100 hp 4,6 m rib - low top speed

[Mauro Brisola]

Sorry for the long post for a simple question. I do appreciate hints from the ribs experts.
It’s the second rib with same hull I have. Zefir G460 (4,6 meters - made in Brazil). The first one had 2014 Mercury 60 hp 4 stroke engine.
As the place I boat are usually flat calm large bay areas I always dreamed with the maximum engine rating for this rib: 90 hp. Because I wanted higher top speed.
I choose the Mercury 2.1 liter 4 cylinder 4 stroke 100 hp Command Thrust (2.38:1) when I bought a new G460 in 2019. Propeller is 15 x 17P (larger gear case/propeller - CT engine version) 163 kg engine
My expectations with the new 100 hp setup considering the prior experience with the 60 hp engine: a very good top speed considering the 66% of power increase and the lightness of the boat (Manufacturer claims just 200 kg – hard to believe. But even if it is 250 kg, is very light for the 100 hp.
But the result are different. Despite very good hole shot, top speed did not improve so much. Even full loaded with five people and full 60 liters midship under center console fuel tank it goes to plane very fast. Lots of thrust, much better than with the 60 hp.
But, in other hand, top speed improvement was marginal.
Before the numbers I’ll advance my theory about it: probably the hull has too little buoyancy. With the 50 kg of the engine weight increase, from de 60 to 100 hp, stern went heavy and buried. Causing a lot of drag of the rear of tubes in high speed. Not enough stern lift underway.
Engine height could be raised. Anti ventilation plate is now flush will keel. Two holes rising could be done. But my concern is that raising engine maybe could improve speed/top rpm but will not solve the buried stern/tubes drag.
I use hydrofoil. Used in the other boat with same hull too. Because this hull is prone to porpoising even with zero trim. Stingray Classic hydrofoil solves porpoising tendency.
While cruising or in top speed it is not possible to see ventilation plate or hydrofoil. They are too deep in the water! Despite hull keel is flush with ventilation plate. Engine trimming is limited. Will help speed increase just up to a small angle.
The top speed is about 30 knots/5500 rpm with just myself (featherweight 57 kg) in the boat. With a 90 kg guy in the front bow seat, helping the stern lift, it goes to 32 knots/5650 rpm. Proving the theory of the buried stern. Too slow for the power. Looks like hull has a semi displacement behavior. Planes easily but kind of stalls after some speed.
Returning to my dreams, I would expect this 100 hp engine over-revving the 6000 rpm engine limit with the 17 pitch prop, and using a 19P to reach at least about 38 – 40 knots at about 5800 rpm. There’s lots of examples of much heavier 5 – 5,5 meters ribs with better performance using 80 to 100 hp engines.

Beyond the engine rising, I have two ideas to try to help decreasing this problem:
1- Would trim tabs help? Will the drag of them to raise the stern compensate any decrease of the tubes water drag? Will them have effect to increase “effective hull length”?
2- Could make and adapt hull flotation pods (see picture example). Hard work and not cheap. Not sure about the possible results too. Like the function of boarding step.

Videos of boat underway: light blue tube band = with 60 hp dark band = 100 hp. The last picture is a flotation pod example.

https://youtu.be/a1hZ0RHkTrE

https://youtu.be/f7seUNt0W98

[Limecc]

You should optimise engine height and then reassess the prop.

You can't do anything about the extra weight, it's always going to sit lower at the stern unless it has a lighter engine. Your short hull simply can't provide the flotation, I'm suprised it has the engine rating it has.

Raising the engine will reduce the drag and improve performance providing it's not too high that it introduces ventilation.

I have the same length boat, probably more wetted area because it's hard sided (doesn't have rear sponsons) and it's rated at 120kg max engine, 300kg hull weight. The Doel fin works well to lift the stern at displacement and doesn't introduce negative handling reported by others. I started with a level AV plate like you and have recently found the perfect height by raising it two holes (40mm) and performance is much improved. DF50 is only 50hp/109kg but gives just 1kn less top speed than you, solo and running a 14p four blade Solas. Before I raised the engine it was so deep it was impossible to fully trim out due to water flow above the hydrofoil.

[jonp]

I fully understand your disappointment and like you quite surprised by those figure, I would also of expected those other figures that you expected to get.

Yes raising the engine might improve things slightly but as you would already know that's only marginally.

Are you fully sure you have full adjustment in both the throttle leaver and throttle cable as something just doesn't seem right at all. Also is the engine fully run in yet, many 4 strokes seem to start performing at best after the first 100hr service (Ive found) but even so that's only marginally improved, not 10kts.

My daughter has a 2014 zodiac pro 500 with a merc 60 4 stroke. Like yours it has 60llt + spare 20lt underfloor fuel tank, heavy duty battery and anchor etc. Same big gearbox yet she gets 33kts flat out and 29 with 5 friends onboard. She has a mercury 3 blade stainless 14 pitch, which doesn't make sense when you put those speeds into a prop calculator. The boat I just sold had a 140 4 stroke suzuki (5.2m glass boat) which had a 21 pitch prop but was slightly slower than my daughters rib (to my annoyance!!). I now have a 14ft glass boat with a 22 degree deadrise hull and after much prop changing Im now getting 33kts from the Honda 90 (same as my daughters 60hp).

When you are up and running full throttle do the rear tubes drag water much ?

One of my mates upgraded to the 115 merc on his 5.5m Chinese rib which did well over 40knots but unfortunately the transom split, tubes pulled away from the boat and the hull became covered in stress fractures.

