Prop Advice

[Sea Zar]

Hello everyone.
I am needing some advice on choosing a new prop for my boat. I have gone from blowing 3 engines due to overstraining it with the wrong jet setup. I have since converted the boat to a sterndrive setup. So now i want to be careful as its getting expensive.

My questions are directed towards a 4 blade prop (unless someone thinks otherwise). I am wanting an easy plane due to offshore fishing (riding swells etc). I also want better fuel economy. I dont plan to drive the boat at WOT all the time, I am hoping for a really good cruising speed without having the secondaries open up too much & guzzle fuel. I think it can be done...hopefully with your help.

My questions are:

Ofcoarse the first question is...what size prop is the best for my specs?

What speeds could I expect to do at cruising speed. My cruising speed at the moment is about 20 to 25 knots at 3000rpm

Is 3000rpm classed a efficient cruising speed. If not, then what is?

What is the absolute minimum rpm at WOT that my engine can run at before I start straining & possibly detonnating my engine?

What speed at WOT should I be aiming for for optimal speed efficiency

What is the difference between 3,4 & 5 blade

Would a 24p 4 blade be over optomostic?


Current Setup

Hull Length 21 foot
Type Fibreglass
Model Savage Marlin (Australian design)
Deadrise 22 degrees
Hull Weight Between 2.3 & 2.5 tonne
Engine Mercruiser 350 Chev Slightly worked & balanced. Rotchestor 4 Barrel
Horsepower Around the 300 to 330 hp
Gearbox & Ratio Alpha one with 2:1 ratio
Current Prop Size 3 Blade, 19pitch, 14 inch diam Stainless
WOT 5000rpm
Speed at WOT 40knots

Cheers
Sea Zar

[gonzo]

Are you sure of the drive ratio? It should be 1:50 to 1 for that engine.

[Sea Zar]

Aren't they the same thing Gonzo??? Your '50' meaning '.50' (1/2).

My engine revolves twice to every one full revolution of the prop.

The prop definately doesn't revolve 50 times per one engine revolution.Assuming that that's the way it's calculated???


Cheers

[gonzo]

You posted a 2:1 ratio. That is different from 1.50:1. That is 1 1/2 turns of the motor for 1 on the output shaft.

[Sea Zar]

I'm getting confused...so what I'll do is have a look at the manual & let you know the exact numerical combination of the ratio. Sorry about that.

Sea Zar

[Sea Zar]

You were right Gonzo. It is 1.50:1...Sorry about that

[Jango]

5000 rpm is probably slightly high. Depending where your torque curve is you could go to a 21 pitch 3 blade or posibly a 21 pitch 4 blade.
WOT rpm would probably drop well below 4000 with a 24 pitch which will be less efficient.

[gonzo]

4200 RPM is the minimum WOT. The rule of thumb is 300 RPM change for 2" of pitch or 400RPM for 1" of diameter.

[Jango]

With a 1.50 to 1 gear, 500 Engine rpm change per 2" pitch change is more correct.

300 rpm change is OK for a 2;1 gear

See http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

[Sea Zar]

Thanks guys.
I've been searching & speaking to a few people & they suggested that 4600rpm would be a good balance at WOT...which will give me 2600 at cruising? OR it will give me about 30knots at 3000rpm cruise. I'm unsure which will be best as I've noticed that you can actually feel where the best rev range is in respect to speed. It sits in the pocket at 3000rpm at aprox 22knot give or take weather conditions.

I've also been interrested in high fives & searching on the net for them (ebay) but can't seem to find one that's 14inches in diameter. most of them are around the 13 inch. I'm assuming this is for an outboard??? Or is that 1 inch not as important due to the amount of blades.

I was also looking at a 2 speed 'Quicksilver power 2' 3 blade prop. Adjusts from 17" to 23" but I'm unsure if it is a better option than 'let's say' a 4 or 5 blade??? I don't even know if there are issues associated with them.

Cheers
Zol

[Jango]

Increasing the number of Blades has the same effect as increasing Dia. Keep in mind that MORE blades also equals LESS efficiency. Three Blade props are probably the best compromise, unless you need more blade area and can't increase dia because of space.

Since your boat is fairly heavy, you may have to increase Blade area ( increase Dia. OR no of blades)and stay with a 19 pitch.

Since you're looking for efficiency, it would be better to stay with 3 blades.

[Sea Zar]

Do you think a 2 speed 3 blade would be a good option? I dont think I have more room than for a 14" or 14.5" prop.

