Propellor advise

[comitan240]

I own a 1986 Cabo Cuddycon 216 (21.6 feet) with a 2012 Honda 150. When I bought the boat it came with two propellors but they were both used when the boat was a fishing boat. I am using it for scuba diving and am carrying a lot more weight and have stern lift issues (6-8 scuba tanks, 2 adults, 2 coolers). Top end speed is not critical. When fully loaded I have to hit 22-24 MPH to get up on plane.

From what I have researched it sounds like a 4 blade would be better for me but I don;t know hoe to convert from a 3 to 4 blade properly. Every prop selector I have tried gives me a different answer.

I need to select a prop that gives me faster stern lift and hopefully improved mileage. Most of the time I am only running around 4000 rpm moving between dive sites. The boat is operated in La Paz, Mexico on the Baja at 0 elevation, saltwater and average 6 inch to 2 foot chop.

The stats for both props are : 1) 15/17p three blade. WOT only gets to 5000. max speed 32 MPH.

2). The second prop is a 15.25/15p WOT hots 6000. Max speed 28 mph.

WOT for my motor is recommended between 5000 and 6000 so each prop is one one end of the limit or the other.

Thank you for any advise you can provide.

 

[jimmbo]

I would verify that the tach is functioning correctly, 1000 rpm difference between 2" pitch change is not normal

 

[Sea Rider]

When achieved 6000 wot rpm with the 15 pitch prop, how loaded was the boat ? Was it loaded as per your usual skin diving load description ? If so, OB is at the very top of its max wot rpm range factory stated. For combo to perform better need to distribute deck weight evenly. Boating with variable loads on a constant basis is a serious wot rpm killer when going per top prop maximizations.

For a OB with ideal OB-Transom height matched, well trimmed along a healthy tach and prop it's quite possible to gain + 500 wot revs per one pitch decrease. The formula of just + 200 wot revs is very conservative and does not apply to all applications. In many prop maximizations done in different boats models with a healthy and accurate tachs, OB has achieved + 400 to 500 wot revs per one less pitch with 3 blade OEM props.

If possible test same with a different tach to check if achieved same mentioned wot rpm with both props and safely go from there.

Happy Boating

 

[comitan240]

The boat was loaded the same with both props and tested 4 days apart in similar water conditions. The tach is a Honda tach installed new when the engine was installed in 2012.

I am going out this afternoon on a friends boat with the same motor. I will take my tach and and switch it back and forth with his to compare them and will post the results later tonight.

Thanks for both of your inputs.

 

[floater212]

an aluminum prop will flex more than a stainless would, more power goes through than aluminum. when you find a good prop think s/s, i have a 14.25"x 19 pitch 5 blade s/s on my rinker 21' 350 mpi and it has great holeshot and good topend, the aluminums props i have are JUST for emergencies. if your marina has props you can try out, that would be a good way to find what you're in need of faster.

 

[Sea Rider]

The flexing issue which seems all blade props achieves when at hole shot, full wot revs is pure BS talk as many here thinks same. In reality is the prop shop that wants to sell you a more expensive SS prop. For that need to beat up the equivalent alum one.

Have consulted different alum and SS prop manufacturers and none of them stated such flexing thing. Assume than on very large HP engines the only available props are all SS ?

Prop right your engine with alum or SS props to rev at least middle to max wot rpm range factory stated as usually loaded to achieve an outstanding water performance compared to doing nothing.

Happy Boating

 

[jimmbo]

an aluminum prop will flex more than a stainless would,

Some people seem to think that AL props flex like a bird's wing flapping. The flex, if any, is about 1/200", insignificant. But it has been lapped up by the masses and has helped sell millions of SS props

 

[comitan240]

I tested my tach today and it reported the same RPM readings as the one that was mounted on the boat we took out. So I believe that eliminates any tach issues. Unfortunately here in La Paz Mexico where my boat is we do not have places with props that we can take out and test. In fact I will have to buy my prop in the U.S. and bring it down on my next trip.