Are you able to try a 17 or 19 pitch prop from anyone ? If you can and the boat still reaches that same 5650rpm you will then know its either throttle travel or cable as the rpm should drop from increased pitch.

[Limecc]

Originally Posted by jonp

Yes raising the engine might improve things slightly but as you would already know that's only marginally.

Are you fully sure you have full adjustment in both the throttle leaver and throttle cable as something just doesn't seem right at all.

I have to disagree from own experience that raising the engine will only improve things marginally. Good point about the throttle travel.

Like a flap on an aircraft wing, trimming up with water flow on top of a hydrofoil will try and bury the stern. Such force the hydraulics couldn't raise the leg beyond a certain point when on the plane. Now the height is optimised it's mainly aerated water above the fin and I have full travel in the tilt ram.

When the engine height is optimised, only now do you start looking at prop selection, and then trim tabs.

[Matt]

It's all setup specific - it can be a game changer (I can name a large raceboat that needs to raise the drives a bit!!), or make little difference - all part of the "system optimisation"

[jonp]

Hydrofoils here in Aus can effect engine warranties on new engines, which I really cant be bothered to explain why as those who like them always come back with their arguments.

On a normal hull without safety wheels (tubes) foils can be downright dangerous in rough conditions, hence why you don't see people using them in area's like where Im from with dangerous bar crossings and big ocean swells and constant daily sea breezes from hot land temps drawing cooler air from the ocean.

[jonp]

Originally Posted by Limecc

I have to disagree from own experience that raising the engine will only improve things marginally. Good point about the throttle travel.

Like a flap on an aircraft wing, trimming up with water flow on top of a hydrofoil will try and bury the stern. Such force the hydraulics couldn't raise the leg beyond a certain point when on the plane. Now the height is optimised it's mainly aerated water above the fin and I have full travel in the tilt ram.

When the engine height is optimised, only then do you start looking at prop selection, and then trim tabs.

What sort of improvement have you seen from adjusting engine height to be more than marginal. When I say marginal I would say that almost every boat Ive bought I have moved my engines 1 or 2 holes up. This has improved rpm from 50-150rpm and freed up 1 to 2 knots maximum on engines in the 100-150hp for me.

Back to Mauro, moving your engine up or down by yourself is a pretty easy job if you have a trailer and jockey wheel.

[Mauro Brisola]

Thanks for the answers

In fact I assume that the 90 hp rating for this hull maybe could be related to the 90 2 strokes. Sold in Brazil until about 2016. In this case weight was about the same 120 kg of the 60 hp 4 stroke I used before. The 50 plus kg of the 4 stroke I'm using now is really heavy for such small hull

I'll start by raising the engine. It will take some time because I'll be traveling next two weeks. As soon the change is made I'll post results here.

[dhaglund]

Even if the engine may be a bit overweight I don't think that should affect performance that much when on plane. And the added horsepower should more than make up for its weight.

The hydrofoil will slow you down because of added drag all the time. I would try without that. Perhaps smart tabs or real tabs would work better in this case? Getting the engine at the correct height is important (but it may be there already)?

The right propeller can give you more stern lift. Typically a 4-blade. That may also allow you to lift the engine a little because of added grip with 4 blades.

[Limecc]

Originally Posted by dhaglund

The hydrofoil will slow you down because of added drag all the time. I would try without that.

Mine's only 1kn less with half the horsepower. Same size hull, same load, same hydrofoil.

Originally Posted by dhaglund

The right propeller can give you more stern lift. Typically a 4-blade.

Yes but it's the shape of the thrust what matters and this comes in the design. Like a hydrofoil, has more effect at displacement and semi-displacement than flat out.

[dhaglund]

I watched you videos now. It is really hard to say anything by looking at the second video. You had so many people in the boat and who knows how that particular boat handles that. It looks like you just need to go faster.

But something is really off with this setup. I had a very light 6 meter RIB, 350 kg. It had a 115 hp Mercury Optimax with a 3-blade 22P stainless propeller and that was just fine giving me a top speed of 47 knots.

Given that (I think) your engine is geared higher than the Optimax and your boat is lighter with a little less hp you should have a 22-pitch propeller too. Or something in that vicinity at least.

[Limecc]

Originally Posted by dhaglund

Given that (I think) your engine is geared higher than the Optimax and your boat is lighter with a little less hp you should have a 22-pitch propeller too. Or something in that vicinity at least.

Whatever. The prop choice may change when the engine height is optimised. There's an order of progression.

[Dankoozy]

Originally Posted by jonp

One of my mates upgraded to the 115 merc on his 5.5m Chinese rib which did well over 40knots but unfortunately the transom split, tubes pulled away from the boat and the hull became covered in stress fractures.

Sh1t, dawg. Was he alright? Did he lose the engine?

[Swordfish]

What was the conclusion on this Mauro? I have a 5 meter RIBCRAFT with a lightweight 90hp Yamaha two stroke. It was initially a little sluggish. I changed the 17" to a 19" and had the engine raised two holes. It feels much faster and better poised but I haven't checked top speed, I'm guessing over 40kts as it can keep up with some larger boats (much to their surprise) From my own experience I'm guessing you need to raise the engine and go with a 19" or more likely a 21" (given how small the boat is). As you raise the engine you may experience more tendency to cavitate on tight cornering at lower speeds. It's a bit of a trade off so be careful not to raise the engine too much.