Horsepower

[Rawai]

Another wondering I have about the boat Im going to order is size of engine.
22 feet, about 1700 lbs net weight, 21° deadrise. Standard is 200hp max but they can build it for 250hp. I had a lot of boats before but never owned a rib, I want it to be fast but not dangerous fast. Would be nice to be able to cruise in 45-50 mph without going full throttle. You think 200hp will be enough?

[treerat]

I run a 135 on a 6m which is 20' more or less and happily cruise at 30knts / 35mph.
Top end on the current prop 36 knts / 42mph. My maximum rating is 150 hp so a 200 on a 6.6m will be entertaining.

Its as much to do with hull shape, chines, transom design, weight as it is size of engine but to answer your question - yes, 200hp should achieve what you're looking for with ease.

[Kristoff]

A 50mph cruise is fast for a rib. It becomes an aerodynamic issue as much as weight and deadrise. The front tubes push a lot of air.

Most ribs with correctly spec'd motors can cruise at around 35mph. A 40mph cruise is pushing it.

Any rib that can cruise at 50mph is a fast rib, unusually fast I'd say. There's only one boat in my yard that could do that, and it is a 7m Brig with 300hp. I would think you'd need all of 250hp to have a 50mph cruise on any 6m+ rib. But I'm only guessing based on my personal experience. I suppose it depends what revs you define as cruise. Having it sat on the rev limiter isn't my idea of a cruise.

[Rawai]

Originally Posted by Kristoff

A 50mph cruise is fast for a rib. It becomes an aerodynamic issue as much as weight and deadrise. The front tubes push a lot of air.

Most ribs with correctly spec'd motors can cruise at around 35mph. A 40mph cruise is pushing it.

Any rib that can cruise at 50mph is a fast rib, unusually fast I'd say. There's only one boat in my yard that could do that, and it is a 7m Brig with 300hp. I would think you'd need all of 250hp to have a 50mph cruise on any 6m+ rib. But I'm only guessing based on my personal experience. I suppose it depends what revs you define as cruise. Having it sat on the rev limiter isn't my idea of a cruise.

cruise might be streching it but on calm days when water is like an mirror it can be fun to speed up a bit and do around 45mph for 20 min or so (one island to antoher) and I don´t like to push engine with full throttle for that long. I want atlest 50-52mph as topspeed but that would be enough I think.
My gut says 200hp might not be enough for that, but in the same time I feel that the weight of only 1700 lbs net weight for the boat is light for 200hp, so if hulldesign is decent it would be enough but as I said I`d never owned or even drove a rib before. My last boat was a 28 feet CC in fiberglass, netweight 4500 lbs and it topped at 45mph with 250hp and not to much fuel in the tank.

[Kristoff]

Originally Posted by Rawai

cruise might be streching it but on calm days when water is like an mirror it can be fun to speed up a bit and do around 45mph for 20 min or so (one island to antoher) and I don´t like to push engine with full throttle for that long. I want atlest 50-52mph as topspeed but that would be enough I think.
My gut says 200hp might not be enough for that, but in the same time I feel that the weight of only 1700 lbs net weight for the boat is light for 200hp, so if hulldesign is decent it would be enough but as I said I`d never owned or even drove a rib before. My last boat was a 28 feet CC in fiberglass, netweight 4500 lbs and it topped at 45mph with 250hp and not to much fuel in the tank.

You won't know for sure until you try, but I suspect 50mph will be doable, just, with 200hp. My boat also happens to weigh 1700lbs (plus engine, plus kit) and with 135hp I'm seeing 40mph flat out give or take 1 or 2 mph either way for wind and tide. I recon I'd need 200hp to see a genuine 50 mph.

As I said, its more an aero thing once you get above 40mph or so. Ribs take more shove to get to 50mph than fiberglass boats. My last Fiberglass boat would nudge 60mph with its 190hp mercruiser, when propped for speed. But in any sort of sea at 30mph it wouldn't see which way my rib went after a few minutes.

[Peter_C]

Just go max HP. You sure won't ever regret it

[Matt]

Originally Posted by Peter_C

Just go max HP. You sure won't ever regret it

I completely agree with Peter - as long as the engine isn't "too" heavy. It all comes down to balance.

What you want is a boat that has enough hp so you can run it at your target cruise speed without having to have it fully trimmed out & aired out. This tends to mean the boat will maintain a nice neutral pitch going up and down waves, doesn't go bow high, slamming etc quite so much. AND also making sure the engine isn't too heavy either - since moving the weight to the stern usually helps with top speed, but also can reduce stability & usually isn't ideal at slower speeds.

For example, my rib would max out at 43 knots, but I would have to be properly "driving" it to maintain 40+ knots as it was all a bit maxxed out. So I tended to cruise it at 25-30 knots which was nice and relaxed.

My other boat goes a bit quicker - and will comfortably cruise at 40 knots and be nice and relaxed.

What I would say as a general bit of advice is to go with the max hp within a given engine class.
So if you're looking at the Merc V6 class of motors, I'd try and go for the 225.