To restate my position I am looking for better back end lift so I plane easier. Top end speed is not critical.

One fitem I left out on my first post is the estimated weight of my boat with 85 gals of fuel, scuba gear, coolers, motor , my girlfriend and I is most likely around 4300 LBS. The dry hull is 2800 lbs.

Thank you
Dave

 

[Faztbullet]

The older thin blade aluminum props DO FLEX !! The newer thick blades due to manufacturing process deflect very little. Mercury had a video of blades flex at service school years ago along detonation video with engine with clear cylinder head. Also due to way S/S props are made you can have 4 identical props and 1 or 2 will perform better than the other ones.

 

[ahicks]

I agree you can get an alum. prop to flex, but it's going to take a lot of power. I was reading somewhere where the author made that distinction, and said he thought props that are 21" of pitch or more were good candidates to be stainless. At those power levels, that makes sense here. When we are talking speeds of 30mph more or less, those are NOT speeds where you are flexing an alum. prop enough to make any sort of difference.

What makes this dive boat different, is the large amount of weight involved. Thinking here we could make a good case for stainless as well.

My experience with the smaller, 90hp or less, Honda's is that they like to run at about 5500rpm. Not knowing any better, I would use that for a target rpm. here. Knowing the 15p 3 blade prop turns up 6k, my best guess would be for a 4 blade 15p.

I'm pretty sure there are some larger prop dealers that will allow you to swap 4 blade stainless props if the one you select doesn't work out on the first try.

Geez, have fun with that rig. So jealous of your location and how you're using it!

 

[Sea Rider]

Good to read that tach is performing accurate. Recapitulating.

If with a 17 pitch OB achieved 5K wot rrpm and with a15 pitch OB achieved 6 K rpm, with both scenarios as currently loaded, then a 16 pitch same 3 blade alum model should dial around 5.5K wot rpm. If suggested pitch is no market available, take it to a prop shop and have it repitched to that size. Inquire if in need to add more cup too. Need to go that routet when only 2 or 1 full size prop pitches are available.

Some issues : If asumming max wot range factory stated for that OB is 5 - 6 K For any OB, ideal is to rev at least middle to max wot rpm range, that's 5500 to 6000 rpm.

With current 17 pitch it's already revving at the max wot range, with this prop achieves better combo
acceleration, be on its way faster along better hole shot with slight top end speed decrease.

With a 16 pitch shpould have slight top end speed increase, but will worsen best hole shot which I personally like for heavier set ups as in your particular case. Keep the 15 pitch and use it as a paper weight or have it repitched to a 16 too.

For me it's much faster going towards a correct prop maximization while maintaining same 3 blade alum or SS configuration and oly playing repitching 1 or half less pitch sizes for any given prop at a prop shop. Need to do that when only 2 or 1 size full prop pitches are market available. Very few go towards this nice alternative route.


Happy Boating

 

[comitan240]

I want to thank everyone for your responses. Your information has been very helpful. Unfortunately there are no props shops here locally so I will pick up a 16p when I am in the U.S. first week in October.

 

[ahicks]

So you are giving up on the idea of getting some "lift" with your prop? The 4 blade is your plan there, and it will have slightly more drag than a 3 blade of the same pitch.

 

[Sea Rider]

Going blindly for a 4 blade prop without a starter one to go from there will take more time testing and money spent in at least 2 props (a starter one and usually a second fine tuned one) than going per nex 16 pitch 3 blade one.

OP already knows the max wot rpm range achieved with both mentioned 3 blade props boating while hevily loaded.

Can take the max out of his 150 along best prop thrust if happens to be seated at the sweet engine-transom height spot, a matter to check...

Happy Boating

 

[Sea Rider]

Hey Comitan,

Here's your best deal on a alum 3 blade 16 pitch prop at Iboats. Check if needs a hub and if it's sold separately ?

Quicksilver Black Diamond 16" x 16" pitch Standard Rotation 3 Blade Aluminum Boat Propeller

Happy Boating