Do I need a new prop?

[fotis64]

Hi there

I own a 12 ft rib (inflatable rigid boat), 200 lbs net weight, 480 lbs gross with outboard, fuel, cockpit, seat etc. Usually two adults on board (2 x 180 lbs). Thus, total weight with passengers is about 840 lbs.

The outboard is a new Yamaha F15C, 4 strokes, with power tilt and remote control (5000-6000 full throttle operating range).
Currently using a Yamaha 3 Blade Alloy propeller 9 1/4 x 9 pitch (I have changed the 10 pitch that came with the outboard because it was over-prop for the boat).
All the weight is back (driver seat and fuel tank).
The boat gets on plane with two adults on board but never with three. (If the two adults are sitting on the driver’s seat I have to stand up and make a small step forward in order to get on plane. If one of us is sitting in front everything is OK, no need to step forward.)

With two adults the numbers are as follow:
RPM @ WOT: 5600 and the top speed just below 19 mph.
At 5400 rpm, 17 mph
At 5000 rpm, 15 mph
If I drop from 5000 rpm the boat loose planning and the speed falls to 8 mph. So, my useable range is just between 5000-5600 rpm.

With three adults: the max rpm are 4800 and no planning.

With just me the max rpm are 5800 and the top speed 21 mph.

I tried a solas 8” 4 blend but there was not any improvement.

1. I would like to get on plane with no difficulty and also to get to plane with three people on board. Should I replave propeller? Do you suggest 8 or 7 pitch? Should I try a stainless one?
2. My propeller is mentioned as J1 hub. Is there a difference between J1 and J hub? This is crucial because the shorter J1 propeller is 9 pitch. The available propellers with shorter pitch are with type J hub.
3. Should I replace propeller anyway? What about if I decide finally to travel only with two adults on board? In that case I should remain at 9”?

Looking forward to your advice. Thanks in advance!

 

[alldodge]

I think you need more HP or less weight.

Sounds like your normal load is with two folks and planning between 5 and 6K rpm. If you reduce the pitch your rpm's go up but now your going slower and your rpm's need to go up higher. While the 8 pitch will help with three folks, I'm not so sure you can get it on top the water.

WOT Prop slips is around 15 percent with one, 20 percent with two and 60 percent with three. Reduce the rpm to 5K and two folks your at 29 percent. The boat does best running on top.

15 HP 4-stroke 113 pounds
15 HP 2-stroke 81 pounds

The 2-stroke is a bit lighter and since it hangs off the stern the 32 pounds may be enough to get it up. Otherwise maybe a 20 HP which current shows up at 111 pounds

 

[fotis64]

Thank you AllDodge

I can not replace the outboard engine (at least for 3-4 years ahead... )
So, your suggestion is to run the engine at 5000 rpm or at 5600 for cruising with two folks?

.

 

[alldodge]

So, your suggestion is to run the engine at 5000 rpm or at 5600 for cruising with two folks?
.

From the numbers you have provided, I'm seeing that your current setup provides best overall performance with no more then 2 folks. The prop slips reduces as the RPM goes above 5K, so 5600 is where your getting best gas fuel mileage and performance, and so long as you don't go below 5K your still doing ok. You just need more HP to move more weight IMO.

Would be interesting to use a 8 pitch to see what it does with 3 folks, but I think it may not even get on plane with it

 

[fotis64]

If it is not getting on plane with three folks anyway, should I better use the initial prop (10" pitch) which will go up may be max to 5400 rpm WOT?
The top speed will be better than the current and the rpms lower.

 

[alldodge]

If it is not getting on plane with three folks anyway, should I better use the initial prop (10" pitch) which will go up may be max to 5400 rpm WOT?
The top speed will be better than the current and the rpms lower.

Actually no, the 10 pitch slows the rpm down more but causes the motor to work harder. Using a higher pitch prop does not increase speed, in most cases it reduces the total speed. Since you have the prop you can prove it out. Use the 9, then change and use the 10 over the same course.

 

[fotis64]

Thanks again!

The 5600 rpms WOT are enough or it would be better to go up to 6000 in terms of engine performance and engine life?

And what about a 7" pitch prop? Should I try this for three folks on board?

