at slow speed I simply mean it wanders more than my old I/O , nothing major just a bit awkward at idle speed

D

 

[porscheguy]

A 150 optimax has a 1.87 gear ratio. Your 21p lets it turn 5600rpm @42mph. You have 30% slip. That’s very high. I don’t know if there are any particulars with an aluminum hull or your style of boat, but even so, your slip is high.

You also said it has squirelly handling.

What’s your motor height setting? I would say it’s too low, but I’ll wait til you report back with that info.

Are you certain about the speed and rpm? I’m asking because your numbers are so far out there.

Id check the motor height and make sure that’s correct first. Then I’d confirm your performance data.

 

[Sea Rider]

If OB sits too low will produce back or over transom water splashes at speed, a matter to check what's going on there. Water flow is not passing right under small upper splash plate where the sharp edge is located to cut water as cheese. Will need to raise OB a bit till water splashes cancelles out.

Happy Boating

 

[HT32BSX115]

Thanks for all your replies - I get 5500-5600 RPM at about 42MPH (GPS) on flat water.

When I say the boat is squirrely at slow speed I simply mean it wanders more than my old I/O , nothing major just a bit awkward at idle speed

D

All boats (without a "real" keel) wander. It's a characteristic of pushing a huge log through the water. Once you get the boat to a speed where the you begin to get more laminar flow of the water over the hull, it will stop. no amount of stern drag (like "not-so-smart" tabs or double sea-anchors) will fix it...

The only way to stop it is to speed up a little so the bottom of the boat begins to act more like a "keel".....but that unfortunately might be a speed that begins to produce a wake.

If you're getting 5500-5600 RPM at WOT (that's the correct RPM for your engine?) determine your lower unit ratio and use a prop slip calculator to estimate your slip.

http://www.go-fast.com/Prop_Slip_Calculator.htm

It should be somewhere around 12-15%

Don't know what your boat should do with that engine, but if 42 mph is right (and your Tach is verified) you might be right where you want to be.

Do you also have a hydraulically adjustable transom mount? You might get more efficiency out of changing the tilt and height of the engine

 

[porscheguy]

Optimax uses a 1.87 gear ratio. Coupled with a 21P prop, running at 5600 rpm and a speed of 42 mph yields 29.5% slip. Way too high.

I’m thinking the motor is too low which causes drag, coupled with an aluminum prop that’s no frills. Alternatively i wonder if the motor is too high and is aerating the water that’s going through a plain Jane aluminum prop that lacks bite. Having the motor too high or too low can cause odd handling.

The boat appears to have a 17 degree deadrise, and it’s not very heavy. A 150 optimax is more than adequate power, especially since many 18’ boats with O/Bs seem to have a 115. If his engine wasn’t making full RPM, I’d question if he had a mechanical issue. But while I never rule out anything, I can change its position on the priority list. And mechanical issues are pretty low at this point.

i think a 12-15% slip is well within reason on this type of boat. And if you go from a well used, no frills aluminum prop, to a stainless performance prop with lots of cup and rake, you can probably lower your slip 8-10%. This would take you down to 19.5% slip. A significant improvement no doubt, but there’s still room for more. This is why I mention motor height. This isn’t something I’m too familiar with as those of us with i/Os don’t have that luxury, but it’s a straightforward concept. It’s also what I would check first, before spending a dime on a prop.

 

[HT32BSX115]

Optimax uses a 1.87 gear ratio. Coupled with a 21P prop, running at 5600 rpm and a speed of 42 mph yields 29.5% slip. Way too high.

I’m thinking the motor is too low which causes drag, coupled with an aluminum prop that’s no frills.

Yeah, I got 30% slip from the go-fast calculator!


This might be the "key" here. Slip can be caused by several things to include too much weight in the boat! (or a waterlogged boat) If you have tabs (of any kind) adjust them all the way up (they cause drag)

And by the way, if your TACH is wrong (as many are in the upper RPM ranges) it will give you high slip numbers.

If you did not verify your tach with a known one, you are literally wasting your time fooling with props


//

 

[porscheguy]

There’s no “might” about it. Excess slip is the reason for his issues. But it could be caused by excess drag, or excessive ventilation if the motor is too high. Too high will allow him to turn max rpm without making a lot of speed or overworking the engine.

OP, if you’re still checking this, is your boat considered a walleye boat?

 

[Sea Rider]

If you think OB sits too high go for a wot spin with deck weight evenly distributed on flat calm water cond and when boat planes out trim OB to run neutral, make sharp close turns, if there isn't the slightest prop aeration whatsoever, OB sits right.

OTOH, if thinking OB sits too loq, do same and visualñly check what's going at back transom. Ideal is for lower leg to achieve no water splashing up, just a flat middle flow outting lower leg, If not, fine tune raising OB a bit till that cond is achieved.

Then can go play with props models, pitches all you want provided that boat is in excellent shape and not waterlogged.

Happy Boating

 

[dmanmtl]

Guys - thanks for all your replies. I believe I made one small mistake in my conversion of KM to MPH the boat tops at 75KMh which calculates to almost 47MPH.

Also, with regards to the tach - I am using the Faria tach installed in the boat, so it may not be super accurate. I have had the boat for a year and when I had a marine repair shop check it out for me last year (very experienced mechanic) he did lower the motor 1 hole as he said the prop was ventilating in cornering when he test drove it.

My sense is that the boat runs really fast, I never really considered the top speed to be a problem, was really just looking to get a spare prop so I thought an upgrade to SS would make sense. But all your responses have me thinking about whether I have some other problems.

I am not sure if this is considered a Walleye boat or not, but here is a pic. Very popular style up here in quebec and Ontario.

Thanks again for your replies - much appreciated.

 

[Sea Rider]

In reality doesn't matter which boat type you run, it's a matter of running an appropriate HP-OB, sit OB at the sweet transom height, well trimmed and propped right for OB to achieve medium to full wot revs as usually loaded. Which rpm modality is preferred, it's entirely to each boater..

With say same HP-OB powering 2 different same lenght different hulls shapes, the one with least water drag wil perform much better...

Happy Boating

 

[porscheguy]

OP, you’re still at 21% slip with your updated speed. That changes things a lot, but you’re still leaving performance on the table.

Your mechanic dropped the motor height one hole because he said it ventilated when turning. Your boat has an aluminum prop. Aluminum props are fine as basic propulsion. With that said, most have minimal cup and minimal bite. They all blow out when turning.

At this point I’d say the only possible issues you have are motor height and an aluminum prop. The motor height is iffy.

I’d try a tempest or Quicksilver torrent. Same prop, tempest is mercury and torrent is Quicksilver. Just make certain the seller can place his hands on the torrent and confirm that it uses the round PVS vents that can be plugged. If it has square holes, pass on it.

 

[Texasmark]

The hull and the tide/river flow can make for squirrely handling at low rpms, especially with a heavy bow, or deep V. The engine is pushing slightly and the bow is digging in, forcing the direction of thrust from the engine to change direction to keep things lined up. Going from a tri-hull shallow draft to a mono deep V is something to get used to.

A good example is to try to run a boat with a trolling motor from the stern vs getting a front mounted ™ and "pull" the boat. Night and day.

Good info on the prop question. If my memory Laser II has square holes and didn't Quicksilver/Merc. have pluss of various sizes for them? I "think" I recall they did. I do agree on having ports in a high perf prop and being able to tune them per the boat's performance requirement. Course you can plug them with JB weld and drill them back out to a smaller diameter to tune them.