Prop Advice

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

 

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

 

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

 

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.

 

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

 

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

 

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.

 

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

 

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

 

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

 

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.

 

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.

 

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.

 

Its actually 40 knots 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