If you're in the V8 class (more weight on the transom though - make sure you are OK with that, it changes the characteristics of the boat), then I'd try and go for the 300 over the 250.

In all cases, I'd listen to the boatbuilder, they'll know what the hull responds well to. And ultimately, a lot of it comes down to budget and availability, just because an engine is listed doesn't mean there isn't a long lead time to get one.

[Rawai]

Originally Posted by Matt

I completely agree with Peter - as long as the engine isn't "too" heavy. It all comes down to balance.

What you want is a boat that has enough hp so you can run it at your target cruise speed without having to have it fully trimmed out & aired out. This tends to mean the boat will maintain a nice neutral pitch going up and down waves, doesn't go bow high, slamming etc quite so much. AND also making sure the engine isn't too heavy either - since moving the weight to the stern usually helps with top speed, but also can reduce stability & usually isn't ideal at slower speeds.

For example, my rib would max out at 43 knots, but I would have to be properly "driving" it to maintain 40+ knots as it was all a bit maxxed out. So I tended to cruise it at 25-30 knots which was nice and relaxed.

My other boat goes a bit quicker - and will comfortably cruise at 40 knots and be nice and relaxed.

What I would say as a general bit of advice is to go with the max hp within a given engine class.
So if you're looking at the Merc V6 class of motors, I'd try and go for the 225.

If you're in the V8 class (more weight on the transom though - make sure you are OK with that, it changes the characteristics of the boat), then I'd try and go for the 300 over the 250.

In all cases, I'd listen to the boatbuilder, they'll know what the hull responds well to. And ultimately, a lot of it comes down to budget and availability, just because an engine is listed doesn't mean there isn't a long lead time to get one.

Weight is what concerns me with a bigger engine. I was consider going for Suzuki but it seems that Merc 250 is the same weight as Suzuki 200. But after some googleing it seams that Merc has more issues and Suziki is cheaper and have a service center close to my house while Merc service center is 15 miles away. Manufatorer says 45 knots with 200hp so I´ll think I go for a Suzuki 200 but I will probably regret I didn`t go for a 250 when it´s to late. If it where a fiberglass CC I would def go for the bigger engine but what you guyes says about aero issues with a rib concerns me a bit, I like speed but don´t want it to be dangerous.

[Matt]

One final note. On a 6.7m rib, I reckon 200hp will be plenty enough and if going from 200 to 250 is a heavier engine (V6 to V8, say), then I suspect it might negatively impact usability.

Some of the older guys here may remember "Leviathan", a 28' Phantom.
There was another one, "Wentworth Auto & Marine".

They were both identical hulls, twin engine, bravo outdrives and in the early days, same horsepower, 600ish total IIRC.
The difference was Leviathan had much heavier engines. It made a huge difference in how it handled.

Wentworth Auto & Marine ran amazingly, whereas leviathan really had to run WOT to start to settle down, even though they both had virtually the same top speed. Leviathan always wanted to run bow up, but Wentworth ran nice and flat.

Wentworth was so well balanced that he eventually upgraded the engines in it and I recall running 93mph in it up Southampton water (before the speed limit days). Roger (the owner) was driving it one handed, it was that that balanced. I've never been in a boat like it.

And in the rough stuff, Wentworth's neutral balance meant it could keep going, whereas Leviathan required much more commitment to keep the bow down.

Horse for Courses.

[Matt]

Originally Posted by Rawai

Weight is what concerns me with a bigger engine. I was consider going for Suzuki but it seems that Merc 250 is the same weight as Suzuki 200. But after some googleing it seams that Merc has more issues and Suziki is cheaper and have a service center close to my house while Merc service center is 15 miles away. Manufatorer says 45 knots with 200hp so I´ll think I go for a Suzuki 200 but I will probably regret I didn`t go for a 250 when it´s to late. If it where a fiberglass CC I would def go for the bigger engine but what you guyes says about aero issues with a rib concerns me a bit, I like speed but don´t want it to be dangerous.

Local service and spares availability is a key point. I have a merc & tohatsu dealer round the corner from me, so even though I do all my own work, I have easy spares availability.

[Rawai]

Originally Posted by Matt

One final note. On a 6.7m rib, I reckon 200hp will be plenty enough and if going from 200 to 250 is a heavier engine (V6 to V8, say), then I suspect it might negatively impact usability.

Some of the older guys here may remember "Leviathan", a 28' Phantom.
There was another one, "Wentworth Auto & Marine".

They were both identical hulls, twin engine, bravo outdrives and in the early days, same horsepower, 600ish total IIRC.
The difference was Leviathan had much heavier engines. It made a huge difference in how it handled.

Wentworth Auto & Marine ran amazingly, whereas leviathan really had to run WOT to start to settle down, even though they both had virtually the same top speed. Leviathan always wanted to run bow up, but Wentworth ran nice and flat.