 

[Sea Rider]

Before venturing maximizing prop pitches need to know if OB sits at the Sweet OB/Transom height, doesn't produce water splashes out or over transom or prop aeration with current OB. To determine :

-Inflate Rib to its factory recommended working pressure, usually 3-3.5 PSI with gauge.

-Assume it's a tiller driven combo, right ? if so, sit a boating mate up front and driver aft, trim OB to sit at 90 deg, usually 2-3 holes out transom.

Install the prop that delivers max wot rpm with 2 up, if between middle to max wot rpm range much better, third boater can stay at shore zipping cold ones.

Go for a wot spin on flat calm no wind water cond as early in the morning.

Pull head out transom and check at which lower leg height is water flow passing at plane, could be over or under upper splash plate, middle or above lower cav plate. All 3 positions gives different blade bites (thrust), need to dial the best OB height to achieve that cond.

Go for the test and report your findings, can work from there..

Happy Boating

 

[alldodge]

The 8 pitch might bring your rpm 6000 rpm, and if your slip stays at 30 percent you could reach 15 mph, but that means dropping your slip in half. The issue is I see your slip staying around the 45 to 50 percent area because you need more HP. Changing to a 7 pitch gets you in the 13 mph and you could not stay on plane with just two at that speed

 

[QBhoy]

Hi.
Having had a similar sized rib with both a 2 stroke 10hp mariner and a 9.9hp 4 stroke I can perhaps advise on a few things.
I found that there a a few problems with this kind of set up;
Firstly is the short length of the boat. It makes it more difficult to plane than normal, especially where the drivers position is, but also to do with the relatively short area of hull in the water. With a normal larger boat with small power, you have the luxury of leaning forward to help the boat plane, but I found that this had a detrimental effect with the wee rib. It meant that you are actually putting more of the boat in the water and tends to plough heavily at the front.

Secondly, I can tell you that the 4 stroke just couldn?t compare to the 2 stroke at giving the boat enough of a kick in the bum to get up on the plane. To do with the much lighter weight and the torque difference.

I also found that the key is between a careful balance of weight and the right prop. To explain...try not to be tempted to throw too much weight forward like your instinct would tell you...have the passengers try and keep their weight just forward of mid ships and sitting on the tubes leaning inward to the centre whilst getting over the hump. Try and keep the weight over the fibre glass hul and stop the tubes dragging too much.
It a fine balance with the prop. Especially with your 4 stroke. You need a low enough pitch to give you the initial bow rise, but not too low that it will have you screaming over the limit of rpm when you do get up.

As a last resort...and I wouldn?t normally recommend this with any other application, but fitting the doelfins planing fins to the engine would likely help.

Going for a bigger engine isn?t really going to help. The heavy 4 stroke is bad enough on the back of such a wee boat.

I know some aren?t a fan, but ultimately a late model 2 stroke of the same power would easily solve this problem !

 

[QBhoy]

Also. Not sure of the Props on each of the engines I had (sold now), but I can tell you that on the boat I had which was a 3.2m valiant rib with jockey seat and console with remotes...the 10hp 2 stroke achieved 23 point something mph gps 1 up and was capable of towing a teenager on a knee board and inflatable.
The 4 stroke managed 20mph and was not able to get on the plane towing an inflatable with one in the boat.

 

[Sea Rider]

Can you post a nice pic of your combo to avoid speculation and address the issue straight forward..

Yams usually revs 4.5-5.5 wot revs. Double check to be sure before playing with props pitches.

A 3.65 Mtr rib with a 15 HP is more efficient than a 3.20 Mtr Rib, hull drags much less being larger. Does side tubes floats on water, or ride above water ? Poorly inflated tubes will drag more and slide less efficiently on water compared to second escenario.

If assuming OB revs at 6K at wot, Could prop right OB to rev at 6 K with 2 up, when with 3 up wot revs should theoretically descend to middle 5.5 K which still is good. How heavy is third boater ?

You cal always re pitch props in up/down 0.5 sizes at a prop shop for fine tuning less or slight more wot rpm compared to what could be achieved playing with whole1 pitch sizes.

Doel fins will kill the set up, it's a matter of correct weight distribution throughout the rib, correct tube inflation, OB height and trim along a maximized pitch prop. Once you dial this 5 ingredient tech cocktail spot on will start to have fun and enjoy your combo.