[Jango]

With the no's given, 40 Kph @ 5000rpm, your Prop slip is between 23% and 27%, with is extremely HIGH at that speed. This leads me to believe your curent Prop is TOO SMALL, OR, your Trim is completely wrong (Prop slip should be between 10% - 15% for your setup)

If space for a Larger Dia, 3 Blade is not available, you have little choice but to go to a 4 or 5 Blade prop. You need to keep the Dia at, or as close to 14 in. as posible. A 13 dia 4 blade will act nearly the same as a 14 dia 3 blade with next to NO additional benefits. Each additional blade will DROP engine rpm by 400 - 500, or the same as adding about 1" to the 3 blade dia.

[Sea Zar]

Its actually 40 knots Jango

[Jango]

Yes, I know. I wrote Kph meaning 40 Knots per hour, which equals 46 mph.

Sorry for the confusion, I think in terms of mph

[Sea Zar]

Hmmm... I see what you mean. So once the correct prop is installed I should end up with a tremendous difference speed at wot & top cruising speed?! Do you have an idea on how much improvement I could expect?

[Jango]

You probably would see the most improvement at cruising speeds, perhaps a 10 - 20% increase in speed with the same RPM. Acceleration should also be improved.

Since either a larger 3 blade or same size 4 blade will reduce WOT rpm,s to about 4500, the increase may not be as dramatic.

Assuming 40 Knots @ 5000rpm is correct, increasing the 3 blade dia by 1" (15x19 full cup)OR going to a 14x19, 4 Blade (full cup) should produce the following:

AT 10% SLIP ----- 51.1 mph or 44.5 Knots

AT 15% SLIP ----- 48.3 mph or 42 Knots

These figures are calculated using Basic formulas and assume ALL conditions are Ideal - Trim, minimal wind and minimal wave conditions.

[Sea Zar]

Yes...I'm understanding. So am I to assume that a 5 blade with your above calculations that it would reduce top speed back to about 40 knots?

A mercruiser repairer had the opinion (a very loose one) that a 5 blade seems to be a little over efficient in a way. I didn't quite understand what he meant.

[Jango]

If the 5 Blade dia. remained the same as 3 blade (14"), yes speed would be lower due to the further loss of WOT rpm. However, 5 blade props are probably Smaller (13 dia. as you indicated) which could act the same as a 14 dia 4 blade.

Not sure what the Mercruiser person meant, since 5 blades are less efficient than 4 blades. The theory is blades are closer together and "previous blade disturbs water for next blade". Only advantage is Increased Blade area which is better obtained thru Increased Dia.

A 13 x 19, 5 blade should be better than a 14 x 19, 3 blade, but not as good as the previous sizes mentioned.

[Sea Zar]

Jango. Thanks for your replies. You seem to sleep as much as I do...which not much...hehehe

Let's face the fact that the boat will rarerly be driven at WOT for long periods. I would say that it's not healthy for the engine anyway...wouldn't you agree!?

So the first & foremost thing that will be essential is a healthy & easy cruising speed. The only reason why I think we use WOT is just for engine load setting & ofcoarse to get you outta sheit in bad weather. Generally speaking ofcoarse.

I was looking at a highfive 15 1/2 diam x 19 pitch for sale. That could work. A quick hole shot is another thing that I want to make sure of. Particularly riding back from offshore with a full house of fishermen & fish trying to ride a wave back & getting sucked into a wave troph. Very had to pull out at the moment without having to "GUN" the revs for power.

[Jango]

I suspect A 15 1/2 X 19, 5 Blade is way TOO MUCH prop.

[Sea Zar]

Hmmm...OK

[Sea Zar]

I just talked to a professional prop repairer & he seemed to think that I should buy a 3 blade 17" pitch prop which will fix all my needs.....

I find that frustratingly confusing & very hard to beleive. But has posed an insecurity in buying wrong prop. So I have decided to cup my original 3 blader to the maximum to see if will eleviate that slip you mentioned Jango. I want to see what it's optimal result is before I decide on another 4 blader.

I'll let you know what eventuates ounce I test it next week.

Cheers
Sea Zar

[Jango]

I believe you are correct in not purchasing until you are reasonably sure of the outcome.
If your prop is not already cupped, adding Maximum cup will have a similar effect as increasing pitch by 1" which could decrease slip somewhat.

Further increasing pitch will help, but more than a 2 or 3" increase will drop max RPM,s excessively.(below motor peak)

Normally excessive slip is caused by Improper Trim or Inadequate Blade area.
i.e. increase Dia and/or no. of blades.

See if you can borrow a 15(or 15 1/2) x19 full cup 3 blade to try.