Wentworth was so well balanced that he eventually upgraded the engines in it and I recall running 93mph in it up Southampton water (before the speed limit days). Roger (the owner) was driving it one handed, it was that that balanced. I've never been in a boat like it.

And in the rough stuff, Wentworth's neutral balance meant it could keep going, whereas Leviathan required much more commitment to keep the bow down.

Horse for Courses.

Suki 200 is an inline 4, weight is 531 lbs and that is pretty much same as Merc 250. I would def go for the Merc if it wasnt for bad reviews vs Suki and distance to service center is deff a point.

[Kristoff]

There's nothing dangerous about going fast in a rib, you just need more power to do it due to the fact they have the aerodynamics of a large sofa, compared to a fiberglass boat that's shaped like a dart.

[Rawai]

Originally Posted by Kristoff

There's nothing dangerous about going fast in a rib, you just need more power to do it due to the fact they have the aerodynamics of a large sofa, compared to a fiberglass boat that's shaped like a dart.

Ah ok, good info
I thought rib where faster due to lower weight.
How is handling in rough sea compared to a fiberglass boat?

[Matt]

Outside of racing where an extra 0.5mph makes a difference and proper high performance, that's not quite right.

Majority of the drag on any high performance boat is hydrodynamic, aerodynamics play a very small part until you're going very fast. And worse, the hydrodynamic drag is not all lift based drag, ie the hull generating lift, because drag there is basically linear (wetted surface area and AoA reduces with speed), but the appendages - the gearcase. That's one of the reasons why raising the engine on the transom, subject to still being able to carry the bow enough, is such a common thing to tune a boat to go faster.

More of an issue on a rib is to make sure it's designed for the performance regime it operates in, they often don't have the hull design to support proper high performance - deep V, no pad, no steps etc and so, for example, might become susceptible to chinewalking or hooking. Naturally there are also many hulls designed for those speed regimes - in both ribs and hardboats and many of them also cruise nicely too. But I wouldn't want, for instance, an 80mph Humber, I reckon that'd be a right handful. But it'd make a great rescue or dive boat.

[Matt]

Originally Posted by Rawai

Ah ok, good info
I thought rib where faster due to lower weight.
How is handling in rough sea compared to a fiberglass boat?

At normal speeds, the toobs give you nice soft water deflector.

But at high speeds, the toobs don't really do anything - ie at 70mph, water pressure is about 70psi. Tube pressure is about 5psi - so the tubes just get the hell out of the way if you mess up. But then it starts to be more about "recovery" in the hull design.

So many variables, many of which are personal preference and driving style.

I like both, it all depends on what you're trying to do with the boat.

[Rawai]

As it´s pretty much open ocean among the paradise islands around Phuket I want the biggest speedboat possible with a max netweight of a 1000 kg so I can manage it on a trailer and park at my house, dont want it moored in the ocean due to crazy amount of work with barnacles etc.

I have no experiance of rib´s so it will be interesting to see how I feel for it ones I`ll get it. I am importing from China so it will also be interesting to see what I´ll get. Seams to be one of the better manufatorer over there and they actually have some companies producing high quality stuff, ex Huawei and Xiaomi. But China is china so we`ll see.

[Kristoff]

Originally Posted by Rawai

As it´s pretty much open ocean among the paradise islands around Phuket I want the biggest speedboat possible with a max netweight of a 1000 kg so I can manage it on a trailer and park at my house, dont want it moored in the ocean due to crazy amount of work with barnacles etc.

I have no experiance of rib´s so it will be interesting to see how I feel for it ones I`ll get it. I am importing from China so it will also be interesting to see what I´ll get. Seams to be one of the better manufatorer over there and they actually have some companies producing high quality stuff, ex Huawei and Xiaomi. But China is china so we`ll see.

It will feel very stable, it will feel very safe, it will feel very comfortable in a chop, it will also feel very windy and a tad slow. The fact it will feel slow is as much down to the ease at which it will get though through the chop as opposed to the 5 or 10mph less that you are traveling than what you are used to.

That's coming from a long time fiberglass boat owner. Would I go back to fiberglass? No, not for any boat that sits on a trailer anyway.

[jonp]

I drive a 6m boat with a 21 degree hull powered by a 250 mercury which scoots along nicely at almost 50kt. The boat is propped to carry load, with an average of around 1 ton of shellfish per day, 3 people, dive compressor and weights.

My little 4.3m centre console is powered by a Yam f115, again 21 degree hull heavy built small boat (commercial build). 38 kts with one person and light load .
I always power to the max recommended hp and have yet to find it to much in any boat. You don't have to use all the power at full rpm, its having that power ready at lower speeds in big seas where you need to sit on the back of big swells and navigate extremely rough conditions where power comes in handy.

Obviously the weight you are likely to carry will bare influence on the power you instal. The price between the 200, 225 and 250 are also big considerations. If the 200 is a fair amount cheaper, then go with that and prop it accordingly for the loads you most commonly run in the boat. If it's just a fe people touring will be different to carrying 8 people with scuba gear etc.