Happy Boating

 

[fotis64]

No Title

Two photos of a similar boat with 30 hp Mercury and five photos of my setup.

 

[Sea Rider]

We have similar rigged Ribs, the only difference is that yours is a 3.65 Mtr center console Rib and mine a larger 450 tiller driven Rib. Been through extreme experimentation regarding Opt OB/Transom Height and maximizing prop pitch for OB to run at max wot range with 2 up which is the usual number of boaters, but can carry a third boater revving around middle wot range which is still good with a very underpowered 2 strokes 18 HP for that lenght Rib.

If wanting to get the max out of your nice combo need to know firstly what's going on at back transom, ideal is when water flow at plane skims right under small upper splash plate as in example, at that lower leg height prop is obtaining its best thrust while combo rides parallel to water level. Check example..

https://www.youtube.com/watch?v=oBeZjaA76VM

What you can do is sit an adult boater at bow seat while you drive, if with manual trim, trim OB to sit at 90 deg (perpendicular to water level), on flat calm water cond, have someone else follow your rib from behind with other boat, ask him to check at which lower leg height is water flow passing by. Only then will know if need to raise or lower OB on transom to match given YT example. From there can go for a prop maximization.

After dialing spot on OB/transom height optimization went through a extensive prop experimentation and maximization, now combo runs as a charm with 2 up. Check how wot rpm changes dramatically on a light watercraft going just one pitch down from a 9 factory delivered prop to 8 and 8.5 props respectively. Click lower link, check Post 140 and 144.

http://forums.iboats.com/forum/gener...i-swear/page10

Perform the test with niner pitch prop as suggested, report your findings...

Happy Boating

 

[fotis64]

No Title

Back transom.
You can see that I have raised the OB as far it could be.
Moreover, a new plate with angle is used in order to have better angle to get at plane easier.
Since I have power trim, I adjust trimming angle when at plane. Best results in terms of speed when parallel to sea level or just a click up.

 

[Sea Rider]

The new plate with angle is the one that's placed on transom's top, right ? Don't know if have made the test to determine at which lower leg is water flow passing by, only achievable by visual inspection. Anyway, this is the Sweet OB/Transom Height match for any Sib/Rib, as in pic. Try to dial OB height for water flow at plane to pass right under yellow line. Prop will have the best thrust ever while OB rides at 90? with excellent water bed height.



Will assure you that after a spot on prop maximization for OB to rev to their max wot range, you won't need to play with trim as the Rib will begin to plane by its own once OB revs up provided that deck weight, passengers are well distributed. Mine is set immovable at 90, can sit 2 boaters at rear seat and Rib will plane near instantly without the need to step forward or move weight up front.

Happy Boating

 

[fotis64]

No Title

The newer plate with angle is this one:

 

[fotis64]

I have not made the test to determine at which lower leg is water flow passing by, since it is not easy to move up or down the OB (screws... )
I think that it is OK as it is now because in the Yamaha's user guide is being mentioned that the lower splash plate should be 0"-1" lower than the lower part of the boat. Now it is exactly between 0 and 1".

I made new tests with two people on board.
One boarder must be in front in order to plane. Then, the max rpm are about 5600 and the max speed 19 mph.
BUT: If the above boarder moves back at rear seat (when the boat is allready at plane) the rpms are going up to 5800 and the speed up to 21 mph! The only drawback is that the bow moves up - down more than before.

I do not know yet if I should buy a 1" shorter pitch prop in order to get to plane without the need to have a boarder in front and in order to plane with three-four people on board. I do not know if the speed will be enough to get to plane.

 

[Sea Rider]

To determine where water flow is passing at speed doesn't mean you have to move OB upwards or downwards, the idea is to just have a look. What stated on Owners Manuals is relative, there are 2 completely different scenarios, a dry installation and a wet one, that's when combo planes at water level. It's normal for OB to gain slight more wot revs when 2 souls sits way aft, due to less hull drag, the only down issue is that need to throttle more for bow not to rock once on plane, although its fun ride, will consume more fuel dealing with that issue.

Going one less pitch would be too much rpm when 2 up, a 0.5 re pitch at a prop shop will suffice. If with 4 souls often then go for one less pitch, just throttle back when with 2 as OB will certainly over rev.

Happy Boating

 

[fotis64]

Very helpfool!
Many thanks!
I will check.