Let us know how it works out.

[Sea Zar]

The shop said that stainless props are very hard to pitch up as they seem to have a 'memory' & don't like bending. That's why I thought that cupping is the next option. it actually had a small amount of cupping on it, so... as you said Jango, we will se the difference.

I also might mark the prop & sleave to make sure the rubber is not slipping as well.

Sea Zar

[Sea Zar]

OK...I picked up the prop on Monday & took it for a spin.

They only increased the cupping to it's maximum. The results are a little different except for cavitation. THAT hasen't changed, though minor trimming has increased lift & drop. The speed has increased by about 2 knots at cruise (24 to 26 knots). Speed at WOT is still around 40 knots but revs have dropped to 3800rpm.

Would I be correct in saying that a 19P 4 blade prop would be the best prop or should I be looking at a 20p? I DO want to bring the revs down another 200rpm or 300rpm. I'm thinking that the extra load will be absorbed a little easier with a 4 blader anyway????????

SeaZar

[Jango]

Sea Zar, you need MORE blade area which only can be obtained by a Larger Dia 3 Blade or going to a 4 Blade. I,m a little supprised that RPM dropped from 5000 to 3800 with Cupping ONLY. Make sure your Boat Loading is the same as originally tested. Stay with the same 19 Pitch unless RPM DROPS below 4000 with no load. (Minimal fuel and driver only) Otherwise you may need to drop down to a 4 blade 17 pitch.
With the 4 Blade, you probably don't need max. Cup.
Jango

[Sea Zar]

Sorry Jango I typed the wrong RPM. It dropped only 200 rpm. 4800rpm & not 3800rpm.....sorry about that.
Would do you think?

SeaZar

[Jango]

That's what I would expect.
Go with a 14 x 19, 4 blade - better hole shot, acceleration. WOT should be between 4400 - 4600 rpm depending on Cup.
Also Higher top speed and cruise.

[Sea Zar]

Jango. Thanks for your help & patience with nutting out this issue.
One of the big issues that I'm sure that alot of readerswould experience is that you get stuck with buying a second hand prop. So making a correct decision with which pitch etc can be a stab in the dark. The only way to exchange props (with some companies like Solas) is to buy a new one. But geez that can get expensive.
Even though we speculate with theory, we end up with a better chance in choosing the right prop than taking a chance with a "guestimate"

Thanks again & I'll post again once I find a prop.
Cheers
SeaZar

[Jango]

Sea Zar, Happy to help out.
Be sure to try and stay with a 14 dia. or perhaps 14 1/2 max. A larger dia (15+) will require a smaller pitch.
Let us know how it works out.
Jango

[Sea Zar]

OK
I've decided not to muck around & have decided to purchase a new prop which I will pick up on wednesday. I've decided this because I think that I really need the option of exchange.

I'm going with a 'Power Tech offshore series' Model OFS4R. Any pro's & con's on this prop are very welcome please.

I have given them all the specs & they have decided that the best option in my case (slippage, deep 'v'..etc) would be a 4 blade, 15.25" diameter, 16" pitch model. A moderate high rake & full tip... which they recommend for offshore intentions. The say that I won't beleive the difference. They also recommended I take on this new rubber hub that apparently never needs replacing. Don't know much about that. I'm still trying to find it on their site.

I would have thought a 17p would be more appropriate but we will see when I take it out for a run.

Seazar

[Sea Zar]

Picked up prop Monday & took it for a run. Though the control & handling was superb the pitch was way off. I lost 5 knots all round & engine maxed out in revs.
I exchanged it for an 18" pitch.That brought the revs down at WOT to 4950rpm. Cruise speed is 25knt at 3000rpm. Still not quite what I want. I'm trying to acheive 30knt at 3000rpm & WOT at 4500 to 4600rpm. So they have decided to up the pitch to 21. It's been ordered & should arrive next week from the U.S hopefully.

I would have thought that the equivalent pitch (19") to the 3 blade would have been correct- like we discussed here. I'm very surprised that it is going to manage 21" in pitch?! Any ideas why?

SeaZar

[Jango]

Sea Zar,

If the blade area remains the same, (Dia) you will drop 300 RPM for EACH inch of pitch added.(with a 1.50:1 gear). If it doesn't drop 900 Rpm, 3" x 300 RPM it is because the blade area is smaller.
Quite often Prop manfacturers DECREASE Dia slightly as Pitch increases. Smaller Dia will offset some RPM drop, but with added slip. (decreased effeciency)
Good luck, you're probably getting close.